Hello Guys,
As some of you may already know, I've got my late brother Apt Holman preamplifier. It has gone through an chemical caps change and some repair (the phono entry died on me because of a faulty transistor).
Today I discovered that the "Stereo Mode" is behaving strangely.
If I turn the pot fully CCW I've sound on both the left and right channels.
If I turn the pot fully CW I've sound only in the left channel .
So IMHO this device has a fault.
I would very much like to hear from you about the probable cause before opening it.
Many thanks in advance for your answer !
Have a nice day and stay safe !
P.S. : If I'm not mistaken, turning this pot CW should SUBSTRACT left from right channel so if the input is mono, no sound, and if stereo, a "faint" sound on both channels from the part of signal common in both channels. If I turn this pot CCW, this should ADD the left to the right channel so a higher volume perhaps but nothing more than that.
As some of you may already know, I've got my late brother Apt Holman preamplifier. It has gone through an chemical caps change and some repair (the phono entry died on me because of a faulty transistor).
Today I discovered that the "Stereo Mode" is behaving strangely.
If I turn the pot fully CCW I've sound on both the left and right channels.
If I turn the pot fully CW I've sound only in the left channel .
So IMHO this device has a fault.
I would very much like to hear from you about the probable cause before opening it.
Many thanks in advance for your answer !
Have a nice day and stay safe !
P.S. : If I'm not mistaken, turning this pot CW should SUBSTRACT left from right channel so if the input is mono, no sound, and if stereo, a "faint" sound on both channels from the part of signal common in both channels. If I turn this pot CCW, this should ADD the left to the right channel so a higher volume perhaps but nothing more than that.
Did you swap out C41 and C42 (22uF electrolytic) in the Mode circuit? Did you replace any of the resistors or op-amps in the Mode circuit?
What source signal, Stereo or Mono?
Does the right channel sound change gradually as you turn the knob, or does it cut off abruptly at some point in rotation?
In L-R mode (CW) the left channel signal comes from IC 6a, the right channel from IC 7a. With a good stereo signal, for right channel to have no output would seem to imply a fault in the IC 7a circuitry: either a bad op-amp or bad input/feedback resistors (R53/54) or output cap (C42)/resistor (R149), or bad solder joint. Also possible is bad pot wiper contact.
Read section 4.8 in the manual, it describes how the Mode function works. It keeps constant volume.
With a TRUE mono signal, L-R(CW) should result in almost no sound from either channel. With a GOOD stereo signal (wide sound stage), it should almost eliminate any central (mono) sound while leaving non-central sound still coming from left/right speakers.
Hello Seanc,
Thank you very much for your answer.
I'll answer your question here in the order you asked them.
Yes C41 and C42 were replaced. No I did not replace any resistor nor Op amp. *
The signal was from the tuner, switched to mono
The right channel decrease a bit when I move the pot from the central position and then cut out completely approximately at 2 o'clock pot position. Volume on the left channel does not change (by ear ... ). Tested on people speaking or with music.
I tried this mode with a phono input using a mono record (I own a few) and nice stereo records. Behavior is the same. Left channel output is unchanged and right channel shut off when pot hits around 2 o'clock position.
I should add that I tested this mode when I got the preamp and it was fun and running like it should. So either something went wrong since or I did something wrong when I switched the caps...
Thank again !
* : I refrained to change any op amp or install sockets for them. I was afraid to break or ruin a printed board when de-soldering....
Seanc understands the operation of the Apt Holman better than anyone I know, so can probably figure this out without photos, but I find they are often useful when some work has been done. Can you post some photos, at least of the areas you worked on?
The fact that it worked properly before you did the upgrades is telling. Suggest you double check capacitor polarities and solder joints. Look for lifted/broken traces too.
OTOH, cutting off abruptly at 2 o'clock would seem to imply loss of wiper contact at that point. You might wash out the pot with a suitable cleaner.
It's difficult to diagnose remotely. It's not a physically complicated circuit, but the interactions between components in "L-R" mode is important.
There's no need to source a mono signal. To test in "true" mono, just flip the "L>R" switch up: that disconnects right channel input from the line stage, and simultaneously connects left channel input to BOTH left & right channel line stages. ie. you have identical signal in both channels.
In that configuration, with full "L-R" mode, the left channel should also go to zero (or nearly so), even if right channel cuts off abruptly.
thank you Seanc, now I've some leads to follow. And a lot to check.
I've tried to flip the L>R switch and turned the pot to the full CW position. Same as above. No right output after the 2 o'clock position and full output to the left channel. So maybe the pot is not the only culprit here.
I do not think I would be able to open it in the next fortnight..... So I'll keep your diagnostic in mind when I open the preamplifier.
Have a nice day and merry Christmas !
I've tried to flip the L>R switch and turned the pot to the full CW position. Same as above. No right output after the 2 o'clock position and full output to the left channel. So maybe the pot is not the only culprit here.
I do not think I would be able to open it in the next fortnight..... So I'll keep your diagnostic in mind when I open the preamplifier.
Have a nice day and merry Christmas !
I've also tried just connecting one or the other left or right rca inputs. That way you're guaranteed to have no signal on one.
I've run the circuit thru spice simulations in an attempt to replicate your symptoms.
The only condition I can find that still allows full left channel output is if capacitor C41 is open-circuit (either bad solder joint or bad cap).
This doesn't account for sudden right channel cutout, but it's the first place I would check.
I would also check the block diagram in the service manual. It should help track down where the fault is. IIRC, the signal follows left to right when looking at the controls.
Also test with the headphone output - it follows a slightly different path than the line out.
But first, only hook up one channel (via the rca input in back) and see what happens. Then swap it to check the other.
Also test with the headphone output - it follows a slightly different path than the line out.
But first, only hook up one channel (via the rca input in back) and see what happens. Then swap it to check the other.