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Studiomaster 2000e repair help

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URL: https://forum.speakerplans.com/forum_posts.asp?TID=107118
Printed Date: 03 March 2024 at 10:13am
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Topic: Studiomaster 2000e repair help
Posted By: Randy Bohannon
Subject: Studiomaster 2000e repair help
Date Posted: 29 May 2022 at 9:39pm
Hi all, 

I'm bringing one of these back from dead and would appreciate some advice from anyone experienced in fixing them, as to whether a few substitutions would be viable please?

Both modules blew but one much worse than the other as it seems to have been previously repaired but without fusible resistors.

The first question is on the drivers for the HV/LV switching section, the original Rohm 2SB1186A/2SD1768A are out pf production and although a company in the US purports to have NOS, it will be expensive and I have been burned before. Can I therefore be fairly confident in using the MJ1503x type TO220 type driver as they seem fairly similar in terms of gain and Ft except that he MJ1503x seems more robust?

For the drivers for the main amplifier section, I think I have found newly manufactured parts from Profusion in the UK: 

http://https://www.profusionplc.com/parts/2sc4793lb-tf3t - http://https://www.profusionplc.com/parts/2sc4793lb-tf3t

http://https://www.profusionplc.com/parts/2sa1837l-tf3t - http://https://www.profusionplc.com/parts/2sa1837l-tf3t

I appreciate that using the original parts where possible is the safest approach but wonder if these too might not be substituted for MJ1503x parts, again on the basis that the latter is tougher, or does this role need the higher Ft. and lower Cob of the original part?

The last question is on replacing the output transistors, all of the MJ15003/4 from the switching section seem to have survived, all of the MJ15022/23s from the amp section have blown. If I have understood how class G amplifiers work correctly, this isn't altogether unexpected in the case of a short as the "inner" transistors do most of the shorting as the voltage drops but should I change them as a precaution? I am only asking as they are crazy expensive and like main filter capos, if i don't need to change them I'd rather not.

The 15022/23s I'm going to replace with MJ21195/6s as the seem cheaper and better specified than the latter but if I am to replace the 15003/4s, should I use these or go with still-available originals? 

Apologies for the lengthy post and thanks for your help.








Replies:
Posted By: jbl_man
Date Posted: 30 May 2022 at 8:26am
"all of the MJ15003/4 from the switching section seem to have survived, all of the MJ15022/23s from the amp section have blown."

Why change transistors if they are working well? As long as all the 15003/4's all test fine,(personally i would remove them and test individually) then they should be fine. No point in adding to the repair bill when no need.




-------------
Be seeing you.


Posted By: kedwardsleisure
Date Posted: 30 May 2022 at 8:44am
Any ROHM or toshiba drivers you can buy new are likely to be fakes, I would go with the MJ's, they are better and work fine in these.

No problem with upgrading the main outputs, the circuit will remain stable and biased.

Lucky the board doens't have a hole burned in it if the fusible resistors have been discarded, when these blow they can blow badly!

Sure you want to spend money on an oldie like this? I'd think the collectors market is a bit limited....



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Kevin

North Staffordshire



Posted By: Randy Bohannon
Date Posted: 30 May 2022 at 9:39am
Thanks JBLman, you’re a man after my own heart.

My worry was whether this is like the situation when one or more output transistors blow in a class AB amplifier’s bank; in that case it is usually best to replace the rest of transistors in that bank as they are usually “strained” by the blow up and can go quite soon after.

I pulled and tested them individually and they seem ok, but a low current multimeter test can sometimes be misleading and they can test ok but fail under load.

On the other hand the 15003/4s are almost a tenner each… :D


Posted By: Randy Bohannon
Date Posted: 30 May 2022 at 9:42am
Thanks Kev’

The board was scorched in places but not burned through thankfully.

This is more of a labour of love than a commercial exercise, the bill for parts so far has come to 120 quid and that’s not including the main filter caps, which are reading a little low and should probably be replaced. 

Just so I’m clear then, the MJ1503x drivers can be used for both the amplifier and rail switching sections? 

