Crazy High 15v DC Offset!

[josephdaniel]

Working on completing a Pioneer QX949 rebuild. Everything was going smoothly and working well but on the very last part that I was changing (out of the hundreds) something went wrong.

Both power amp boards were rebuilt and working fine and within spec. However, whenever I changed the four large main caps the receiver would not come out of protection. I measured a DC offset of approximately 15.5V on pin 5 of each power amp. On pin 23 for each board the offset is correct, and bias is on spec for all four channels.

Triple checked wiring, reflowed solder, subbed in old capacitors, all power supply voltages and protection voltages are just about dead on. I am stumped - especially since this was supposed to be the very last step in what has been a *very* long project. I'm out in west Texas so no shops nearby to visit for repair. Thanks in advance!

 

[saabracer23]

Hmmm so two channels have an offset of 15.5v and the other two channels have good offset, and this only happened after replacing the filter caps?

What about the voltages on all of the other pins, 1 through 26. If they are correct that would point the fault being on the board, but if both boards exhibit the same fault that’s unlikely. Sounds like a fault in the rails maybe, you’re certain you installed the filters in the right orientation? Maybe some pics could help. I’ve seen people use filter capacitors with dummy pins and they accidentally solder to a dummy pin which caused a fault.

Dan.

 

[gadget73]

Whats your voltage input measured to the ground pin on the power amps? I'm suspecting you'll find they are not equal but opposite. May have lost a ground connection somewhere.

 

[josephdaniel]

@saabracer23 & @gadget73

These are the the current voltages on both power amp boards 1-26. Each board is divided into two channels with the issue somewhere between pins 1-12. Voltage readings from before any work was done seem to show that 13-26 are more or less in the same neighborhood as before any work was done. Looking at these numbers I am more convinced that there is nothing wrong with the four large capacitors and somewhere on the amp boards I installed a transistor incorrectly the same way on each side I'm going to check datasheets again and see if I missed something what was BCE rather than ECB. Could board markings ever be wrong? That's what I was following.

Pin #	A Closest to Front	B Closest to Back
1​	15.25V	16v
2​	15.25V	16v
3​	- 34.2 V	-34.2v
4​	15.5v	16.08v
5​	15.5v	16.08v
6​	0v	0v
7​	21.7v	21.9v
8​	-43.6v	-43.7v
9​	-43.6v	-43.7v
10​	21.7v	14.02v
11​	29.1v	29.4v
12​	29.2v	29.4v
13​	34.0v	34.1v
14​	34.0v	34.0v
15​	29.2v	29.4v
16​	29.2v	29.4v
17​	2.5mv	2.5mv
18​	-43.6v	-43.7v
19​	-43.6v	-43.7v
20​	2.3mv	2.3mv
21​	0v	0v
22​	-5mv	-4.9mv
23​	-5mv	-4.9mv
24​	-34v	-33.9v
25​	-16.8mv	-15.6mv
26​	6.9mv	6.2mv

 

[josephdaniel]

For reference, before I started work I recorded voltages on one board - the notable changes seem to be contained to pins 1, 4/5, 7, & 10.
These were the recorded values on those pins previously:

1: -3.8mv
4: -8.3v
5: -8.3v
7: 5.1v
10: 5.1v

Just visually looking at the schematic Q1 & Q3 seem to be at the center of the issues but I can see that they are installed correctly according to the board markings? I can't think of what would have taken these out in the process of changing those four caps, but they are cheap so I'll order some KSA992's and see if that helps?

 

[Nashou66]

Some times the schematic and or the board markings are wrong. I always remove and replace one component at a time going by how it was installed. I hardly ever remove all parts and then reinstall due to this very reason.

Check the schematic for where the pins go to of each tranny then see if they follow the same path on the board to determine if they are correct.

Athanasios

 

[josephdaniel]

Some times the schematic and or the board markings are wrong. I always remove and replace one component at a time going by how it was installed. I hardly ever remove all parts and then reinstall due to this very reason.

Check the schematic for where the pins go to of each tranny then see if they follow the same path on the board to determine if they are correct.

Athanasios

Went through one side and traced pin continuity and everything is installed correctly and it appears that board markings / service manual all are matching up here.

Can one test transistors in place with some carefully placed grabber probes? Usually I pull parts out to test them, but I hate to pull these boards out until I have a better idea of what's going on or a few more people chime in 38 finicky solid core wires in total of both sides of each board.

 

[ecluser]

Ref voltages on post #5, I see 8.3V between pin 1 and 4. That means R49 is open. I see 5.1V at pins 7 and 10, that means a bad ground connection at pin 7.

 

[josephdaniel]

Ref voltages on post #5, I see 8.3V between pin 1 and 4. That means R49 is open. I see 5.1V at pins 7 and 10, that means a bad ground connection at pin 7.

Checked R49 on both boards that have the same readings. R49 measures at 0.8 and 0.7 ohms respectively with continuity traced out to the board pins.

 

[ecluser]

Do you realize that 8.3V in a 0.8 Ohm resistor means a current of 10A?

