Reverb Transformer impedance Measuring

[keithb7]

Please see attached info for a Fender style reverb transformer. Impedance values are shown.
If I run a DMM set to ohms from Blue to Red wires, will the DMM show about 25K ohm resistance value?
Measuring green to black approximately an 8 ohm resistance value? Just wondering if this a quick easy way to test a reverb transformer?
I am suspect this is not the case. Do I need to run a signal through the circuit and scope it before and after the transformer?
Reverb on a 1972 Deluxe Reverb is not working. V3 & V4 pin voltages test good. Hum increases as reverb pot it turned up. Reverb circuit resistors test within spec. Cathode caps
have been changed by someone, unsure when. Suspect within 10 years.
Cleaned all RCA jacks. Swapped in known good reverb tank, as well as good RCA jacks. No improvement. Suspecting transformer.

Thanks, Keith

http://www.classictone.net/40-18034.pdf

[Enzo]

Your meter measures resistance, not impedance. Those figures are impedances.

The transformer really has no way to change its own impedance, pretty much it is open or it is OK.

Did you replace the pan with the proper type? Should be a 4AB2C1B (or 9ABxxxx or 8ABxxxx) A 4EB2C1B won't work there, even if it works in other amps.

There are two cables to the pan. Pull them from the pan. Turn the amp on and the reverb up part way. Now touch the tip of each of the two cables. One should make hum out the speaker. If neither does, you have a problem in the return circuit, which has nothing to do with the transformer. But one should, and that one plugs into the pan OUTPUT jack.

The remaining cable goes to the reverb pan INPUT jack, but if you copnnect it instead to a small speaker, you should hear your music.

[J M Fahey]

Please see attached info for a Fender style reverb transformer. Impedance values are shown.
If I run a DMM set to ohms from Blue to Red wires, will the DMM show about 25K ohm resistance value?
Measuring green to black approximately an 8 ohm resistance value? Just wondering if this a quick easy way to test a reverb transformer?

No, your meter will measure DC resistance, probably about 1/10th of expected impedance.
So if you measure around, say, 1k primary DCR and about 1 ohm on the secondary, you at least know the transformer is not open and "should" work in a Fender style circuit.

I am suspect this is not the case. Do I need to run a signal through the circuit and scope it before and after the transformer?

No need to scope, just connect primary to a 24VAC transformer you have lying around and expect to measure around 1/2 VAC at the secondary.
Turns ratio is 55:1 .
Of course a scope can also show you that.

Reverb on a 1972 Deluxe Reverb is not working. V3 & V4 pin voltages test good. Hum increases as reverb pot it turned up. Reverb circuit resistors test within spec. Cathode caps
have been changed by someone, unsure when. Suspect within 10 years.
Cleaned all RCA jacks. Swapped in known good reverb tank, as well as good RCA jacks. No improvement. Suspecting transformer.

Reverb is a small power amp driving the tank, the tank itself sith a "speker" at the drive end and a "microphone/pickup" at the other, feeding a signal/reverb preamp , a volume/reverb pot and a simple mixer, so reverb signal can get back in the main audio stream.

So test all those blocks one by one.
* Touch reverb recovery input with a fnger, do you get hum? With reverb pot on 10 and it unmuted by the footswitch.
* Now gently brush the tank springs, do you hear that? ... it should be a LOUD noise.
* now inject audio straight in the reverb drive amp: do the springs vibrate?
* aren´t transducers at either end open?
* to test driver amp and transformer: connect secondary to *any* speaker you have around, it *is* a 1W tube power amp after all: does it sound loud and clear?

[keithb7]

Thanks kind folks. I will try these suggestions after work and follow up. I was wondering:

Looking at the transformer wiring, it appears that I should not be able to get any continuity reading if I measure across the two windings.
For example from blue to green, or red to black. Correct?

[Enzo]

Right, primary and secondary are separate. However, if it is wired into the circuit, outside circuit paths may fool your meter.