Edit: the parts I linked to from Profusion are not NOS Toshiba but newly manufactured Unisonic parts?

http://www.utc-ic.com/upload/2011/1101/20111101111250426.pdf - http://www.utc-ic.com/uploadfile/2011/1101/20111101111250426.pdf


Posted By: kedwardsleisure
Date Posted: 01 June 2022 at 12:57pm
Quote it is usually best to replace the rest of transistors in that bank

yes

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Kevin

North Staffordshire



Posted By: Randy Bohannon
Date Posted: 08 June 2022 at 4:48pm
The parts arrived and I’ve rebuilt one channel.

I’ve had this amp since day dot and it’s seen some partying, hence the sentimental attachment to it and the repair/restoration.

I’ve foolishly not taken a phot of the back of the amplifier bird and forgotten where the two little 470pF capacitors go, between the base and collector of the driver transistors? 




Posted By: cravings
Date Posted: 08 June 2022 at 5:06pm


These? Have this broken one in bits on my broken shelf for years now..

https://imgur.com/a/iaF5Uh1


Posted By: Randy Bohannon
Date Posted: 08 June 2022 at 6:49pm
Legend, thank you mate.

Mine doesn’t have those box film capacitors though. 

These are a bit of a bear to fix…


Posted By: cravings
Date Posted: 08 June 2022 at 7:59pm
i'm really liking all the amplifier repair related threads lately. i don't know how to do it. some day i'll dust off the test equipment and start trying to figure things out.


Posted By: Randy Bohannon
Date Posted: 08 June 2022 at 8:05pm
I did the same, perhaps best to start off with a few home amplifiers as it’s easier and cheaper usually, the principles then scale up.

I’m not an expert by any means but enjoy getting stuck in.

I’ve got the module back in and it power up without any fault lights.

Matters of concern are around 1.5v of DC offset and the amplifier half of the module gets burning hot with no signal and no load.

I didn’t replace the differential input pair as they tested ok on a Peak DCA 75, I can’t see any other way to adjust the offset on this amp
And so was thinking of trying pairs of KSA992s to see if I can get the offset down.

The heating of the amplifier module is concerning though, I don’t seem to have oscillation as I monitor the output on the ‘scope, just the offset.

Any ideas chaps? 


Posted By: Randy Bohannon
Date Posted: 08 June 2022 at 11:29pm
Doing some more trouble shooting, when the Module is connected to just power, ground and speakers it does not draw excessive current or heat up or show any appreciable dc offset.

When the ribbon cable is connected it begins to heat up, again with no sign of oscillation on the output, just the offset.

Disconnecting the input board from the main pcb does not affect it, it still heats up.




Posted By: jbl_man
Date Posted: 09 June 2022 at 10:23am
"Matters of concern are around 1.5v of DC offset and the amplifier half of the module gets burning hot with no signal and no load."

That's not right at all. It's not a mosfet design, so shouldn't be any heat as such at idle. Are you sure you have checked every transistor around that half of the module? Even tiny signal ones?
Is there a bias check- adjust anywhere on this design?

Edit, you say the problem disappears when you disconnect the ribbon cable, but re-occurs the moment you connect again, so this would suggest is an issue with something at maybe the other end of said cable. Suggest you check again for shorted/leaking/dead components.


-------------
Be seeing you.


Posted By: kedwardsleisure
Date Posted: 09 June 2022 at 12:33pm
the ribbon cables carry the mute line which essentially biases-off the output stages until the power supply has stabilised, as there are no speaker relays on this design.
So disconnecting the drive is simply putting the modules into standby.

I suspect you have a bit more repair work on the output modes. The faulty side is turning on and the other side is in a tug of war with it trying to keep the output at zero - the result is alot of heat and bother.

WHen the outputs fail on a studiomaster it takes resistors, diodes and transistors out by the handful. They never fail gracefully. Somewhere I have a diagram showing what normally fails marked in marker pen, I can dig it out if you like

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Kevin

North Staffordshire



Posted By: kedwardsleisure
Date Posted: 09 June 2022 at 12:39pm
PS I have the diagram but it's probably easier to list what normally fails or needs changing as a matter of course.:

R34 R32 R30 R33 TR10 TR11 TR22 TR1 TR15 R21 D22 D21 TR14 TR12 TR13 TR17 TR18 TR19 TR20

(relates to amp pcbs)

For the rail board it is

D1 D2 D5 D6 TR1 TR22 R18 R21 TR12 TR17 TR18 D14 D18 TR13 TR19 TR20 R30 R35 R36 R32 R37 R38

the list is not exhaustive.

