Microphonics driving me nuts!

[Satamax]

Hi guys!

Not been here for a while, i'm working on a laney LC50-2 http://satamax.free.fr/LC50-2R.pdf I have microphonics around the V1 and drive circuits. It's driving me absolutely, uterly, completely nuts. I've changed C2, R4, C11 and 12 (mislabelled C11 too) P2 shielded wire. I have static on the drive dual ganged pot. The wire from P7 to P17 is verry noisy when i touch it with my hands, like an antena. I've chopsticked the whole amp, and it's soo microphonic that it's hard to tell where it comes from! Changed V1 twice, keep on taping it, and seems sound, V2 as well, i've been tapping them all. I'm wondering if it's not RL1A which is going astray. Taping it doesn't make too much noise, but same as the whole amp. Anybody could give me any advice on troubleshooting this?

Thanks a lot.

Max.

[dai h.]

some types of ceramics can be microphonic. How about the 270p and 4700p?

[Satamax]

some types of ceramics can be microphonic. How about the 270p and 4700p?

Thanks a lot for your reply Dai.

Which ones are you talking about? It's late for me now and can't seem to be able to find a 270p. 4700, that's C11 and C12? If yes, i've changed thoses for two polyester ones. To me it seems that this amp has far too much gain. I'll do a voltage check tomorow.

Thanks a lot again.

Max.

[dai h.]

270p. C16. (The signal goes to the clean ch. path so maybe it has nothing to do with the microphonics, though it's still connected to the V1 output.) Have you tried physically damping the input tube? Mesa/Boogie uses heatshrink, some people report success with silicon rubber rings plus some
additional weight with things like washers (changes the resonance I guess).

[Satamax]

Thanks a lot Dai.

Well, ok i see why i couldn't find it, i wasn't paying attention to the normal chanel, since this only happens when drive chanel is engaged. I'll sure check teh cap tomorow. I'll also check voltages. I know i have also HF oscilation since it wisthles before it goes grunting and crackling etc. When i tap the linking around R4, C2 R 12 and P7 that's one of the two parts of the circuit where the chopstick tapping is the loudest. And all the path between P17 to drive P31 all that path is extremely microphonic. A verry gentle tap of teh chopstick can be heard. I was wondering if C1/R1/R3 couldn't have gone mad and increased the gain soo much that it gets real microphonic. I'll check thoses tomorow. Thought, it doesn't do much in normal mode!

Thanks again.

Max.

[Satamax]

Well, might have found it. When i was hitting the valve side lead of R4 it was going into a 160 something hertz rumble. Every time, after changing all wires resistor 3 times etc, it was still doing it. Then i did check the voltage, 194 normal high, then went up to 339 when rumbling, 339DCV??? wtf! Then checked all resistors in the HT path, all seem all right, i don't know about the choke. But then checked the rectifier, 1N4007, and when touching in that area, it sends the amp into that rumble bad time. I gonna check the trany , diodes and snubbing caps, but i suspect one of them is gone! Or something along thoses lines.

I'll report later.

Bye.

Max.

[Satamax]

Hello again.

Well, did change the diodes, removed the snubbing caps. To no avail. Well, i have the node around P7,P8 and R4 which is extremely microphonic, i touch the solder with the chopstick, and i get static. I tap around thoses components and i get a thump thump. I've reflowed all the solder joints in the 2 inch around. I tap on the components, and none seem to make noise on a consistent basis. I have 297v on the R4P7 node, instead of 150 as stated on the schemo and 205 on the other plate. I don't get it. Ho can the voltage be different on teh two plates which have the same resistors when they're fed by the same B+ node???

Well, if anybody can help.

Thanks a lot.

Max.

[dai h.]

max are you getting the unwanted noise when you play?

[Satamax]

max are you getting the unwanted noise when you play?

Dai, thanks a lot for your reply. Nope, it's too unstable to have a guitar pluged in for the moment. IIRC, a tube going high on voltage can be a problem with the cathode circuit no? That's the only one left i could think of! Voltages are about 1.5 instead of .9v

[dai h.]

so it's making a bunch of noise by itself without anything plugged in?

[Satamax]

so it's making a bunch of noise by itself without anything plugged in?

Yes Dai.

Well, i switch it on, still in drive mode, and it pops, like a cap noise, then it's not silent but not extremely loud, then if i touch the area or some components around C2/R4/P7 it goes into a rumble, a mad rumble. At first, i thought about microphoncs, bad ones like a dry solder joint or broken leads, but i've bent all the wires and reflowed all the solders, and to no avail. I have something like 8.9 vac on the plate of V1A. And 297VDC instead of 150vdc and 220mv vac. On V1B i have 204vdc. Sometimes on V1A plate, goes back to 194 vdc. What could cause the increase in voltage. Whenever i measure R12, it measures within specs. and voltage on V1B is far closer to specs than V1A. Cathode resistor or cap which is going astray? That's the only one left i can think of!

Thanks a lot for your help!

[dwhutchens]

Too much gain maybe? I've been playing around with a Princeton Reverb based circuit with no reverb or tremolo. I could touch various components or wires in the preamp circuits and get crazy microphonics and ocillations untill I knocked the gain way down. I'm still working on it.
David

[Satamax]

Too much gain maybe? I've been playing around with a Princeton Reverb based circuit with no reverb or tremolo. I could touch various components or wires in the preamp circuits and get crazy microphonics and ocillations untill I knocked the gain way down. I'm still working on it.
David

vid.

Thanks a lot for your reply. Yes that's the case, too much gain, and i know why, 297vdc on the plate instead of 150. As for why there's that much, duno! Since V1B had only 50 volts over, and not 150. They're fed by the same node.

Thanks a lot.

Max.

[dai h.]

sorry I don't think I'm really helping much. Since you mentioned popping, here's a section from R.G.'s tube debugging page on pops:

http://www.geofex.com/ampdbug/popping.htm

other than that you said it goes into a "mad rumble" which sounds like oscillation and maybe the two are related. A part intermittently disconnects (maybe a ground reference), then that contributes to the setting off of the oscillation.

also, how about the socket pins? Good contact?

[Robert M. Martinelli]

Sometimes on V1A plate, goes back to 194 vdc. What could cause the increase in voltage.
Thanks a lot for your help!

Hi Satamax,
a voltage increase means little ( or no ) current is flowing, so my vote goes to a bad/dirty valve socket contact as per dai h.' s advice ( or a defective plate or cathode resistor which could go open ). Try to clean/re-tension the socket and then measure again voltages and currents under static and dynamic conditions.

Hope this helps

Best regards

Bob