[xtian]

Agree with above. Though I had a Fender OEM reverb tranny that shorted only when HV applied. Measured OK at low voltages.

[g1]

For reference, there is a chart in this article that gives the approx. DC resistance values for the various tank impedances. It also explains what the other numbers in the tank number codes mean.

https://www.amplifiedparts.com/sites...compared_1.pdf

[keithb7]

Following up here for future readers:

I pulled a good known Reverb transformer from my 1971 Vibrolux Reverb and took some comparison measurements.

OEM Fender transformer part number 022921

Good known transformer measurement with transformer wires all removed from the amp circuit:
Blue to green wires: infinity
Green to black wires: 1.2 to 1.3 ohms
Blue to red wires: 1.92 k ohm
Red to black wires: infinity

I found the transformer was the problem in this deluxe Reverb discussed in post 1 of this thread.
The failed transformer showed infinity reading when measuring between the blue/red wired.

[Tom Phillips]

Following up here for future readers:

I pulled a good known Reverb transformer from my 1971 Vibrolux Reverb and took some comparison measurements.

OEM Fender transformer part number 022921

Good known transformer measurement with transformer wires all removed from the amp circuit:
Blue to green wires: 0 ohms
Green to black wires: 1.2 to 1.3 ohms
Blue to red wires: 1.92 k ohm
Red to black wires: 0 ohms

I found the transformer was the problem in this deluxe Reverb discussed in post 1 of this thread.
High impedance winding is open.

Let's get oriented:
The black & green wires are the secondary winding leads. Your 1.2 Ohm reading is reasonable.
The blue and red wires are the primary winding leads. Your 1.92 k Ohm reading is reasonable.
You say that the High Impedance winding is Open. Your readings don't agree with that statement.
Your 0 Ohm readings would indicate that the primary is shorted to the secondary but that is very unlikely from my experience and if true your amp would have been making smoke or blowing the fuse.

Are you sure about the 0 Ohm reading? I.e. same display on the meter as when you touch the test leads together?
Or is the display something like "-O.L-"? I.e. the display you see when the test leads are not connected to anything in the resistance measuring mode.

[Enzo]

Tom, he said he took those readings off a good transformer he pulled from a working amp. The open primary was on the amp he was trying to fix.

[Tom Phillips]

Tom, he said he took those readings off a good transformer he pulled from a working amp. The open primary was on the amp he was trying to fix.

I see that now. So...the known good transformer's primary and secondary are shorted together per the readings?

[Enzo]

I suspect that is his meter indication open. O as in overscale, rather than 0 as in zero. I could be wrong. No, thinking about it, if the red and blue were shorted to the secondary, then the primary would read the same resistance as the secondary - they'd be in parallel.


HAH! I just got back from breakfast at the Big Boy, the bacon is kicking in, I am having a lucid moment.

[Tom Phillips]

I suspect that too. It is important for others who will trip across this discussion to get clarification to the whole thing makes sense.

[Leo_Gnardo]

When you get a reverb transformer that's intermittently arcing, what a pia. Of course it measures good, but when you rev up the voltage, get random pings thru the reverb that sound like someone's hammering at the far end of the subway tunnel. Sometimes a bad reverb drive tube will do this too.

I find it's good practice to have a new reverb transformer on hand at all times - and recommend this to any repair person who handles amps that have this kind of reverb drive. They don't take up a lot of space nor cost much, not going to break the bank to keep one in inventory. And you can look like a hero for knocking out a quick fix.

Recently had a VibroChamp with a similar intermittent arc problem in the OT, made a heck of a racket at full line voltage. Luckily the speaker survived getting blam blam blammed a couple thousand times before its owner dropped it off for repair. And a few more times before I understood what was the problem.

[keithb7]

Sorry folks, as I was unclear. I edited my post to show infinity instead of the mis-leading reading of 0 ohms.

Is this correct?