Nov 18, 2008 03:01 pm Ha, at least you found it, without replacing tons of stuff.
Good tid-bit to keep in mind.
Thanks.
Electronic repair
Posted on Nov 18, 2008 02:54 pm
Rob Stemple
The Czar of BS
Member Since: Dec 31, 2007
Here is a good one for all of you that do repairs.
I am working in a Hot rod Deville amp. The complaint is that it flutters on the drive channel, when it's selected. But clean on the normal channel.
When I brought it in to my shop, I could see that there two harmonics on the channel.
My first thought was the tube. So I replaced the 12AX7. Same results.
OK, I moved on. Thinking that I had a bad diode somewhere. All of the didoes where good.
So, I kept probing to see what could be causing this. And man this took a while to find out what was going on.
After three days of coming to it, then walking a away to deal with other things. I finally found it.
Turns out, it was a resistor. "A resistor?" Yeah, a resistor. When the amp is cold, the resistor test normal. It's in spec., and behaves the way a resistor is suppose to.
When the amp gets warm however, the said resistor starts to behave like a diode. Never seen anything like it. It only have conduction in one direction. And you can only measure resistance in that direction. It also oscillates at 60hz, and 10khz.
The resistor sits at pin one of the 12AX7 (Valve 2). It's the 100k (R16) at a 1/2 watt. You don't see that everyday.
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Nov 18, 2008 03:01 pm Ha, at least you found it, without replacing tons of stuff.
Good tid-bit to keep in mind.
Thanks.
Nov 18, 2008 05:04 pm Instrument Repair Bill
Resistor.............. $0.01
Labor................ $900
just like fixing a car :)
Nov 18, 2008 05:18 pm That's why I hate working on people's computers now. Spend 4 hours to find a problem, and they only want to pay 20$ for the 5 minutes it took to actually fix it.
bah.
Nov 18, 2008 05:23 pm Hey, don't be showing discounted labor cost to everyone. Next thing you know, people will only want to pay that low of a rate.
Gesh, a guy has to make a living you know. =)
Nov 18, 2008 09:42 pm That sounds like a Snubber Resistor, but they are usually packaged like the TO2 type package and don't look like a common resistor at all.
But the oscillating does sound like a Snubber circuit that is going bad. But still it is only a lowly resistor.
OK, I'm over thinking this.
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