I bought a few projects to entertain myself during COVID-19 'stay home' time.
NAD C356BEE - earlier thread
Next up: Cambridge Audio P500
Symptom:"Won't power up."
Diagnosis was easy - no voltage on power transformer secondaries, primary was open.
My guess was the thermal 'fuse' which was buried inside the toroid transformer.
It's an AUPO 4F 130C device.
AUPO official website, thermal fuses, pptc, manufacturer of bi-metal thermal protector
The transformer didn't look like it got very hot, but who knows? Note to designers- protecting the fuses on the circuit board by burying a fuse inside the power transformer isn't a good idea!
IMHO, of course.... 🙂
Unwinding and rewinding a toroid isn't difficult but it is a way to spend some time..
Check YouTube for videos - the key is to make a bobbin by cutting 'U's out of the ends of a stick.
Wind the tape and wire on bobbins as you work.
I added a 12v secondary for possible use with a speaker protection board.
These Cambridge units have undersized heatsinks. I re-did the thermal paste on the power transistors/micas/heatsink as there never had been much, and it was quite dried up.
Powered up with a dim bulb and everything looked OK, so I plugged the amp into power and checked the idle bias. It was close to spec (13mV between E and S on power transistor) ....a bit below, which is good.
Thanks to everybody who has posted info and schematics on this amp and also the A500 (next up on my bench here); it was a great help. This forum is a fantastic resource.
Checking the bias- notes to other beginners: The hints I found indicated connection to the two leads on the right on the power transistor - these are the S and E leads on the NPN (SAP15 N). For the PNP, it's the two leads on the left that are the S and E connections. Connect to either on each channel for setting the bias with the variable resistor on the board. Those resistors need very little movement to make major changes in bias, so be gentle!
Also, I wouldn't adjust the bias without using 'mini grabber' leads for the voltmeter.
NAD C356BEE - earlier thread
Next up: Cambridge Audio P500
Symptom:"Won't power up."
Diagnosis was easy - no voltage on power transformer secondaries, primary was open.
My guess was the thermal 'fuse' which was buried inside the toroid transformer.
It's an AUPO 4F 130C device.
AUPO official website, thermal fuses, pptc, manufacturer of bi-metal thermal protector
The transformer didn't look like it got very hot, but who knows? Note to designers- protecting the fuses on the circuit board by burying a fuse inside the power transformer isn't a good idea!
IMHO, of course.... 🙂
Unwinding and rewinding a toroid isn't difficult but it is a way to spend some time..
Check YouTube for videos - the key is to make a bobbin by cutting 'U's out of the ends of a stick.
Wind the tape and wire on bobbins as you work.
I added a 12v secondary for possible use with a speaker protection board.
These Cambridge units have undersized heatsinks. I re-did the thermal paste on the power transistors/micas/heatsink as there never had been much, and it was quite dried up.
Powered up with a dim bulb and everything looked OK, so I plugged the amp into power and checked the idle bias. It was close to spec (13mV between E and S on power transistor) ....a bit below, which is good.
Thanks to everybody who has posted info and schematics on this amp and also the A500 (next up on my bench here); it was a great help. This forum is a fantastic resource.
Checking the bias- notes to other beginners: The hints I found indicated connection to the two leads on the right on the power transistor - these are the S and E leads on the NPN (SAP15 N). For the PNP, it's the two leads on the left that are the S and E connections. Connect to either on each channel for setting the bias with the variable resistor on the board. Those resistors need very little movement to make major changes in bias, so be gentle!
Also, I wouldn't adjust the bias without using 'mini grabber' leads for the voltmeter.
Attachments
Unwinding and rewinding a toroid is a tough job and you do it just like the toroid never be repaired, good job.
The schematic can find here
CAMBRIDGE P500 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
The schematic can find here
CAMBRIDGE P500 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
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Thanks, Patrick.
I was lucky that there was no damage to the windings, just the thermal fuse was bad. So I could re-use all the wire.
Also lucky that there is lots of empty space in that chassis, so even though the transformer is a bit bigger now, it still fits!
🙂
Patrick-
Thanks for that link to the main schematics for the P500.
I'll try to attach a schematic .pdf to this message.
Thanks for that link to the main schematics for the P500.
I'll try to attach a schematic .pdf to this message.