Reverb Hum

[Chuck H]

Hmmm... Right. Was the input power checked at the send transistor output or the signal that feeds the send transistor? If there is less than a volt feeding the send transistor that would be waayyy too low. The send transistor would be changing signal into power to drive the tank. Maybe check the voltage right on the input transducer itself under what should be operating conditions.

[Shawnobi]

It's only around 0.5v AC I'm seeing, is that enough? That is measured at the end of the send connector that would plug into the pan, by strumming the guitar.

I have rechecked the input to the pan. Between the centre of the connector and the shield I am seeing 60 ohm.

[Shawnobi]

It's only around 0.5v AC I'm seeing, is that enough? That is measured at the end of the send connector that would plug into the pan, by strumming the guitar.

I have rechecked the input to the pan. Between the centre of the connector and the shield I am seeing 60 ohm.

I've noticed that if I do a resistance check across R26 I get 3.7k. I know this could be decieving in circuit, but it's supposed to be 22k resistor. (Clutching at straws now!)
Here is the reverb circuit btw
http://music-electronics-forum.com/a...c30-reverb.jpg

[Shawnobi]

Could anybody tell me the DC resistance on input and output of the original Accutronics C30 reverb pan?
Although the mojotone is supposed to be a direct replacement some have suggested the values I got look wrong.

[Chuck H]

What we use to do was go to the accutronics site. It was the bomb for reverb tech info. Unfortunately the company seems to have sold to an overseas competitor and I'm no longer able to find good technical info on their web site. Unless someone was real slick and saved that info... I don't think the resistance of the transducers was a figure they gave anyway. Do you have another amp around? If you have another amp, a male to two female 1/4" Y splitter and a 1/4" female to RCA female adaptor (plus a few carefully chosen resistors) you could probably test the tank independant of the input circuit in the broken amp.

[Shawnobi]

I've managed to find a site which has proved useful. Seems the pan resistance checks out good. http://roymal.tripod.com/accutron.htm>

I'm struggling to find anything wrong with this reverb circuit yet it still doesn't work!

[Chuck H]

Did you check continuity on the cable?

[Shawnobi]

Did you check continuity on the cable?

Yeah, all the way back to the IC (except the one to ground).

[Shawnobi]

I don't suppose anyone could offer any further suggestions? Today I checked every component on the input side of the reverb. (actually removed them one by one from the board). Couldn't find anything wrong. I'm completely at a loss for ideas.

[Chuck H]

I'm having a hard time coming up something else besides everything. You say that every single component in the chain is fine and that signal is getting to the tank, which is good, and if you jostle the amp you do hear reverb at the output, so all that is good. Nothing is broken, there is no lack of signal, so no shorts or faults, and all readings check out... It's fixed

Seriously though, we know the output is good because you can hear it working. The tank is new and the input ohms out correct (though that doesn's mean it's working). You do have a voltage reading at the output of the driver that is signal dependant. So...

Something your reading is leading you false. Put a 600 ohm resistor in the circuit where the tank should be and take some readings to see if you actually have any drive voltage AND current to the tank. If you do then the tank input is bad.

This may seem insulting, but were you sure to remove any packing materials from the springs used to stabilize them for shipping?

[Enzo]

The resistance of the transducers won't be wrong, they will either be open or they will be OK. COntinuity check is all you need.

A few posts ago you mentioned the cable feeding the INPUT jack of the pan was grounded to the chassis. WHy? The schematic shows the return from that cable - the shield - not grounded at all, but connected to C28 and pin 2 of the op amp.

Your op amp runs on -30v with reference to -15. That means with respect to chassis, I would expect about -15v on the output of the op amp. And that means on the input of the pan. The return - shield - is therefor also sitting at -15v to chassis. C28 blocks that DC. If you ground the shield, then you are grounding off the output of your reverb drive op amp.

Correct the cable wiring back to the correct configuration. J6 gets the hot and J29 the shield from the INPUT cable to the pan.

Only the shield of the return cable - from OUTPUT jack on pan - is grounded. And that ground is J30.

[Shawnobi]

Hi Enzo,
The cables to the pan were the wrong way around initially, so I corrected that. So J6 & J29 are the correct send to the pan. Neither screen nor signal is grounded.
I'm not seeing a -15v at the end of those wires though. Around half a volt when I strum a guitar. I'm still suspecting the drive circuit but I'm strugglnig to see what's wrong. I'm sure something is pulling that signal down somewhere though.
Borrowing a scope and also a signal generator this weekend so will retake all the measurements again. Hopefully will use the signal generator to feed the drive circuit direct. Would you know what kind of signal the reverb drive should be seeing from the 12AX7's that drive into it?

Chuck - thanks for the 600ohm tip. I'll also scour the pan for any hidden packing material. I'm not ruling anything out!

[Chuck H]

Hi Enzo,
The cables to the pan were the wrong way around initially, so I corrected that. So J6 & J29 are the correct send to the pan. Neither screen nor signal is grounded.
I'm not seeing a -15v at the end of those wires though.

Re-read:

The schematic shows the return from that cable - the shield - not grounded at all, but connected to C28 and pin 2 of the op amp.

Your op amp runs on -30v with reference to -15. That means with respect to chassis, I would expect about -15v on the output of the op amp. And that means on the input of the pan. The return - shield - is therefor also sitting at -15v to chassis. C28 blocks that DC. If you ground the shield, then you are grounding off the output of your reverb drive op amp.

Correct the cable wiring back to the correct configuration. J6 gets the hot and J29 the shield from the INPUT cable to the pan.

Only the shield of the return cable - from OUTPUT jack on pan - is grounded. And that ground is J30.

I never even looked at the schem. Enzo did a lot of work for you here. Reading the thread and deciphering, then looking at the schematic to search for the hidden problem. If you don't have it wired just as Enzo described the drive amp will not work.

I'm having some trouble deciphering your descriptions, but, if you DO have it wired just as described and as the schematic shows then there may be a problem with your -15V source reference.

[Shawnobi]

Re-read:

I'm having some trouble deciphering your descriptions, but, if you DO have it wired just as described and as the schematic shows then there may be a problem with your -15V source reference.

Just re-read Enzo's post. Misunderstood the -15v. Thought it meant between the shield and the signal. I get -17v between shield and ground and -17v between signal and ground.