DC-2 problem

DC-2, 3, 5, 10
50 Caliber

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Pilotltd
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DC-2 problem

Post by Pilotltd » Fri Jul 18, 2014 9:17 am

Hi Guys - first post here, hoping someone can help.

I picked up a faulty Studio Caliber DC-2 cheap. Full of dog hairs and spider webs and been left for some time by the looks of it. It had been turned on by previous owner otherwise I'd have run it up on a variac to help the caps.

Symptoms are very weak and very distorted output.

Pulled it and gave it a good clean out also cleaned all sockets and pots. Looks like somebody had attempted fixes before as many of the caps had cut and badly resoldered leads, some weren't the spragues fitted originally. I've replaced all the sprague caps and one 220uF 63V cap. Nothing looks burnt or swollen anywhere and on retrying output is slightly better but still way down and very distorted.

Valves and sockets are all OK (Valves tested in another amp and on valve tester)

Weird that turning the FX mix volume pot alters the output to the speaker and master volume on each channel has no effect whatsoever on output vol. Mute speaker switch works but also no output to phones socket. All channels switch OK.

Next item I'm going to replace is the 5532 in the recording circuit to see if that does anything, that should be delivered tomorrow.

Has anybody any ideas on what could be causing the problem?

Thanks

Steve

Orion_99
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Re: DC-2 problem

Post by Orion_99 » Tue Jul 22, 2014 12:15 am

Could be the 5532. If you pull it out and everything but the recording circuit works, that was the problem. My dc-3 was sick and it ended being the op amp that was shorting out. Also try running another amp into the fx return from its fx send, then the dc-2 send to its return to see if you can narrow down the issue to the preamp or power stage.
Another very common issue with these is bad contacts in the fx jacks. Jumper them and see if that makes a difference.

Pilotltd
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Re: DC-2 problem

Post by Pilotltd » Tue Jul 22, 2014 2:28 am

Orion_99 wrote:Could be the 5532. If you pull it out and everything but the recording circuit works, that was the problem. My dc-3 was sick and it ended being the op amp that was shorting out. Also try running another amp into the fx return from its fx send, then the dc-2 send to its return to see if you can narrow down the issue to the preamp or power stage.
Another very common issue with these is bad contacts in the fx jacks. Jumper them and see if that makes a difference.
Thanks for the reply.

I replaced the 5532 - no difference. I'll try pulling it altogether and see what happens. Jumpered send and return but no go there either. Also tested resistance on contacts with no plugs in and send was 16 ohm - cleaned them and all now less than 1 but still nothing.

All the controls except reverb, presence and masters on either channel seem to work. Looking at the block diagram the problem "seems" to be after the tone stacks and before the masters - but doesn't explain how the output level still works.

I've spotted a few things in the eq fx loop. Q1 and Q3 have been replaced, but with MPSA92's - I'll replace them with a 63 and a 70 as per schematic and hope they are the correct ones - This is a Rev C board and there are few differences, that and the odd layout and access doesn't help one bit.

I'm not bad with valve circuits, but when you throw transistors in the mix, I'm screwed. I cant find anyone around here in the UK who will look at it and Mesa importers in the UK say " we don't have any approved techs - you'll have to send it to us" - They want £48 ($82) per hour, no estimate or fix guaranteed.

Steve

Pilotltd
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Re: DC-2 problem

Post by Pilotltd » Tue Jul 22, 2014 11:06 am

Orion_99 wrote:Could be the 5532. If you pull it out and everything but the recording circuit works, that was the problem. My dc-3 was sick and it ended being the op amp that was shorting out. Also try running another amp into the fx return from its fx send, then the dc-2 send to its return to see if you can narrow down the issue to the preamp or power stage.
Another very common issue with these is bad contacts in the fx jacks. Jumper them and see if that makes a difference.
Pulled the 5532 - no difference. Jumpering fx - no difference. I'll try a pedal into fx return tomorrow, haven't got another amp here with fx on it at the moment. The MPSA's didn't come today but will replace them as soon as they turn up.

Steve

Pilotltd
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Re: DC-2 problem

Post by Pilotltd » Wed Jul 23, 2014 8:14 am

Getting somewhere - Open jack inserted into input and guitar plugged into fx return - nice clean sound. Looks like that may exclude power section and V3b - I think :)

No MPSA's arrived yet though :(

Pilotltd
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Re: DC-2 problem

Post by Pilotltd » Wed Jul 23, 2014 5:54 pm

Thanks to Orion_99 for the pointer - there is something weird going on in the low voltage section. Should be -15V to pins 3 & 5 of the 5532 - there isn't - It's 0. Between the zeners it's 0 too. Hope it's just a shorted zener, but I've a nasty feeling that the 220uF 35V cap may be shot. That means removing the pcb from the chassis and not something I'm looking forward to. I'm going to replace the zeners first, and see what I get. If the board has to come out I might as well replace all the electrolytics and maybe the relays as well :)

Steve

Pilotltd
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Re: DC-2 problem

Post by Pilotltd » Fri Jul 25, 2014 8:53 am

Success!! It lives!

Both Zeners in the low voltage supply were broken, in different ways! One would allow any voltage through, the other was open circuit. One 1N4007 was way off and allowing some voltage back. I've changed all 7 of them in the power supply. One of the 220uF caps was way off too - only 204uF - changed. The 150uF is also a bit down at 140uF but it means lifting the board to change it - I'll see how it behaves after an 6 hour jam session on Sunday ;)

Strangely someone had clipped the 5nF cap in the presence circuit. Didn't have one so stuck a 3.9 in - that works now too. Even the reverb works which does surprise me! Not very strong, but I'll find the mod for that.

Again a big thanks to Orion_99 for pointing me in the right direction. I assume the same fate could befall the DC-3's so one to bear in mind.

Steve

Orion_99
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Re: DC-2 problem

Post by Orion_99 » Fri Jul 25, 2014 9:34 am

I'm glad that I could help
Something to try after you have played with it for a bit. There's a 22k resistor after the treble pot on the lead channel. You can either snip it or, as I did, desolder one end and lift it. This will make it sound like you have removed a blanket from the speaker. I also did the output volume mod which does help a bit.
Have fun with it!

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