SF Vibro Champ, no vibrato

[skydog613]

I picked up a used Vibro Champ with a defective vibrato circuit & no pedal. The seller was up front about it and I figured I could fix it.

So far:
- I got a no-name switch pedal which gave no audible effect. It has an LED which oscillated when switched on.
- Curious about the LED being unexpected in the original circuit I hacked a jumper by cutting the wire and twisted the leads together. No difference.
- I swapped tubes. No difference.

Next up is circuit analysis. I built an amp kit some years ago but I was (un?)lucky enough that it came up perfect on the first powerup, so I didn't learn much in the debugging department.

Noob questions:
- to "test voltage at a point in the circuit" I place my multimeter leads on (a) the circuit point and (b) the chassis (ground), correct? Does the positive or negative lead touch the circuit point?
- to check a capacitor I first check the resistance across it with pos/neg meter leads on the pos/neg leads of the cap. I expect very high resistance, correct? What next for capacitors?
- There is a 470K resistor bridging pins 1-6 on the second 12AX7. I've read that increasing this to 1M boosts the intensity. Is this correct?
- I'm open to any suggestions or debugging steps that any of you have to offer.
- Will it function if I swap a compatible 12A(T/U/Y)7 into the vibro circuit?
- This isn't a prized collectors item. Do people replace the speaker/pedal jacks with standard 1/4" or is drilling vintage chassis Simply Not Done?

Thanks y'all!

[Tom Phillips]

...I hacked a jumper by cutting the wire and twisted the leads together. No difference...

First off note that a short across the foot switch jack of a Vibro Champ will turn the tremolo OFF. A working Vibro Champ does not need a FS for the tremolo to operate.

..This isn't a prized collectors item. Do people replace the speaker/pedal jacks with standard 1/4" or is drilling vintage chassis Simply Not Done?

Not commonly done.


Do you have a copy of the schematic and parts layout and know how to locate the various parts?

[skydog613]

Thanks Tom for straightening out the vibe on/off for me. I was perplexed why only one lead of the jack was wired and now that makes sense. Unfortunately the vibrato knobs have no audible effect either way.

I've got the schematic & layout and can read them. I'm going to spend Saturday under the hood, jotting measurements in my notebook to see what doesn't add up. I'll post what I find.

[riz]

IIRC no drilling/reaming required to change to 1/4" jacks from the stock RCA.

[skydog613]

So I've finally gotten around to debugging this. If anyone can advise on the vibro circuit, I'd appreciate it. I have no audible evidence of vibrato. All tubes are functioning, no components look burned/melted. Both input jacks are sounding as expected.

Working with the schematic & layout I recorded the following. All knobs are on 10 and the vibro pedal is NOT shorted (vibro ON). I tested spots in the schematic that listed a voltage. I understand that measurements can be +-%20 but I listed everything in case one happens to be unusually off spec.

12AX7 (preamp) pins:
1 - 240v, spec 205v
3 - 1.6v
6 - 161v, spec 200v
8 - 2.2v, spec 1.5v

6V6GT pins:
3 - 390v, spec 342v
4 - 390v, spec 340v

12AX7 (vibro) pins:
1 - 160v, spec 170v
6 - 395v, spec 340v
8 - 175v
3 - appears to oscillate between 0.00 and 2.00v

voltage sources (unlabelled, just left of center in layout diag):
415v, spec 355v
404v, spec 340v
368v, spec 320v

Lug 3 of the intensity pot goes to pin 8 of the preamp tube. This seems to be where the oscillating signal rejoins the path. I'm not sure what to look at next. Can someone suggest next steps?

[Leo_Gnardo]

So I've finally gotten around to debugging this. If anyone can advise on the vibro circuit, I'd appreciate it.

There are 3 capacitors in the oscillator bridge, replace all of them with mylar/polyester or polypropylene film caps, bet your vibrato starts wobbulating properly. Over the course of time old caps somehow lose their ability to work in these circuits though they may test good. Got lots of malfunctioning vibrato/tremolos working this way.

Good question about alternate oscillator tubes. You really need the high gain of a 12AX7 for the oscillator to work right. I've found it just barely gets going with a 12AT7, loses depth and often quits at low and high rate extremes. Stick with the 'AX/7025/ECC83.

