Hi - Brand new to this forum and pretty new to audio stuff generally but... Have had a Cambridge A500 for a while, 15 years or so. Recently got it out of storage, replaced some capacitors to eliminate a nasty buzz, and seemed to get everything working fine, at least through a pair of 4-8 ohm MiccaMB42 bookshelf speakers (aka something cheap on Amazon). Then I went out and bought some really neat looking Knight speakers. Great cabinets. Fun vintage stuff. But the tweeters were a little less than enthusiastic sooooo....
This morning I was cleaning, doing my Saturday morning routine, and had the cambridge running through those nights, a little Roosevelt Sykes, with some extra volume to enliven the tweeters a bit. The cambridge then emitted a large pop, the left channel dropped dead, and everything smelled like roasted electronics. The top of the amp was quite warm, far warmer than I'd ever felt it.
Opened it up and identified the corpse of a cap and several resistors that look rather barbecued. I've tried to identify the problematic areas in the photo.
I've read on here that Cambridge Amps of this vintage weren't always so good at managing heat and that the SAP15 power transistors might have gone bad? If anyone has any experience doing CPR on a cambridge amp, advice would be much appreciated. Thanks -
This morning I was cleaning, doing my Saturday morning routine, and had the cambridge running through those nights, a little Roosevelt Sykes, with some extra volume to enliven the tweeters a bit. The cambridge then emitted a large pop, the left channel dropped dead, and everything smelled like roasted electronics. The top of the amp was quite warm, far warmer than I'd ever felt it.
Opened it up and identified the corpse of a cap and several resistors that look rather barbecued. I've tried to identify the problematic areas in the photo.
I've read on here that Cambridge Amps of this vintage weren't always so good at managing heat and that the SAP15 power transistors might have gone bad? If anyone has any experience doing CPR on a cambridge amp, advice would be much appreciated. Thanks -
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Cambridge have used a wide range of designs of all configurations in their amplifiers, from discrete parts to completely integrated. I agree that one common factor is the limited cooling but some of their competitors are just as mean with it.
You are probably correct in assuming the darlington output transistors have failed because the bias controller cap C238 and current limiter resistor R227? are associated with U201, the SAP15N darlington transistor. Other marked parts I can't identify. These SAP15 parts have been obsolete for many years, replaced first by SAP16 pairs and the present replacement is actually a new design, STD02/03 pair that unfortunately, aren't drop-in replacements.
You are limited to NOS or used components for these Sanken SAP pairs, of which there are plenty, because most that you find on the web seem to be fakes but these can only lead to more failures and damage. Yes, you do have to think why China is the largest source of so many desirable NOS parts. Choose a reputable supplier with a sensible refund policy for these old parts. Other semis before the output stage have possibly been shorted too, so it will pay to check them with a digital multimeter. These are inexpensive US On-Semiconductor MJE340/50 & MPSA43/93 types, so you shouldn't have difficulty sourcing locally.
I assume your other channel is still working which will be a great benefit when you can can compare voltages at the various nodes to track down problems. Before powering up again, fit a bulb limiter (a 60-100W incandescent lightbulb for this application, wired in series with the mains supply to the power transformer or the whole amplifier) to limit current, provide an indication of shorts and thus prevent further damage. A basic test on a working amplifier is done without speakers (possibly the original problem) or input signal connected. Measure for DC which should be <100 mV at the output terminals.
Here's a shortform service manual link - requires registration: Cambridge Audio A500 Manual - Stereo Integrated Amplifier - HiFi Engine
Build Your Own Current Limiter for Protection when Repairing and Testing Electronic Equipment - YouTube
You are probably correct in assuming the darlington output transistors have failed because the bias controller cap C238 and current limiter resistor R227? are associated with U201, the SAP15N darlington transistor. Other marked parts I can't identify. These SAP15 parts have been obsolete for many years, replaced first by SAP16 pairs and the present replacement is actually a new design, STD02/03 pair that unfortunately, aren't drop-in replacements.
You are limited to NOS or used components for these Sanken SAP pairs, of which there are plenty, because most that you find on the web seem to be fakes but these can only lead to more failures and damage. Yes, you do have to think why China is the largest source of so many desirable NOS parts. Choose a reputable supplier with a sensible refund policy for these old parts. Other semis before the output stage have possibly been shorted too, so it will pay to check them with a digital multimeter. These are inexpensive US On-Semiconductor MJE340/50 & MPSA43/93 types, so you shouldn't have difficulty sourcing locally.
I assume your other channel is still working which will be a great benefit when you can can compare voltages at the various nodes to track down problems. Before powering up again, fit a bulb limiter (a 60-100W incandescent lightbulb for this application, wired in series with the mains supply to the power transformer or the whole amplifier) to limit current, provide an indication of shorts and thus prevent further damage. A basic test on a working amplifier is done without speakers (possibly the original problem) or input signal connected. Measure for DC which should be <100 mV at the output terminals.
Here's a shortform service manual link - requires registration: Cambridge Audio A500 Manual - Stereo Integrated Amplifier - HiFi Engine
Build Your Own Current Limiter for Protection when Repairing and Testing Electronic Equipment - YouTube
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