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
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
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
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
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.
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.
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.
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.
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
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
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
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 Hi, I know this is a very old thread but I have the same problem with my X-A1. The left channel has blown and I'm sure it is the SAP15's. Given these are discontinued, looking at the thread it seems the STD03's are a replacement for these. My question is, the STD03's don't have the emitter resistor so there is no connection for the pin which the SAP15's used. Can I simply replace with the STD03's and have nothing connected to the pad on the PCB which was used to connect the emitter resistor?
Also, how do I check for the correct bias current when I replace with the STD03's using the trimpot?
I've had no luck finding a cct diagram for the X-A1 despite searching the web.
Any help would be much appreciated. Thanks!
Mark
Also, how do I check for the correct bias current when I replace with the STD03's using the trimpot?
I've had no luck finding a cct diagram for the X-A1 despite searching the web.
Any help would be much appreciated. Thanks!
Mark
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