Echoplex Transistors

[TASBrendan]

I do see 35V and 22V at the correct points, just the zener is off. The circled points on the schematic are wires. After looking at the resistance from point 8 to ground, I was getting weird readings and then I noticed that there was no continuity from the switch to point 8- should just be a wire. I found the wire that was there was a 2 wire plus ground and the wrong wires were connected at either end and the whole switch was wired backwards. So the work that was done before I got it is proving to be not great. Once that was all set straight, I have echo, but it's still really noisy. I have to get the right zener and voltages to that board before I can do any more troubleshooting. And the .0015 filter cap to record level is two .033 in series, which gives me really .0165, kinda high.

[Helmholtz]

I do see 35V and 22V at the correct points, just the zener is off. The circled points on the schematic are wires. After looking at the resistance from point 8 to ground, ...

Where do you see 22V? You should have 26V at point 13.

Resistance from point 8 to ground should be 395R in both Echo and SOS modes.

Noise with running tape could be due to:
- dirty heads
- old/dirty tape
- magnetized playback heads.

[TASBrendan]

Sorry, typo there- I do see 26V at point 13. The only low voltage it at point 12, running at ~ 12.5V. Echo head reads 390 Ohms, Record reads ~65 Ohms and Erase is at 25 Ohms. Thanks!

[Helmholtz]

Not sure if the zener is bad. Also check the 1k dropper resistor and the 500µ filter cap.

[TASBrendan]

1K reads good and all the caps in the supply read good ESR. Since I'm finding a lot of mistakes in the previous work, I suspect the diodes. There's two glass diodes with no markings in series. Might as well put in what I know is right.
As far as the noise goes, it happens with the tape off the heads but only as the Volume knob mixes in the echo side- the dry signal from the preamp is quiet. The tape does need to be replaced. I think I'll pick up a demag tool and see if that helps- probably time for it anyway. I think that the low voltage on the amp board is causing bad signal to noise ratio, too in the playback. I'm not even going to try calibrating the bias and record levels until that's sorted. Right now I do have record and playback, but at low distorted levels and also an oscillation/ring modulation happening as I adjust the bias oscillator or sustain amount as well as the constant noise.

[Helmholtz]

1K reads good and all the caps in the supply read good ESR.

Low ESR doesn't mean low leakage current. Increased leakage might pull the voltage down.
Nevertheless, best thing you can do is sub the 22V zener.
If the voltage is still too low, lift the 500µ cap for a test.

[TASBrendan]

That's great, thanks Helmholtz! I see what you mean with the leakage now. I have someone close by loaning me a leakage tester and a demag tool- good timing!

[TASBrendan]

It's alive and well! The 500uF cap tested good on the leakage tester. Put a 24V zener in and replaced the 1k carbon comp resistor with a carbon film while I was in there, too. All the voltages looked good (+1V due to the higher zener) except at the collector of Q2 was low. I replaced the 35/25V and started checking the resistors. The two 10k resistors on the power rail had been replaced with 12k and the 22k had drifted up to 35k, so they all got pulled, too. Then I noticed there was no .003 cap. I had a .0047, so that went in and that cleaned up all the high end noise.

Thanks all for your help on this! Still waiting for the new tape and SOS head, but at least it's working again.