[skydog613]

There are 3 capacitors in the oscillator bridge, replace all of them with mylar/polyester or polypropylene film caps, bet your vibrato starts wobbulating properly. Over the course of time old caps somehow lose their ability to work in these circuits though they may test good. Got lots of malfunctioning vibrato/tremolos working this way.

Thanks Leo - a huge help! While I have your ear... There was another forum discussion regarding the same issue in a Princeton, and the vibro section looks the same. The suggestion was to increase/double the 470k resistor bridging pins 1-6 and the result was a comparable boost in intensity. Does this make sense to you?

[Enzo]

Is it now working? Fix an amp first, make it work, THEN go about modifying it.

If by lug 3 you mean the center one, which we call the wiper, then it does not connect to pin 8 of the tube, there is a resistor in between. But you mention pin 8 is where the trem signal emerges, so is there a varying voltage at that point? You seem to have one at pin 3. Does the signal at pin 3 change frequency when you adjust the speed control?

[Justin Thomas]

Just for clarification, because I made the same mistake, are you sure you're looking at a SF schem? I know the model neve changed, but the later Champ schem lists a B+ of 420V on the 6V6 plate. Your voltages mostly seem right for a SF schem, but too high for a BF.

Look for a Bronco schem - same exact amp as a SF VC but different label. Those all list 420V B+.

Justin

[Leo_Gnardo]

Thanks Leo - a huge help! While I have your ear... There was another forum discussion regarding the same issue in a Princeton, and the vibro section looks the same. The suggestion was to increase/double the 470k resistor bridging pins 1-6 and the result was a comparable boost in intensity. Does this make sense to you?

What Enzo says, get it working right first then let's see what about modding. Increasing the value of the 470K you mention may strengthen the vibrato but no point in sticking on a bandaid if the oscillator isn't running strongly in the first place. One more thing to check/change, if the bypass cap across the cathode R 4K7 on the first triode of the oscillator isn't healthy, it won't work very well or not at all. Value isn't so critical, the schematic shows 25 uF, I'm sure anything from 10 to 100 would work fine, whatever you have handy.

Having a blink at the AA964 Princeton there's a single-triode oscillator. In that amp I've gotten a deeper vibrato by reducing the value of the 1 meg resistor off the osc's plate. Typically I bridge the 1M with a 470K or 330K but that works for the Princeton not Champ.

[skydog613]

Thanks guys.

- the amp is working, just no vibro. In the other article I mention, the vibro was inaudible until boosted with the new resistor, which is why I ask.
- The one I'm using is from the Tube Amp Book CD and it matches vibrochamp.org. Switching to http://ampwares.com/schematics/bronco_ab764.pdf
- I understand testing voltage to ground, but how do I test capacitors?
- Thanks for the correction on the pot lug. In the VC/Bronco layout diagram it is the far-right lug, connected to the yellow wire. I'll test this further.

[Leo_Gnardo]

Well there *is* just a little vibro, 2V as you say, which means the oscillator is working but weakly. At most you might double that by increasing that 470K to 1M but it's still weak. That's why I'm directing your attention to replacing the trio of caps in the oscillator bridge plus the cathode bypass cap. Yeh I used to test 'em, of course they always measured good, but when replaced - most of the time that oscillator would jump right back into action. Now I skip the test-the-caps part of the exercise to save time & frustration because it never got me anywhere. So you can fiddle faddle until daybreak, or just change the suspect caps and get 'er done. Worst that can happen is it still won't vibrolate but you know you have fresh caps in that circuit so it isn't them.

Yes I know it's an empirical method, those who wish to spend hours figuring out "why?" may continue their scheduled activities... sometimes ya just gotta get things fixed. One experienced tech passed this phrase to me & all too often it's true: "analysis is paralysis."

[skydog613]

Wise words! I'll post again once I hit the local shop for capacitors.

[g1]

In the other article I mention, the vibro was inaudible until boosted with the new resistor, which is why I ask.

If it was really inaudible, that amp must have had a defect that was not cured. Instead, a "band-aid" solution was applied. Did the guy really think it never worked right as designed?
I'm pretty sure that if he had properly fixed it, he would have not needed to change the resistor.
This is why most folks around here will say the same as Enzo, "fix first, mod later".