Fender reverb circuit diagnostics/repair - diyAudio
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Old 2nd April 2013, 06:10 PM   #1
cancon is offline cancon  Canada
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Default Fender reverb circuit diagnostics/repair

Hello again.

I've been meaning to get an amp on the bench which needs attention in the reverb section...today's the day! I've always been in the dark about this circuitry but after reading a bit, I think I'm getting more comfortable...Basically the input signal does this: input signal preamp > reverb driver stage > output transformer > tank > preamp recovery stage > mixing with dry signals and onto the phase inverter. Sound about right?

I was wondering if someone can point me in the right direction of diagnostic tips for typical dead reverb problems.

Today I'm dealing with a non-dead reverb..well..sort of. I've got a re-issue Deluxe Reverb here which does not reverberate the dry signal. However, the tank does work, as it is able to pass mechanical vibrations (tapping) into the amp. When the reverb is activated the tank picks up the idle noise and hiss from the amp, or mechanical vibrations from playing at loud volume. It reverberates all these noises but not the input signal. Furthermore, when you crank the reverb control the unit starts to feedback. The feedback ramps up very slowly, maybe over 15-20 sec. This feedback can only be silenced by reducing the reverb control.

The tubes test OK, and I've got a scope on hand if that helps.

Thanks!
adam
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Old 2nd April 2013, 08:06 PM   #2
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i'd pull the pan and look at the leads going from the rca jack to the driver coil (input) these often break if they're not broken check to see if there's any continuity through the coil they often go open.
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Old 2nd April 2013, 08:32 PM   #3
Enzo is offline Enzo  United States
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You described the circuit well enough. The preamp sends its signal to a small powr amp that drives one end of the reverb springs. The output from the spring unit is then connected to a sensitive input stage for recovery.

If you have no reverb, isolate the problem. It is one of three things: the reverb pan and its wiring are bad, the drive circuit is bad, or the recovery circuit is bad.

You have already determined that the recovery circuit works by shaking the pan and hearing the result out the speaker. That tells us the recovery stage works AND that the output end of the reverb pan works. SO that leaves the drive end.

As suggested above, do inspect the reverb pan. AT the input jack, unplug the cable, and measure resistance across the jack. It should be as low resistance. The resistance won;t be wrong, it will either show continuity or open. Open is bad. And do look inside the pan, the little black and green wires can break off. Sometimes that can be repaired with solder, other times a new pan is needed.

If the pan is OK, then the drive is simple enough. A tiny power amp is made of a dual triode in parallel feeding through a small output transformer. You can look for signal at the output of that, or even connect the drive cable to a small speaker. Otherwise it is a simple tube stage. Signal on the grid? Signal on the plate? B+ on the plate? A couple volts on the cathode? Tube lighting up? Both windings of transformer intact?
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Old 2nd April 2013, 08:49 PM   #4
cancon is offline cancon  Canada
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Thank you for the advice.
I've pulled the pan and found that one spring has come apart at both ends (the magnet on both side have fallen out). The 2nd spring looks intact. I read about 25ohms on the input coil, about 700k on the output coil.

I have another deluxe reverb pan here, the input coil reads 1.4ohm and 210k on the output. Is there anyway to test continuity between the coil and the spring?
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Old 2nd April 2013, 08:59 PM   #5
cancon is offline cancon  Canada
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Thank you enzo, I'll get the amp back together and do those tests you've described. Great tips!
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Old 2nd April 2013, 09:21 PM   #6
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there shouldn't be continuity to the springs the springs are only there to transfer the driver coil output to the receiver coil (the old school guys used to do things like change spring types or modify lengths to change the sound quality) so if you do come across a pan that has continuity to the springs i would not advise you to use it.

oops what am i saying i am an old school guy!
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Old 2nd April 2013, 09:21 PM   #7
cancon is offline cancon  Canada
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Bad wiring in the input of the reverb pan. Subbed in another pan and reverb is revived! Thanks again.
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Old 2nd April 2013, 09:40 PM   #8
cancon is offline cancon  Canada
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How does the coil transfer the incoming AC signal as vibration to the springs?
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Old 2nd April 2013, 10:12 PM   #9
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those little brass nubs with the "wires" that the springs attach to are transducers (read really small speakers without cones) so the transfer is mechanical the spring type length and tension all play into the reverb sound the receiving end just reverses the process.
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Old 3rd April 2013, 04:03 AM   #10
Enzo is offline Enzo  United States
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It works, great.

Be aware there are different types of pan. There ought to be a number on the pan in the format: 4AB2C1B or something similar. That would be a number used in the Fender system with the little transformer. Other systems, mainly solid state use a higher impedance, so the pan type is something like: 4EB2C1B


The 4ABxxxx has a low impedance drive coil, and it measures very low resistance. 1.4 ohms sounds about right. 25 ohms sounds like a higher impedance type. Look for the type number adn see if ir is a 4EB instead of 4AB. (Or 8AB or 9AB versus EB)

I wonder if your original pan was the wrong type.
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