Vactrol Vactec sources?

[nevetslab]

I'm presently servicing an Ampeg SVT4-Pro that has significant distortion in the Vactec-based Limiter in the front end of the Power Amp Circuit. It uses a Vactec VTL-5C7 in the feedback loop around an inverting 6dB gain stage ahead of the op amp that drives the output stage. The LDR portion of the Vactec is switched in using a J176 P-Ch FET. When I turn the limiter off, allowing the amp to clip, it sounds fine, other than you can drive it into clipping. With the limiting stage in circuit (normal operation), it's preventing full output and does appear to be where the distortion is occuring.

I just ordered replacement TL074 op amps and replacement J176 P-Ch FET's, but can't find any source for the Vactec VTL-5C7 anywhere. I haven't yet contacted Yamaha to see if they're still using that Vactec in the power amp stage (it's also used in the compressor circuit.

Power Amp PCB Schematics (428xxh0_).pdf

VTL5C6, 5C7.pdf

VTL5C3, 5C4.pdf

Anyone know a source for the VTL-5C7? I do have a few VTL-4C4's on hand.

[Jon Snell]

2SJ176 jugFets in this employment are prone to fail, not the Vactrol though.
Why are you replaceing the TL074? Throwing parts at it won't tell you what the problem was just possibly give you a headache with man made faults.
If the TL074 was faulty, there would be no output.

Did you monitor the gate voltage?
Have you proved the vactrol is not changing impedance with current fed through the led?

[nevetslab]

2SJ176 jugFets in this employment are prone to fail, not the Vactrol though.
Why are you replaceing the TL074? Throwing parts at it won't tell you what the problem was just possibly give you a headache with man made faults.
If the TL074 was faulty, there would be no output.

Did you monitor the gate voltage?
Have you proved the vactrol is not changing impedance with current fed through the led?

No on both accounts. I too doubted replacing the TL074. Thus far, I haven't removed the power amp assembly from the chassis. I also doubt if the problem is the Vactec, but just a thought. I do have the replacement J176 fets coming in next week. I already have to pull the power amp to deal with the LM35 Thermal sensor that controls the Fan thru it's TIP 125 P-Ch Darlington. Fan turns on as soon as the amp is powered up.