JCM800 No Sound

[MadMax64]

I'm new to the forum but have been here looking for a solution for a few days. I found a post from 15 years ago with the same title but it doesn't seem they posted/confirmed the solution. I have some experience building a 50W Plexi kit and have made some repairs to my 65 Deluxe Reissue, so a buddy asked me to take a look at his JCM800 that "has been dead for a few months". It powers up but makes absolutely no sound, I can't hear anything through it. I've checked all the voltages (see below) but other than some higher than expected readings, I can't find anything wrong. Fuses are good, tried known good tubes from my amp, known good speaker, all the input and output jacks check out. I've tried some continuity checks on the OT but haven't wanted to desolder anything yet.

V1
1: 310.2V
2:
3: 2.5V
4:
5:
6: 363.5V
7:
8: 3.8V
9:

V2
1: 415V
2:
3:
4:
5:
6: 421V
7: 415V
8: 3.8mV
9:

V3
1: 417V
2: 24.9V
3: 28.8V
4:
5:
6: 231V
7: 14.8V
8: 28.8V
9:

V4
1:
2:
3: 453V
4: 444V
5: -41.7V
6: 453V
7:
8:

V5
1:
2:
3: 454V
4: 449V
5: -41.8V
6: 453V
7:
8:

There are a couple things about this amp that are different. One of the output jack wires runs through the first jack, to the second jack, and then returns to the first. It appears to have been soldered at some point but is not connected now.

IMG_3574.jpg

There is a pot on the back panel marked D.I. that can be seen to the left of the output jacks in the picture above. Here is a shot of the back panel. It appears to be factory installed. I've found a little info online about it but not really sure what it does.

IMG_3579.jpg

[pdf64]

So this is a 2204? JCM800 is a product range, dozens of models.
What year?
Are the valves in V2&3 actually getting warm?

[Helmholtz]

Three tubes in V3?

[MadMax64]

So this is a 2204? JCM800 is a product range, dozens of models.
What year?
Are the valves in V2&3 actually getting warm?

Based on the SN it was built in 1988. V2 doesn't seem to be but V3 is getting hot.

[MadMax64]

Three tubes in V3?

Copy/Paste...forgot to change the power tubes. It's corrected now, thanks!

[romberg]

V2 doesn't seem to be but V3 is getting hot.

The voltages look off (especially for V2). If it were working correctly it would be conducting and you should have a volt or two on pin 3. See this page for a very general voltage chart:

https://mhuss.com/MyJCM/vchart.html

Note that this is for an 82 amp with a power transformer that puts around 400v on the plates. Your amp is later and has higher voltages. None the less, it still looks like V2 is suspect as you see nothing on the pin 3 cathode.

I'd check the *AC* voltage between pins 4/5 and 9 on V2 and V3. These are the heater pins. You should have a bit under 7vac on them. If not that is the problem.

Mike

[Reeltarded]

The wire that wraps at the output jacks used to ground the FIXED DI. That dual pot is added where a cap, resistor, and jack used to be.

Where do the wires go? Looks like some attempt at a master volume.It appears to still be installed. It looks like it has the output tube grid feeds attached but it has no safety resistors. [ppimv]..

Get real close with your nose to that pot. Do you smell ozone metallic? Where the wires go will be the empty places on the board marked for 220k or 150k on the controls side of the board between the two output caps, on the other end of those caps an 82k and a 100k make a triangle with a 50p-100p small cap that crosses at their tube side end.

I see two orange and a white. The orange are the returns to where the board used to have output grid resistors and I am imagining the white is your bias feed. Bias is measured in milivolts. (mV) I don't like the looks of that dual pot with no safety resistors. The bias signal with these types of things usually is shield from the sends.

An image of the output end of the board is required to make any other judgement.

[pdf64]

V2 doesn't seem to be but V3 is getting hot.

The voltages look off (especially for V2). If it were working correctly it would be conducting and you should have a volt or two on pin 3. See this page for a very general voltage chart:

https://mhuss.com/MyJCM/vchart.html

Note that this is for an 82 amp with a power transformer that puts around 400v on the plates. Your amp is later and has higher voltages. ...

That voltage chart is good, note that the HT is up near 470V, you may have thinking of the earlier JMP era 50 watters?
I think a 1988 2204 would had similar voltages on V3-5, but may have higher voltages on V1&2, as the design changed mid 1988 and a HT node was removed, such that V2 was fed from the same node as V3.
https://el34world.com/charts/Schematics ... 4_mv50.gif
https://el34world.com/charts/Schematics ... w_2204.pdf

So this is a 2204? JCM800 is a product range, dozens of models.
What year?
Are the valves in V2&3 actually getting warm?

Based on the SN it was built in 1988. V2 doesn't seem to be but V3 is getting hot.

A valve that doesn't warm up can't work. Just to be clear, if either triode of V2 wasn't operational, the amp wouldn't pass any guitar signal. Though there should some background hiss hum with your ear close the the speaker.

I suspect only one triode in V3 is warming up and working, the pins1-4 triode may as well not be there, no current seems to be flowing.

What's strange is that V1 is warming up and working.

Is the valve in V3 getting aa warm as the valve in V1?
Is an orange glow showing in both triode sections of ghe valve in V3?

Have you eliminated the possibility that there's a couple of faulty valves?

[MadMax64]

V2 doesn't seem to be but V3 is getting hot.

