2040 hot chassis and other problems

invalidbuffa

Super Member
I'm working on a 2040 and I can't for the life of me determine why the chassis is hot. I checked the ESR on the filter caps and everything looked fine, and I couldn't see any obvious problems that would cause the issue. The chassis has more potential when the unit is plugged in 'backwards' as in the prong that first goes to the fuse and switch is plugged into neutral.

The other problem is when I turn the unit on, there's no wait for the protection relay to energize the amplifier and it produces a big scary pop as soon as the power switch is pressed.

I don't know if this is relevant but at one point while I was working on it I had the dial face lamp assembly removed, and the two connectors that went to it touched while it was energized. It blew the 3A fuse and killed the diode at D912 (shorted)...I replaced the diode with a 1N4004.

So far I've performed the following work on the receiver:

I replaced the "death" RF capacitor across the power switch with a X/Y safety cap

Replaced diode at D912 with 1N4004

Replaced lamps in tuner face and pointer

Adjusted DC offset and idle current to manufacturer spec

Any ideas that could help me troubleshoot the problem are appreciated, thanks.
 
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I'm working on a 2040 and I can't for the life of me determine why the chassis is hot. I checked the ESR on the filter caps and everything looked fine, and I couldn't see any obvious problems that would cause the issue. The chassis has more potential when the unit is plugged in 'backwards' as in the prong that first goes to the fuse and switch is plugged into neutral.

The other problem is when I turn the unit on, there's no wait for the protection relay to energize the amplifier and it produces a big scary pop as soon as the power switch is pressed.

I don't know if this is relevant but at one point while I was working on it I had the dial face lamp assembly removed, and the two connectors that went to it touched while it was energized. It blew the 3A fuse and killed the diode at D912 (shorted)...I replaced the diode with a 1N4004.

So far I've performed the following work on the receiver:

I replaced the "death" RF capacitor across the power switch with a X/Y safety cap

Replaced diode at D912 with 1N4004

Replaced lamps in tuner face and pointer

Adjusted DC offset and idle current to manufacturer spec

Any ideas that could help me troubleshoot the problem are appreciated, thanks.

Reverse the AC plug and recheck.
 
The other problem is when I turn the unit on, there's no wait for the protection relay to energize the amplifier and it produces a big scary pop as soon as the power switch is pressed.

There is a bad cap in the protect circuit or a shorted relay driver transistor.
 
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How much voltage AC and DC do you have riding on the chassis ?
I believe there is a 2.2 meg Ω resistor tied from the neutral side of the AC to chassis ground.
 
A shorted transformer winding or and internally shorted power switch can pass AC current onto the chassis. Pinched wires can also be the culprit.
 
see how many AC volts from chassis to the earth pin at your electrical outlet .
then flip the plug and see how many volts again .
 
I should also mention it's hot when it's switched off.

Here are my chassis V measurements:
When the unit is off:
"right way" (hot to switch/fuse first)=3V
reversed=100V

Unit on:
"right way" =36V
reversed=90V

No DC present

On the protection circuit I tested the transistors, and they all check out. I tested ESR on the capacitors... Both caps are 16v 33uF...C949 is .9 ohms, C917 reads 1.7 ohms. According to the docs that came with the meter, C917 should probably be replaced...
 
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what is your mains voltage ?
the readings are opposite to what i was expecting . neutral/cold should be close to earth potential .
 
what is your mains voltage ?
the readings are opposite to what i was expecting . neutral/cold should be close to earth potential .

My utility voltage is 124-126V

I removed the 10A fuse, chassis still hot when plugged in backwards.
 
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looks to me like your utility is wired in reverse .
way i check is volts hot to earth should be line voltage . cold to earth should be very low ..mine is 10 volts as its faulty .
this is for uk so maybe you don't use the same terra terra system .
 
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AAArgh sorry, I'm confusing myself and everybody here...

I think I was reading ohms through the transformer. The chassis voltage is low when plugged in correctly and high when the plug is flipped. I'll edit my previous post...

And no dirt or any signs of a high resistance short to ground...except for that 2M resistor.

Thanks for the assistance so far
 
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too tired for the maths to work out voltage drop over the 2.2 meg resistor to work out the current available at chassis . if memory serves right 0.5ma is the max allowable for safety .
 
The caps came and I put them in...unfortunately they did not fix the problem :sigh:

When I measured resistances through the transistors (in-circuit) they were all in the kiliohm range except for TR911 (the one right before the relay coil) which was 340 ohms between the collector and emitter. Could that transistor possibly be bad?

I'm struggling a little with this repair because I don't quite understand how the protection circuit works. What exactly tells the relay coil to wait a few seconds before energizing? I don't see any kind of timer...
 
Ahh, the circuit makes a lot more sense now.

...I replaced both 33/16 caps.

Am I even looking in the right place? Is there something else that could make the relay come on immediately?
 
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