Rossi

Reverb Riddle - help appreciated
« on: June 21, 2008, 09:56:30 AM »
Here's a strange problem. I have an old Laney AOR-30R tube amp. Actually it's the first guitar amp I ever bought back in the 80s. At some point the reverb stopped working, so I checked the reverb tank's coils for continuity with an ohm meter. Suddenly it worked again - but only for 5-10 minutes or so. Whenever I touch the output plug of the reverb tank, I get reverb for a while and then it dies again.

And so it has remained for the past 20 years. But now I finally want to solve the problem. So what could it be? Here's the reverb section of original schematic - sorry it's a bit fuzzy:



This is a later version of the same amp, some values have changed and the opamps are now TL 071/72 instead of LF 351/353. But otherwise very similar and a sharper image:



The PCB is a real ***** to work on as the leads to the pots and tubes are very short. So I don't feel like experimenting too much. :?

Any ideas?
"I am not a number, I'm a free man!"
"Hahahahaaaaaa!!!!!"


clintrubber

Reverb Riddle - help appreciated
« Reply #1 on: June 21, 2008, 10:21:34 AM »
Hi,

Is it possible to check if the continuity of the coils is maintained when the reverb stops working ?


Apart from that... it makes me think of a little practice amp that had a non-intermittent speaker voice coil _and_ a fine working power-amp section... drove me nuts...

Finally I discovered the cone & voice-coil of the speaker were no longer properly attached to each other...

Maybe an alike thing with your reverb-tank, can you swap it to see if it's the tank or the electronics ? Or how about the cabling ?

Bye,

  Peter

hodad

Reverb Riddle - help appreciated
« Reply #2 on: June 21, 2008, 10:41:36 AM »
My thought would be to check the wires connected to the transducers in the reverb tank first & see if you have a dodgy connection.

Rossi

Reverb Riddle - help appreciated
« Reply #3 on: June 21, 2008, 10:46:37 AM »
I'm pretty sure it's the electronics. I think I tried other reverb tanks with it in the past 20 years or so. Continuity seems fine: Input is 24 ohms DCR, Output is 193 ohms DCR.

If it wasn't such a ***** PCB, I'd take a wild guess and try replacing the opamp connected to the output. But I'd rather wait for some more opinions, perhaps someone has encountered a similar problem.
"I am not a number, I'm a free man!"
"Hahahahaaaaaa!!!!!"

AudioJohn

Reverb Riddle - help appreciated
« Reply #4 on: June 21, 2008, 11:41:26 AM »
Clean up the phono sockets on the spring reverb tray - this is a common cause of intermittent connection problems - they corrode and produce a curious oxide coating. This is almost certainly causing your fault.

On occasions I have found open circuit (or intermittent) phono cables - but it is very rare to get problems with the drive or receive electronics......

JG

rodabod

Reverb Riddle - help appreciated
« Reply #5 on: June 21, 2008, 12:06:39 PM »
As boring and simple as it may seem, I'd agree and say spray some contact cleaner where necessary first.
Quote from: tv
punchy fat bastard chip

JohnRoberts

Reverb Riddle - help appreciated
« Reply #6 on: June 21, 2008, 06:41:26 PM »
From the symptoms another vote for flaky RCA... not most robust connectors.

JR
It's nice to be nice....

Rossi

Reverb Riddle - help appreciated
« Reply #7 on: June 21, 2008, 07:57:13 PM »
Well, I resoldered the plug connections and made sure there's good contact, but things didn't change. The funny thing is the DCR-setting of my multimeter has influence on the reverb strenght. When I measure ithe output coil n the 200 ohms setting, I get a light reverb for a while, when I use the 2k setting, I get a much stronger reverb effect - just like it used to be. But of course the reverb dies after a while.

