Weak reverb, or...what's weak this week

[bassistbobby]

I bought a 1993 Marshall JCM 900 HGDReverb 50watt 4500 head used about 10 years ago. I recently have been realizing that my reverb has been weak as long as I remember. I have to crank the reverb all the way to 10 just to get any real effect.
So far I've taken out the reverb pan, a 1993 8DB2C1D original factory part.
I checked the resistance of both input (32ohms) and output (200ohms) RCA Jacks. And cleaned up the jacks and jills.
I discovered the interior input leads (green and black) were leaning against the spring suspended metal bar, and although it didn't look like it had damaged anything, I did fix it by moving those wires (too no effect).
When I touch the output RCA connector, I do get a hum from my amp
I connected another reverb tank, from another amp, and got almost the exact same weak response

That makes me figure it has to be on the preamp circuit board.

I have the schematic, a multimeter, I'm a hobbyist and am currently trying to be a total DIYer with my gear (AMAP)

I read on this site that, 8 years ago, there was another guy with a Crate VC50 amp with a weak reverb responce, and it turned out to be one of the ICs on his preamp board. He said ENZO the GREAT had helped him, greatly, and the problem got solved (thanks Baddog [Keith])

So,.. the reverb tank looks fine, and the resistance is measuring as expected. I have no idea what to do next.
Please help (I have some good photos of my reverb tank and of all the sections of both of my amp's circuit boards, and could post them, if that's going to help at all)

Thanks in advance

[g1]

I always thought the reverb on those was pretty weak anyway, nothing like classic Fender surf type. The fact that you don't remember it ever being better lends credence to that theory.
Have you heard another amp of the same model that has more reverb?

[bassistbobby]

Actually, not in person. I'm basing my impression on what I've heard on youtube. There's basically no noticeable reverb until it's up to 9 or 10. That can't be right. Maybe I'll go hunt down one in a guitar store this week, and check it out.
Thanks g1

[g1]

That does sound like it may be worse than normal.
Attached is the schematic. If you have a good level of hum when you touch the return cable, then chances are the tank is not getting enough signal into it. IC2 & IC3 are part of the tank drive circuit.

[bassistbobby]

Thanks for responding.
Looking at the schematic, I see three of those IC chips in the reverb recovery unit. There's that IC2, then further down the signal path is IC3, marked with an "a", and another below it marked only with a "b".
And it looks like they've identified IC2 as TL071, and IC3 (a & b) as MC1458.

So you're saying I should check those ICs 1st?
Can I check them without needing to remove them from the board?
Do you know what readings I should get at those pins?

[guitician]

Does the line from the tank to the amp have a loud hum when you touch the tip of it(While not touching anything else)? Like G1 asked. That should let you know if you should concentrate on the drive side of the circuit. If you hear anything, chances are that the IC chips are fine. I'd bet anything that the coupling capacitors need replacing, especially if they are Electrolytic types. C41 & C45 are since they're 2.2uf. And that controls the recovery stages gain.

[bassistbobby]

Yes, it does hum, I don't know if it's loud, but the amp wasn't turned up much..
So you think it's one or more of C16, C17, and/or C18 ?
Can I test those with a multimeter.

Thanks

[guitician]

No, not as much trouble with film caps like those. If the reverb control is up all the way, then the hum should be at least as loud as the guitar. But C18 is 100uf and could be a problem in the drive circuit. But the other test you can do is rap on the tank with the reverb control up. If you hear the springs rattle then the recovery may be fine.

[bassistbobby]

So, I need a meter that measures micro fereds (sp), right?

[bassistbobby]

Hope you're still there, do you mean to drop the pan hard enough so the plate hits the pan, and if I hear that cymbal type splash sound through my speaker, then the drive unit is probably okay?

[guitician]

No need to measure, I just swap it out, they're cheap. The springs in the tank move easily when the amp is bumped, and make a crashing sound. If the tank is on rubber mounts then it should move when bumped too.

[bassistbobby]

Okay,... I dropped the tank (it's still out of the amp) and I had to drop it hard enough for the plate to smack against the pan, it did absolutely make that splash sound through my speaker cab. Does that mean my reverb drive is probably just fine?
Or should I replace that cap anyway?
Is it possible these reverb tanks just get weak over time with all the banging around they get when amps get moved? I've read this horizontal, open end up, configuration my amp head uses is actually the worst way to mount a reverb tank.

[guitician]

You have to gauge the intensity of the reverb by how loud the guitar plays. It's relative to that, and the crash should have been as loud, or louder, than a full chord strummed hard. If it was, then the drive stage is what needs attention.

The tank is the last thing that I would suspect. They're just two tiny transducers and springs, and if you hear anything through it, it's probably fine.

[g1]

Just to clarify, the IC's being discussed are in the drive circuit, not the recovery circuit (see schematic in post #4). So if it is a drive problem, the IC's (2&3) are still suspect.

[guitician]

If he's going in with a desoldering tool, sure why not RR a $2 chip along with the cap, but the cap is easier, and I don't know how good he is with an iron.