JCM900 SL-X ...Bad IC at Fx Loop?

[Smiles]

hello,

I have some sort of problem at the pre-amp section of my JCM900 SL-X that leads it to act almost like a metronome. It has a continous click. I've followed the signal path throught de 3 12ax7 from the preamp and the clicking begins right before the 1st IC that leads the signal into the fx loop path.

Could this be a bad IC or a bad CAP that supplies bad current to it causing this metronome-like-clicking?

Thank you!

[Mars Amp Repair]

Hello,
Looks like you had no takers on this one...I don't have any direct experience with this issue, but I've looked briefly on the schematic that I believe is close to your unit on Schematic heaven http://www.schematicheaven.com/marsh..._100w_2100.pdf, tho I don't see an effects loop on this schemo.
1. Is the clicking consistent like a metronome?
2. The click remains when all but the Phase inverter ax7's are removed?
3. the Click disappears when you interrupt the effects loop (you did say that)
4. I'm guessing the answers are all yes, hence the thought that the issue is in the IC's.
5. I see that there is some channel swithcing on the last IC before the PI. Does switching channels change anything?
6. This schemo doesn't show an effects loop, but I'm guessing that's how you traced where the click stops & also by pulling the AX7's & breaking the effects loop.
5.5 do you get the click when feeding the 'send' line to another amp? If so then so far you're correct about where the click originates.
7. It looks like the only thing the + & - 15volt supply feeds is those 2 IC's. You might be certain the votages there are correct. If you have a scope, you could monitor there & see if they are stable. Still would be very weird for the supply to cause this.
8, Other wise tapping around with an plastic tool on the pcb might reveal some poor grounds.

***Watch out for lethal voltages in there!***...Sorry can't be more specific...g

[Enzo]

Do you sit your cell phone on top of the amp?

[Smiles]

Do you sit your cell phone on top of the amp?

sorry, I'm not that dumb.

Yes, When I pull off the PI valve the clicking stops (no signal going to the power-amp stage) and yes...the clicking also gets out throught the line out output jack. The clicking is just like a metronome, very constant.

[Mars Amp Repair]

Hello, been off for a while...
Boy, I really have no answers to a constant clicking. Seems to be generated in the 2 ic's that precede the send jack. As I mentioned, those 2 ic's are fed exclusively by the +15 & -15 regulators...you might see if there is any noise coming from there...Freeze spray on the + & - 15v regulators as well as the IC's might shed some light....glen

[oc disorder]

neg f/b?

Hi
Just some food for thought based on observation...Smiles ..you don't seem to state
specificialy that the clicking comes out the send jack ..but it comes out
the line out jack..
From the diagram Mars amp Repair found ,page 3 of the PDF
shows an efx loop and a direct and record out.
(Pardon my ignorance but where is the "P2 A3 C7' point in the schematic that
connects to the 10k and thus the direct out..? Was it a cross reference
that refers to the edge of the schematic that has been cropped ?)
I will assume the direct outs and fx send are post and pre respectfully.
Now Smiles says that removing the phase splitter/inverter stops it.
My point being that if direct out is post Pi obviously the clicking will go..
As Smiles says the signal path has been followed through the 3 12ax7's
Does this mean with a cro ? or simply those valves have been removed to no avail ?
So if i try to keep my tuppence worth simple I would do this and report back to the gurus..
1.Using an external self contained amp plug the send (from efx loop) in the 900 to the external amp.
Click or no click ?
2 same again only from direct or record out...
click no click?
3.After letting the amp cool pull out the o/p tubes and repeat the above..
click or no click?

As the subject title suggests I'm guessing some weird fault in the neg f/b loop
which would also explain why it goes when pi is pulled....

Hope that is of some help or at least food for thought.....

I shall be watching the experts reply with trepidation....

My big fix for the day was the washing machine which tripped the earth leakage when it got to drain..... turns out bit of a leak on the pump motor
Blood oath what an effort to get out of the laundry and undo the crudded
up screws etc etc I know i much prefer small chassis.......
sorry not much to do with guitar amps..... took most of the day up though

cheers

[Amp Kat]

To be honest I haven't seen many op-amp failures in these or the Fender amps. It's always a connection or voltage supply problem. The Marshall's are notorious for that Rectifier from the heaters giving out and leaving burnt marks on the board from heat dissipation. When that thing goes all sorts of things happen not only from a voltage standpoint but from a ground standpoint which many components have their ground tied to that common point. If you have verified that your +/- 15 volt rails are solid and the regulators working fine then I would suspect one of the many jacks on the amp may be bad and as the above poster mentioned you may have to sub them out in isolation style to find the one it is. Footswitch jacks on Marshalls are a common thing that cause problems but yours sounds like a jack or even a cold solder joint which is a little more time consuming to find.

