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Haze Caravelle (barn find!)

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Category: Other Chat
Forum Name: Golden oldies
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URL: https://forum.speakerplans.com/forum_posts.asp?TID=102867
Printed Date: 20 September 2019 at 10:13pm
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Topic: Haze Caravelle (barn find!)
Posted By: vital spark
Subject: Haze Caravelle (barn find!)
Date Posted: 16 November 2018 at 2:15pm
Good Afternoon.
 
Since its my first post, i'll give a little background.
 
My name is Allan, and i live up in NE Scotland. Electrics and basic electronics are well known to me, as is more modern day Audio equipment, however DJ'ing and Disco Equipment are a bit Alien to me.
 
I recently stumbled across a facebook marketplace ad for an old "Haze Caravelle console". Since i have young nieces very interested in DJ'ing, and my continuous lust for tinkering, i snapped up the console with a view to getting it full up and running and getting use of it again.
 
Googling "Haze", Caravelle", "Bournemouth" isnt so successful, obviously due to to pre-dating the interweb, and a company probably long past existence, however the only reference i can i find was this post on these very forums:
 
https://forum.speakerplans.com/old-skool-triple-deck-console_topic44325 - https://forum.speakerplans.com/old-skool-triple-deck-console_topic44325
 
 
The unit itself isnt in too bad a condition. the decks (Garrard 125sb's) seem to functioning from my testing so far, with exception to a missing centre spindle on one, new belts required for both. a clean/regrease/oil etc and a tone arm weight for both.  Not got as far as cartridges/needles yet, but there does look theres option out there still avaliable.
 
Onto the console itself. I'm as yet to get either an input or ourput hooked up yet, but from my initial findings, i noticed that one amp has been bypassed completely. This had me assume that the Right channel amp had popped and was bypassed, however, testing individual components and input voltages etc, all seems to check out ok.
 
What i can only assume (until i dig further), is that it could possibly be right channel inside the mixing console thats been the problem.
 
I'm making progress with providing an input source to test further etc, and will likely fire her up as is, see what's good, and work backwards to nail down the issue.
 
Sorry for the long winded intro!
 
What i'm interested in, is any history on these units. how rare are they, how many would have been sold originally, how good were they in there day (the turntables arent world beaters from my googling, but not the worst by far, and can be pretty good with the right cartridge).
 
Also, are there any wiring digrams in existence (anything Haze console related would be a start), and potentail spares lying around, either for the decks or console. Anyone to speak to thats an expert on all things Haze. Any still in existence?  Generally, any assistance/info appreciated!
 
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Replies:
Posted By: djeddie
Date Posted: 17 November 2018 at 6:31pm
Got to admit, I've not seen one in the flesh before. Are you on Facebook? Cuz there's an awesome group (mainly consisting of the old farts from here), that will be able to help with all the questions.


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Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB


Posted By: jbl_man
Date Posted: 17 November 2018 at 9:23pm
Ditto Eddie,also not seen one in the flesh. Haze were only around for a few years back in the late 70's and early 80's.

Besides the parts already mentioned above,the crucial sculptured rubber turntable mats are also missing? Locating 40year old spare parts is becoming increasingly difficult...and expensive. Just keep your eye on ebay, but be prepared to pay a high price for the missing items.

I would also say there is almost no chance of ever finding a schematic for the mixer or amp modules, but that wont matter,as will all be quite simple basic stuff inside. If you get stuck, just take a photo of the PCB's (both sides) and post it,im sure will help with the electronic side of things.


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Be seeing you.


Posted By: jbl_man
Date Posted: 17 November 2018 at 9:33pm
Re your question Allan "What i'm interested in, is any history on these units. how rare are they, how many would have been sold originally, how good were they in there day"

Quite rare...Citronic were the market leader and the most popular, probably the best-all-round units, Soundout were next in the list along with Cloud and Discosound. FAL were the budget option,and considered the bargain-basement of disco consoles.

