JCM 900 Preamp woes

[beasleybodyshop]

Hello again folks, I have myself a real problem.

I have been working on a early 90s Marshall JCM900 Dual Channel Reverb, and im not getting signal into any of the transistors in the preamp. I worked backwards from the Low Tension power supply winding that feeds the power to the M5201 transistors. Attached is a photo. The problem i am getting is this: Voltage from the LT is 20-0-20 or 40v across both positive wires respectfully - this is correct as per what ive researched. It feeds into the FWBR and comes out two ends - one positive voltage, one negative. Negative gets the ripple smoothed out by C32, Positive C31. Before it hits the dropping resistor (R33, R34,) i am getting 30/ -30 volts. on the other end, i am getting like 3/-3 volts....huh? Also when the amp is on, i can smell something burning on the LT board. I am sure it is not the transformer.

What the hell is going on? My guess is that I have a short somewhere, that is, voltage is being shunted to ground therefore not much is actually passing through the dropping resistors? I am stumped. All the highlighted stuff is components I have physically replaced with more robust versions than what it calls for. I am really pulling my hair out with this one...any help would be amazing right now

[JazzGuitarGimp]

My guess is that one or more of the amplifier transistors powered by the LT bipolar supply have failed. When transistors fail, the silicon chip melts, and causes a dead short from emitter to collector, in almost all cases.

[beasleybodyshop]

My guess is that one or more of the amplifier transistors powered by the LT bipolar supply have failed. When transistors fail, the silicon chip melts, and causes a dead short from emitter to collector, in almost all cases.

But would this cause the abnormally low voltages on the "other" side of the resistor? Thanks for the insight.

[beasleybodyshop]

Update:

After swapping out several ICs, im no longer having LT Issues! Thanks!

Now for some nitty gritty:

Attached are several photos from my Scope. I am using a 1Khz reference tone at approx 2.30 V PP. Gain control is dimed. I am just looking for signal at this point.

Second photo is the signal output on IC1 in the schematic. Its the first "gain stage" this amp sees. I will assume this is normal.

Moving on, the signal looks relatively similar to photo 2 until I get to photo 3. This was taken before R13 on the schematic (its the resistor right before the slope resistor, R62) Now, this looks ok to me. measurement taken on the other side of the resistor (closer to the tone stack) gets me to photo 4.....what does this mean? The signal shouldn't weaken that much right? Looks like the cathode follower feeds into the signal here via C11, should be a bigger waveform instead of a smaller one, right? As i check down the path, the signal just goes non-existent. I am certain that the issue at R13 is the culprit, but what do i do at this point? I checked C11, shows proper capacitance...I even swapped in a good, known functioning tube, still the same. Any thoughts? I have also attached the preamp schematic for anyone to follow along with.

[beasleybodyshop]

Double Update - Signal wasnt passing because of a poor grid connection on V1! Slowly but surely....


So photo attached shows my preamp out signal....this looks pretty ugly. All EQ controls are dimed, channel A gain at noon, master volume control off.


Ugh. Ive replaced every IC in the preamp, plus the three transistors/FETS.

At this point all it does is make shrieking noise regardless of the volume control.

Im about to give up at this point....any and all thoughts and ideas would be appreciated.

[beasleybodyshop]

Ok, almost got this thing sorted out.

It has several IC OPAMPs in the pre acting as gain stages. Something happened to the power that supplied the voltage to those ICs, and subsequently fried them. I got in a order of IC sockets today, and just finished installing them.

I have a question - with the IC sockets installed and no ICs in them, i am not getting any voltage past the dropping resistors in the LT power supply. Whenever I start to stick a bunch of the old ICs in, i see waay too much voltage on one side (like -30v, 10v) or (-14, 20v)

So my question is, do the IC's "load" the power supply and that is how it's supposed to work? I am assuming that the bad ones are causing some kind of short, which causes some kind of voltage imbalance. - Someone please correct me if I'm wrong, i would just like to know.

[roberto]

My guess is that one or more of the amplifier transistors powered by the LT bipolar supply have failed. When transistors fail, the silicon chip melts, and causes a dead short from emitter to collector, in almost all cases.

But would this cause the abnormally low voltages on the "other" side of the resistor? Thanks for the insight.

Of course. The lower the load, the higher the current, the higher the voltage drop across the resistors, the lower the voltage downstream the resistors.

[roberto]

im not getting signal into any of the transistors in the preamp. ...the M5201 transistors.

M5201s are not transistors, they are ICs.
This caused the misunderstanding with JazzGuitarGimp.

If you have no voltage after the dropping resistors, you have some shortcircuit around. Are the zeners ok?

[beasleybodyshop]

im not getting signal into any of the transistors in the preamp. ...the M5201 transistors.

M5201s are not transistors, they are ICs.
This caused the misunderstanding with JazzGuitarGimp.

If you have no voltage after the dropping resistors, you have some shortcircuit around. Are the zeners ok?

With my Diode check function on my fluke they are showing .3 v drop both ways.

They are still in circuit. I would imagine this means they are bad?

[beasleybodyshop]

I see these are 15v zeners. Any harm in using say 20v or 30v ones?

[roberto]

Yes. Max voltage supply of cmos is +-18V.
I think you should call a technician.

[beasleybodyshop]

Yes. Max voltage supply of cmos is +-18V.
I think you should call a technician.

You were right. I had no knowledge of bi-polar power supplies.


So I did, I found a tech who was happy to explain to me how to troubleshoot it.

I was checking voltage from chassis to component. I didnt understand why i was getting really odd voltages from -30 to 10v or none. I replaced everything - 15v zeners, all caps, and socketed all the IC slots. Still very little signal. Now i check continuity from -/+ LT power to each IC...two of them have shorted. A little sliver of the PCB trace was missing.....i wouldnt have even been able to tell if i wasnt heavily checking continuity every few inches.



Now im getting 0 volts with my negative lead on my meter to ground. Now I check from 20v LT winding center tap to the -/+ terminals on the ICs. now i am getting 16v +/-!

I check continuity from preamp board to ground. nothing. Huh? I wire a ground from preamp to chassis. Now it makes noise! Checking signal on the scope looks as good as anything thats clipped hard with those ICs/diodes.

So happy it works now! Sorry for the long winded posts, but maybe it can help those who have to do these kinds of repairs in the future! FWIW, the amp sounds terrible. lol. I would have gutted it and built a JCM 800!

[M Fowler]

JCM900 you mean one of these amps? This one sounded okay after I repaired the pots lifted from the board, easier fix then yours.