JCM 900 SLX Feedback

[Tage]

I have a a JCM900 SLX that begins too feedback a lot once the master, that is master "A", is turned up. Now I know that these amps are kinda anomalies but there are 2 master volumes that are op amps with 1M pots in there feedback loops to set their gain. I don't have a pedal to try switching to the "B" volume to see if it does the same thing. Could the op amps be causing this feedback themselves? I checked the pots and they seem alright. all I have tried so far is upping the value of the 10pF cap to 100pF to help snub some even higher frequencies in case it was oscillating and that seemed to help a bit.

Here is the schem

http://www.drtube.com/schematics/mar...slxa-61-02.gif

[Mick Bailey]

Changing component values in an amp that's got a fault isn't generally advisable. The amp worked when it was new with the original values, so the best course of action is to replace like-with-like until the fault is fixed.

I suspect you may have a preamp valve that's gone sufficiently microphonic to feed back. Gently tap the ECC83s to see if this is the case. You can switch them round to move the offending valve to a lower gain position if this is the problem.

[Tage]

Yeah it's defiantly not any of the tube as all have been swapped with known good tubes. what it really does is feeds back at high volume settings and then when you strum a chord it clips hard and a volume drop occurs and then the volume returns to normal levels with feedback, well a high pitched squeal. the hard cutoff has got me I'm not sure what is causing that to happen unless the op-amps that control the master volumes have a fault and they are clipping the signal.

[Chuck H]

Sounds like a ground fault to me. Have you traced the signal through the amp to isolate the problem?

[jmaf]

Marshall wanted to please the Grunge crowd of the early 90's with that amp, it's probably their highest gain hybrid tube/opamp circuit out there so it's expected it should give much feedback.

About the guitar feeding back, that is just how that amplifier works on maximum gain. Using long plate preamp tubes on a SL-X will increase noise and feedback. I prefer short plate JJ tubes on that amp, I learned how to tame the beast that way.

EDITED/ADDED:

By the way, the volume disappearing problem is common on SLX 2100 heads. I've found it was these common causes:

- There is a resistor feeding the inverter tube, which fries. I used to replace it with two 3 Watt resistors in series, sticking up and out of the PCB. It doesn't simply fail, it works so hot it may develop a bad solder and symptoms were various, like AM radio noise from arcing, volume disappearing or lowering for no reason, etc.

- The preamp tube, or tubes(don't recall), heaters use a rectified supply. The factory diode bridge there on 1993 and 1994 models simply fries, it's under-dimensioned for that job, reckless calculation from Marshall engineers. If this happened, you'll need to have a look under the PCB, it normally burns that track too which goes into the 10000uF cap.

- The factory potentiometers are xing ling no brand and simply fail. I don't know why Marshall thought it'd be a good idea to save 30 bucks on the price tag by using those pots. Your volume problem could be there too.

As you said this circuit is an aberration, Marshall jumped on this bandwagon in the early 90's, producing an extremely high gain amp with bad parts and poor engineering. Expect anything from a SLX head.

Hope this helps.

[Tage]

Thanks for the replies. the BR3 had fried earlier I replaced that and upgraded it. I will check that resistor out too, I suspected the pots too...

-Oh and I have yet to get myself a scope so I could track the spot down, that will be a post christmas investment.

[Tage]

Hey Jmaf I've finally gotten around to looking at this amp and I can't seem to see the resistor you refered to. in my schematic there is no resister feeding the phase inverter, only capacitor C7. Would you be able to help me out a bit more on this?

[jmaf]

Hey Jmaf I've finally gotten around to looking at this amp and I can't seem to see the resistor you refered to. in my schematic there is no resister feeding the phase inverter, only capacitor C7. Would you be able to help me out a bit more on this?

Hi. Can you take a photo of the board and post it? Also please post the schem you are using as reference, so we're sure to be talking about the same thing.

Edit: On the schematic you posted above it'd probably be R18. Have you checked it?

[g1]

JMAF, sorry to put words in your mouth, but I think you meant R30 (shown on power amp schematic). It feeds the phase splitter plate resistors. For the EL34 versions it is 22K1W, for 5881 version it is 10K1W. As you suggested, best to replace it with a higher wattage resistor.
http://www.drtube.com/schematics/mar...2500-60-34.gif
http://www.drtube.com/schematics/mar...2500-60-58.gif
For 100W models, R30 is the same.

[jmaf]

You're right, g-one. I recall the last time I fixed this I used two 10K resistors in series, so the 10K value on this schem had me puzzled.

The photo for "resistor failure modes" on Wikipedia was mine and it was a R30 from a JCM900 repair I did. Here it is published under Matarese.com: http://matarese.com/fotos/1/images/476.jpg

[Tage]

Awesome and yeah I did replace that with a 3 watt 22k. it seems to be fine but I'm gonna double check that the traces are all good. I've also replaced both of the op-amps, althugh iv'e never really had an op-amp fail on me before, but I figured it wouldn't hurt.

[g1]

Did you try ch.B yet? Looks like plugging an open plug or any patch cord into the footswitch jack should make it switch. However, it looks like all that does is change the master pot, so it's not really a different channel.

[Tage]

Yeah I used a cable to get it to switch to the b volume and it still had the same issue. So I played it harder and all I got it to do was sag out really bad, and then got a severe volume drop. I didn't have my meter on it but I'm thinking the B+ dropped way off, like a cap failed or something. I'm gonna change out the caps, they are very old, and see it that helps. I never got it to act up this much before but it seems logical as only with large signal hitting the PI and Power tube sdoes the condition occur. I'll update once it's back together.

[jmaf]

Yeah I used a cable to get it to switch to the b volume and it still had the same issue. So I played it harder and all I got it to do was sag out really bad, and then got a severe volume drop. I didn't have my meter on it but I'm thinking the B+ dropped way off, like a cap failed or something. I'm gonna change out the caps, they are very old, and see it that helps. I never got it to act up this much before but it seems logical as only with large signal hitting the PI and Power tube sdoes the condition occur. I'll update once it's back together.

I assume you have it open in front of you. Nothing obvious is showing up on the boards? Have you tried poking resistors, the infamous heater diode bridge, etc with a chopstick? No funny smells? Nothing working too hot aside from the tubes?

Changing those LCR cans is good practice anyway, but if one of them was causing that much sag on a transformer like the JCM has, it'd be pretty hot or would've bulged or leaked. Go for the low hanging fruit first: tap the pots, boards, tubes, etc, see if it's a mechanical problem first. Then my other guesses would be the usual suspects we mentioned earlier.

You've got to eliminate....touch the inverter grids see if it's loud, touch the previous stage and so on until it fades out or is too distorted.... That amp has solid state gain stages as well(Grunge crowd, remember ), maybe an opamp is gone. Just throwing ideas at you, hope it helps.

[g1]

Start with simple stuff first. The volume drop could be unrelated to the feedback problem. Sounds like a classic case of dirty FX return/power amp in jack, or a bad solder connection. If it's not that then start looking for missing DC supply voltages. Not likely a bad filter cap would cause this problem without blowing a fuse or some other major event.