Haze were in that middle group,your unit with its built in amps would have been a good one back in the day,and very heavy too. Probably quite expensive too, Maybe around £500 in the late 70's,which would have been a fortune then. If yours was complete and fully working,would be worth a fair price today too.


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Be seeing you.


Posted By: njw
Date Posted: 17 November 2018 at 10:29pm
I love seeing these old consoles and learning the history of them. In the mid to late 90's when I was a lad and being a mobile dj was my dream(!) I used to take note of what all the local djs were using at school discos and family parties and more often than not it was a twin deck console like this but with a cd player or two plugged in, when I then started doing the odd gig with a mate in '98 we were given two Sound Creations consoles by his uncle, we did the same thing and only used them for the in-built power amps (which at the time seemed VERY loud), I've since learned that the Sound Creations stuff was at the 'cheap' end but but those consoles did us well at the time!


Posted By: concept-10
Date Posted: 18 November 2018 at 7:43am
Nice to see, made round the corner from me, I used to go into the factory from time to time, never owned one but borrowed one twice when the Citronic needed work, really nice consul if i remember correctly. I think we tried to buy the hire one and trade in our Citronic but no deal was struck, I would keep hold of it and do your best to restore it to original condition, I will speak to a few friends from that time and see what they can come up with for you, good luck Smile


Posted By: colint
Date Posted: 18 November 2018 at 8:30am
Originally posted by djeddie djeddie wrote:

Got to admit, I've not seen one in the flesh before. Are you on Facebook? Cuz there's an awesome group (mainly consisting of the old farts from here), that will be able to help with all the questions.

Old Fart's? bloody cheek Tongue


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Never criticise another man until you've walked a mile in his shoes. Once you have, call him what you like, you're a mile away and you've got his shoes!


Posted By: vital spark
Date Posted: 19 November 2018 at 3:40pm
Thanks for the info people, its great to learn stories/associations and a bit of background info on finds.
 
When "fair price" is mentioned, how much would be a fair price?!!
 
I dont intend to sell, i have a good use for it, and a bit of retro fun to be had by me, the daughter with her electric guitar, and the neice with her DJ aspirations.
 
Ref turtable platters, yes i recieved two, but both completely mishapen after lying scrunched up in the box. One seems ok, but the other has quickly gone hard/brittle and destined for the bin!
 
I found one complete turntable from Ebay Italy, so await its arrival, and i'm discussing another from Ebay Germany, although the first one should free enough parts to make a good one, the one in Germany is far better cosmetic condition.
 
While i await parts, i've been busy with the carpentry aspects over the weekend. When i state barn find, i really do mean that. Its been sat in the corner of a Barn for neigh on 20 years, and the bottom of the transit case had rotted out, and the console suffering from a bit of mould, and the appauling stench associated with it.
 
Unfortunately, it was unbearable to have this sitting inside the house, and although the wood can be cleaned and demoulded, the exterior covering, as well as ripped, bashed, screwed into etc over the years, stank also. There was also some damage to the bottom right corner where the mixing console sits (plywood delaminating), so as much as i would have liked to keep the original covering, it had to go. Also, no disrespect to Haze, but the openings on the front of the case looked like an afterthought, and cut out using a hammer, so needed a good tidy up also.
 
Some pics below of progress so far.
 
Heres the internal amps. Note the right one bypassed, also they look two different amps. Note sure how this will affect its sound or appeal?!??
 
 
 
 
Box after a de-covering, de glueing,  repair, resmooth and a paint to seal it. I'm thinking of using a leatherette type of covering since i cant find anything close to the original covering.
 
 
Transit case getting back together :) . a lick of matt black paint will hide the repairs i think.
 
 
Thanks for all your help so far. I did look on Facebook, but couldnt see anything that had been active since 2016??
 
