Marshall JTM30 bias mod problem & IC3 (LM348) problem

[Tone Meister]

JTM30 has two issues. 1) excessive hum and 2) bias mod problem

Problem 1: The amp powers on but has substantial hum that overrides any amplified audio signal. I can unplug CON3 and that noise is eliminated - of course a slight hum is introduced with CON3 disconnected but that is expected. Neither channel volume effects the hum, but it is eliminated when the MASTER VOLUME is all the way down. Tested IC3 op amp and there is negative DC voltage on every pin.

IC3 LM348N - all voltages DC
pin 1 (-4.7) ... pin 8 (-16.5)
pin 2 (-5.0) ... pin 9 (-15.4)
pin 3 (-5.6) ... pin 10 (-14.6)
pin 4 (-5.9) ... pin 11 (-14.7)
pin 5 (-7.4) ... pin 12 (-12.4)
pin 6 (-8.5) ... pin 13 (-11.5)
pin 7 (-9.5) ... pin 14 (-10.3)

Problem 2: The power tubes are redplating and bias idle current is excessive. The bias circuit was previously modded - the zener string (ZD101, ZD102, and ZD103) was removed in favor of a 56K R - 50K pot - 56K R string. I'm not able to adjust the bias with the bias pot. I have confirmed that both 56K resistors are good and the pot itself works fine in isolation. The stock JTM 30 has a zener diode regulated bias supply that is "non adjustable", so it runs hot because Marshall calls it a "class A amplifier" with two 5881s producing ~30 watts. This amp still has what appears to be the stock power tubes.

I've done this mod myself to a couple of these in the past and have had no problems, but I had taken working amps and performed the mods entirely myself from start to finish.

Can these two problems be related?
Is that LM348N op amp toast?

[Enzo]

The 348 could be defective, but not from anything you have shown us. Clearly the +15v rail is missing. Are the channel switching LEDs working? And the relay? They also run on +15. Is the 78L15 getting input voltage?

I see no involvement in bias from the +15v. The bias supply is just s simple supply.

So what bias supply voltage is on the grids? The zeners add up to 39v, do you have anything like that?

I am not sold on the adjust mod. After all, what it amounts to is a 150k resistance in parallel with R132. The pot will dial it down to 100k. So I think the range of adjustment is roughly 50k-60k.

[MarkusBass]

Please take a look at the schematic. On pin #4 there should be +15V. Most probably, the REG2 (78L15) has failed, or R35 is open. Do you have voltage on the input of REG2?

Mark

[Tone Meister]

Will check those things with the +15v rail today.

I guess I'll try to figure out why the bias has run away and isn't adjustable with the R string first before trying to restore the bias circuit to stock. Otherwise I'd have to order the correct value zeners. We've modded these for bias adjust before and the amps work well.

[Tone Meister]

UPDATE

Problem 1 - This was easily solved. The two metal standoffs for the tube PCB is ground, and without them screwed tight there is no ground. Both screws were missing, so when I replaced the screws the +15v rail was restored. Voltages at the LM348 are what they should be. Hum is gone.

Problem 2 - There is -17.8 volts at the grid of each power tube, measured at R127 and R129. The bias is still not adjustable with the Bourns 50k multi turn pot. I removed and tested the pot and both 56k resistors and checked each solder pad for integrity. Reinstalled the bias mod resistor string and still unable to adjust the bias. There are 0.0 volts at one end of the bias R string and those 17.8 volts at the other. Checked and verified C127, R133, R132, C126, D108, D109, F1, D2, and D3.

No doubt this will be as simple as the +15v rail but right now I don't see it. Any insight?

[g1]

Did you check R107?
There are checks you can make with power tubes removed. AC voltage on either side of R107 and C127. DC voltage at anode of D107.

[Tone Meister]

Did you check R107?
There are checks you can make with power tubes removed. AC voltage on either side of R107 and C127. DC voltage at anode of D107.

AC voltage on either side of R107 196 & 190 VAC
AC voltage on either side of C127 190 & 92 VAC
DC voltage at cathode of D107 52.5 VDC
DC voltage at anode of D107 -22.6 VDC

[g1]

That would be minus 52.5VDC ?
So you have -52VDC across your string of 56K resistor, 50K pot, and 56K resistor?
What are your voltages at each those 3 components, and how is the pot wired?

[Tone Meister]

+52.5 at the cathode
-22.6 at the anode

at the top of the 56K where the 9V ZD101 once was = 0.0 vdc
Between the top 56K and the pot = -7.9 vdc
Other side of pot where it connects to bottom 56K = -14.8 vdc
Bottom of 56& where ZD103 once was = -22.6 vdc

[g1]

You have 92VAC going to D107, you should get a lot more negative at the anode than what you are getting (-22.6).
I'm guessing D107 anode is no longer connected to C126.
You said you have done this mod before, I'm sure you will find this has not been done the same way.
You need to draw out what you have for the bias circuit there, and compare it to the drawing for the way you have done it before.
No drawings, no go.

[Tone Meister]

That's what's funny, this one is wired the same way as the ones I've done. It's really as simple as removing 3 zeners and replacing them with 3 resistors, one being variable. There is continuity from the anode of D107 to C126.

Here is a crude drawing that shows the same thing as the annotated snapshot from the schematic in post #9.

[Enzo]

I don't see where you replaced or tested D107.

[g1]

And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.

[Tone Meister]

I don't see where you replaced or tested D107.

D107 tested good as did R107, R105 and R106. There is something simple here that I am missing

And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.

G1, I've done this mod myself twice before and each time was able to adjust bias with the pot. Certainly my circuit prowess has a lot of room for growth, and perhaps my drawing is flawed. This one is wired the same way I recall doing it in the past. I am missing something very simple here.

The series zener string is replaced with the series resistor string, which is in parallel with R132

56K resistor replaces ZD101
50K pot replaces ZD102
56K resistor replaces ZD103
series string in parallel with R132

[Tone Meister]

And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.

Will you explain why?