-------------
Kevin

North Staffordshire



Posted By: Randy Bohannon
Date Posted: 09 June 2022 at 2:46pm
Thank you, very much appreciated. I suspected the module was still not quite right as on the dim bulb tester it dimmed and then slowly glowed as if the bias current was excessive. I wondered if there was a problem in the components fed by the HV auxiliary line, which is on that ribbon cable, and your explanation makes sense considering the offset. 

I replaced all of the bav20 and zener diodes but not all the small signal transistors. I will replace them and try again.

I lifted one leg of all the resistors and measured them, replace as a matter of course you reckon? 

The problem with this amp is that it’s impossible to probe the thing running without either a collection of precarious jumper wires it by basically rebuilding it outside it’s case…

A thought, there aren’t any mistakes on the silkscreen, are there? I might have put something in the wrong way around if there are as I foolishly went through and removed damaged parts waiting for the mouser order to arrive.


Posted By: kedwardsleisure
Date Posted: 09 June 2022 at 6:43pm
never noticed any mistakes in the silkscreen, no.
Should be enough to check the signal transistors with a meter, and most of the resistors will check in circuit.

You can of course narrow it down to a single pcb by doing a swap with the good channel. If using a dim bulb method, pull the fuses from the good side so you get a true indication of current draw from the patient under investigation.

I've never felt the need to test anything live on these, first of all as you know the boards are sort of sandwiched and the wires are very short anyway, it would be easy to slip with a probe or knock a wire off or let the live heatsinks touch something and then it all gets very smokey

-------------
Kevin

North Staffordshire



Posted By: Randy Bohannon
Date Posted: 12 June 2022 at 5:45am
Originally posted by jbl_man jbl_man wrote:

"Matters of concern are around 1.5v of DC offset and the amplifier half of the module gets burning hot with no signal and no load."

That's not right at all. It's not a mosfet design, so shouldn't be any heat as such at idle. Are you sure you have checked every transistor around that half of the module? Even tiny signal ones?
Is there a bias check- adjust anywhere on this design?

Edit, you say the problem disappears when you disconnect the ribbon cable, but re-occurs the moment you connect again, so this would suggest is an issue with something at maybe the other end of said cable. Suggest you check again for shorted/leaking/dead components.

Thanks JBLman, following your advice I did some checking around on the input and front LED/gain control PCBs and found the cause of the offset; a half dead 4560 opamp.

This seems to have been caused by the power supply as whilst the positive rail measured around +18v the negative rail was an unregulated -28v… This is right on the ragged edge of what these opamps can take, according to their data sheet, and I’m amazed only one half of one op amp died. I’m not sure if they have been “stressed” but they seem to pass signal fine now.

I traced this back to a dead lm7915 in the power supply and replaced it with a lm337, weird to use a 7915 and not a 337 from factory but I suppose they must have had their reasons… swapping R8 and R9 around to suit gave -14 volts, but channeling my OCD I ended up trimming both rails to +\- 17v with metal film resistors… :D

This removed the offset when the ribbon cable was plugged in but sadly the amplifier half still heats up very quickly with no signal and no load.






Posted By: Randy Bohannon
Date Posted: 12 June 2022 at 5:53am
Originally posted by kedwardsleisure kedwardsleisure wrote:

never noticed any mistakes in the silkscreen, no.
Should be enough to check the signal transistors with a meter, and most of the resistors will check in circuit.

You can of course narrow it down to a single pcb by doing a swap with the good channel. If using a dim bulb method, pull the fuses from the good side so you get a true indication of current draw from the patient under investigation.

I've never felt the need to test anything live on these, first of all as you know the boards are sort of sandwiched and the wires are very short anyway, it would be easy to slip with a probe or knock a wire off or let the live heatsinks touch something and then it all gets very smokey

This is the good half…OuchLOL

This is bizzare, I pulled and tested all the transistors on a Peak DCA 75, all the diodes are new and the resistors checked.

Unless it’s the triac ( don’t think so because the fuses would blow), some film or ceramic capacitor ( highly unlikely) or an error of component orientation I’m a little stuck.

The only thing I can think of is that the old zener diodes between the driver transistors measured a reverse voltage of ~2.7v ( says 3.3v on the schematic ) and the new ones are almost bang on at 3.3v; could this be “overbiasing” the driver transistors? 