Al
 
 


Posted By: jbl_man
Date Posted: 19 November 2018 at 3:53pm
If the amp on the right has been by-passed,is probably dead. Shouldn't be to hard to repair. The one on the left looks like a later up-graded module, (maybe the original died that side as well) with dual TO3 outputs per side,as opposed to the single ones on the right. Left hand amp also has larger caps. Will probably sound the same if you can fix the right hand one, as both use the same size transformers.


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Be seeing you.


Posted By: vital spark
Date Posted: 19 November 2018 at 4:03pm
I've ran over all transistors, resitors, diodes etc of the disconnected amp and they all test out ok, its two green LED's light up the same as the left one, and its got the same input voltage as the left side. Will soon find out though.
 
I did break into the mixing module for a quick look, and found some broken wires on the pot for the right hand mid tones, so i'm thinking it could be there that the problem lies. Will find out soon enough though.
 
Al


Posted By: 4D
Date Posted: 19 November 2018 at 7:57pm


I am not sure DJ niece will be to ecstatic beat mixing without any pitch controls though the belt drive should add some interesting after effects 


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DMZ. "The bass was intense. Girls were literally running up to stand next to the subs"


Posted By: vital spark
Date Posted: 19 November 2018 at 8:43pm
Originally posted by 4D 4D wrote:



I am not sure DJ niece will be to ecstatic beat mixing without any pitch controls though the belt drive should add some interesting after effects 

Yup, no cross fader either! To be honest, it’ll probably spend most of its time hooked up to an iPad and the dj app. Nice to be able to play records, amp a guitar, overlay music microphone and so on though. She’s 9, so Calvin Harris can sleep easy for a few months at least!


Posted By: vital spark
Date Posted: 20 November 2018 at 10:39am
Originally posted by jbl_man jbl_man wrote:

If the amp on the right has been by-passed,is probably dead. Shouldn't be to hard to repair. The one on the left looks like a later up-graded module, (maybe the original died that side as well) with dual TO3 outputs per side,as opposed to the single ones on the right. Left hand amp also has larger caps. Will probably sound the same if you can fix the right hand one, as both use the same size transformers.
 
Transformers look the same, but they arent unfortunately. I'll do a proper diagram later, but the one on the right is as follows (as i remember it):
 
240v ac to the transformer input, Ov and 30v used on the secondary, through the bridge rectifier at 30v DC and on to the two caps. line voltage at the input fuse terminals 87v DC
 
Left: 240v ac transformer primary. Transformer secondary +30/-30v tappings. Centre tapped to caps/earth. AC voltage at bridge rectifier 60v, line voltage at the input fuse terminals 87v DC.


Posted By: vital spark
Date Posted: 27 November 2018 at 9:23am
Good Morning.

A bit of progress being made. All powered up, and the mixer seems to be mostly working fine. Only really the second mic input has a proper issue, and will investigate later.

Looking into the amps, and wired up the right hand amp again, and found its output weak and sounding pretty bad. Definately the amp itself.

Cant see any dry joints, fried components or broken tracks, but what i can see from voltage testing, is that one of the power transistors show 44v to its opposite DC line, where all the others (both amps) have 87v DC, so with my basic knowledge, i'm assuming i have a duff power transistor!?







Posted By: concept-10
Date Posted: 27 November 2018 at 9:56am
Loving this thread Smile


Posted By: vital spark
Date Posted: 27 November 2018 at 2:45pm
Originally posted by jbl_man jbl_man wrote:

Besides checking the obvious 3773 outputs, i would check all the small signal transitors too,(probably BC184's or similar). One of them will be faulty.

Same with the power transistors, all the board mounted ones tested ok for resistance and direction. I'm not an expert by any stretch, and my fault finding is merely by comparison. If i voltage test again, would comparison be useful again (ie, all pretty much showing the same voltages each), or do you have any more tips for testing? For all the cost, i might just be as well replacing all of them though?

Thanks for the help, Al


Posted By: APW
Date Posted: 27 November 2018 at 4:33pm
that amp appears to missing its driver transistors.


