TFM-25 shuts itself down

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Jazzman53
M-400t CUBE
Posts: 134
Joined: Mon Oct 05, 2009 10:37 am

TFM-25 shuts itself down

Post by Jazzman53 » Sun Oct 17, 2010 8:13 pm

Hello all,

I have a problem with the right channel on my TFM-25.

For weeks it would spike the right power meter at startup, then play fine. But recently I powered it up and heard popping noises from the right channel and it shut down. Now the popping has stopped and it will play on both channels as long as the volume is set very low. When I push the volume above about 3 watts output the protect circuits trip and shut the amp down.

I have recapped a few amps but that's just removing and replacing parts. I have practically no electronics experience and I'm lost trying to troubleshoot this problem. I checked the idle voltages on both channels and found the left channel was OK at 1.0 mv and responds to the trim pot but the right channel is 2.8 mv and doesn't respond to the trim pot at all. Also, when discharging the big power supply caps (10,000 uf) using my 75 watt lamp, the left cap lights up my lamp brightly but the right cap doesn't light it up and tall (the amp had only been switched off about 1/2 hour earlier so both caps should have retained charge). I figure that big right channel cap is being pulled down by something shorted downstream somewhere but I have no idea how to find it.

I anyone can offer any advice, I'd be grateful

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falcon377
Newbie 50+
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Joined: Tue Nov 04, 2008 2:10 pm
Location: Pittsburgh, PA.

Re: TFM-25 shuts itself down

Post by falcon377 » Mon Oct 18, 2010 2:26 am

Leaking output transistor?? That will put it in protection at just a couple watts output power.

Jazzman53
M-400t CUBE
Posts: 134
Joined: Mon Oct 05, 2009 10:37 am

Re: TFM-25 shuts itself down

Post by Jazzman53 » Mon Oct 18, 2010 3:56 am

Thanks, I should be able to check those output transistors with my DMM (I think). I also have the issue of the right channel idle voltage sitting at 2.8 mv (s/b 1.0mv) and not responding to pot adjustments.

angelod307
SILVER-7 TUBE AMP
Posts: 2654
Joined: Sat Jun 06, 2009 10:56 pm
Location: manning, sc

Re: TFM-25 shuts itself down

Post by angelod307 » Mon Oct 18, 2010 7:28 am

well let me tell you. i have been deeply involved in some tfm35's. and have had a simular issue. i am no master tech by any means, but have got to know this amp model line as they all share the same basic design. there are indeed some transistors that have shot their load off. i would venture to say that some diodes and resistors had crapped out as well. after about 40hours hands on, tracing parts to see what the problems may be, i can tell you that you may get it in a moment or two, or 30 hours later. worst part, some of the diodes like the ones labeled "175 wiv" are on the extinct list. the info is so vague, that i am having a time getting the right replacement part. so, with that said, i would think that the bias transistor or multiples in that channel are bad as well as something giving some dc to trip the protection circuit as well. good luck on fixing it for sure, and i am willing to work on it provided you pay for shipping both ways and the parts. manning 29102 if the urge strikes you. cheers, angelo
CarverFest: I just go for the arts and crafts

Jazzman53
M-400t CUBE
Posts: 134
Joined: Mon Oct 05, 2009 10:37 am

Re: TFM-25 shuts itself down

Post by Jazzman53 » Mon Oct 18, 2010 12:32 pm

Angelo,
Thanks for the helpfull info and your generous offer too. For now I will keep tinkering with it myself and see what I can learn and do. If it comes to having someone else fix it I'd rather find a local tech than burden you with it. I've just purchased another TFM-25 (waiting for it to arrive) so it's not a show stopper if I don't get this one fixed right away. I will keep posting on what I find. Thanks again for your help!
Charlie

Jazzman53
M-400t CUBE
Posts: 134
Joined: Mon Oct 05, 2009 10:37 am

Re: TFM-25 shuts itself down

Post by Jazzman53 » Wed Oct 20, 2010 6:08 am

Update:

Symptoms:
* Initial popping on right channel has stopped-- amp plays on both channels with volume set low but shuts down when volume if increased beyond 3 watts output.
* When discharging the large power caps about 30 minutes after the amp was powered off, I found that the right channel cap had already drained.
* Right channel bias voltage is 2.8 mv and does not respond to pot adjustment.


I've checked the right channel output transistors for shorts and found no continuity leg-to-leg or legs-to-heat sink. I'm wondering if I need to lift all their base legs so I can check their diode modes out of circuit?

Also pulled the ribbon cable from the small ECB transistor screwed to the heat sink, which I believe is the biasing transistor(?). It's checks good out-of-circuit.

Also, the bias pot and that 4.7uf / 50v cap next to it check OK.

For now I need to learn the current paths and educate myself a bit more so I'm not just checking stuff randomly. This is gonna take some time.

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