Fender reverb circuit diagnostics/repair

cancon

Member
2011-03-06 2:58 am
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
 
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?
 

cancon

Member
2011-03-06 2:58 am
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?
 

turk 182

Member
2012-10-26 3:03 pm
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!
 

turk 182

Member
2012-10-26 3:03 pm
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.
 
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.
 

cancon

Member
2011-03-06 2:58 am
Thanks again, that's interesting.

The old pan is 4EC3C1B. I think you're right - this was a wrong replacement. I learned a lot from these two links:

accutronics_products_and_specifications
spring_reverb_tanks_explained_and_compared

So the important details from the code translate to Type 4 length (17"), 600ohm input impedance, 10k output impedance.

Delving further into the reverb domain... I notice (more often than not) that reverb circuits are noisy, commonly adding lots of hiss as you bring up the control knob. I did a quick test, unplugging the pan input did not reduce the hiss, but removing the output did...so would the pan or recovery stage be suspect?
 
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Hiss is a companion to gain. The reverb recovery deals with weak signals and so has a lot of gain. That can be noisy. You can unplug the reverb cables from th baack of the amp and listen to the hiss from the circuit itself. The long wires down to the pan will pick up noise. They are shielded, but that is not 100% efective. Also, the footswitch for reverb is simple, it grounds the return signal. The reverb footswitch is a shielded cable, parallel the pan output, and in the foot pedal it simply shorts teh shield to teh signal. So unplugging the footswitch potentially reduces noise. Anothewr factor, the metal reverb pan has an open side. One might get some improvement by putting a metal cover over the open side.