Posted By: odc04r
Date Posted: 27 November 2018 at 4:56pm
Yes they certainly are missing, you can see on the underside that there has been rework around where they should be. Getting originals might be difficult even if you knew what to go for. This is the kind of thing you might get some good help for on diyaudio.com

If you could spend some time tracing out and drawing the circuit people there would probably tell you exactly what to substitute in.



Posted By: jbl_man
Date Posted: 27 November 2018 at 6:15pm
Follow the circuit back,and see what is fitted on the other (working) amp PCB as drivers. Should be able to get a transistor number from the working one.


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Be seeing you.


Posted By: jbl_man
Date Posted: 27 November 2018 at 8:39pm
It will be the two that are mounted on the alluminium strip in the middle.





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Be seeing you.


Posted By: vital spark
Date Posted: 28 November 2018 at 8:37am
interesting stuff  and thanks for your input. I did notice the potential missing transistors, but dismissed it, due to the fact there are other differences, such as the left amp has 4 power transistors, and other subtle differences, such as more resistors, and some other additional components connected to the heat sink.

Main thing is, both PCB's and enclosures are identical (Haze POWAMP MK3).

Question is, do i fit the two missing transistors and heatsink only, or change the right amp to match exactly the left (add 2 more power transistors), or a 3rd option which would help greatly with my OCD, which is to change the caps and tranny's also so they match. 

Third option might be deemed as sacralidge, since it would likely be torroidal transformers, and 21st century caps?

Whats your thoughts guys?

Job is growing slightly, but not outwith the realms of my capability, and i'm all for doing it right (and once only!)


Posted By: jbl_man
Date Posted: 28 November 2018 at 9:28am
First thing would be to get the number off the existing driver transistors,and see if they (or an equivilant) are still available.


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Be seeing you.


Posted By: odc04r
Date Posted: 28 November 2018 at 12:31pm
Yep, follow the priorities. Also check to see if you can find spare power transistors for the missing slots. Beware of sales on eBay etc of harder to find devices, transistor fakes are rife if there is a profit to be made reprinting cheaper devices to pass off as older/rarer/more expensive. Power transistors are especially bad for this.

Personally I would draw out the circuits, figure them out and then substitute sensible modern components as required/necessary to bring them back to life. I'd definitely replace electrolytic caps, but I'd leave the small signal components alone if they tested well.

For me vintage kit is 99% the outside appearence physical functionality. I have no problem making sensible modern part swaps as necessary to amplifier modules and PSUs etc. Appreciate that other peoples opinion can differ.


Posted By: vital spark
Date Posted: 28 November 2018 at 12:52pm
Those transistors are  TIP41C's, and it looks like its readily available from a quick google.

Will definately draw out both circuit diagrams and take it from there.

Thanks for your input/opinions/advice so far. it's really appreciated.

My concern is that the differences between the two amps is to accommodate additional power transistors, possible a higher rated amp, so in my limited opinion, it makes sense to replicate the working amp with the same components, rather than take a stab in the dark as to how the non working one should be.

Al






Posted By: kedwardsleisure
Date Posted: 28 November 2018 at 4:23pm
are those resistors burnt out by the fuse?

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K. Edwards Electronics Engineers

North Staffordshire



Posted By: odc04r
Date Posted: 29 November 2018 at 9:51am
I thought possible the same, but after zooming in the markings are legible. I think for whatever reason they are just black resistors.

If you draw out the circuit and indicate components then post it up here+DIYAudio you will end up with many suggestions regarding circuit improvements/stability/ability to drive the putput stage etc. Driving a pair of output transistors should not be an issue for a single driver but some attention may have to be paid to heatsinking/bias.




Posted By: Pinyorouk
Date Posted: 29 November 2018 at 3:44pm
Usaually if there are 2 output transtors it is only good for 8 ohm operation minimum. With 4 output transistors, 4 ohm operation minimum. That was the case with amps I worked on in the 80's.