The voltages look off (especially for V2). If it were working correctly it would be conducting and you should have a volt or two on pin 3. See this page for a very general voltage chart:

https://mhuss.com/MyJCM/vchart.html

Note that this is for an 82 amp with a power transformer that puts around 400v on the plates. Your amp is later and has higher voltages. None the less, it still looks like V2 is suspect as you see nothing on the pin 3 cathode.

I'd check the *AC* voltage between pins 4/5 and 9 on V2 and V3. These are the heater pins. You should have a bit under 7vac on them. If not that is the problem.

Mike

Heater voltage is 6.64VAC

I put another 12AX7 in V2 and and still got no reading on pin 3.

[MadMax64]

V2 doesn't seem to be but V3 is getting hot.

The voltages look off (especially for V2). If it were working correctly it would be conducting and you should have a volt or two on pin 3. See this page for a very general voltage chart:

https://mhuss.com/MyJCM/vchart.html

Note that this is for an 82 amp with a power transformer that puts around 400v on the plates. Your amp is later and has higher voltages. ...

That voltage chart is good, note that the HT is up near 470V, you may have thinking of the earlier JMP era 50 watters?
I think a 1988 2204 would had similar voltages on V3-5, but may have higher voltages on V1&2, as the design changed mid 1988 and a HT node was removed, such that V2 was fed from the same node as V3.
https://el34world.com/charts/Schematics ... 4_mv50.gif
https://el34world.com/charts/Schematics ... w_2204.pdf

So this is a 2204? JCM800 is a product range, dozens of models.
What year?
Are the valves in V2&3 actually getting warm?

Based on the SN it was built in 1988. V2 doesn't seem to be but V3 is getting hot.

A valve that doesn't warm up can't work. Just to be clear, if either triode of V2 wasn't operational, the amp wouldn't pass any guitar signal. Though there should some background hiss hum with your ear close the the speaker.

I suspect only one triode in V3 is warming up and working, the pins1-4 triode may as well not be there, no current seems to be flowing.

What's strange is that V1 is warming up and working.

Is the valve in V3 getting aa warm as the valve in V1?
Is an orange glow showing in both triode sections of ghe valve in V3?

Have you eliminated the possibility that there's a couple of faulty valves?

One of the first things I did was put all known good tubes in from a 1987 clone that I built a few years ago. The 1987 worked before and after I placed the tubes into this amp, so those tubes were not damaged by what ever is going on here. Even with those tubes there was still ZERO sound coming out the amp. Yes, V3 is as hot as V1 and is glowing as expected.

[MadMax64]

Thanks for the quick responses!

[MadMax64]

The wire that wraps at the output jacks used to ground the FIXED DI. That dual pot is added where a cap, resistor, and jack used to be.

Where do the wires go? Looks like some attempt at a master volume.It appears to still be installed. It looks like it has the output tube grid feeds attached but it has no safety resistors. [ppimv]..

Get real close with your nose to that pot. Do you smell ozone metallic? Where the wires go will be the empty places on the board marked for 220k or 150k on the controls side of the board between the two output caps, on the other end of those caps an 82k and a 100k make a triangle with a 50p-100p small cap that crosses at their tube side end.

I see two orange and a white. The orange are the returns to where the board used to have output grid resistors and I am imagining the white is your bias feed. Bias is measured in milivolts. (mV) I don't like the looks of that dual pot with no safety resistors. The bias signal with these types of things usually is shield from the sends.

An image of the output end of the board is required to make any other judgement.

Here are photos of the wires at the pot and where they land on the board

IMG_3586.jpg

IMG_3587.jpg

Here is a photo of the power section

IMG_3583.jpg

[pdf64]

So there's definitely a glowing heater at the top of the V3 pins1-3 triode (ie as well as the one at the top of the pins 6-8 triode)?
Whatever, a non functional V3 pins1-3 triode might not completely mute the amp, so your task is to identify the bad connection which is preventing the valve in V2 from warming up.
Realistically, given the valve and heater supply are reported good, it can only either be a dry joint at V2 socket lugs or a bad contact between V2 pin to socket.

If there's no hiss / hum, then maybe the OT secondary is open circuit somewhere. What resistance do you measure at the speaker output sockets?

[MadMax64]

So there's definitely a glowing heater at the top of the V3 pins1-3 triode (ie as well as the one at the top of the pins 6-8 triode)?
Whatever, a non functional V3 pins1-3 triode might not completely mute the amp, so your task is to identify the bad connection which is preventing the valve in V2 from warming up.
Realistically, given the valve and heater supply are reported good, it can only either be a dry joint at V2 socket lugs or a bad contact between V2 pin to socket.

If there's no hiss / hum, then maybe the OT secondary is open circuit somewhere. What resistance do you measure at the speaker output sockets?

Yes, definitely a glowing heater at the top of the V3 pins, both triodes. Measuring the speaker output jacks using the speaker cable tip/sleeve I read 0 ohms. Is there another way to check them? I've suspected the OT from the beginning because the amp is so quiet, but wanted to eliminate everything else. I checked the cathode resistor for P3 and its 826 Ohms which matches the schematic.

[sluckey]

Measuring the speaker output jacks using the speaker cable tip/sleeve I read 0 ohms.

That's about what you should read.

The voltages you listed for V2 are messed up and would account for no sound. V3 pin 1 is wrong also. It should read about the same as pin 6. Recheck the voltages for V2 and V3.