Posted By: jbl_man
Date Posted: 12 June 2022 at 9:42am
It can be frustrating when it should work, and all components check fine, but still wont work....might be worth removing the offending boards, take outside into good strong daylight, with you best glasses on, and just check that everything is actually "in curcuit", almost a continuity check on every component. On several occasions having done this, i have found a tiny fracture in a pcb track, that you simply cant see indoors.

Worth a try if all else fails. It can sometimes just be a simple thing that is causing the issue.


-------------
Be seeing you.


Posted By: Earplug
Date Posted: 12 June 2022 at 6:51pm
Originally posted by jbl_man jbl_man wrote:

It can be frustrating when it should work, and all components check fine, but still wont work....might be worth removing the offending boards, take outside into good strong daylight, with you best glasses on, and just check that everything is actually "in curcuit", almost a continuity check on every component. On several occasions having done this, i have found a tiny fracture in a pcb track, that you simply cant see indoors.

Worth a try if all else fails. It can sometimes just be a simple thing that is causing the issue.


Definitely - or get out the magnifying glass!  Smile

Or sometimes something looks like it's soldered in, but wigglying it about shows that maybe it isn't.

Also, the symptoms so much point to some unwanted oscillation. That's definitely not happening? Maybe check those ceramic caps. Embarrassed




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Earplugs Are For Wimps!


Posted By: Randy Bohannon
Date Posted: 13 June 2022 at 9:47am
Thanks chaps, I walked away from it for a few days out of frustration but I think I’ll give it another bash this evening.

I agree that it is showing the classic symptoms of oscillation but can’t see any probing the output with the ‘scope. It’s has 200MHz bandwidth and so I’d expect to see something, but nothing, at least at 0v this time.

I would like to remove the outputs and probe the base voltages but this thing is a bastard to work on, as well as having 200v backed by 85 joules worth of capacitance to help vaporise things if I slip with a probe…LOL


Posted By: Earplug
Date Posted: 13 June 2022 at 11:01am
Yes - sometimes it´s better to just walk away & take a break. Then attack again when refreshed.

Let us know how it goes.  Smile




-------------
Earplugs Are For Wimps!


Posted By: Randy Bohannon
Date Posted: 14 June 2022 at 4:31pm
Ok…

Schematic in one hand 

https://audio-circuit.dk/downloads/studiomaster/Studiomaster-2000E-pwr-sch.pdf" rel="nofollow - https://audio-circuit.dk/downloads/studiomaster/Studiomaster-2000E-pwr-sch.pdf

And beer in the other time.

The amp section does not get hot until the control signal (thank you Kevward) is present.

But having dealt with the DC offset, this doesn’t make any sense to my slightly tipsy-in-the-sunshine self, that control signal seems to mute the amp if not present by turning off the input differential pair.

Given the DC offset was caused by an op amp upstream and not the diff pair ( which I pulled and gained matched, let alone tested), I am struggling to how it could
Cause the output transistors to turn on.

The other connections on the ribbon cable seem fairly harmless other than the auxiliary high voltage line that feeds the front end.

Would it be logical to assume that the problem therefore is somewhere in this auxiliary powered circuitry? 

I will thoughtfully grill some teriyaki chicken and drink some more beer while I ponder and get feedback on my half cut theorising… LOL


Posted By: kedwardsleisure
Date Posted: 14 June 2022 at 5:00pm
no tipsy amp repairs please, it will end in tears.

1. ascertain if its an output module or something else.

(swap entire module from one side to the other)

2. either repair the 'something else' (pre amp) or

3. ascertain whether the faulty module is the switching or amp pcb

(swap boards with well-marked good side)

4. Go over the faulty board part by part. On a simple amp like this its quicker to cold check components rather than get in an endless loop with signals and offsets and long tailed pairs and whathaveyou.




-------------
Kevin

North Staffordshire



Posted By: Randy Bohannon
Date Posted: 14 June 2022 at 5:10pm
Thanks Kev, definitely no tipsy amp repairs. The weather is so glorious I’m going to be bbq’ing in the garden and not hunched over that infernal amplifier.

Your methodology makes sense, this was the better of two blown modules and so I had hoped that it would be more likely to work.

