a/d/s PQ10 dead channel

Jump to Latest
lt_texan

lt_texan

Member
Joined 2005
#1
I happen to have one of these great amps and love it. Well, until channel 3 died.

I bought it a year or so ago off ebay (!!!) and had no complaints. The seller recapped the amp (delivering all the old caps) and is much more talented than me with a soldering iron and circuit boards!

The amp stated going in and out on channel 1 and 3 (hooked up to front and rear left speakers). Now channel 1 sounds fine (don't you love irratic) and channel 3 is dead.

I'm more comfortable around tube amps.

Then looking at the circuit, I see IC603 (LA6458S). Is the purpose of this IC to invert the phase of channel 1's input signal? And there is another on channel 3? Is this something that has to do with the amp's bridged function?

I'm thinking that could be the culprit. Maybe jump pins 6 and 8 to bypass the IC and see if that works? (I don't operated it bridged, so are these IC's needed?)

I have it hooked up to a terrible 12v power source that can just manage 11.9v at idle. And it seems like the PS is just fine.

All the output transistors measure volts to ground:
B 0.6, (0.6)
C 23.6, (23.6), +0.2, -0.1
E 0, 0

Bypass LA6458S?

Thanks in advance,
Dan Perez
 

Attachments

  • IMG_2269.JPG
    IMG_2269.JPG
    485.5 KB · Views: 373
  • ads_pq10_schematic.pdf
    ads_pq10_schematic.pdf
    442.4 KB · Views: 139
#2
Have you operated all pots and switches through their entire range while listening for audio to see if it every played through (even just for an instant)?

With a speaker and signal source connected, did you operate all pots and switches in all combinations to see if any audio/static/noise ever played through?

Do you have a scope?
 
#3
Have you operated all pots and switches through their entire range while listening for audio to see if it every played through (even just for an instant)?

With a speaker and signal source connected, did you operate all pots and switches in all combinations to see if any audio/static/noise ever played through?

Do you have a scope?
Click to expand...
I think the only pots are the input attenuators, and yes to those. No noise/static. Seem to operate correctly. I've switched the bridge mode switches back and forth, but while the amp was off.

I've switched the inputs from the source and no matter the input, channel 3 doesn't play.

I've switched the channel 3 speakers (left rear) and they play with a different channel.

And I don't have a scope.
 
#4
Then looking at the circuit, I see IC603 (LA6458S). Is the purpose of this IC to invert the phase of channel 1's input signal? And there is another on channel 3? Is this something that has to do with the amp's bridged function?
Click to expand...
Oops, I see that IC603 is on channel 1 and 4, not 1 and 3.

So that cannot be dead channel 3's culprit.
 
#6
Perry Babin said:
With a non-conductive probe, push on various components. Do you find any that make ch3 intermittently play?

If you use a heat gun or blow dryer, can you find any areas that when heated, make ch3 play?
Click to expand...
Thank you for the advice.

With a non-conductive probe, it looks like my problem is the input level pot for channels 3 and 4. When I push on the pot's casing, channel 3 starts playing. It also plays at some settings, but mostly doesn't. The channel 4 is just fine. Channel 1 is shorting too, but not near as much, and not when I press on the pot's casing.

On the bench, I can hear the static as I adjust the pot, but I couldn't while adjusting it in the trunk.

Thanks again.
IMG_2278.JPG
 
#7
I'm thinking about just removing the Alps input pots and bypassing them (not finding replacements via search). It looks like I have to remove the board to get to the screw that holds on the pot's board and the connection to the main board is the pin connectors on the sides and the pot's board will just lift off. I'm not looking forward to this, but I can't find anyone local in Austin to work on it.
 
#8
If you set the gain pot fully clockwise, you will see that two pins are shorted together (0 ohms). Do you read 0 ohms on the ch4 pot but something else on the ch3 pot?

There could be a broken solder connection on the header for that board. Does pushing on the board do the same thing as when you touch that pot?
 
#9
Perry Babin said:
If you set the gain pot fully clockwise, you will see that two pins are shorted together (0 ohms). Do you read 0 ohms on the ch4 pot but something else on the ch3 pot?

There could be a broken solder connection on the header for that board. Does pushing on the board do the same thing as when you touch that pot?
Click to expand...
Yes. 0 ohms on 4 but open circuit on 3. When I push (mostly) sideways on the pot's shaft, channel 3 starts working.

I've pushed and probed around on the board and the solder joints and the pot's body but that doesn't give me static or get the channel working.

The top three pins are channels 2 and 4 and the bottom are 1 and 3. The pins are In, Out, Ground from the left looking at the pot. Maybe just short the In and Out pins?
 

Attachments

  • Screen Shot 2022-01-29 at 5.58.28 PM.png
    Screen Shot 2022-01-29 at 5.58.28 PM.png
    319.7 KB · Views: 71
  • Screen Shot 2022-01-29 at 5.50.20 PM.png
    Screen Shot 2022-01-29 at 5.50.20 PM.png
    574 KB · Views: 65
#10
To use the amp, you could solder a bridge on the ch3 pot that shorts the pins that are shorted on the ch4 section with the gain at full CW.

If you want a gain setting that's less than full, you could replace it with resistors.

These pots look fairly generic. Mouser may have something that would work.
 
#11
Perry Babin said:
To use the amp, you could solder a bridge on the ch3 pot that shorts the pins that are shorted on the ch4 section with the gain at full CW.

If you want a gain setting that's less than full, you could replace it with resistors.

These pots look fairly generic. Mouser may have something that would work.
Click to expand...
Thanks much. I think I will short out "in" to "out." And our the pots fully CW.

I couldn't find replacement pots on Digi-key our Mouser. These are Alps pots btw.
 
You must log in or register to reply here.
Top Bottom