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Studiomaster 2000e repair help

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Randy Bohannon View Drop Down
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    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.







Edited by Randy Bohannon - 30 May 2022 at 10:51am
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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: 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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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....

Kevin

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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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


Edited by Randy Bohannon - 30 May 2022 at 10:46am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 01 June 2022 at 12:57pm
Quote it is usually best to replace the rest of transistors in that bank

yes
Kevin

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


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

Edited by cravings - 08 June 2022 at 5:10pm
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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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…
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cravings Quote  Post ReplyReply Direct Link To This Post 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.
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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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? 
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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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.


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