Ylli
Super Member
Good info, even though I haven't figured out how this processor controlled unit actually switches to mono
Ok I do get sound out of other channel when I jumper p105, p106...but not on r191, r192
Ylli
Super Member
There are a couple more places we could put the jumper to localize the issue, but let's check the bias voltages on Q119 and Q120 first. Break out that voltmeter and measure the base voltage of both transistors (circled in blue), the emitter voltages of both (circled in red), and the collectors (both collectors are tied to the same point, circled in green). All measurements referenced to chassis ground. I circled points on resistor leads instead of directly on the transistors to reduce the chance of a probe slipping and shorting pins.
One thing I noted is heatsink q551 is fairly warm even when the unit is powered off. Plugged in but powered off.
Ylli
Super Member
Q551 is part of the 'standby' power supply, and will be active as long as the unit is plugged in. It is a actually a 78M15 voltage regulator chip, and these 78xxx chips are made to run a bit warm. That's why the heat sink is there. Q557 (78M05) is also a voltage regulator chip and may run a bit warm.
Leave the unit plugged in for a while, then unplug it. Moisten your finger and touch the heat sink(s). Just hot or does it 'sizzle'. Just hot is OK, sizzle is not.
Leave the unit plugged in for a while, then unplug it. Moisten your finger and touch the heat sink(s). Just hot or does it 'sizzle'. Just hot is OK, sizzle is not.
oops missed that one...had to head out will check late tonight, sorry
Ylli
Super Member
Based on the base voltages, I suspect R129 will be right around 0.68 volts. If so, the numbers are good, and the two transistors are probably OK. If the voltage on R129 is not around 0.68 volts. Stop and let me know.
So we jumpered the inputs to the filters (R191/R192) and the left signal did not get through. Let's try jumpering on the outputs of the filters. Jumper bottom of R127 to top of R128.
So we jumpered the inputs to the filters (R191/R192) and the left signal did not get through. Let's try jumpering on the outputs of the filters. Jumper bottom of R127 to top of R128.
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You were right on r129. .68 ....jumpered 127,128 and got sound on left
Ylli
Super Member
Review..
Jumpered filter inputs together, no sound on left.
Jumpered filter outputs together, sound both channels.
[Just noticed the layout and schematic have a minor inconsistency. The designators for Z111 and Z112 are reversed. On the schematic, Z112 is shown as the left filter, but on the board layout Z111 is shown as the left filter.]
Has to be a bad filter, or crack in the solder/foil between our R191 test point and the filter, or from the filter to our R127 test point.
Get out a magnifying glass and a good light source and look very closely for any cracks. Then I'd like you to reflow the solder connections on the Low Pass Filter Z112 (Layout designator Z111).
If the above does not fix it, then we will need to confirm a bad filter. Try a jumper between low end of R125 and low end of R127 - red circles. You *should* have audio on the left channel. If you do, then disconnect the R125/R127 jumper and add a jumper directly between the input and output of filter Z112 (Z111 on the layout) - blue circles. Audio on left?
Jumpered filter inputs together, no sound on left.
Jumpered filter outputs together, sound both channels.
[Just noticed the layout and schematic have a minor inconsistency. The designators for Z111 and Z112 are reversed. On the schematic, Z112 is shown as the left filter, but on the board layout Z111 is shown as the left filter.]
Has to be a bad filter, or crack in the solder/foil between our R191 test point and the filter, or from the filter to our R127 test point.
Get out a magnifying glass and a good light source and look very closely for any cracks. Then I'd like you to reflow the solder connections on the Low Pass Filter Z112 (Layout designator Z111).
If the above does not fix it, then we will need to confirm a bad filter. Try a jumper between low end of R125 and low end of R127 - red circles. You *should* have audio on the left channel. If you do, then disconnect the R125/R127 jumper and add a jumper directly between the input and output of filter Z112 (Z111 on the layout) - blue circles. Audio on left?
Some of the solder did look suspect but no luck, solder wasn’t drawing in to joint well though...I couldn’t find flux, so maybe that was issue. Anyway did best I could w/ no luck, did get sound jumpering 125 127.
Z112 (marked z111 on board) I jumpered underneath as no access from top. I got sound on left, pretty sure hit correct points there.
Ylli
Super Member
Do I understand correctly that with a jumper soldered across Z112 (marked Z111 on board) you are getting audio out of both channels?
Have you got any way to desolder the part? Solder sucker or solder wick? If you do, carefully remove solder from the pin that does not seem to be wetting with the solder. See what you can do to clean the pin - I typically would use an exacto knife to gently scrape the surface to remove any oxidation. Now try to resolder - we need to have the solder flow smoothly into the joint, wetting both the lead and the pad.
I'm hoping it is a solder connection, as I have not been able to find any information as to a replacement if it is a bad filter.
What kind of solder are you using? Should be electronics grade rosin core - extra flux should not be needed.
Have you got any way to desolder the part? Solder sucker or solder wick? If you do, carefully remove solder from the pin that does not seem to be wetting with the solder. See what you can do to clean the pin - I typically would use an exacto knife to gently scrape the surface to remove any oxidation. Now try to resolder - we need to have the solder flow smoothly into the joint, wetting both the lead and the pad.
I'm hoping it is a solder connection, as I have not been able to find any information as to a replacement if it is a bad filter.
Ylli
Super Member
Good on the solder. The pins that are on pads with no runs do not need to be soldered. One input pin, one output pin, one ground, then the two mounting lugs. Looking at the picture... Confirm there is not a short in the red circled area. Then, did you reflow the solder on the blue circled pins? Hard to tell from a picture, but it looks like there may be some cracking on the blue circle pins.
Got it I see no runs on those...no short on that red circle, reflowed others. No luck
Yes it plays when I jumper them. That sucks that the part may be impossible to get, but hey I had a fun going though this and learned a few things...been a while since I did this stuff. I really appreciate the help again, pretty amazed at your knowledge here. I will try to find part!