Hi,
I've got an old Cambridge audio 540A amplifier. It has worked well for years, but now has a crackle from one channel. It happens about 5-10 minutes after switching on, lasts for a few seconds and then it clears back to normal.
No noise from the pots/switches/cables etc so from what I can gather it's probably one of the SAP15 output darlingtons on its way out. Googling suggests that I should replace the SAP15 with STD03 (with added 0.22ohms resistor). The plan would be to swap all 4 (SAP15N and SAP15P).
It doesn't look to be a hugely difficult job to do but I would like to know if anyone has done this mod and whether there are any pitfalls I'm going to experience (like how to set up the bias preset etc).
Or....is there another diagnosis/solution to this problem?
Many thanks
Ron
I've got an old Cambridge audio 540A amplifier. It has worked well for years, but now has a crackle from one channel. It happens about 5-10 minutes after switching on, lasts for a few seconds and then it clears back to normal.
No noise from the pots/switches/cables etc so from what I can gather it's probably one of the SAP15 output darlingtons on its way out. Googling suggests that I should replace the SAP15 with STD03 (with added 0.22ohms resistor). The plan would be to swap all 4 (SAP15N and SAP15P).
It doesn't look to be a hugely difficult job to do but I would like to know if anyone has done this mod and whether there are any pitfalls I'm going to experience (like how to set up the bias preset etc).
Or....is there another diagnosis/solution to this problem?
Many thanks
Ron
From what you describe --"after 15minutes it crackles on one channel ---lasts for a few seconds ,then clears back to NORMAL" .
Classic symptoms from the Audio Hymn Book of Audio Faults # 1
Down to cold -then heat - then hotter .
RE-solder the output devices and associated resistors first before you try anything else.
HR soldered joint typical symptoms.
Classic symptoms from the Audio Hymn Book of Audio Faults # 1
Down to cold -then heat - then hotter .
RE-solder the output devices and associated resistors first before you try anything else.
HR soldered joint typical symptoms.
Wow.....now that was a tip a very useful tip.
You wouldn't have thought it possible, but there it was, and presumably from when it was first assembled....err, what quality control?
One of the output transistor legs wasn't pushed into the board correctly, lifted the track and the track was actually broken.....quick fix....scrape...dab of flux and bridged with solder...test this evening.
What a tip....well done.
Thanks
Ron
You wouldn't have thought it possible, but there it was, and presumably from when it was first assembled....err, what quality control?
One of the output transistor legs wasn't pushed into the board correctly, lifted the track and the track was actually broken.....quick fix....scrape...dab of flux and bridged with solder...test this evening.
What a tip....well done.
Thanks
Ron
Attachments
2 weeks later, no more crackles. That was definitely the fault.
Thanks for the tip Duncan.
Cheers
Ron
Thanks for the tip Duncan.
Cheers
Ron