Posted By: vital spark
Date Posted: 29 November 2018 at 10:29pm
hi, I actually thought the same about those resistors, since there’s a little heat damage against the fuse. I removed them and tested individually. They are resistors, and a bit of searching their part numbers etc and they are 0.47 ohm fused resistors. Very low resistance, so assume they are effectively just a fuse?

Progressing along with the rest. I’m 50% there with a circuit diagram on the first, and with the milder weather I’m finishing painting the transit case. Also lost two starboard side tyres commuting tonight, to a pothole,  so that’s a bit of a setback! 



Posted By: odc04r
Date Posted: 30 November 2018 at 11:48am
Originally posted by vital spark vital spark wrote:

hi, I actually thought the same about those resistors, since there’s a little heat damage against the fuse. I removed them and tested individually. They are resistors, and a bit of searching their part numbers etc and they are 0.47 ohm fused resistors. Very low resistance, so assume they are effectively just a fuse?

Progressing along with the rest. I’m 50% there with a circuit diagram on the first, and with the milder weather I’m finishing painting the transit case. Also lost two starboard side tyres commuting tonight, to a pothole,  so that’s a bit of a setback! 



They're possibly both fuses and also act as emitter balancing resistors when using multiple output transistors to make sure currents stay roughly balanced.

Good work on the diagram, be interested to see it. I'm sure it is a very classic circuit


Posted By: Muckerbarnes1
Date Posted: 14 December 2018 at 9:44am
I fixed lots of these in the late 70's, early 80's. 

The same amp modules were in a slave box too. 

Lots of manufactures made similar. They all used very similar components and are simple to work on. 2 O/P transistors is fine on 4 ohms in such, but that does of course depend on the psu. In general terms 2 2N3773's  is 100W and 4 2N3773's is 200W. TIP31/2 and TIP41/2 drivers is very common. 

I refurb lots of consoles like this. 


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Billy Dawg.


Posted By: vital spark
Date Posted: 19 December 2018 at 2:40pm
Hi there, i've been quiet, but that means less browsing, more tinkering!

As a general update (and pics to follow), i've finished the transit case, wrapped the unit itself in a faux leather plastic wrap, stripped the bottom door holding the amps, and refurbed, all ready for the rebuild.

Completed the schematics and hopefully managed to share my findings below 

uploads/22347/Haze_amp_-_non_working_right.pdf" rel="nofollow - uploads/22347/Haze_amp_-_non_working_right.pdf

uploads/22347/Haze_amp_-_working_left.pdf" rel="nofollow - uploads/22347/Haze_amp_-_working_left.pdf

Hopefully i've managed to transfer a PCB into some form of readable schematic. Its tricky trying to keep a schematic with some form of geographical resemblance to a PCB to make it simple to cross ref, but still keep the schematic readable.

I'm not an electronics expert by any stretch, so if anything looks odd or cant work, just say!

I havent sought any advice regarding any tweaks or upgrades as yet, i'm just keen on getting both working and the same. using the schematics i've ordered the additional/changed components to make both the same as per the "200w" amp.

progress/updates:

Found some new unused caps of a similar vintage on Ebay (1986). capacitance checks out ok, and probably better than what i had, but not 2018 units.

transformers, they do actually seem to be the same units, but a different newer model. Both MT125ft's from the same manufacturer. Also found the secondary wired incorrectly (not centre tapped). the link to combine the two 30v output winding was in the wrong place. the bridge was getting 30vac, and the centre tap was the other 30vac, so possibly explains (without too much thought) why the DC readings were ok, but actually the amp was only using half the secondary winding!?

So i've wired them both the same with the correct tapping, and kept them both.

De-annodised the heat sinks on the newer amp to make them look the same.

Full rewire and tidy up!

Also under the amp casings revealed their ages :)

more pics to follow!




