I need a little help with my QX-949A

[topgun]

Hello AK,
I decided to go ahead with the repair and restoration of my Pioneer QX-949A. Condition wise it’s in exceptionally clean condition. But it I has a couple of issues. As most people would agree, I’ve been working on repairing the issues that it has. First, the scope display wasn’t working properly. Only one channel would light. I found that the little shutters inside were working fine and it was that the other three bulbs were out. The dial pointer wasn’t working. Bulb replaced. Two of the indicators were not working. Blown bulbs. Fixed those too.
Now, the only issue that I’m having a difficult time with is that the rear channels are working, but the front channels are not. If I hook up speakers to the front, I get nothing. If I hook them up to the rear channels, I get sound. This thing sounds pretty nice BTW. In any case, I just can’t seem to figure out what is going on here. If I turn it on, both relays( it has two) click. I’ve cleaned the heck out of every switch and potentiometer in the unit, twice now. And still nothing. I ohm-ed out the volume pot and it seems to be working properly. Voltage’s at the power supply are good. Voltages at the amplifier are also good.
I then decided to just start carpet bombing this thing, starting with the amplifiers, which need to be completely removed in order to do anything to them.
I recapped them, replaced all the transistors except the outputs since they were all still good. Checked all the resistors, all good. And even replaced the bias and offset pots with new bourne ones. Started it up, adjusted the bias(20mv), and offset to zero. Still, nothing from the front channels. Next, I turned to the protection board. I’m thinking that it was not likely to be the problem. I figured I would just rework it anyway as part of the restoration process. As expected, still no change.
At this point, I’m starting to believe that my issue lies elsewhere. I’m thinking maybe in 4 channel modulation board. Not the CD-4 board because if I’m not mistaken, that section only affects phono playback?
Before I tear into the modulation board. Is there anything else I should look at before I start on that section of this receiver? Anything I have overlooked? I’m hoping that you all can help me figure this one out. As always, I do appreciate any help. Rich……….

 

[Buck Rogers]

You need to be able to "Trace" the signal and for that you need a signal tracer.

These are the handiest and most useful piece of test gear you will ever own.

Between this and a meter you can fix 95% of all electronics!

Signal injector...

 

[Buck Rogers]

Usually you start by injecting a signal into the power amplifier driver board input.

If the tone comes out of the speaker, Amp is good.

Now we inject a signal into the Tone control board and volume control and work our way backward.

When you get stuck it's time to try front to back...

 

[topgun]

Usually you start by injecting a signal into the power amplifier driver board input.

If the tone comes out of the speaker, Amp is good.

Now we inject a signal into the Tone control board and volume control and work our way backward.

When you get stuck it's time to try front to back...

Wouldn’t using an oscilloscope and a signal generator accomplish the same thing?

 

[Buck Rogers]

Sure, But then you have to keep looking at your scope... Zap~~!

Plus the scope doesn't tell you what it sounds like.

Use what you have.

 

[Tom B]

Follow what Buck Rogers is telling you. Signal tracers are the best for troubleshooting problems such as yours. Shotgunning as you are doing will only cause more problems, and make it more difficuly in fixing the unit (Did I insert more problems by replacing a bunch of parts that may have been OK?)
As a FYI, those units are notorious for having really dirty switches that are real difficuly to get to in order to clean properly (Tape monitors, 2 ch, 4 ch, push switches etc) The relay contacts also get dirty and produce little or no sound output.

 

[topgun]

Follow what Buck Rogers is telling you. Signal tracers are the best for troubleshooting problems such as yours. Shotgunning as you are doing will only cause more problems, and make it more difficuly in fixing the unit (Did I insert more problems by replacing a bunch of parts that may have been OK?)
As a FYI, those units are notorious for having really dirty switches that are real difficuly to get to in order to clean properly (Tape monitors, 2 ch, 4 ch, push switches etc) The relay contacts also get dirty and produce little or no sound output.

I understand. I am taking in the advice from Buck Roger’s. I’m going to take a couple of days to educate myself on this piece of equipment. I’ve never used one before. So I think I’m just gonna hold off on any further work on this unit for now while I look into a signal tracer. I’ll just spend some time at the YouTube signal tracer training center. Thank you for your feedback.

 

[topgun]

Sure, But then you have to keep looking at your scope... Zap~~!

Plus the scope doesn't tell you what it sounds like.

Use what you have.

Thanks for your help. Since I’m a big fan of Heathkit, maybe I’ll look into the ones they had produced. Any opinion on them?
I have tried to find the issue with a signal generator and my scope but my experience with the scope is limited. Plus like you say I have to constantly take me eyes off the work and I’m afraid of releasing the magic smoke.

 

[Buck Rogers]

Heathkit perhaps the greatest of all time~~!

I own three of these. Just in case!

This amazing unit gives you an input to a speaker, An input to an output transformer.

Line level gain input, and High gain RF inputs.

Operates Better than a "Dim Bulb" tester because it actually Measures AC Wattage.

Last item it produces a noise signal for tracing.

Being made with tubes it's impervious to most technical mistakes.

When my business bought out the Heath service center in 1991 it was 7 large van loads.

Hundreds of amps, preamps, speakers, crossovers and about 75 pieces of test gear including 12 of these.

To top it off there were 48 boxes of manuals each one was a two man carry! Holy Crap.

HEATHKIT~~~ Don't leave home with out one!

Edit: As Tom_B mentioned! Tape monitor switches are the Bane of all receivers. I heat them up with a Hair Dryer (Not a heat gun!) and clean the crap out of them Otherwise they need to be removed and placed in an ultrasonic cleaner... ARG!

Inspect and reflow the solder connections of all switches mounted on the circuit boards.