 

[josephdaniel]

Do you realize that 8.3V in a 0.8 Ohm resistor means a current of 10A?

Any chance that voltage is coming from somewhere else? Those voltages you are referencing in post #5 are from before I touched anything. Right now Pin 4 is at 15.5v and pin 1 15.25v as mentioned in post #4. I feel like if there was that much current a fuse would have blown, I would have some sort of indication on the DBT, or something would at least get hot.

I'm planning to pull one of the boards out this evening and search for bridged solder pads, test transistors, and check for any open circuits and see what I come up with.

 

[josephdaniel]

I have checked out the board - everything is looking how it should according to the schematic. No cold solder joints, broken traces, or bridged pads. Transistors appear to be installed with the correct pin-outs per datasheets (everything I changed is all ECB) and board markings verified with the schematic. In circuit the transistors appear to test fine, but that can be problematic from what I am reading. I'll pull them one at a time and test them out of circuit and see if anything pops up.

 

[ecluser]

I think you have a bad ground connection at pin 7. Pin 7 should be grounded (0V)

 

[josephdaniel]

I think you have a bad ground connection at pin 7. Pin 7 should be grounded (0V)

Thanks for this, I think I am on the right track here finally!

I had checked the wires for breaks between pins 7 and their termination on the tone control board (pins 5&6) previously but didn't go further, however following the circuit those connect directly to pin#2 of the tone control board. Pin #2 of the tone control board is the ground between the TCB and volume control which is where I lost my ground signal. If I wiggle the wire between the TCB and the volume control I can get good conductivity all the way back to pin 7 on the power amp.

Ergo, there was a break in the wire running from the tone control board to the volume control which caused two of the channels that shared that ground to lose it. I am going to try putting everything back together and seeing if were back in business! I never would have found that on my own.

 

[josephdaniel]

Update success!

Reinstalled the power amp board with a jumper between pin two of the tone control board and the receiver immediately came out of protection! Adjusted bias & DC offset. All voltages are right where they should be!

Previously before this grounding issue the rear left channel is about 50% of the output which is still an issue. I haven't cleaned the big 4-channel / 2-channel plug on the back of the receiver yet, but if that doesn't fix it I'll make a separate thread for that issue.

 

[w3jn]

Congrats on your success!

Please don't start a separate thread.... you can edit the title or have one of us moderators do it by hitting "report".

WIth multiple threads it's REALLY hard for those trying to help to keep track of all you did, and subsequent troubles can be connected to past work or symptoms.

 

[josephdaniel]

Now that the power amps are working as they should it seems like the next best step would be to track down the source of the weak rear left channel in all modes

I don't have a signal generator, but I do have an older Tek oscilloscope that used to work last time it was used 5 years ago if that might come in handy. Where my probes are is another story.

Sending in a constant level 440hz tone from a digital source to the aux inputs at a mid volume results in a voltage of 0.100V on each pin 17 and one of the pin# 10's of the two power amplifier boards. The channel that is weak reads a voltage of 0.020V at pin 10 of the power amps.

I am assuming that this is my weak channel and it is somewhere around the Matrix and Decoder boards since the front channels are unaffected in any mode. Switches have been cleaned and checked, bridge switch, and controls cleaned.

With that said, would anyone be able to point me to some test points on the decoder / matrix boards to narrow where the signal attenuation is happening? That seems like the best starting place to me.

 

[w3jn]

I took a look at the schematic and quickly gave up. That has to be the WORST schematic I've ever seen.... sorry and best of luck with it.

 

[josephdaniel]

I took a look at the schematic and quickly gave up. That has to be the WORST schematic I've ever seen.... sorry and best of luck with it.

Thanks for talking a look. Those are my thought's exactly, This receiver brought my AK account out of it's many year hibernation!


I found my the oscilloscope and a set of probes. It's a Tektronics 2213A - don't think I've ever used it before. I don't have a signal generator, but I can get a signal from my laptop on the scope coming from the speaker outputs. Not sure yet how useful that will turn out to be, but it's worth a try! It would be helpful if I could inject a signal right where the receiver splits from 2-ch to 4-ch, but I've got to work with what I have.

I'm going to see if I can play a game of circuit marco polo with some 'educated' guessing. Maybe I'll get lucky and blindly stumble on the problem!

 

[josephdaniel]

Can someone check my logic here?

Using my oscilloscope as a basic "signal visualizer" I followed the signal back to the tone control board (schematic below). The weak signal seems to be coming from Pin #7 which is fed from pin #4 if I understand correctly.

Pin#4 on this board reads 0.051v and I compared that to pin #13 (a front channel) which measures .048v. This would mean that the signal coming out of the weak channel *should* actually be slightly stronger than the other channels at this point, however where the signals exit I am seeing pin#7 measure 0.033V (the problem channel) and pin#10 read 0.162V.

Therefore, it would stand to reason that the signal attenuation is happening somewhere in the TCB between pin #7 and pin #4? Does this sounds like I am on the right path here?