Heathkit TA-16 tremolo

[pontiacpete]

With the lights off completely in the room tremolo is still weak. I measure 6k across the photo resistor.

[Helmholtz]

. I measure 6k across the photo resistor.

With or without light? What is dark resistance?

[pontiacpete]

With the unit on the resistor reads from 3.8k to 5k. Measuring with it on right?

[Mark Hammer]

Following this closely. I also have a TA-16 whose tremolo is nonfunctional. The LDR is fine but the bulb needs replacing. I'm wondering whether the circuit could be adapted to use a white LED whose lifespan would be better.

I'll note that the amp had annnoying hiss when I bought it (2nd hand). Replaced the input transistors and that improved matters considerably.

[Enzo]

Is the light going completely out when in dark cycle?

[Enzo]

An LED may well outlast the existing type bulb, but considering the amp is over 50 years old, what life span are we comparing?

[pontiacpete]

It’s hard to tell. It’s an LED and it is flashing, but it’s flashing fast enough that it doesn’t appear to be completely off.,in the dark cycle.

[Helmholtz]

With the unit on the resistor reads from 3.8k to 5k. Measuring with it on right?

No, I meant measuring LDR resistance, i.e. one end lifted, unit off, complete darkness.

[pontiacpete]

With one light bulb on in my shop I measure 1M to 1.5M. With it off it goes open, it keeps going up to 230m and more.

[Helmholtz]

With one light bulb on in my shop I measure 1M to 1.5M. With it off it goes open, it keeps going up to 230m and more.

Ok, the LDR is good.

[pontiacpete]

I put the 1k resistor on the cathode of the LED. Is that the correct place for it to be?

[Helmholtz]

I put the 1k resistor on the cathode of the LED. Is that the correct place for it to be?

That's fine. Doesn't matter if the resistor is connected at the anode or the cathode. Purpose of the resistor is to limit the LED current to around 10mA.

Ok, we know the LED is flashing and lowers the resistance of the LDR from megohms to kiloohms.

What changes if you add a current bypass resistor (e.g. 330R) across the series arrangement of LED +1k, i.e.between the 13V and the collector of Q13?
The idea is to lower LED brightness hoping that gives more variation of LDR resistance.

If that doesn't help, try increasing LED brightness by lowering the series resistor from 1k to 560R.
All tests with depth control fully up.

And check C32.

Do you have a scope?

[pontiacpete]

I added the 330R this way and then I put it on a resistor combination box. As I increase the value the trem gets a little deeper but not so much. Might be good enough.
Yes I have a scope

[J M Fahey]

Way to go but you need to tweak values a little so a HIGHLY nonlinear LED mimics better a still nonlinear but much flatter tungsten filament, specially within the current/voltage supplied by that transistor.

Let me experiment a little and suggest some values, better than throwing parts at random

You sure 6V 25mA fits?

So I kludge something which drops 6V and gets brightest with 25mA through it and gets almost dark with, say, 2V or less than 5-10 mA .... "just like the proper bulb would".

Also use a *good* Led, not some mystery junkbin part.

Please post the full schematic, jon snell only shows the oscillator.

[pontiacpete]

The schematic is in my first post under the photo of the transistors. Thanks for looking at this