The Heath construction manuals are so good you can build almost any heath product from scratch just from the manual! Electronic engineers have been doing that for 75 years!

 

[topgun]

Heathkit perhaps the greatest of all time~~!

I own three of these. Just in case!

This amazing unit gives you an input to a speaker, An input to an output transformer.

Line level gain input, and High gain RF inputs.

Operates Better than a "Dim Bulb" tester because it actually Measures AC Wattage.

Last item it produces a noise signal for tracing.

Being made with tubes it's impervious to most technical mistakes.

When my business bought out the Heath service center in 1991 it was 7 large van loads.

Hundreds of amps, preamps, speakers, crossovers and about 75 pieces of test gear including 12 of these.

To top it off there were 48 boxes of manuals each one was a two man carry! Holy Crap.

HEATHKIT~~~ Don't leave home with out one!

Edit: As Tom_B mentioned! Tape monitor switches are the Bane of all receivers. I heat them up with a Hair Dryer (Not a heat gun!) and clean the crap out of them Otherwise they need to be removed and placed in an ultrasonic cleaner... ARG!

Inspect and reflow the solder connections of all switches mounted on the circuit boards.

The Heath construction manuals are so good you can build almost any heath product from scratch just from the manual! Electronic engineers have been doing that for 75 years!

Wish I could have been in on that haul. I’m a huge fan of Heathkit. I just recently finished restoring two AA-1640 power amplifiers. One for my son and one to add to my collection of other Heathkit stereo gear. I’m looking at an IT-12. What is your opinion of this model?

 

[Buck Rogers]

It's fine and usually lower priced than Hickoks.

Try to look around locally for a Ham or repeater club, They always have some cool Heath stuff laying around.

 

[Tom B]

Paco and Eico made nice signal tracers as well. I may have an extra Paco I'll see if I can find it. I'll send you a PM if I find it. Paco is the tracer I have used semi regular since 2009, and also used it from 1973 to 1980.

 

[topgun]

I have an update. I have done some research on signal tracers, and I’m keeping an eye on a Heathkit IT-12 or an Eico 147A. There is a ton of them out there but I’m a stickler for condition, so I will not rush into it until I find a nice clean one.
And now for some good news. I think. I was finding it hard to give up on my 949a project and so after I finished replacing the lamps that were blown in the scope and indicators , I decided to pull out the preamp board to take another look. Right above the preamp board sits the switch board that houses all the push button switches. Under the switch board there is an access panel held on with two screws. So I figured, what the heck , let’s have a look. My heart immediately sank when I saw what was behind the access panel. The switch board had a huge crack in it that was nearly halfway across the board. Right underneath the two switches for the front an and b speakers selector. And the scope attenuation buttons. I pretty sure that I have found my problem big time. At this point I have no choice but to remove the entire board and either replace it which will likely be nearly impossible, or repair it. I have done this type of thing before but removing this board requires removing the entire front of the chassis and almost 30 wires. I was up till almost 4 in the morning and I just had to step away from this thing for awhile and get some sleep before I get up the enthusiasm to dive into this endless pit. Pictures to follow follow.
Rich……,,

 

[topgun]

Yesterday I went ahead and removed the damaged board. I then had to decide to replace the damaged board and likely wind up with this unit in this state of disassembly for who knows till I can source a replacement. So attempting a repair makes better sense to me. Worse thing that could happen is that I fail and have to do this job twice.
So I jumped right in and went to work. I did not realize till it was too late that I had forgotten to take pictures of the damaged board still in place and after being removed. I first checked continuity on all the traces and made a note on the ones I need to address. I also checked all the resistors and diodes, and all checked out as good. This is a double sided board so I had to be careful here. After a couple of hours I think I was able to successfully repair the board. I also think I discovered why it broke in the first place as I’m pretty certain that no one had ever had been this deep into this unit before me. One other thing, when I was rebuilding the amplifier boards, the outputs were still shiny and new as if they had never even had gotten hot. I would bet that this receiver was sitting for many years somewhere which also explains its like new condition. Anyway, the board is attached to the chassis with a bracket that supports all four corners of the board. When I removed it from the chassis, the board was only held in the bracket with only two screws in the front of the board and had nothing in the back which left the rear of the board unsupported. It is really crowded in this area so it kinda makes sense to me why this happened. Somebody at the Pioneer assembly plant probably had to much sake the night before I guess.
Here are some pictures of where it is currently. I’m now going to start reinstalling the board, button it up and let the dim bulb decide if I win this round. More to come. Rich………

 

[topgun]

After sitting for several hours at my bench, I finally finished reinstalling the “repaired” swith board. While I had the board out I took the opportunity to really clean and lube the switches on the board. I also gave all the other controls on the unit another once over . With the whole front of the chassis wide open, it was much easier to reach everything. I also recapped the tone control board also.
Hooked it up to the dim bulb and it stayed dim. Plugged it in the mains and, Success! It’s alive and everything works now. With a sigh of relief, I will now take a break and continue on it tonight. I did suffer one setback though. After I had already replaced the bulb in the pointer, I accidentally placed my hand in the wrong place and broke the damn thing. I now have to try and source an another one. Everything considered, I’m in good shape now. What’s left is the power supply , tuner and quad boards which I will probably take on some other time. I must say that , even though I have not given this thing a proper listen, what I hear so far sounds great. Rich…….

 

[topgun]

This will be my last entry to this thread. The Pioneer is up and running. I was listening to it for an extended period today and I am satisfied with the outcome so far. I did also manage to repair the broken dial pointer also. I still have more work to complete on this unit before it is finished but I’m going to leave it alone for now as I’ve got other things to work on and we have another trip to Oklahoma coming up next month. I’m posting a few more pictures of the unit. Rich………