Heathkit TA-16 tremolo

[pontiacpete]

This amp came to me with one or both of the transistors changed in the tremolo circuit. The 2n3692 looks to me not to be the original(I broke a leg trying to get one out to test, i know) and looks like it could be original but with different numbers.
The voltages except that I’m getting more like 13v on the collector of Q13 and Q12 has fluctuating voltage, as if it is oscillating. When I probe the caps the meter pulses


but not tremolo. I put it on a foot switch and still nothing. Anyone have an idea?
. ta-16_schematic.pdf

[Helmholtz]

Measure the DCR of LMP2 (one end lifted).

[Jon Snell]

Q12 is the Colpitts oscillator.
Q13 is the driver.

The oscillator will oscillate on its own, producing a sine wave. If it doesn't check C29, 30 & 31 for phase shift.

[pontiacpete]

Forgot to mention that the original opto the light measures open with the ohm meter. So I put another 6v lamp and photo resistor. The light measures 12.3ohms.
All caps are new in the trem circuit

[Helmholtz]

Forgot to mention that the original opto the light measures open with the ohm meter. So I put another 6v lamp and photo resistor. The light measures 12.3ohms.

The original opto lamp is specified as 6V/25mA (150mW). That means a hot resistance of 240R, cold resistance might be lower by a factor of 5 to 10.
So if your lamp measures lower than 24R , it's a higher wattage lamp requiring more current than Q13 might be able to deliver.

[pontiacpete]

I bought two of these and both measure 12.5R. Aren’t these the correct lights?

[Helmholtz]

I bought two of these and both measure 12.5R. Aren’t these the correct lights?

Well this is a 50mA (300mW) lamp, i.e. twice power.

Can you see the lamp flashing/pulsating?

[pontiacpete]

I took it out of the shrink wrap tubing and sure enough it isn’t flashing. I need to find the right light.

[pontiacpete]

A bulb with these specs seems to be rare. Can I use this?https://www.newark.com/mcm/25-080/fu...%2025ma%20bulb

or is there a way of adjusting the circuit to use an LED or some other light.

[Helmholtz]

A bulb with these specs seems to be rare. Can I use this?https://www.newark.com/mcm/25-080/fu...%2025ma%20bulb

or is there a way of adjusting the circuit to use an LED or some other light.

The fuse type bulb is 250mA, not 25mA. So no.

A red LED should work, but might require a little tweaking.

Easy to try: Wire a 1k resistor in series with the LED and connect in place of the lamp. Watch polarity (cathode of LED goes to collector of Q13).
Does it flash?

[pontiacpete]

The LED is flashing but no tremolo. I was expecting it work now that the light is flashing.

[Helmholtz]

The LED is flashing but no tremolo. I was expecting it work now that the light is flashing.

Is this with the shrink tubing? The LDR/LED assembly must be completely shielded from ambient light.

[pontiacpete]

Yes, it’s completely covered.

[pontiacpete]

Ok I did the covering over with electrical tape and I can faintly hear the trem. It’s very weak.

[Helmholtz]

Yes, it’s completely covered.

Make sure no light can enter at both ends of the tube. If necessary turn off room lights.

If still no tremolo, check the LDR with your Ohmmeter.
Resistance should be much lower when exposed to light compared to absolute darkness (at least by a factor of 10).