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Old 28th June 2015, 12:16 PM   #1
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Default Need help setting idling current on old Philips amp

Hey guys

I have a very old Philips amp (66rh891) that doesn't work too well. I've almost fixed its various issues, but I don't know how to set the idling current.

I tried measuring DC on the four 1 ohm resistors near the four power transistors, and I get about 40 mv on each single resistor on one side, and 0 mv on the other. Both channels sound bad, the 0 mv especially, it probably has blown outputs (I plan on replacing them all anyway).

What is a good value to adjust to here? 40 mv is probably too little. Remember it has 1 ohm emitter resistors.

Also, the amp has something called centre adjust, one for each channel, and I do not know what this is supposed to do. Any clues there? I know these are common on single supply amps, but this one has +-28.5 V.

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Old 28th June 2015, 12:29 PM   #2
Mooly is online now Mooly  United Kingdom
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Centre point will be DC offset. So adjust to give 0.00 volts across the speaker terminals. If one amp sounds 'bad' then something more than just bias adjustment is wrong. 40 ma is plenty to eliminate obvious crossoer distortion, in fact I'd say that 40ma was about right give or take. Anything over even a milliamp or two will seem to eliminate audible distortion.

The channel with no bias has a problem that needs fixing.
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Old 28th June 2015, 01:25 PM   #3
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Scratch that. Now it has bias. Not sure why I wasn't seeing anything earlier. It might have some intermittent fault. Now it's 130 mV from the start, then it sinks slowly to about 30 after some minutes. Checked the outputs too, they seem fine, all of them.

Thanks, I don't know why it never occurred to me that of course that's DC offset.

The "good", 40 mv channel has another fault: at startup it sounds horribly distorted with lots of white noise, which fades away into decent sound after about 30 seconds. And right when I turn the unit off, both channels sound just lovely for a few seconds. Full, nice sound.

I have another question: how do I reset bias to nothing? The wiper is connected to one side of the VR, do I turn it towards this side or away from it?
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Old 28th June 2015, 01:46 PM   #4
Mooly is online now Mooly  United Kingdom
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To turn the bias down the wiper of the preset has to be turned so that it connects to R988. It might be designed so it doesn't fully reduce to zero.

White noise points to a defective component such as a transistor or perhaps a leaky cap, typically a ceramic or tantalum.
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