Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jjackson7 on September 04, 2011, 12:33:21 pm
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Hello all,
I have been perusing this forum for a little while and have been amazed at the depth of knowledge here. I am an engineering student and I'm taking a circuit analysis class right now. I am still not putting all of the pieces together but I have some basic understanding of what components do and how to work on tube amps safely.
So... when I play my traynor at high volume the sound will eventually fizzle out. If I stop playing it will usually come back, but if I really try to abuse it it will stay in that state (blocking distortion?). I took the amp apart and started checking components. I found a transistor (BC550) that was shorted from C to E. I replaced it and the amp played fine for about 15min before the problem came back. I checked the same transistor and it was shorted again. Here is the service manual for the amp http://www.traynoramps.com/downloads/servman/smycv40.pdf (http://www.traynoramps.com/downloads/servman/smycv40.pdf). The transistor I changed was Q12. The service manual shows the latest version. Mine is revision 10.01 so some of the schem doesn't match. I can post voltages but I wasn't sure if I should change the transistor again before I post voltages. I am trying to minimize the number of times I have to put the iron to the pcb because I feel like I could eventually damage the board (maybe a noob fear). I will keep going through the suggested reading but any advice would be welcome. Thanks. -Jordan.
EDIT by sluckey... fixed broken link
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Still reading up. Saw something similar in the debugging article...
"Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note
You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics."
Don't have an Oscope but i'm looking into getting one. Also, I've changed pre and power tubes and it didn't fix it.
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I would check the power supply filter caps or cathode bypass caps. Sounds more like a filter cap though /
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Would that be C39, C40, C60, and C41 on the switching/supply schem? I have some 10U/450V so I may just try it.
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Replaced those four and put a new BC550 in. Same problem. I'm going to order the 82uf/450V that is immediately after the High voltage rectifier. If anyone sees anything else I should change while i have it apart, I'm willing to give it a try. Thanks.
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> shorted from C to E
Without a full fault-analysis, I'm thinking R100 or its joints opened-up. Then the full 2x6L6 current (200+mA) flows in Q12 base-emitter, C28 C84 discharge to load through collector-emitter, and the huge current could also over-volt Q12.
I can't see how else Q12 could be damaged. (Or ever do much good... the current-limit is 1.8V/2r or 0.9 Amps, 360 Watts to the amplifier, which is about four times what a 40W amp could ever need.)
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Checked r100 this morning and it is currently showing ~2ohms. I will replace it when I change the 82uf cap just to be sure. I really feel like I'm chasing my tail at this point, but I guess I've got to start somewhere. Also starting to really dislike thus amp since the tone wasn't that spectacular anyway. Thanks for the help
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You may have to remove R100 to properly measure it,but at that point it's best to change it anyway.
Did you check the bias?If it's too hot that might have something to do with it.
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> currently showing ~2ohms.
"I'm thinking R100 or its joints opened-up."
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Gotcha PRR. I'll change it and be sure and check continuity on surrounding traces when I get my next parts order. From memory I think the schem says to check bias across 90 and 91 to 75mv but that schem shows a bias pot which is not in my revision. I checked across those resistors while the amp was not functioning and I was looking at around 200mv each... Scratched my head and decided to disregard for now. Once I get new parts I will revisit that. Thanks for the help. I think what I like most about posting here is the reason not to give up. I'll update soon. Thanks!
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The guy's here will help you get it working right. I don't think they know the meaning of the words give up. Don't think I've seen a circuit these guy's can't lick! :BangHead: (well as in fix, licking a circuit is highly discouraged haha..) Good luck with it (and thanks for the Traynor manual, I save that stuff in directories, ya never know what you might run across!).
Regards,
Don
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Well changed R100 and that 82uf cap and put a new transistor in. Played it for about fifteen min. at high volume with no problems. I'm keeping my fingers crossed. Also, one more question. The caps were orginally glued to the board with some kind of silicone. What is the purpose of that and should I get some more to smear in there. If so, what type of silicone is it? Thanks again for the help and encouragement. I still don't know which part failed since both the cap and resistor test fine with my fluke dmm, but I'm happy to have it running.
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Son of a bitch! I played this thing for two days with no problems and buttoned it up last night. I turned it on after school today and the problem came back. I think what is happening is the transistor shorts and drops the HI+ through the mosfet and I'm essentially in standby mode. I get a slight crackle of a signal like I would in standby with this amp (not really sure why that happens either). Anyone have a good plan for a 40watt build i could cram into this chassis?
:cussing: