Accutronics Reverb problems. Like Nothing (old Randall) SOLVED!!!

[ToadMeister]

Multimeter sez voltage happens when guitar is plucked when multimeter probes are on the RCA jack, drive side.
Shaking amp produces spring sound through the speaker.

Leads from inductors in reverb tank are re-soldered, and both inductors read ~170 ohms, there is no wire break.

Have the little magnets lost their poles from the speaker magnets up above?
Also tried different positions of the tank, up, down, sideways.

I have hit the wall, I need help.

The amp is an old 1970's Randall Commander 1, the reverb unit model # is 4fb2a1a.

Thanks.

[phatt]

Sounds like you only checked the reverb output at the amp socket.
With the amplifier off, unplug the Reverb drive side RCA lead from the amp then set DMM for resistance and check if you have a reading close to the 170 Ohms on that plug, those RCA leads are a common fail point and often need replacing.
Phil.

[ToadMeister]

The RCA jack and cable (in and out) is hard wired in, i also tested those leads + and -, the RCA jacks are fine, both drive, and pick-up.
I tested them for continuity, but perhaps not with the reverb plugged in?

The RCA jacks themselves seem clean, I even gave the RCA male and female (wire end, reverb tank end) a buff with sandpaper,

Something is wrong, I will recheck some of this 'phatt'.
(PS, I have overlooked the blatently obvious before, I say this because it is blatently obvious I should have reverb by now, and i don't!)

Thanks for your suggestions.

[ToadMeister]

I copied and pasted the following from galaxiex's post over at the schematics section.
I measured mine,  ~170 ohms  on both inductors.
As is typical with 'netformation' some say the inductences must be equal, that inductors work, or don't work. Am I measuring my ohms wrong? Is this where the problem is?
Why are my readings different?
Do I need a new tank?
------------------------
'Well, reverb tanks are relatively cheap so ordered up a 4FB2A1A.

4= Type 4 - 17" long, 4 springs (ToadMeisters note; mine has 2 springs, but the same model # ?!?!)
F= 1475 input impedance
B= 2250 output impedance
2= Medium decay
A= input/output grounded
1= No lock
A= horizontal mount, open side up "

[g1]

The impedance is not the same as the DC resistance your meter measures.
Sounds like your transducers are ok.
See the section on input and output impedance in the following link, approx. DC resistances are shown in smaller numbers.

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

[ToadMeister]

So that's what the smaller numbers are for.
My drive is rated at 10v according to the schematics, and indeed the numbers fly up to 10 with a hard pluck of a string, softer plucks are between 0-5,6v.
(measured at reverb side of RCA jack)

I have boiled down the problem to 2 possibilities;

I have the wrong model number of tank.
The spring magnets have lost polarity.

[ToadMeister]

Update;
Hooked up a little speaker to the drive side as the internet suggested.
Nothing came from the speaker, yet the multimeter shows voltage.

WUT?
Seems I am getting the wrong signal?
Should I recap the reverb dept.?
In particular C43 in the dotted line area of schematic.
https://www.ssguitar.com/index.php?topic=4236.0

[Enzo]

That works for the transformer drive low impedance pans like on all Fender tube amps.  The high impedance drive circuits won't drive a speaker.  But you can probably listen to it with a signal tracer.

4FB should work, but make sure the two sides have complete circuits to the chassis.

[ToadMeister]

Couldn't my multimeter be used like a signal detector?
I know it is reading volt output, even up to 10 volts if the guitar is picked hard.
But maybe the Voltage is not as strong as it should be?

Is the capacitor just before the drive output (3.3mfd) is that used to block DC voltage, and if so, if my meter is set to DC I should get a steady '0V'?
But if it does read any voltage in DC, that would interfere with the drive side of the tank?

How wrong am I?
And I am just fresh to the world of signal tracers, so I know almost nothing except they can send and receive signals.

[ToadMeister]

Dammit! It's C45.......?
It's acting as a negative V signal dump (ie filter cap) going to the drive output.
If it was a bad cap, it would allow -V through, confusing the little reverb tank magnets.
(not C43, I assume that is for smoothing out the signal, to take out harshness)

http://www.ebay.com/itm/4PCS-Japan-NICHICON-470uF-50V-Most-High-end-MUSE-KZ-Audio-Electrolytic-Capacitor-/111568521088?hash=item19fa008780:g:SWwAAOSwRLZT5yov
Should I get these and replace them all?
They do look iffy, like I suspect that that what could be crust at the leads could be electrolyte. (the ones in my amp, not the ebay ones)
I also notice sometimes hum during each note plucked, but nothing major (like those 6000uf cans)
Sorry about the crazy posting, but my mind has been stuck on this issue all day and all night.

Let me know if my figuring is on track.
Thanks.

PS, also, if you follow the path at tab 'c' from the tremolo side, it also leads to a 500uf cap (C18, which in my amp is 470uf) with one resistor in it's path (R19).
My tremolo also does not work.

[Enzo]

A signal tracer is nothing more than another amplifier with a probe plugged into it.  Like a scope, but makes sound instead of pictures.

C43 is the output cap for the drive circuit.  It needs some form of load to charge up.  Normally the pan input coil does that, but a resistor would work for testing.  The collector of Q24 is the output, but is not at ground potential in terms of DC, so that cap blocks the DC but passes the signal to the reverb pan coil.

C45 is a filter cap in the B power supply node, I don't see it preventing reverb.

[ToadMeister]

Do I need one of those ESR meters to test the cap?

So, is that the purpose of an RC circuit, for charge to build in the Capacitor, because it needs a load to charge? Until something like a transistor opens things up to electrical flow to dump it's charge. Is C43 charging from R75's direction?
So what if C43 did not charge because of high ESR (note, must learn more about ESR)?

Trying hard to understand here, I know how a lot of the components work, just how they work together confuses me (being ADD doesn't help either, dealing with it for almost 5 decades now)

[Enzo]

Nothing that complex.  Since the transistor sits at some voltage, if we just hook a cap to it, the far end of that cap will want to sit at that voltage.  The same voltage on both ends of the cap means it is not charged.   SO when we put some path to ground on the righthand end, that allows a charging path for the cap, now the left end charges to the voltage in the circuit, while the other end stays at no DC volts.

C43 is doing the exact same job as C42 one stage earlier, it is just a larger value so it can pass enough juice for the reverb transducer.  Single supply power amps use the same thing - a big cap - at the speaker output.

You can check it for ESR, but to stop working completely, that ESR would have to be sky high.

[phatt]

With transducer connected and you have continuity from hot to ground at the cable termination inside the amp then the circuit should work. If it does not work then pull the tank apart and check the springs are not broken.
The tiny wires that run through the magnet often break which is not normally fixable.
If the drive end is broken but PU end is still intact then the pickup end will still make a clang if you shake the amp, giving the impression that it still works.
I've had a few Amps with that issue, especially if stored for long periods in humid/damp environments.
Phil.

[ToadMeister]

Enzo - Probably because it's morning and I have to go to work soon, so I feel hasty.
Having trouble visualizing your explanation.
The transistor sits idle, hook up a cap, voltage still the same. But the same voltage on both ends, while the far end stays the same?
I have to pound my head with info until knowledge sticks (I'm thick) then I become learned, but before that I get bogged by details and racing thoughts.
I will probably understand better later on after work after the influence of 8-10 cups of coffee.
So far every time I read it, it's getting clearer. It's just some details.....like left and right, no DC on this end and......
Pictures ARE worth millions of words.

What is causing my tank to not work? There is nothing physically wrong with it! The tank is not accepting the power into it (ie bad spring magnets), or the power is weak.