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Philips power amp bias current

clivem2

clivem2

pfm Member
Due to the forum outage my posting seems to have been lost. I'll provide an update for Jez and Paul who are helping.

The Philips 380 power amp circuit:
380 by Clive M, on Flickr

The full manual: http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Philips-AF-380-Service-Manual.pdf

Where I'm up to - the bias on the right channel was set at around 3mA; I've reset this to 20mA. It is temp sensitive and needs about 10 mins to stabilise.

The left channel starts at 65mA and climbs past 80mA. Twiddling the preset does nothing to change the bias. The tech who looked at it last week said a driver transistor had a dry joint. I wonder whether heating up a transistor made it work properly for a short time. Any ideas or is the amp in need of a pro to fix it?
 
The preset seems to operate correctly in that the resistance between it the components nearby check out as expected when I alter it. This is starting to look to be beyond my capabilities.
 
I've checked the diodes feeding the preset on both channels, both read 0.54V and 1.2V either side of them (cathode/anode).
 
Success! My local tech guy to be fair to him said there was a dry joint on a driver transistor. He reflowed the joint and this seemed to work but only temporarily. I've done the same and can now set the bias. I'll leave the amp open for a while and check again from cold startup, maybe the joint was the issue or could it be a dying transistor helped by heating the joint? Usually though isn't it freezing components that helps?
 
Excellent....

(Didn't notice an outage.)

One thing always worth doing is checking voltages around the place. The service manual and schematic give you plenty of examples so even if the problem returns I don't think it is necessarily over.

Knowing that if a transistor or diode is 'on' then it has between 0.5 and 0.7 volts between base/emitter or anode/cathode and the almost everywhere else V=IR takes you a very long way with a circuit that should be good.

Paul
 
The amp is still running to spec. I've optimised the DC offset and bias; I've even got the power meters running in sync. I believe it's sounding better...but I would having done this work. The bias was so far out - 80mA and 3mA instead of 20mA - there should be some improvement in SQ.
 
If it is a readily available part I'd replace the bias transistor as a precaution, you don't want it failing the "wrong" way and frying your output transistors.
 
It's a diode in this amp.

There are decent separate protection circuits, so hopefully it would fail overall safe.

Paul
 
It's full of Japanese parts, might not be easy to source though a diode shouldn't be too special.....not a typical Philips product. It's running cool so hopefully it was a dry joint. This amp is so darned good that I've just purchased a second one.
 
Now....the 2nd amp I purchased was supposed to work. Not surprisingly the first thing I did when I got it home was to check the dc offset and bias. The dc offset was close to right and I was able to improve it. The bias was very low on one channel - I have it close to correct now. The other channel has 2 issues:
1) the bias won't adjust
2) more importantly I expect - I only get a meaningful mV reading across one emitter resistor; it reads 13mV, the rest read 1.0mV, 1.1mV and 0mV.

I tried an old car speaker - the faulty channel plays music but is quite distorted.

What's likely? Blown output transistors or is it hard to say as it could be many things?

Update 1
I thought I'd measure the emitter resistors. I've simply done this in circuit so it's not terribly meaningful I suppose. 7 of the 8 measure 1.2R to 1.4R - their value is 0.85R. one resistor on the faulty channel measure 0.6R so there's something different with that resistor or a component near it. It would be great if it's just a faulty resistor but that's too much to hope for.

Update 2
I've lifted one leg of the questionable emitter resistor and a leg of a good one. The good one still measures around 1.2R so it could be 0.85R (5% tolerance) plus my meter wires = 1.2R. The questionable one measures open circuit. Aside from getting a new resistor is there anything else I can check?

I have a 1R 11W resistor. Would it be sensible for me to install this in place of the o/s 0.85R or might I cause more damage if say a transistor is blown? Probably replacing a 3W resistor with my 11W one wouldn't be too smart as this time the resistor may not blow but something else might.
 
I bit the bullet by installing a low wattage suitable emitter resistor to replace the blown one. Predictably it didn't help. There was no heat or drama. The resistor survived. Presumably at a minimum the amp requires a new output transistor. I see Cricklewood have them: https://www.cricklewoodelectronics.com/2SD732.html

Given that the problem channel did play albeit with distortion, is there a decent chance all that's needed is a new output transistor?
 
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