Fender TFL5005 vibrato not working

[Fred G.]

Hi All,

I have a Fender Bandmaster reverb TFL5005 I'm working on that came in for a rehab. I've got the amp sounding sweet, and everything is working fine, except the vibrato. The roach lights up and pulses at a variable rate according to control variation with a grounding plug inserted, but...no vibrato. Here are the V5 tube pin voltages with the grounding pin REMOVED:

Pin 1: 414V
pin 2; -46 V
pin 3: 0v
pins 4/5 - heaters
pin 6: 362v
pin 7: -46v
pin 8: 0V

I'd love to get some insight into sorting this out. One thing that does leap out at me is the 0 volts on pins 3 and 8 - shouldn't these be at about 1.8V? Also, those plate voltages seem awfully high, compared to the schematic. I'm suspecting the 12AX7 is not biasing correctly, but...I figured I'd ask. I've replaced the cathode bypass caps, but I haven't looked into the cathode resistors just yet. Duh, I know.

Thanks in advance, everyone!

Fred G.

[Enzo]

OK, let's think about the circuit. Your bulb blinks. What blinks the bulb? The V5 circuit. One half of the tube is your LFO - low frequency oscillator. and the other half directly drives the bulb. Point being, if your bulb is blinking, that tells us V5 is working and doing its job. I will discuss the tube in a moment. But for now, the issue is not the tube, your bulb blinks, so that means for some reason your trem bug is not working or the circuit it is in is faulty. Get an ohm meter on the photocell side of the trem bug, and run the trem slowest. Can you see the resistance dancing? It might be simple as needing a new bug.


Back to the tube. The tube works, so we know all the voltages are getting there. But you are worried about pins 3 and 8 being zero. But you pulled the tube!! SO of course they would be at zero. Look at the circuit. The only thing connected to pins 3 and 8 (the cathodes) are in each case a resistor to ground. How doe a resistor get a voltage across it? By Ohm's Law, a voltage appears when current runs through the resistor. If you pull the tube, no current can flow through the resistors. And by Ohm's Law, zero current means zero voltage. And likewise, your plate voltages, pins 1 and 6, are going to sit up at B+ voltage with no tube. The plate voltage will be lower than B+ by the amount of voltage dropped across the plate resistor due to current flow. Pull the tube, no current flow so no voltage drop.

[The Dude]

https://el34world.com/charts/Schemat...1069_schem.pdf

Is this the schematic for your amp? If not, please link or upload the one that matches your amp. Do you have a scope?

[Fred G.]

Enzo - those voltage readings are with the tube INSTALLED. I did NOT pull the tube prior to taking measurements. Not sure where you got that idea?

Dude - that's the closest schematic I can find, the Bias circuit had 80 uF caps, though (before I converted the amp to adjustable bias).

[Enzo]

I am sorry, I saw that REMOVED in your post and assumed it meant the tube. meanwhile, your zero on cathode voltages tell us no current is flowing, as in the tube is off, yet you seem to have function in the tube. Please explore the signal path side of the trem bug.

[Fred G.]

OK...when I changed the bias circuit to adjustable bias I removed the TWO 80uf E-caps, converted to the adjustable bias circuit in the schematic the TheDude provided (also borrowed from the Hoffman amps site), but opted to go with a 100uF bias cap instead of the 50 uF. I have read that dicking around with the bias circuit can sometimes hose up the Vib circuit - is that perhaps what I might have "accomplished"?

[Enzo]

All that affects the oscillator side, which if the bulb blinks, is working.

The bias voltage gets applied to the trem oscillator, but only to keep it off. When you shunt the footswitch jack to turn it on, you are grounding off that bias input.

[g1]

When you removed the shorting plug, you disabled the oscillator. That's why you had 0V at the cathode pins.
As Enzo said, the oscillator is working. So either the LDR side of the bug is bad, or it's not getting connected to the circuit properly. Have you checked the ground connection of the bug, and the intensity pot?