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Texas Star DX35O Smoked

nbsr

wdx 3277
Mar 29, 2014
35
18
18
Hudson Valley, NY
A friend came by and said while he was keyed up and yappin, there was a power surge (sounded like a slight delay power switch from the electric company) and suddenly the amp started smoking. Power supply did not suffer any issue nor did the radio.
Without hooking it up, I took the cover off and R5, R7 and R8 were toast. Fuse didn’t blow (30A). I replaced the 3 resistors, reconnected to power and OFF, R8 toasted immediately. I pulled one leg of D4 even though the fuse was intact, and it tested to still be a diode. Then for shits and giggles I checked the resistance between the positive and negative rail which gave a 189 ohm reading. This gentleman is on a fixed income and disabled and without saying it, there is no way he can replace it financially. He’s a true radio head and has been since the 60s and I’m trying to help him out. I’m trying not to disassemble the entire amp but if the board has to come out, I won’t be lazy about it. Both he and I would appreciate any assistance you all might have.
 

nbsr

wdx 3277
Mar 29, 2014
35
18
18
Hudson Valley, NY
not sure if this will help
IMG_2620.JPG
 

DoctorAudio

Well-Known Member
Sep 11, 2009
165
227
53
Once R8 opened, straight 12 volts went to the bases and would have blown both transistors and 10 ohms.
It seems strange that R8 smoked again when you powered it back up after replacing it. Were you using your friends power supply? If so you might want to measure the output voltage and make sure it's not putting out 24 volts.
 
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nomadradio

Analog Retentive
Apr 3, 2005
5,560
8,191
573
Louisville, KY
www.nomadradio.com
Lifting JUST the base lead of each transistor so that it's isolated from the circuit board allows checking for the most-common failure.

A short from base to collector. Causes the full power-supply voltage to come out of the base terminal, feeding into what should be a 6/10 of a Volt circuit. This is a 20-to-1 overload factor for all the parts connected to the base of the transistor. Smoke and discoloration are the signs.

But if you just check each transistor for a short from base to emitter it saves a lot of wasted effort, hoping for a cheaper explanation.

73
 

nbsr

wdx 3277
Mar 29, 2014
35
18
18
Hudson Valley, NY
There's an old Jewish saying, "Man plans, God laughs".
Nothing too serious, but I have a surprise surgery coming up this week and all the prep bullshit. Not looking for sympathy, but if someone asked me for help and I don't hear back, that's the last time I help. So just wanted to let you all know I dig the shit out of the info you gave me and will be diving in when possible.
Thank you.
Mark
 
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nbsr

wdx 3277
Mar 29, 2014
35
18
18
Hudson Valley, NY
Nomad, I had some time to kill and got into it.
Pill #1
B-C 346 ohms, 435K
B-E 362, 447K

Pill #2
B-C 16 ohms, 16
B-E 4.6 ohms, 4.6

Thanks for the down and dirty!
Mark
 
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