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.
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.
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.
The channel with no bias has a problem that needs fixing.
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?
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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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.
White noise points to a defective component such as a transistor or perhaps a leaky cap, typically a ceramic or tantalum.
Still cannot figure this amp out. It's seriously misbehaving, and I've tried quite a lot.
Here's a 1 kHz sine wave (no load). The yellow channel is having major problems.
Here's what it does: it has a white noise at startup, which goes away after half a minute. I cannot figure out why this happens.
The gain also seems to be way higher, and it seems to invert the signal. How can this be?
Some of the thing's I've tried so far.
Replaced all electrolytic capacitors, and the four small signal transistors (per channel) in the early stages, all eight totally matched with the same hFE. This fixed the yellow channel partially, before it would play white noise constantly. Now it only does in the beginning.
I have tried to swap the four later transistors between channels (T447/448, T449/450, T451/452 and T453/454), and this did not cause the hissing noise to move, so the fault shouldn't be in them.
I have tried to replace the output transistors in the yellow channel. Still no change.
I have not yet touched the driver transistors at all.
Here's a better schematic if anyone's interested:
Thanks for looking. 🙂
Here's a 1 kHz sine wave (no load). The yellow channel is having major problems.
Here's what it does: it has a white noise at startup, which goes away after half a minute. I cannot figure out why this happens.
The gain also seems to be way higher, and it seems to invert the signal. How can this be?
Some of the thing's I've tried so far.
Replaced all electrolytic capacitors, and the four small signal transistors (per channel) in the early stages, all eight totally matched with the same hFE. This fixed the yellow channel partially, before it would play white noise constantly. Now it only does in the beginning.
I have tried to swap the four later transistors between channels (T447/448, T449/450, T451/452 and T453/454), and this did not cause the hissing noise to move, so the fault shouldn't be in them.
I have tried to replace the output transistors in the yellow channel. Still no change.
I have not yet touched the driver transistors at all.
Here's a better schematic if anyone's interested:
Thanks for looking. 🙂
Direct link to way bigger (readable) schematic: http://i.imgur.com/VXVAHMg.jpg
To clarify/summarize:
One channel hisses white noise upon startup, which fades away after a couple of seconds. This noise can be heard EVERY time I turn the amp on - even if it has only been off for less than a second!
The hiss is independent of volume knob position, unless it is turned all the way down - during which the hiss can only faintly be heard.
This channel also has about double the gain compared to the the normal channel. It is way louder with speakers, and measured voltage clearly way higher.
To clarify/summarize:
One channel hisses white noise upon startup, which fades away after a couple of seconds. This noise can be heard EVERY time I turn the amp on - even if it has only been off for less than a second!
The hiss is independent of volume knob position, unless it is turned all the way down - during which the hiss can only faintly be heard.
This channel also has about double the gain compared to the the normal channel. It is way louder with speakers, and measured voltage clearly way higher.
You need to measure the signal directly at the power amp inputs to confirm both channels are seeing the same applied signal.
Turning the volume to minimum suggests the hiss problem is probably before the control although its not absolute definitive proof.
As it stands at the moment assuming you are applying the test signal to a standard user input on the amp, then the problem appears to be preamp rather than power amp related. That should be easy to trace with a scope.
Turning the volume to minimum suggests the hiss problem is probably before the control although its not absolute definitive proof.
As it stands at the moment assuming you are applying the test signal to a standard user input on the amp, then the problem appears to be preamp rather than power amp related. That should be easy to trace with a scope.
Hi,
I actually checked this right before I read your post... 🙂
I applied the signal directly to the 4.7 µF capacitor C689/690, and removed the 82K resistor R943/944 to get the pre-amp out of the picture.
Now the signal is the same on both channels, and sounds fine. I tried playing real loud and no problems.
However, my biggest concern - the hiss noise still happens - and it's still affected by the volume control (but the input signal obviously isn't).
I actually checked this right before I read your post... 🙂
I applied the signal directly to the 4.7 µF capacitor C689/690, and removed the 82K resistor R943/944 to get the pre-amp out of the picture.
Now the signal is the same on both channels, and sounds fine. I tried playing real loud and no problems.
However, my biggest concern - the hiss noise still happens - and it's still affected by the volume control (but the input signal obviously isn't).
There seems to be a mute switch, the one with a '1' marking that show that it shunts the signal to ground.
Does it hiss when that is closed ?
You could isolate C677/678 and then working further towards the inputs, C667/668.
I would refit the 82k's as they don't fully isolate things. And refit anything isolated before you move on to the next step.
Does it hiss when that is closed ?
You could isolate C677/678 and then working further towards the inputs, C667/668.
I would refit the 82k's as they don't fully isolate things. And refit anything isolated before you move on to the next step.
Thanks man, I got it sorted. 🙂
It was R916, which was open. No juice to T444 then, I guess?
My original plan was to just shotgun all the supply resistors from the get go, but never got around to it. That would have fixed my problem, but I wouldn't have known what it was.
Thanks again. 🙂
Oh, and the bias thing again. A little embarrassing but the cause for the no-bias on the one side, was that one fuse was intermittent (it was green!). Never actually checked those, somehow missed that obvious part. Now that's sorted too, and I've set the bias to 40 mV on each side. Think I should raise it to 50? More? Less?
It was R916, which was open. No juice to T444 then, I guess?
My original plan was to just shotgun all the supply resistors from the get go, but never got around to it. That would have fixed my problem, but I wouldn't have known what it was.
Thanks again. 🙂
Oh, and the bias thing again. A little embarrassing but the cause for the no-bias on the one side, was that one fuse was intermittent (it was green!). Never actually checked those, somehow missed that obvious part. Now that's sorted too, and I've set the bias to 40 mV on each side. Think I should raise it to 50? More? Less?
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