I’ll go through the other one, swap halves back and forth and see if I can get any further. Thumbs Up


Posted By: jbl_man
Date Posted: 20 June 2022 at 3:27pm
Any update on this Randy? 

-------------
Be seeing you.


Posted By: Randy Bohannon
Date Posted: 21 June 2022 at 9:44pm
No update as yet I'm afraid.

I got mixed up in a Yamaha M85 and the active crossover in the car has just died and so it's gone on the back burner for now.

I'm going to rebuild the other module so I can do some swapping around, hopefully in the week.


Posted By: Randy Bohannon
Date Posted: 31 October 2022 at 2:40pm
Suitably emboldened by the success of my other two restorations, I thought to tackle this treacherous malingerer, erm... I mean favourite old amp again...

Part of the problem with this amp was that it was badly repaired in the dim and distant by some arse who buggered up a lot of the PCB tracks. I've tried to repair what I can but given that pretty much every component was changed for brand new, I took a close look at the back of the board.

With the help of the brilliant photo that Cravings kindly posted earlier, I realised I has made a small mistake with a connection of a small compensation cap but nothing that would seem likely to cause the problems I am facing.

I would be very grateful if @Cravings, or any other kindly soul for that matter, might post large photos of the backs (and fronts if it's not too cheeky) of both amp and switching PCBs so ?I can do a sanity check and make sure my PCB repairs are correct?

The issues I'm having are:

a) 2v DC offset on the output, amplifier passes and amplifies a clean 1kHz sinewave on this offset however

b)Consequently, or by coincidence, the amplifier half of the two-PCB output module heats up like made when off the dim bulb tester. From dead cold, the fans will spin in about 30 seconds or so and go straight to full speed.


@Kevin, thanks for your help as always.

Thanks for the roadmap, replies as per below:


1. ascertain if its an output module or something else.


(swap entire module from one side to the other)

Tricky as I have two blown modules and this one is the better (!) one...

2. either repair the 'something else' (pre amp) or

I don't think bad opamps could do this, the amp would be railed if the op amps were? I've been over the other circuits and they all seem to work, I'll check each opamp output and check that they're at 0v DC.

3. ascertain whether the faulty module is the switching or amp pcb

The amp PCB is the one getting hot, the switching PCB stays ice cold, I'm thinking that means it's the amp side but class G is fairly new to me and I'm just not sure.
 
(swap boards with well-marked good side)

don't have one sadly.


4. Go over the faulty board part by part. On a simple amp like this its quicker to cold check components rather than get in an endless loop with signals and offsets and long tailed pairs and whathaveyou.


Fair shout, I mean, I think I have but it can never hurt to double check, it was the comment about the mute signal biasing them off that got me thinking whether it was them and the fact that they are original. The only amp I've seen that managed to take out it's LTP from and output stage blow up was a Yamaha M80; perhaps I should have changed them too?






Posted By: cravings
Date Posted: 01 November 2022 at 1:46pm
https://imgur.com/a/fcgdVMO" rel="nofollow - https://imgur.com/a/fcgdVMO

they're obviously not disassembled, and i didn't really look at the photos, just threw them up quickly. included bits that obviously fail "visual inspection" on mine.

let me know if you want anything else (that's relatively easy to do)


Posted By: luton_soundman
Date Posted: 01 November 2022 at 4:27pm
I miss my 2000e's Cry

-------------
Sound Hire/Sales new/used equipment.


Posted By: Randy Bohannon
Date Posted: 03 November 2022 at 9:28pm
Originally posted by cravings cravings wrote:

https://imgur.com/a/fcgdVMO" rel="nofollow - https://imgur.com/a/fcgdVMO

they're obviously not disassembled, and i didn't really look at the photos, just threw them up quickly. included bits that obviously fail "visual inspection" on mine.

let me know if you want anything else (that's relatively easy to do)

Thank you mate, very kind of you.

I'm going to give the other module a go and come back to this one.


Posted By: Randy Bohannon
Date Posted: 10 November 2022 at 8:35pm
I rebuilt the other amp module completely with new semiconductors in all positions.

Plugged into the same switching board as before it Gives the same symptoms as the other amp board, I.e. gets got and oscillates. 

I note that my board doesn’t have the extra film box caps on the bottoms that appear in the photos Cravings posted above, could that be the missing piece of the puzzle?

Does anyone know their value? 





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