Silvertone 1484 Tremolo Help

[misterc57]

I have been using an isolation-transformer-variac. Since I do not have the 68K resistor I think I can do a quick test and clip in a 330K in parallel at R58 to see if it changes-fixes the issue.

[tboy]

Where was the amp plugged in before?

From post #5:

Testing done running chassis on a isolation variac at around 115 VAC setting.

[misterc57]

Still not working but more info.

Turns out that R58 does read 68K, I triple checked to be sure.

C25C voltage is 162. After it passes through the roach the voltage is 79. Too large a drop? Bad roach?

The background tremolo pulse I hear through the speakers only happens when my meter probes are connected to read voltage. If I remove the probes the pulse in the speakers is not present.

[Helmholtz]

Turns out that R58 does read 68K, I triple checked to be sure.

Fine.

C25C voltage is 162.

Still low. As said said I would expect around 200V.

After it passes through the roach the voltage is 79. Too large a drop? Bad roach?

A lit neon typically dops around 80V. So 79V at the plate looks normal with a supply of only 162V.

The oscillator side has 530 ohms across its leads.

Was this measured across the LDR? If yes, it's contradictory to your statement: Resistance is pulsing from 830K to 900K, this was about the max I could dial in.


Do you have a scope?

[Helmholtz]

Question to others having more experience with neon tremolos: Is the neon always kept on (trickling) with brightness modulated by plate current or is turned off and on (including periodic re-ignition splikes)?

[Enzo]

In general I expect the neon to be flashing.

[misterc57]

"The oscillator side has 530 ohms across its leads." That should have read 530K ohms and that was with the amp off.

Was this measured across the LDR? Yes.

My readings have changed. Perhaps because I raised the voltage a bit, and I am more aware of what I am checking.

Amp OFF LDR = about 540K ohms.

Amp ON LDR pulsing from about 530K to 570K. I read that the LDR resistance goes down with the light on. My readings indicate otherwise?

I have a scope, but not much scope experience. Tell me what to setup and I can give it a go.

Thank you.

[g1]

Question to others having more experience with neon tremolos: Is the neon always kept on (simmering) with brightness modulated by plate current or is turned off and on (including periodic re-ignition splikes)?

Fender (classic) circuit is neon, and as far as I know, the ignition spikes are what causes the 'ticking' issue.

[Helmholtz]

If the neon is actually turned off and on, plate voltage must periodically jump by more than 20V. Actually between 0V (neon off and non-conducting) and 80V to 120V.
But I don't see how this would allow for much depth variation with this circuit. Maybe duty cycle effect?

Anyway the lower ("on") LDR resistance is much too high to produce an audible effect..

[misterc57]

My V6 pin 1 voltage jump is only 1 to 2 volts.

Looking at the C25C supply again. Schematic shows that this also feeds V1 pin 1 and pin 6. Earlier readings showed 96 V, probably 100 V now that I have turned my variac up slightly. This would imply proper supply from C25C, right?

I just looked at R39 connected to V4 pin 6. I read 205 V on one side of R39, 143 V on the other side. This is also supposed to be supplied by C25C according to the schematic. Turns out it is not, R39 is supplied by C25B.

Makes me think the schematic is wrong here and there. Only way to see 200 V at the LDR would be to connect to C25B. That would bring pin 1 down to 120 V per the schematic. Thoughts? I know that I can be wrong but can both the schematic and wiring be wrong here?

[Helmholtz]

Amp OFF LDR = about 540K ohms.

Amp ON LDR pulsing from about 530K to 570K. I read that the LDR resistance goes down with the light on. My readings indicate otherwise?

Resistance is pulsing from 830K to 900K, this was about the max I could dial in.

Inconsistent readings my be caused by different amounts of stray light from outside entering the roach with exposed chassis.

[misterc57]

Oh, Oh, Trem-Oh-Low... You still elude me.

I moved the bulb feed from C25C to C25B. I now have 120 V at V6 pin 1. Pulsing is only varying by 1 or 2 volts. Still no tremolo.

What else to check?

Thank you!

[Helmholtz]

Pulsing is only varying by 1 or 2 volts.

No wonder if your neon is always on as the neon stabilizes the voltage.
You might have too little grid (pin2) drive or a low emission/gain V6 tube. Did you try a new 12AX7 in V6?

[misterc57]

I did swap that tube with another in stock. I have another I will go try.

[misterc57]

I have tired 3 different 12ax7 tubes in V6. No changes.

V6 VDC readings

pin 1 ; 118 - 119
pin 2 ; pulsing slightly above and below 0
pin 3 ; 1.4 - 1,6

pin 6 ; 168 - 180
pin 7 ; - 0.6
pin 8 ; 1 mV (connect to ground)