I received this amplifier for service. No documentation can be found (hopefully someone has some insight).
I got it for service in a failed condition. What I wasn't told was someone else tried to fix it and it failed again - taking more stuff out. The repair was done improperly, incorrect transistor substitutions, incorrect pin-outs, incorrect resistor values and blown capacitors. Lovely.
At this point I have replaced dead / incorrect transistors and matched the pairs. Resistors are now correct as far as I can tell. The bias circuit works, but at low voltages, but the bias current quickly become excessive as supply voltages are raised. Right now I can run it at +/- 16 VDC or so. Normal supplies sit a little lower than 50 VDC. Outputs have been tested for leakage in all modes, one needed replacement with the original part I got years ago (it's a real semi). High bias is expected as the bias circuit voltage is higher than it should be. So outputs and drivers are operating as they should. Current through the current mirror (Q2, Q3 using the A75 schematic) is high and runs much higher with increasing voltage. These parts were burned out when i got it. Same for the other phase (channel).
The circuit has been checked for oscillation (it isn't). I expect this to be a simple problem caused by the previous technician.
Any assistance would be greatly appreciated!
-Chris
I got it for service in a failed condition. What I wasn't told was someone else tried to fix it and it failed again - taking more stuff out. The repair was done improperly, incorrect transistor substitutions, incorrect pin-outs, incorrect resistor values and blown capacitors. Lovely.
At this point I have replaced dead / incorrect transistors and matched the pairs. Resistors are now correct as far as I can tell. The bias circuit works, but at low voltages, but the bias current quickly become excessive as supply voltages are raised. Right now I can run it at +/- 16 VDC or so. Normal supplies sit a little lower than 50 VDC. Outputs have been tested for leakage in all modes, one needed replacement with the original part I got years ago (it's a real semi). High bias is expected as the bias circuit voltage is higher than it should be. So outputs and drivers are operating as they should. Current through the current mirror (Q2, Q3 using the A75 schematic) is high and runs much higher with increasing voltage. These parts were burned out when i got it. Same for the other phase (channel).
The circuit has been checked for oscillation (it isn't). I expect this to be a simple problem caused by the previous technician.
Any assistance would be greatly appreciated!
-Chris
Hi huggygood,
Thanks ...
Yes, that is what I am using. There is a great deal more circuitry in this on the main PCB. The entire voltage amp section is on the PCB, not in the "hybrid" PCB. That is a phase inverter on this unit used to bridge the two sections.
There are 8 additional small signal transistors in each channel on this PCB assy. It is in fact marked "A75" on the PCB underneath.
Thanks ...
Yes, that is what I am using. There is a great deal more circuitry in this on the main PCB. The entire voltage amp section is on the PCB, not in the "hybrid" PCB. That is a phase inverter on this unit used to bridge the two sections.
There are 8 additional small signal transistors in each channel on this PCB assy. It is in fact marked "A75" on the PCB underneath.
Not sure about Bonanza, but Vintage Audio in Portland OR seems ok. "Now over 19,000 manuals!"
Should be able to go by 1st class mail, or media mail.
Well, Stan Curtis wasn't able to offer any information or insights. I'll slug through it and report back if I get anywhere.
Hi Chris.
I just got hold of a pair of these to rebuild. I haven't even opened them yet, but if I can be of any assistance along the way, please reach out.
Cheers,
Glenn.
I just got hold of a pair of these to rebuild. I haven't even opened them yet, but if I can be of any assistance along the way, please reach out.
Cheers,
Glenn.
Hi Glenn,
Many thanks!
What I really need to do I guess is have a good one apart beside the bad one and compare voltage readings between them. A proper schematic would have avoided all this grief - that an a law preventing all but qualified people from working on things!
-Chris
Many thanks!
What I really need to do I guess is have a good one apart beside the bad one and compare voltage readings between them. A proper schematic would have avoided all this grief - that an a law preventing all but qualified people from working on things!
-Chris
How much of a hurry are you in? I have also sent a message to Cambridge asking for the service manual.
Hi Glenn,
I'm not. I already asked the factory, and the designer for information. No joy.
-Chris
I'm not. I already asked the factory, and the designer for information. No joy.
-Chris
Hi Chris. Did you get anywhere with this. I am just starting to work on mine. Cleaned them thoroughly yesterday. Look to be in decent condition. The Guys I got them off say both are oscillating wildly, so a little journey ahead I suspect. I have emailed Vintage Audio to verify that the manual is complete and has schematic and board layout. It's a little expensive when you [ay postage and convert to AUD, but probably worth it to get these babies up and running. Also found another random post about what caps to replace/upgrade.
The problem with Cambridge Audio is the number of hands it went through, summed up in a Wikipedia page https://en.wikipedia.org/wiki/Cambridge_Audio
So what particular era a design was manufactured, and whether anyone involved at the time that was the case is a bit of a problem. I'm surprised that Curtis can't help - he was involved off and on from the earliest days working for founder Gordon Edge, through to owning the company, and ultimately selling it.
But given the number of owners of Cambridge Audio over the years, who can say what happened to anything resembling a product archive.
So what particular era a design was manufactured, and whether anyone involved at the time that was the case is a bit of a problem. I'm surprised that Curtis can't help - he was involved off and on from the earliest days working for founder Gordon Edge, through to owning the company, and ultimately selling it.
But given the number of owners of Cambridge Audio over the years, who can say what happened to anything resembling a product archive.
I am hopeful that Vintage Audio is legit and will send me the manual. If not, I'll just have to use the A75 circuit and trace the rest. The place I got these off said they are both doing a good imitation of CW RF transmitters at present, so not just a simple fault. Odd that both are doing it though. One has been worked on, one set of outputs has been swapped (2SD 1047 in place of 2SD1046) along with sundry resistors. The main filter caps have bulged plastic tops, but I am pretty sure it is cosmetic as they test fine as far as I can tell. The trimpots are a bit ordinary, so I will order some Bournes replacements. Anyhow, once they are completely dry after their bath, I'll wind them up slowly on a variac and see what happens.