lectrolab trem woes

[funkmeblue]

I have what I believe to be a 1964 lectrolab r600c on my bench. I can't seem to get the trem to work. It's bias modulating with a 6au6 tube. What should I look for? I just started learning about trem circuits, but this one confuses me a bit. the schematic can be found at http://lectrolab.wordpress.com/lectrolab-models/r600/

[martin manning]

I see an R600B schematic there, is that the same tremolo as yours? If so it's a pretty standard phase-shift oscillator, except that it's using a pentode that is grid-leak biased instead of the common 12AX7. Do you know if the tube is good (shown as 6SH7)? You can measure the voltage across the plate load (R32) and calculate the current to see if it is conducting. From the schematic it's 50V/150k = 0.3mA. Also check that the grounds are good. The most likely failure is one or more of the three phase-shift caps (0.03 and two 0.02) is leaking, so if all else looks ok replace all three.

[funkmeblue]

it's the r600c schematic on the same page. Actually it's a picture of a schematic stuck to this amp.

[martin manning]

Same basic trem oscillator using 6AU6, three 0.02 phase-shift caps, 90V/150k = 0.6mA.

[funkmeblue]

all caps have been replaced and still not working. I'm getting a 100 volts across the 150k plate resistor

[martin manning]

OK 100V sounds good. Measuring voltage on pin 8 of the 6AU6, what do you get, and is it steady or varying (adjust speed control to see if it has any effect)? Trying to find out if the oscillator is oscillating or not. Check voltage on pin 6, should be about 80V. Do you have another 6AU6?

[funkmeblue]

there is only 7 pins

[martin manning]

Oops, sorry, pin 5, the plate. Hard to read that from the schematic!

[funkmeblue]

I'm getting a solid 170 volts on pin 6

[funkmeblue]

250 volts on the plate pin 5 the schematic is numbered wrong and I have tried 3 tested tubes

[martin manning]

So I guess I did read it right, then. You've got 250 on the plate and then +100 for 350 on the other side of the 150k plate load, right? If so then 170 on pin 6 doesn't look right. you have a voltage divider there with 470k and 150k so the junction should be ~85-ish (but less than that due to screen current). Check those resistors. You'll have to disconnect the 470k from the B+ to get a good reading.

[funkmeblue]

it was the resistors in the voltage divider.....one of them.....I replaced them both and it works.....thank you much

[martin manning]

OK cool. So you've got new caps in there too... Should be good for a while now!