Pioneer sx-1050 no voltage at output caps problem

[lopezluna]

Hi everyone. I just purchased a beautiful pioneer sx-1050 receiver and when I received it I plugged it in and the sound was terrible. Very low volume output and distorted. I tested all t03 outputs and they all tested good. I just took a reading with my dmm on the big output caps and am not getting not even 1v and yes this is while powered on with a dbt 75w bulb. When i turn on the switch to power on the dbt the bulb doesn't even get bright at all as it should initially. Anyone know why I am not getting any voltage at the output caps or what I should look for and where? Any help would be greatly appreciated. Thanks, Ben

 

[Big Ben]

I suppose you mean the main filter caps? Maybe an open bridge rectifier on the power supply board (the one at the bottom of the amp)? I’d also check the voltages on the transformer secondary.

 

[zebulon1]

Are the input function and speaker selection lamps working?
Soft-start relay is located on the chassis near the large resistor.
Is it energizing when you power up the receiver?
You can see it and hear it operate. It normally energizes a second after the power is turned on.
If is not operating, shut it down and look for an open micro-temp fuse. Located on that big resistor.

 

[lopezluna]

I suppose you mean the main filter caps? Maybe an open bridge rectifier on the power supply board (the one at the bottom of the amp)? I’d also check the voltages on the transformer secondary.

Big Ben, Yes the main big filter caps. Is the bridge rectifier under heatsink on power supply board under the receiver?And how would I go about testing bridge rectifier and checking voltages on transformer secondary? I appreciate your input Big Ben. @merlynski

 

[rcs16]

when I received it I plugged it in and the sound was terrible

I just took a reading with my dmm on the big output caps and am not getting not even 1v and yes this is while powered on with a dbt 75w bulb.

Is it even possible to have no sound with no voltage?
But Zeb is correct on what to check.
The unregulated power supply operates the soft-start relay coil, so it could be fuses or a PS ecap issue that caused the thermal fuse to blow. If the PS is original, probably time for a re-build as a start.

 

[lopezluna]

Are the input function and speaker selection lamps working?
Soft-start relay is located on the chassis near the large resistor.
Is it energizing when you power up the receiver?
You can see it and hear it operate. It normally energizes a second after the power is turned on.
If is not operating, shut it down and look for an open micro-temp fuse. Located on that big resistor.

@zebulon1 yes all selection lamps work and both relays do click

 

[zebulon1]

Good.
Check all fuses.
Check power supply pin voltages.
Rick is on to something.
Use the schematic for reference and pin readings.
Report your findings.

The big filters are checked with the meter on DC and the black meter probe connected to the chassis. Looking for +-62 volts on the leaded filter connections.

 

[lopezluna]

Is it even possible to have no sound with no voltage?
But Zeb is correct on what to check.
The unregulated power supply operates the soft-start relay coil, so it could be fuses or a PS ecap issue that caused the thermal fuse to blow. If the PS is original, probably time for a re-build as a start.

Yeah pretty weird. Both relays are clicking. I do have a rebuild kit for this receiver but would like to figure out the issue with the very low voltage at the big power supply caps before I do a restoration. Thanks for your input, I do appreciate it. I am pretty sure with you guys help this will get sorter out

 

[rcs16]

LV on the big ecaps means check the rectifiers for opens, check the secondary voltages on the transformer leads, you checked OPS devices already? a few things

 

[lopezluna]

Good.
Check all fuses.
Check power supply pin voltages.
Rick is on to something.
Use the schematic for reference and pin readings.
Report your findings.

The big filters are checked with the meter on DC and the black meter probe connected to the chassis. Looking for +-62 volts on the leaded filter connections.

I will check the power supply pin voltages after work this evening. Now on wich power supply board do I check pin voltages? Both top and bottom ps boards? I do have a Service manual. Will post readings on ps boards pins. Thanks guys

 

[lopezluna]

LV on the big ecaps means check the rectifiers for opens, check the secondary voltages on the transformer leads, you checked OPS devices already? a few things

Hi @rcs16, is the rectifier under the heatsink on bottom power supply board and how do I check it? Secondary transformer leads I will have to find out wich ones those are. Sorry but what does OPS stand for?

 

[rcs16]

page 21 of the service manual is the theory on the s/s ckt
page 98 for AWR-104 which is the rectifiers, fuses, filters. On the bottom
check with your DMM (DCV) black "-" to chassis ground.

OutPut Stage or the TO-3 bjt (transistors on the heatsinks

Good luck

 

[merlynski]

You may get some (bad) sound from the power amp section, even if the main supplies are down, because the diff amp and drivers get their power from a different set of power supplies. The main supplies (on the big caps) provide the current to feed the output transistors for drive the speakers.
The power supply that feeds the Soft-start relay is not the same one that charges C1 and C2, the main supplies.
The secondary winding that feeds the main supplies has a grounded canter tap, so you can measure AC voltages with your black lead clipped on the chassis ground. Measure AC volts on pins 2 and 4 of the AWR-104 power supply PCB, Should be 45 to 50vAC there using the DBT. If the AC is there and the DC on C1 and C2 is missing or low you likely have a rectifier problem. That bridge rectifier is a pain to get its board out to where you can easily work on it.

 

[lopezluna]

page 21 of the service manual is the theory on the s/s ckt
page 98 for AWR-104 which is the rectifiers, fuses, filters

OutPut Stage or the TO-3 bjt (transistors on the heatsinks

Good luck

Oh yes I did remove each TO-3 transistor and checked with peak transistor checker. None were faulty/open. Now that I have a lead on what to check I cant wait to get home and do some measurements on voltage. I will read pg 98 on sm and see what I find. Thank you very much

 

[merlynski]

Leave the output transistors (TO-3) uninstalled for now, if they are still out, but be sure you know which one went to where.
I marked mine on the 'sink and the transistors.

 

[lopezluna]

You may get some (bad) sound from the power amp section, even if the main supplies are down, because the diff amp and drivers get their power from a different set of power supplies. The main supplies (on the big caps) provide the current to feed the output transistors for drive the speakers.
The power supply that feeds the Soft-start relay is not the same one that charges C1 and C2, the main supplies.
The secondary winding that feeds the main supplies has a grounded canter tap, so you can measure AC voltages with your black lead clipped on the chassis ground. Measure AC volts on pins 2 and 4 of the AWR-104 power supply PCB, Should be 45 to 50vAC there using the DBT. If the AC is there and the DC on C1 and C2 is missing or low you likely have a rectifier problem. That bridge rectifier is a pain to get its board out to where you can easily work on it.

Will get those readings as soon as I get home this evening. Where Is this bridge rectifier at? I have been searching for it. Thank you marlynski I appreciate your help. All you guys are being very helpful and now I have a lead on what to test

 

[lopezluna]

Leave the output transistors (TO-3) uninstalled for now, if they are still out, but be sure you know which one went to where.
I marked mine on the 'sink and the transistors.

I reinstalled them after I tested them. I can remove them and mark them if that would be the best thing to do

 

[merlynski]

The outputs are expensive parts to replace, and don't need to be in place until you are ready to actually adjust and test the power amp.
Some may think I overdo my documentation, but it helps me stay on track.

 

[rcs16]

a man with a 1050 to the rescue

 

[merlynski]

Where Is this bridge rectifier at? I

It is under/attached to this heatsink (in the red circle) on the AWR-104 Power Supply (rectifier) PCB.
It is soldered to the board, and has big lugs that will make it had to unsolder, if it is bad. Mine was fine so I left it unchanged after testing it.