[Enzo]

Aside from the cell phone idea, I thought about two things that might make ticking sounds - a cap breaking down and arcing over inside, or a tube socket or other point arcing.

With the amp running on the bench so the symptom occurs, turn out ALL the roomlights and look closely at the amp circuits. IF you see a tiny flash of light anywhere, it is an arc. COuld be underneath, or could be on top or a socket.

[Smiles]

Again

To be honest I haven't seen many op-amp failures in these or the Fender amps. It's always a connection or voltage supply problem. The Marshall's are notorious for that Rectifier from the heaters giving out and leaving burnt marks on the board from heat dissipation. When that thing goes all sorts of things happen not only from a voltage standpoint but from a ground standpoint which many components have their ground tied to that common point. If you have verified that your +/- 15 volt rails are solid and the regulators working fine then I would suspect one of the many jacks on the amp may be bad and as the above poster mentioned you may have to sub them out in isolation style to find the one it is. Footswitch jacks on Marshalls are a common thing that cause problems but yours sounds like a jack or even a cold solder joint which is a little more time consuming to find.

ok...
if the problem (constant metronome-like clicking) relay's on the burn tracks of the heater rectifier (it blown up some years ago but i did replaced it) how can I bring it back to normal?

2)
I followed the signal throught the pre-amp sinal path with a pc speaker, connected the ground input of the speaker to the chassis then I soldered the hot wire to a multimeter probe and followed the signal throught out the stages with the gain pot turned full (max gain, goes out to 20 in the SL-X).
- Until the first resistor before the IC everything's ok! When I connect the probe (connected to pc speaker to get a sound output) at the end of the tone stack right before that R27 (680k) I get the metronome-like clicking.

3) someone ask me about the signals coming in and out of the fx loop:
- If I connect an external pre-amp at the fx loop's return I get no click, but remains some kind of a high-pitch hiss (or like a "piiiiiiiiiiiiiiiiiiiiiiiiiiiii") and it's pitch get's higher, slowly, while the amp is on.

4) also, before this goddamn amp broke, I used to work with it at night plugging a Behringuer's D.I Box with Speaker Simulator at the "Send" of the amp's fx loop. Did this hurt the amp? (yes, the amp was always connected to the cab, and by the way...great sound camed out from that D.I).


So... after all these what's your advice? Substitute the rectifier and caps from the IC's powersupply?
Can I bypass that fx loop since I don't use any kind of effects with this amp?

Thank you very much for all your help!
Cheers,

[Amp Kat]

ok...

So... after all these what's your advice? Substitute the rectifier and caps from the IC's powersupply?
Can I bypass that fx loop since I don't use any kind of effects with this amp?

Thank you very much for all your help!
Cheers,

Well I would do a couple of things. First measure the rail voltage on the TLO-72 at pins 4 & 8 and make sure your getting your +/- 15 that is coming from the BR. If that is good then it is possible that you fried the op-amp with the DI box if the signal got to large after being transformed. That connection shows a blue wire there from the output of the TLO-72 to the send jack. If you can remove that connection and test the signal again at the same place (R-27) then I'd say yes replace the TLO-72. As far as bypassing the effects loop you can do this by patching a cable from send to return and your done. While your at it you can also test the send and return circuits which is actually a preamp out of the TLO-72 pin 1 and returns on the next TLO-72 with the stereo jacks and variable level pots in beween. Take a cable and come out of the send to the front of another amp or your speaker simulator and see if the clicking is there. If it is your problem is most likely that op-amp or before the tone stack which only really leaves the tube 12ax7. Then take a signal from anothe amp or signal generator and go straight into the return. If no clicking is there then you know it's in the preamp and the output section is ok. Don't disregard that send jack as the culprit because it could be the problem. That's why I wanted you to disconnect that blue wire which is going to it.

[Smiles]

send/return

Ok.. it goes like this..

Haven't tested the voltages yet but...

The clicking comes out of the "send" output too. If I connect the sl-x's pre-amp to another power-amp the click remains. So it's a pre-amp failure.

If I connect an external pre-amp to "return" I get no click but I get a really high pitched noise at low volume.

So taking the output of a valve amp pre-amp throught it's fx-loop straight into a D.I may "grill" the op-amp? How can that happen since we're dealing with and output...shouldn't be the D.I to fry !?

[Amp Kat]

Well remember that a DI box is nothing more than a transformer to either switch a low impedance to high or vice versa. If the signal coming in already matched impedance and you drove it even higher with the transformer it could have blown the input of the op-amp which also explains why it sounded good because it was louder which tells me the signal in was pretty hot. Notice that connector P2 to C7 is going directly right at that power point. I'd check R-16 too and make sure it's reading 10k. I'd change the TLO-72 no matter what.