JCM900 reverb tank

Without tubes. No standby switch on. No operate.
V1 pin2 3,4Vac.
Pin5 -37,2 Vdc.
Pin7 3,4 Vac.
Pin3 & 4 340Vdc
V2 pin 2 3,4 Vac.
Pin5 -37,2 Vdc.
Pin7 3,4Vac.
Pin3 & 4 340Vdc
V3 pin 2 3,5Vac.
Pin 5 -37,2 Vdc.
Pin 7 03,5Vac.
Pin3 & 4 340Vdc
V4 pin2 3,5 Vac.
Pin5 -37,2 Vdc.
Pin7 3,5 Vac.
Pin3 & 4 340Vdc

Doing this mesuraments I saw R30 from rear board with voltages arc flashes on It.
:confused:
 
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Without tubes. Standby switch on.
V1 pin 3 505 Vdc.
Pin4 495 Vdc.

V2 pin 3 505 Vdc.
Pin4 495 Vdc.

V3 pin 3 505 Vdc.
Pin4 495 Vdc.

V4 pin 3 505 Vdc.
Pin4 495 Vdc
 
Without tubes. Standby switch on.
V4pin 3 492Vdc.
Pin4 493 Vdc.

V5pin 3 493Vdc.
Pin4 493 Vdc.

V6pin 3 492 Vdc.
Pin4 493Vdc.

V7 pin 3 492 Vdc.
Pin4 493 Vdc
 
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Without tubes. No standby switch on. No operate.
V4 pin2 3,4Vac.
Pin5 -70,2 Vdc.
Pin7 3,4 Vac.
Pin3 & 4 136Vdc

V5 pin 2 3,4 Vac.
Pin5 -70,2 Vdc.
Pin7 3,4Vac.
Pin3 & 4 136Vdc

V6 pin 2 3,5Vac.
Pin 5 -71,1 Vdc.
Pin 7 03,5Vac.
Pin3 & 4 137Vdc

V7 pin2 3,5 Vac.
Pin5 -71,2 Vdc.
Pin7 3,5 Vac.
Pin3 & 4 137Vdc

Doing this mesuraments I saw R30 from rear board with voltages arc flashes on It.

Edit: it's inside C15
 
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marshalltsl said:
As output transformer it's broken I guess.
Click to expand...
If you have voltage 492DC on pins 3, 4 of output tubes, output transformer looks OK so far.
Power on.
Standby set to "warmup." Do not turn standby to operate position.
Pin 5: turn the bias adjust pot and see if you can set pin 5 for -42 VDC.

If you have 3.4 volts AC on pin 2 and 7, the filament is working OK so far...

What model JCM 900?
What output tubes?

However do not put tubes in or play / operate the amp.
 
Amp Mad Scientist said:
If you have voltage 492DC on pins 3, 4 of output tubes, output transformer looks OK so far.
Power on.
Standby set to "warmup." Do not turn standby to operate position.
Pin 5: turn the bias adjust pot and see if you can set pin 5 for -42 VDC.

If you have 3.4 volts AC on pin 2 and 7, the filament is working OK so far...

What model JCM 900?
What output tubes?

However do not put tubes in or play / operate the amp.
Click to expand...
Its a EL34 JCM900 Dual reverb. I think C15 it's open. Yesterday bias voltages was ok. Today when doing this mesuraments I saw C15 with an intermitent light inside on It. Then negative bias voltage change. - 37 vdc first Then -71dc.
Tomorrow I will try adjust bias pot to see if I got -42 Vdc. You know I'm not an amp tech but I know how use a tin welder. If I not can get -42vdc on pin 5 I will disasemble C15 and test It with a cap meter. Thank you Amp Mad Scientist and 67plexi for help me.
 
marshalltsl said:
Its a EL34 JCM900 Dual reverb. I think C15 it's open. Yesterday bias voltages was ok. Today when doing this mesuraments I saw C15 with an intermitent light inside on It. Then negative bias voltage change. - 37 vdc first Then -71dc.
Tomorrow I will try adjust bias pot to see if I got -42 Vdc. You know I'm not an amp tech but I know how use a tin welder. If I not can get -42vdc on pin 5 I will disasemble C15 and test It with a cap meter. Thank you Amp Mad Scientist and 67plexi for help me.
Click to expand...
We have to identify the amp better.
Is C15 connected from pin 1 to pin 6 of the phase inverter?

There are 2 different schematics, the part numbers on the 2 boards are different.
We need to find out which one you have.

On the rear circuit board, there are 4X 5 watt big resistors.
Each 5 watt resistor connects to pin 4 of a power tube.
Are these 4 large (5 watt) resistors 470 ohm or 2.2 K ohm?


These amps came stock with either 5881 tubes or EL34 tubes.
The resistors tell you which version you have.
People often change them to different tubes....that's why we need to look and see "if" it's been changed.

Once we see which board you have and how it is set up, for which tubes...original or changed?
Then we know more.
 
marshalltsl said:
Its a EL34 JCM900 Dual reverb. I think C15 it's open. Yesterday bias voltages was ok. Today when doing this mesuraments I saw C15 with an intermitent light inside on It. Then negative bias voltage change. - 37 vdc first Then -71dc.
Tomorrow I will try adjust bias pot to see if I got -42 Vdc. You know I'm not an amp tech but I know how use a tin welder. If I not can get -42vdc on pin 5 I will disasemble C15 and test It with a cap meter. Thank you Amp Mad Scientist and 67plexi for help me.
Click to expand...
The bias adjust pot might be cooked.
That explains why the bias reads -71 VDC. The wiper of the pot could be fried.
 
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