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Old 30th March 2009, 06:17 AM   #1
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Default Blown power transistor SAP15 on a Cambridge A500

I recently purchased a Cambridge A500 Integrated amp with only one side working for 25.00
I opened the case to find one of the 4 SAP15 power transistors blown. I was able to locate the part at Farnell. I went ahead and ordered the matching P also of the N/P couplet. I am hoping by re-soldering these in place I will have a working 2 channel amp. Has anybody ever done this before? I am asking for advice, cautions, pitfalls, etc...
It does not appear to have any other damaged parts. Should I go ahead and replace any other parts as well while I am in there. I am a newbee to this kind of stuff.
Thanks,
Jeff
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Old 30th March 2009, 11:12 PM   #2
jaycee is offline jaycee  United Kingdom
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You may want to check the surrounding parts, especially the bias setting trimmer and resistors leading to the power transistors bases.

Search this board and you should find the A500 service manual as a PDF file
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Old 31st March 2009, 10:26 AM   #3
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i'd second what jaycee said about the trim pot, and resistors, i'd also check the small cap next to the pot, and all transistors because when these amps go the sap15's seldom go alone.
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Old 2nd April 2009, 06:22 AM   #4
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Default Beautiful!!!

Well I got the parts from Farnell today. Soldered them in. Powered it up without a source or speakers at first. Both channels' LEDs came on. I checked the resistance across both pots while it was off and the blown side was about 12 ohms higher than the non-blown side. (46 r and 33.5 l) So I adjusted the high side down. Plugged it up to the CDP and speakers and.....it worked perfect! Now I got a very nice amp for 25.00 plus 12.00 for parts. This is my very first attempt at an electronic repair, guess it was beginner's luck!
Jeff
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Old 2nd April 2009, 09:08 AM   #5
jaycee is offline jaycee  United Kingdom
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The high side might well have been correct. This is called bias adjustment, and depends on the characteristics of the transistors installed.

Here's what the service manual says:

"When replacing the SAP15 output pair, reset the bias current by adjusting RV201 while monitoring the voltage between pins S and E of the transistor, to achieve a voltage of 13mV (equivalent of approx. 60mA)"

On the SAP15N transistor the S and E pins are the two pins on the right.

RV201 is the trimpot for the left channel. For the right, it's RV202. You need to adjust, wait for the amp to warm up, say 20 minutes or so, then check the reading and readjust when warm.

The main problem with these Cambridge amp's is poor heatsinking. They are not suitable for party use or extended loud listening - they overheat and the SAPs blow. To make it worse, the SAPs have a design flaw where the inbuilt emitter resistors fail long before the transistor itself does - this is why Sanken have discontinued them. For light use they are fine, however.
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Old 2nd April 2009, 09:25 AM   #6
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Default blown

What about replacing all 4 with the SAP16. Will this also raise the effective WPC as well as eliminate the design flaw? Or does the SAP16 have the same flaw?
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Old 2nd April 2009, 04:41 PM   #7
jaycee is offline jaycee  United Kingdom
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Same flaw. Sanken discontinued both of them, and they now make the "STD03" which is the same transistor but without the emitter resistor.

The only real cure is to improve the heatsinking. The one in the box is just a large L-angle with some folded metal for fins. A solid aluminium type would work better, but would've increased the cost too much for the price point Cambridge aim for.
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Old 2nd April 2009, 07:20 PM   #8
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Default Heatsink modification

I have a nice thick aluminum heatsink that came out of a treadmill. I was actually eyeing last night as a replacement. I think it will fit well and I can modify it to fit. Was also thinking of placing 2 brushless micro-fans out of a lap top inside the chassis for x-tra cooling. Will try this and post some pics, maybe someone else could find it helpful.
Thanks again for everyones input!
Jeff Miller
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Old 3rd April 2009, 01:13 AM   #9
jaycee is offline jaycee  United Kingdom
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A good solid aluminium heatsink alone should provide the needed cooling. I wouldn't bother with the fans.

Cambridge did drop this silly method of heatsinking. Even their LM3886 based amps have good solid black anodised heatsinks in They have more heatsinking than my own LM3886 based amp does, and I have never set the protection off on mine even at booming levels
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