Posted By: jbl_man
Date Posted: 19 December 2018 at 2:57pm
Well done with the schematics.....very interesting, the earlier (non working) amp with the missing parts uses a single 2n3773 on both rails.... wheres the later working one uses two MJ15003 on the + rail, and two MJ15004 on the - rail.

The missing drivers would appear to be TIP42C on the +,and TIP41C on the - side.


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Be seeing you.


Posted By: vital spark
Date Posted: 03 January 2019 at 10:42am
Hi all and happy New year.

Well, we had a New Years party, and the old girl performed well. some of the banks get a little hot for my liking, especially since its not running hard, but this is likely due to the biasing not being setup, and i think I've sussed it (found a very good tutorial last night on class AB amps and how they work).


The mixing console has been getting a good workover. some items fixed/ongoing:

1, there's a hum that increases with volume, regardless of what input being used, and its also apparent on the headphone output, so i replaced all the power supply caps. Still present, but at least the caps are now ruled out. - all suggestions appreciated!
One of the PSU's has a 15v zenner diode to ground, and i assume its for reverse/overvoltage protection. This was one supply on its own from one transfomer and was red hot to touch. The transformer was putting out 20v, so the diode(and power resitor(see below photo)) were being eating up a lot of energy. I reconfigured the tapping on the transformer to a little over 14v, and all seems to be well.
all the slider pots feel nice and smooth with no crackle, so i've left these alone, but most of the rotaries were either tight due to old grease, or crackling, so i went round these and cleaned/lubed and there's now no crackling, however most if not all are just out of spec. the 100k pots range from 120-150k, which is giving some funny effects with the tone controls in particular, where for example, past 100k on the bass pot, and treble dissapears. Managed to find some close alternatives from Farnell, and await their arrival.
The knackered mic channel was an easy find. A 714 op amp was hot enough to cook an egg on, so thats replaced and all working.
Also received a new deck picked up from Ebay Italy. Rust free, nice smooth tone arm and far less mileage that the one it replaced. The other deck is rusty, the tone arm is far too loose, but otherwise works fine. I'll bide my time and pick up another on Ebay.
Both decks treated to an AT95 cartridge, which seems to be working well, but i think might be driving a slightly higher voltage than whatever was fitted previously, or what the console was designed for, since you dont need much gain at all for the deck inputs to be overly loud, and can distort easily and sound a bit tinny at times. Might need to figure out some form of attenuation, or maybe alter the value of the input/loading resistor on the console (currently 46k)
Installed a logitech bluetooth audio adaptor into the 3rd deck input, along with an L pad for attenuation. Sounds great/works great and completely stealth :)
The lights previously robbed 60v ac from the right hand amp, so i've installed a 5v psu on the main feed and replaced the bulbs. (5v PSU also powers the Bluetooth receiver)

Pics below. We'll chuffed with my efforts on rebuilding the right amp, especially considering its working!












getting there!!



Posted By: concept-10
Date Posted: 03 January 2019 at 1:47pm
Excellent work, well done ClapClap


Posted By: Robbo
Date Posted: 03 January 2019 at 2:11pm
It may be worthwhile fitting an earthed shielding plate across and above the amp modules and mains trannies as they are very close to both the turntables and the mixer when in use.


Posted By: vital spark
Date Posted: 03 January 2019 at 2:41pm
Originally posted by Robbo Robbo wrote:

It may be worthwhile fitting an earthed shielding plate across and above the amp modules and mains trannies as they are very close to both the turntables and the mixer when in use.

Robbo, i've thought the same myself. I dont think its the source of my hum though, since theres no difference with the "hood" lifted, or down, but could be from the tranny's to the amps, however from what i can gather, if the hum doesnt vary with the mixer volume, its likely the amps themselves, but the hum does alter with volume.




Posted By: jbl_man
Date Posted: 03 January 2019 at 5:59pm
"there's a hum that increases with volume, regardless of what input being used, and its also apparent on the headphone output, so i replaced all the power supply caps. Still present, but at least the caps are now ruled out. - all suggestions appreciated!"

