Customer brought his 100W JCM 2000 into the shop. One of the power tubes isn't heating. I swap tubes, the problem stays with the one socket of interest ( "V6").
I take out the tube from that socket, stick my multimeter probes into pins 2 and 7 of the socket and measure 7V RMS AC. Looks about right. Then I put the tube back in and measure the voltage again between pins 2 and 7, (but on the PCB, not in the socket since there is a tube in there now). I now measure 0V!
When I look at both pins of the PCB with my O-scope I see that now (with the tube inserted into that socket) both voltages are indeed tracking each other. They appear shorted.
So, I think, ah hah, a short in the tubes heater wiring! So, I swap the tube with its neighbor (V7) and the problem does not follow the tube, it stays with the same socket.
I must admit I am puzzled about this.
Anyone seen anything like this before?
It seems that placing a tube in the socket causes a short between socket pins 2 and 7.
Marshall JCM2000 (100W)- heater circuit failure
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Stevem
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Re: Marshall JCM2000 (100W)- heater circuit failure
You have a bad connection! It takes little if any current flow to get a meter or a O scope to read something, but if the tube can not pull its needed current due to a poor connection it will not light up!
I would re- flow all the solder connections that deal with powering that socket, if that does not do it than replace that socket, or get a scribe ground to a sharp pint and retention pins 2 and 7 on that socket and give all those other pins a good look over also as your heater issue may also be due to arcing from pins 3 or 4 to pins 2 or 7!
I would re- flow all the solder connections that deal with powering that socket, if that does not do it than replace that socket, or get a scribe ground to a sharp pint and retention pins 2 and 7 on that socket and give all those other pins a good look over also as your heater issue may also be due to arcing from pins 3 or 4 to pins 2 or 7!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Marshall JCM2000 (100W)- heater circuit failure
Yep, 2K resistance solder joint to one sockets pin-7
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Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Marshall JCM2000 (100W)- heater circuit failure
Cool!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!