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Russell H.
Russell H., Service Tech
Category: Consumer Electronics
Satisfied Customers: 10787
Experience:  8+ years of professional experience
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Struggling with a Gigrac 600

Customer Question

Struggling with a Gigrac 600
Submitted: 1 year ago.
Category: Consumer Electronics
Customer: replied 1 year ago.
I own an electronic repair shop and repair PA. I've been wrestling with this thing for weeks. When signal is applied, the clip lights come on for both amps in either Main or Mains/Monitor configuration. I have verified, out of circuit, ALL the semiconductors and they are fine as are the resistors and caps.
On the mixer front, I get great signal from the Monitor out and Main out and ran those through a separate amplifier with perfect results. I am getting weird and random voltages with each power up on pins 1,2 (Amp 2 and Amp 1) and pins 5 &6 (Clip 1 and Clip 2). The TL072 is also new. Soundcraft is clueless so I cast my net to see what I can catch.
Customer: replied 1 year ago.
The inputs to the non-inverted pins of the TL072 come off the positive side of a 100uF/25V cap. So both pins 3 and 5 go to a cap each. There is anywhere from 5 to 12 volts on the positive side and typically -.135 on the negative side. I'm seeing about 18 volts on the Amp 1 and Amp 2 pins and about 15 volts on the Clip pins (5 & 6).
Customer: replied 1 year ago.
The bias voltages are also wacky. On the base and collector I have about 82 volts (V+) and basically nothing on the collector so it appears that the bias network isn't working. The condition is identical for both amps.
Customer: replied 1 year ago.
Sorry.....nothing on the emitter
Customer: replied 1 year ago.
Bias transistor for amp 2 has 82 volts on the emitter and base and -.173 on the collector and the same for the Amp 1 bias transistor except there is -.554 volts on the collector.
Customer: replied 1 year ago.
This is with NO signal applied.
Expert:  Russell H. replied 1 year ago.
Hi, thank you for contacting My name is Russell. I will do my best to provide the right answer to your question. Those transistor voltages are utterly wrong. Not so much zero on emitter... that's an emitter-to-ground configuration's norm, more or less.But 80-odd volts?? Transistors do not run on those voltages, they burn out on those voltages. I suspect you need to measure AC on those, also... if the AC voltage is high too, you have a defective power supply, possibly with a bad filter capacitor or a defective rectifier. At least, those are good to check on! Do you have any sort of schematic, or service manual? a service manual for this device, if it could be found, would help immensely with the repair. Shall I search for a (source for) one for you?
Customer: replied 1 year ago.
Verified and cleared the PS. Pulled and checked all the filter caps and they checked good and were stable using a capacitance meter. Bridge rectifiers and regulators are also good. I have 2 units and am using both to get one working so used both PS and got the same results. The other amp module is fried, it was a spectacular light and smoke show!
These aren't common emitter biased configuration transistors but are high voltage (BF423). If anything, they are common collector. The Emitters come right off V+ but have a 470 ohm biasing resistor in line between the transistor and V+. I have pulled all the semiconductors and checked each one and all checked good. I have attached the service manual for this device for you to review. Page 44 is the specific schematic in question. Connector CN1 uses 8 of the 10 pins and the DC voltages are measured with respect to ground at pin 8 or the left end of R86. I have proper voltages at pins 3, 4 and 7 (Phantom power, -15 Vdc and +15Vdc) The Amp 1 and Amp 2 are on Pins 2 and 1 respectively while Clip 1 and Clip 2 are on 5 and 6. The biasing network starts with TR9 for Amp 2 and TR28 for Amp 1.
There is a TL072 op amp used for the clip circuit. The non-inverting inputs go to 100uF/25V caps. I have anywhere from 7 to 10 volts on the positive side and -.135 on the negative leg of each cap. That's causing part of the problem. The other is obviously the biasing issue. The TL072 is brand new. I've even been in contact with Soundcraft but they are limited in what they can do as I am not an authorized factory repair center anymore. There's something I'm missing. It's probably right in front of me.
Expert:  Russell H. replied 1 year ago.
Did you 'verify and clear' the PS, using test points, measuring both DC and AC voltages? "The other amp module is fried" - do you mean that the two PSs are parallel, but one had an amp fried in its setup ? if so, that PS is suspect at best, ***** ***** be used as an infallible standard. -135 volts... what is the voltage rating of the caps subject to that?and is that -135 volts at all typical of the voltages properly produced by the PS in good order?
Customer: replied 1 year ago.
The PS came from 2 separate units. 2 different Gigracs. One was purchased off eBay to be used as parts. That was the amp board that was "fried". Both PS provide all the correct voltages, both AC and DC with V+ at 83VDC and V- at -83VDC. The capacitor voltages were NEGATIVE POINT 135 (-.135) not NEGATIVE 135. Put a scope on the outputs and it is quite clean with very little ripple to speak of. I'm gaining more confidence that this is centered around 2 things, the first being the biasing issue. The second is where the voltages to the IC are coming from as they disappear when I remove the chip. I might typically have 11 or so volts DC on pins 1, 2 3, 5, 6 and 7. The voltage on each pin shouldn't be the same. I have replaced the chip a number of times and even used different varieties (5532, 4558, 2608) and netted identical results. I did recap the entire unit but to no avail. Still getting V+ (83 VDC) on the base and emitter of both biasing transistors and -.something on the collector. It varies when I power the unit down.
I power it down and come back in 5 minutes and the voltages are all completely different. They remain stable the entire time it's under power but is very subject to change again once powered off. A portion of the clip circuit is tired to the bias network so there is very likely a relationship that I need to explore. Now you know why Soundcraft didn't have any ideas! I do appreciate your help. Your suggestions have been good for the most part. As it can be in some cases, this unit just might not be worth fixing. I'm never going to see the money back I already got in it. So I need to cut my loses and tell the customer to start planning funeral services for it.
Expert:  Russell H. replied 1 year ago.
Can you inject a signal into the amp, and see where it... kind of gets lost, or ceases passing through amplifiedly (to use loose terminology) ?