Marshall jvm410h problem

[Rod]

i have a problem im my JVM410h, has you can ear in the clip bellow the amp souds strange, and it seems like the signal its squeezed and distorted, when i play harder in the clean channel.

the gain channels dont have enough gain, something its going on here and i dont know What it is.

I know electronics very well so i can repair the amp but dont have idea where to start, so can someone tell me what could this problem be?

https://drive.google.com/file/d/1Zhf...ew?usp=sharing

[Helmholtz]

Please post schematic and all supply and tube plate and cathode voltages as well as power tube screen voltages.

[Rod]

Please post schematic and all supply and tube plate and cathode voltages as well as power tube screen voltages.

OK finally i had time to measure, voltages, I have notice strange noises and channel switching system noise, when i putted the multimeter proble in R23.

what do you guys think of the measured voltage? is it necessary to measure more voltages?

i have attached amp schematics.










jvm410-61-02-iss1-1.pdf
jvm410-60-02-iss2-1.pdf
jvm410-61-02-iss1-2.pdf
jvm410-60-02-iss2-2.pdf

[Helmholtz]

First of all:
Can you exclude a bad speaker?
Did you try patching Preamp out and PA return jacks with a cable?
Did you check connectors for contact problems?

Please add power tube cathode and individual screen voltages measured at tube sockets and verify low voltage supplies.

Do you have a scope, a signal source/generator and a dummy load?

[Rod]

Hello! Helmhotz.

Ok i bought a scope and i have notice something strange when i put the probe of the scope in the R92 before the first preamp tube, has you can see in the video bellow, the sine wave of the signal goes nuts, after i remove the amp from standby.
There is signal in the resistir R127.

what can be the problem here?


https://drive.google.com/file/d/1_tD...ew?usp=sharing

First of all:
Can you exclude a bad speaker?
Did you try patching Preamp out and PA return jacks with a cable?
Did you check connectors for contact problems?

Please add power tube cathode and individual screen voltages measured at tube sockets and verify low voltage supplies.

Do you have a scope, a signal source/generator and a dummy load?

[Helmholtz]

The grid drifting up to a positive DC voltage might indicate a bad (gassy, poor vacuum) tube.
This is further confirmed by low plate voltage.

Replace V6 with a good new tube.

Also measure resistance of R70 and R71.

[Rod]

I have replaced the tube in V6 with a new good tube, and the same problem happens but it seems that this drifting is more slow, as you can see in the link bellow.

i have measured R70 and R71 and they are ok.

Could this be a problem in the heaters of V6?

https://drive.google.com/file/d/1aBr...ew?usp=sharing

The grid drifting up to a positive DC voltage might indicate a bad (gassy, poor vacuum) tube.
This is further confirmed by low plate voltage.

Replace V6 with a good new tube.

Also measure resistance of R70 and R71.

[Helmholtz]

Please do the following:

- Measure resistance from V6b grid to ground
- Re-measure V6b cathode and plate voltages (without signal) as well as HT5 voltage.
- If your meter has a high input resistance >1M, you might be able to see a positive (rising) voltage at the grid.

Watch for voltage drift over at least 2 minutes.

What is the input resistance of your scope probe?

[Mick Bailey]

I'm wondering if R91 is open and there's no ground reference for V6b's grid.. This could account for the blocking distortion in the first audio clip.

[Helmholtz]

I'm wondering if R91 is open and there's no ground reference for V6b's grid..

Yes, that would show with the resistance measurement between grid and ground I suggested above.
An open grid could also explain the drift especially when using a 10M probe, while a 1M probe (or DMM) would just substitute the grid leak resistor.

[Rod]

-Helmholtz do you want me to measure resistance from V6b grid to ground with the amp on or off? with the amp off it measure 1Mohm.

Im noticing V6b voltage drop in the plate from 160V to 145V after a while with the amp on and on standby.

How to i measure the probe input resistance?

[Rod]

Im noticing alot of sensetivity at R91, and its measuring aroung 0.9 Mohms for a resistor 1% tolerance, this resistor could may be the problem

[Helmholtz]

-Helmholtz do you want me to measure resistance from V6b grid to ground with the amp on or off? with the amp off it measure 1Mohm.



How to i measure the probe input resistance?

Resistance measurements are always done with amp off and caps discharged.
1M between grid and ground looks good.

Im noticing V6b voltage drop in the plate from 160V to 145V after a while with the amp on and on standby.

Do you mean plate voltage? Standby mode is irrelevant here.

Please post all voltages as requested.

How to i measure the probe input resistance?

You should find it in the probe specs (manual).

Typically 1X probes provide 1M (corresponding to the direct input impedance of the scope) and 10X probes 10M.

You can use your Ohmmeter at the probe input with probe connected to scope in DC coupling.

[Rod]

Im using the probe on 10x.

Helmhotlz, what do you think of R91 measuring around 0.91 Mohms, for a 1% tolerance resistor?

[Helmholtz]

Helmhotlz, what do you think of R91 measuring around 0.91 Mohms, for a 1% tolerance resistor?

Should work the same. Exact resistance is not critical.