Try changing that pair of old green Plessey 2200uf caps on the mixer PSU board, they are 40 years old,and cant be helping much. Thumbs Up


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Be seeing you.


Posted By: vital spark
Date Posted: 03 January 2019 at 6:19pm
Originally posted by jbl_man jbl_man wrote:

"there's a hum that increases with volume, regardless of what input being used, and its also apparent on the headphone output, so i replaced all the power supply caps. Still present, but at least the caps are now ruled out. - all suggestions appreciated!"

Try changing that pair of old green Plessey 2200uf caps on the mixer PSU board, they are 40 years old,and cant be helping much. Thumbs Up

Those are gone also. I replaced all the 470uf’s (9 of) and the 3 2200uf’s All the old caps were still within 100uf of their stamped value.


Posted By: vital spark
Date Posted: 04 January 2019 at 9:27am
i should also add, regarding the hum, that i've cleaned up all the earths both on the mixing console and amps, and where possible remade, re crimped etc. i've check for continuity with a meter, and i've also improved the earthing on the mains transformers/transformer screens etc. also tried removing earths systematically with no difference in the hum at all.

as stated, also no difference with the unit opened and the mixing console further away from the mains transformers.

regarding the decks, i get some additional hum from them, but pretty sure its just atmospheric noise from the MM cartridge in open air, which i think would be acceptable on their own if the overall hum was gone. Theres no additional hum from the decks when you power up the motors, so definately not coming from the deck motors.

I'm thinking there must be a component failed/failing or a dry joint somewhere on the common output stages of the mixing console, and when the new pots arrive, i'll spend a bit of time testing/inspecting components in this area.





Posted By: odc04r
Date Posted: 04 January 2019 at 10:19am
First of all apologies for not responding after you took the time to draw out the schematics, but as you seem to be happily making noise I guess we can move on from that!

Regarding the hum, it is probably50Hz earthing. You could check by recording a sample of it and running an FFT using software like Audacity etc, it will show a spectrum peak where the hum frequency is.

When it comes to hum it is probably not so much how well your earth points are connected (well it does matter but...). More importantly is how they are connected. You need to avoid earth loops. Entire book chapters have been written about good grounding practicebut the key to it is that you need to avoid loops. There should be a single point of the chassis earthed to the mains, don't connect it at any other point (you'll make a loop).

Then wire the grounds in either a star or tree-like structure,, sometimes the star is more logical to people. After the resovoir caps of the PSU is a sensible point.

Where a lot of people fall down is the input connectors. Make sure the outer shells are isolated from the metal chassis of the equipment. Wire the inputs with tightly twisted wire to the amp modules, and connect them at one point on the board as close as possible. The ground rail of the board is connected back to your star point post caps. If the outer shell of the phonos is in contact with the equipment chassis, you have a nice big ground loop. If you have long runs of high impedance untwisted wire, you have aerials for 50Hz hum induction.

Also check that the pots are isolated from the chassis, and make sure that the decks are well grounded to a single point at the amplifier or phone stage board input, again using nice twisted wire.

An electric screwdriver and a vice makes for a very quick and good way of making lengths of twisted wire quickly!


Posted By: vital spark
Date Posted: 05 January 2019 at 7:01pm
Thanks very much to take the time to provide above detail its very much appreciated.