I know, when something doesn't work its the connectors 90% of the time, but this problem seems to belong to the other 10%. :?
"I am not a number, I'm a free man!"
"Hahahahaaaaaa!!!!!"

pucho812

Reverb Riddle - help appreciated
« Reply #8 on: June 24, 2008, 12:32:47 AM »
I would replace the RCA connector as opposed to resoldering it. Maybe the connector itself is shorting out.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

mrphotodude

Reverb Riddle - help appreciated
« Reply #9 on: June 24, 2008, 01:25:52 AM »
sometimes one connector is grounded to the case(usually the return) and depending on the jacks...may have corrosion between the tank and connector.
If they are rivoted on, you may have to drill the rivot out and check that out.
If it is a constant thing like the amp will have reverb for the first 20 minutes it is on and then dies...possibly the op amp driving the reverb setup is dying?
another day...another dalek


Walrus

Reverb Riddle - help appreciated
« Reply #10 on: June 24, 2008, 07:01:47 AM »
Why not try soldering a link on the PCB between the input to and output from the reverb?
That will then prove where the fault is.
Kevin.

okgb

Reverb Riddle - help appreciated
« Reply #11 on: June 24, 2008, 08:17:06 AM »
the time out thing makes it curious , [ electronic ? ]
i would have said the little wires going into the transformer ,
but Rossi did say he tried a different tank with the same outcome
GKB Audio / Greg Boboski

Rossi

Reverb Riddle - help appreciated
« Reply #12 on: June 25, 2008, 03:47:18 AM »
As I said, I'm pretty sure it's not the connections. Also, I tested the reverb tank in another amp, and it works dandy.

My guess is the receiver circuit. When I mechanically stimulate the reverb springs, the volume dramatically inceases dramatically after a (temporary) DCR-Meter cure [i.e. measuring the receiver coil]. And as I said, the 2k setting restores the full amount of reverb (for a while) while the 200 ohms settin only restores the reverb partially. So I conclude the DCR meter's measuring voltage does something to the receiver circuit - but what could that be?
"I am not a number, I'm a free man!"
"Hahahahaaaaaa!!!!!"

Harpo

Reverb Riddle - help appreciated
« Reply #13 on: June 25, 2008, 05:08:44 AM »
did you check the shunt arm of IC2b or IC3 for intermitting resistors or broken traces around R44 or R51? would change the amps from non inverting gain-stages to voltage followers. (value of R51 1k5 is maybe a typo, would be 40dB gain)
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

dale116dot7

Reverb Riddle - help appreciated
« Reply #14 on: June 25, 2008, 01:01:58 PM »
Check for offset voltages on the op-amps. Check that the + and - inputs are where they oughtta be. If they drift up or down, check for the bias current sink (R50) being open. Of course, the tank coil should be in parallel with it. Or check for oscillation. Maybe the op-amp is oscillating madly. Both power supply rails there? Maybe one power supply rail is missing off that op-amp? 40dB of gain is probably right from a reverb tank. The signal off the pickup end isn't exactly huge. Check for zero DC volts at the end of R47. When it dies, check the signal voltage there, possibly the output is doing something funny and hitting the tank fixes it up.

For an op-amp circuit, I always measure +in, -in, and out to ensure that voltages make sense. + and - should be the same. You can fix a lot of analogue stuff even without a schematic if you follow that guideline. Also, check C32 for open.

If IC3's + and - inputs diverge, but the output does not rail right away, it's toast. I might suspect that IC3 might have suffered ESD damage given that the gate of its internal FET is nekkid.

JohnRoberts

Reverb Riddle - help appreciated
« Reply #15 on: June 25, 2008, 01:31:13 PM »
It sounds like something simple, they always are "after" you find them...

If touching and probing around makes a difference it seems like there may be a missing or weak DC path somewhere. This will make the most difference around opamp inputs or feedback paths. Logically with the low impedance coils there should be robust DC paths, but something is funny somewhere.

Perhaps make a jumper clip lead with a resistor to simulate your probing of the receive coil, and measure the send and return opamps DC operating points before and after. It could be a bad solder connection, bad wiring to reverb pan, etc...

Measure, clip on probe that makes a difference, and measure what changes. I'd look at receive but don't rule out drive electronics. Once you see what is changing, work backwards from there to why.

JR
It's nice to be nice....


 

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