Cambridge A500 strange fault with one channel - Page 1
Forum section: electronics / repair

author: cowasaki
author_rank: Frequent Contributor
timestamp: August 16, 2019, 08:14:53 pm
content: Just picked up a Cambridge A500 amplifier as faulty for little more than a few pounds. Hoping to get it working but digital is more my bag so it's going to be a mission :-)Anyway does anyone have a circuit diagram/service manual ?Strange fault is that it works perfectly when you switch it on then carries on working for about 20 seconds. After that you get some clipping type cracks and pops which get gradually worse then the channel drops but comes back intermittently for a short time with or without the crackles then dies again.......THEN even if the channel is apparently dead, if you unplug the amp the channel comes back perfectly every time for maybe 0.5-1 second before the PSU stops giving enough energy.All I've done is take off the lid and take a look. No obvious blown caps/loose components or anything like that. I'm not expecting someone to immediately diagnose the issue based on the above but if anyone has any likely ideas to point me in the right direction or circuit diagrams etc then it would be appreciated. I've no time tonight but will be having a look tomorrow. At least I have one channel working to compare with the other, a scope and signal generator so should, at least, have the tools.... if not the experience with analogue.

author: mc172
author_rank: Frequent Contributor
timestamp: August 16, 2019, 08:22:29 pm
content: https://elektrotanya.com/cambridge_a500-int_sch.pdf/download.html

author: cowasaki
author_rank: Frequent Contributor
timestamp: August 16, 2019, 08:23:58 pm
content: Thanks! I just found that exact link and was going to post it :-)Hoping I can work it out tomorrow

author: mc172
author_rank: Frequent Contributor
timestamp: August 16, 2019, 08:38:32 pm
content: Just had a look at the schematics out of curiosity. I've messed around with a few different amplifiers that had the same output devices - the SAP15N/P complementary pair. From what I remember, the built in emitter resistors are quite weak on those and are usually what fails.

author: cowasaki
author_rank: Frequent Contributor
timestamp: August 16, 2019, 09:44:09 pm
content: So does that mean replacing the packages? How can I specifically test to see if that is the fault?

author: cowasaki
author_rank: Frequent Contributor
timestamp: August 17, 2019, 12:46:19 pm
content: Even stranger.....My reasonable thinking was that the two channels have a pair of SAP15 N&P transistor packs connected to a heat sink which are basically interchangeable (I know I need to set the VR) so as they are electrically and physically identical if I swap them round and the fault swaps channel then I know that it is one or the other transistor pack on the faulty channel and all I need is to replace them. So I swapped them over and now...... It's working on both channels and sounds very good. There was definitely no soldering issues so it wasn't just a bad solder connector. I'm going to set the VRs and soak test it later but than is rather strange and not bad for £10. If it never worked again the toroidal 30V -+ transformer is worth several times that on it's own :-)

author: cowasaki
author_rank: Frequent Contributor
timestamp: August 17, 2019, 08:52:03 pm
content: Just been messing and the fault is back but its on the original channel which means the the SAP15 N&Ps are ok. So tomorrow I'll take the board out and check are start following the schematic.To emulate a line level do I create a 2v sine wave and follow it through the amp?I have a couple of 10w 8ohm resistors, can I use these to emulate the speakers?