I spent today investigating/reworking the grounds, and also last night did a bit further reading on the interweb on the subject.

further to below, i've also replaced pots on the main tone/balance controls, tidied up some dodgy looking "modifications", cabling, and also scrubbed/cleaned a lot of crud off the PCB. It really is starting to sound like a nice a bit of kit :)

So (ground)work completed today:

Created a star point on the main (safety) earth - (as per diagram below)
the decks were not earthed as such, and only grounded via the signal shield, which had been purposefully grounded out to everything. Grounding to safety earth obviously worsened things, in creating a loop, so i reworked the signal shield to only shield the signal cable, and provide the ground reference for the cartridge. Then connected system earth to the turntables, tonearm as per below diagram

On the mixing console i created a star point as per below diagram, removed the local signal grounds on the din sockets. The unit is all closed up now, but looking at the diagram i should have connected the second Tranny negative to the star point - doh! (note that theres two transformers - T1 is centre tapped giving a +/-/0 and the second only a +/- supply)

So in summary, what i have now is a much cleaner sound all round (YAY!!). no deck hum at all, and a minor ammount of hum on the output to the amps. I'll describe these problems seperately below, but theres a good chance they are related:

DECKS/INPUTS:

Theres now a fair ammount of what i would consider RFI on the inputs. Even with the decks disconnected, ramping up the volume gives a lot of hiss/crackle, and you can almost hear a radio station in the background! (plugging in the decks and introducing the cartridge makes no difference, neither does it add any additional noise. noting that previously it was definitely a mains hum)

If you look at photos below, you'll see the two things i need a bit of help with (cabling and routing):

1, the mixing console outer track would be considered its own ground bus. mixing signal/power all the way round. Advice seems to be that signal +/- should reside together as far as practical (what i have), but ideally should return to the star point. Whats the best here? take all the signal grounds direct from the input/output din plugs straight to the star point and ignore the mixing console PCB altogether?
2, currently the inputs/outputs are wired with single core screen cable. Both screens are connected on the mixing console PCB for each input, however at the din socket, only one is connected and used, the other left open. I'm assuming this is to avoid a ground loop, however i think the open one has the effect of a radio antennal!! 

So what would be the best type of cable to use for these - a TSP, or the L/R signals twisted and straight to the board, with ground going to the star point? Wolly dependant on the best placement of the grounds.

OUTPUT:

mains hum is greatly reduced, but it does still exist. Isolating the output from the mixing console removes any noise completely - silence. Of course this is just a pair of amps with zero input, however connecting to the console with all volumes at 0 introduces the hum, as does only touching the  signal ground (outer case of the din plug) to the mixing console chassis.

There's definitely a ground loop at play here, and from what i understand, signal ground should be isolated from earth at the amp input, however if you look at my schematics, they are connected directly to the amp ground. Would it be a simple case of removing the amp ground from the input?
Also noting that i have no safety earth/system ground connected after the amp transformers. the transformer secondary is pretty much isolated, and the amp grounds are all common at the reservoir caps.

Whats you thoughts?







Posted By: Robbo
Date Posted: 05 January 2019 at 7:12pm
This may seem a bit daft at this stage of investigation but I can remember vaguely a problem that we had about forty years ago with a Discosound console with a hum problem that was caused by the 12v flexi lights or transformer--I should imagine that you have probably already checked it out but if not, it may be worthwhile isolating that circuit to see if anything changes.


Posted By: vital spark
Date Posted: 05 January 2019 at 8:05pm
Hi, yup, I was well aware that switchmode psu’s Are a no no for audio equipment, but didn’t let that get in the way! 

All above is mostly with it isolated and off, so definitely not having an effect.


Posted By: vital spark
Date Posted: 07 January 2019 at 10:43am
further to my last update, i've been continuing to read up on the subject.

I now have improved my understanding of where/whjat/why etc, and might have missinterpreted locations, and star points etc, so next time i have the mixing console apart, i think i can make further improvements, which better follow convention, especially regarding safety/power grounds.

regarding the signal grounds, i think one of the main drawbacks in this unit is the 5 pin din sockets for the inputs/outputs, and i'm going to change to RCA's, which will allow grounds and sheilds to run continuous and seperate from the PCB signal ground through to the end component without getting commoned in the din plug, and will allow a single signal/ground twisted pair internally. Will also give me the chance to change out the external cabling, which is 40 years old also. 

Al



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