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Old 24th October 2007, 08:07 PM   #1
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Join Date: Nov 2005
Location: suffolk
Default sap15n/p bias

Hi everyone havent been here for a while,

have been working on a cambridge audio a5 amp,keeps blowing fuses.

1 channel was duff with sap15 n with hole in it,had another pair in my draw,popped them in fuses went again,bummer.

Had a check with drivers and pre drivers before powered up all checked same on both channels.

Any way after head scratching noticed the bias pot on duff channel was open,supposed to be 100 ohms value reading 42 ohms on other channel,so found a 470 ohm pot and adjusted it to 41 ohms,

Took the new sap15n and sap15 p out now they are shorted


My question is would this open bias pot write the output stage off


Psu caps checked ,diodes etc...
if you strip it for parts keep them for years as soon as you get rid you will want them .....................
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Old 25th October 2007, 12:53 AM   #2
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
Yes it would - if the bias pot goes open, then the quiescent current through the outputs would be huge and blow the transistors instantly.

This is why I am not keen on the diode string arrangement used by these devices - a conventional VBE arrangement using the on-die diodes as a sensor is much better - if the pot fails, the quiescent current goes to zero.
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Old 25th October 2007, 01:15 AM   #3
GK is offline GK  Australia
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Join Date: Jan 2006
If you are paranoid about it dying like this again, pull out the new trimpot once you have correctly set the bias current. Measure it and replace it with a fixed resistor (or some series/paralleled up ones) of equal value.

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Old 25th October 2007, 02:22 AM   #4
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Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
well. one of the problems with the SAP devices was that the emitter resistor wasn't fully up to the task --- and they couldn't be biased sufficiently. if you notice, the new Sanken's (STD01N or P and STD03N ans P -- cut off the pin !!!!

so you might not have a bum transistor, just a blown incorporated Re -- i would check the value.
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Old 25th October 2007, 06:42 AM   #5
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Join Date: Nov 2005
Location: suffolk
Default sap15 n/p bias

Wow thanks guys for help thats was so fast aswell,

Ibought this amp off ebay very cheap spares or repairs,these amps do sound good but going by how many go wrong ,i would say the design
is not very rugged,i also purchased Arcam delta 90 ,supposed to have a blown channel nothing wrong with it (bonus) but can not seem to source front half of volume knob as in 2 parts,this amp is cracking,
got all original parts a really rocks,better designed than the later c. audio,s,has the t03 mj trannys in output stage very musical amp puts my rotel ra940bx to shame...

Also purchased a cambridge audio a3i from quick test there is 28 vdc on left outputs which is why the protection wont activate,
the outputs are the winged trannnys 2sa1216 and 2sc2922 jobbies,

All of these amps except rotel were bought to be sold on when fixed
if repairable,

I can get i pair of sap15n/and p for 4.00 pouds 10 pence + 2.00 pounds postage which is good,

Ihave purchased from them before and fixed an A4

Mainline Electronics ltd from ebay uk if anyone is interested...
if you strip it for parts keep them for years as soon as you get rid you will want them .....................
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