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Studiomaster 2000e repair help

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jbl_man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post 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.


Edited by jbl_man - 09 June 2022 at 10:40am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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
Kevin

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Post Options Post Options   Thanks (1) Thanks(1)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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.


Edited by Randy Bohannon - 10 June 2022 at 7:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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? 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post 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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post 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


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 14 June 2022 at 4:31pm
Ok…

Schematic in one hand 


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


Edited by Randy Bohannon - 14 June 2022 at 4:32pm
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