Pioneer SX-650 protection circuit troubleshooting

[ViperZ]

Got this Pioneer SX-650 stuck in protection. All power supply voltages are as per manual. Relay itself seems ok. Output modules are not shorted. DC offset is around 30mV in each channel.

Started going through protection circuit. Q11 and Q12 have their 0V on each base (no DC offset in each channel). +21V on each collector is present, so this pair must be ok?

Now get to Q13 and Q14, and voltages are all over the place. Simple junction test in-circuit shows transistors are at least not shorted.

Q13 Vdc readings:
* base: 1.5V
* collector: 0.7V instead of -1.5V
* emitter: 2.1V instead of 17V

Q14 Vdc readings:
* Base: 0.7V instead of -1.5V
* Collector: 0V instead of 13V (so Q15 is turned off)
* Emitter: ground

D7, D8, D9 - junctions seem ok according to my multimeter. I replaced 4 electrolytic caps "just in case" in the circuit - no change. Interesting that C36 was 100uF/16V instead of 220/16 (my parts 650 has 220/16, which matches the manual).

Should I disconnect D7 and see if the circuit works, or replace Q13 and Q14?

I think substitutes are:
Q13 - 2SA823 PNP - KSA992FBU
Q14 - 2SC1647 NPN - KSC1845

Thank you!

 

[petehall347]

AC volts at pin 31 ? q14 appears on and should be off . with -1.5v at it base .

 

[ViperZ]

8VAC on pin 31. Q13 and Q14 are both completely turned on, at least based on the voltages that I'm reading. I haven't checked Q5/Q6 feeds - I think I'll measure them out and compare them to each other?

 

[petehall347]

d13 anode voltage ?

 

[ViperZ]

Here is something interesting. Both q5 and q6 have just over 50mv on base. Emitter of q5 has 600mv, emitter of q6 has 10mv .

 

[ViperZ]

-4.7vdc on d13 anode

 

[petehall347]

try pulling d7 like you said .

 

[ViperZ]

I can see the imbalance now. Q5 has 0.6v across EB, so it's turned on. Q6 has pretty much zero across EB.

 

[markthefixer]

Pull D7 for "subtractive troubleshooting". That will tell that q5 or q6 is the leaky one. I'd just pop in two new ksc1845's
BUT also replace C21 and C22 0.22uf - they BOTH are the dreaded CSSA Sanyo "capsistors" that fail shorted eventually. These caps leak so badly they turn into resistors, and resistors that drop lower and lower in resistance until they read as a dead short. In this circuit, that means the over current protection will not work.

ALL sanyo sky blue CSSA caps should be pre-emptively removed!!! They ALL will EVENTUALLY short out.

 

[ViperZ]

Wow, Mark. I literally just pulled these two caps, and one was white on the bottom. Replaced them, I had some 0.22/50. Still no go. Pulled d7, turned on, relay engaged.
So you're saying q5 or q6 is bad then? I think I have a bag of ksc1845 somewhere. The original transistors are color coded, so I assume they're matched for gain? I don't have a transistor tester .

 

[petehall347]

i am thinking you should be looking at voltages for all pins of the modules .

 

[ViperZ]

Both modules were identical to each other .

I replaced q5 and q6, put d7 in, and now relay engages.

The kicker is that there is not even a whisper on the outputs. I can see the DC offset on the terminals, 33mv and 50mv, I can see tuner pointer showing Fu signal, but nothing comes out of either channel.

I assume it's not a relay since I see the DC offset on speaker terminals?

 

[ViperZ]

Ok, it's an unrelated problem now. Side note, I may have to change a relay, since I'm getting around 8 ohms on closed contacts.

Here is what's going on.

If I crank the volume high, I get very distorted output randomly.
I traced the signal back .Nothing on relay, nothing on microcircuits, nothing on power amp board inputs, tone board, etc.

I put my meter across volume pot with power pff, and I get tons of random readings between 0 and 8k. My parts unit reads 0 to 50k uniformly on both sides. I have never seen a completely shot volume pot. Am I on the right track? Thank you.

 

[zebulon1]

A poor mans signal tracer is my Fluke 117. I set it to AC mV's and play a CD through the inputs. "Tape one" or "aux" ect. I look for the meter to jump with the music while probing for signal at the input and output from the boards. It's not full proof but it works most of the time. I'll even run the amp with speakers - listening as I check for the signal. The drawbacks are: closely mismatched signals are hard to distinguish, The music stops and you lose the signal or you forgot to select the correct input.

I have all the proper equipment but will rely on the quick and dirty when presented.
Mark has a deal where you take an RCA patch cord and connect one end in the tape monitor and the other end a twisted ceramic cap around the positive male plug. You basically regurgitate the signal through the "Tape Monitor" using the other leg of the ceramic cap as a probe. It works well. Not in your case. The volume should be set low at first and increased if needed.
The line level signal will be around 250mV's up to the volume control. After that you may have to increase the volume to increase the signal strength between the volume and the amplifier.

I write this hoping it's what you need. I didn't read the entire thread.

 

[markthefixer]

Wow, Mark. I literally just pulled these two caps, and one was white on the bottom. Replaced them, I had some 0.22/50. Still no go. Pulled d7, turned on, relay engaged.
So you're saying q5 or q6 is bad then? I think I have a bag of ksc1845 somewhere. The original transistors are color coded, so I assume they're matched for gain? I don't have a transistor tester .

I surmise blue dots are noise rating (which would be low leakage), not gain matching. No special precautions for c1845's going in. Never had any problems with that swap.

 

[ViperZ]

I think I figured out the 2nd issue with this receiver. Instead of tracing the signal, I should have checked the voltages going to the boards. I checked the voltages from power supply, but didn't realize that voltages going to tone and phono board go through a resistor first, and I didn't check those pins. Bottom line is +21V and -19V supplies going to tone and phono boards were shorted to each other (80 ohms). I disconnected power to tone board - still shorted. So one of the phono amplifier microcircuits. Desoldered +/- pins of left channel microcircuit, and bingo, supplies are no longer shorted to each other. Now I have at least an hour ahead of me to put the whole front end back together, remove that microcircuit, solder supply pins back into boards, etc. Weeee.

 

[ViperZ]

OK, so two main problems solved. Residual problems:

1). Need TA7136P1 (TA7136P) microcircuit to get phono working again. Retroamplis is the only good supplier I know, and they don't have stock. Ebay is all from China.

2). FM radio takes like half an hour to kick in. I will check voltages - would that be a regulator problem?

A side note - one channel was playing quieter than the other. Relay contact resistance on that channel was 8 Ohms (vs 2 Ohms on louder channel). Replaced relay with MY4 modern relay - fixed that problem.

 

[zebulon1]

Hey, progress.

 

[ViperZ]

Hey, progress.

Yes, definitely some progress. I ordered the microcircuit from a distributor in Quebec yesterday (almost local!). Now just need to figure out the FM problem.

 

[ViperZ]

I don't know why I keep working super-late after I get really tired.

1). Checked voltages to FM board - no problem. Oh, look there is FM Muting switch after supplies. Oh wait, it's in the ON position. Boom, radio works.
2). Oh, I ordered new microcircuits... Oh wait, I have a broken-down SX-650 sitting under my desk. Why can't I just pull a microcircuit from it???

So basically I can do everything tonight, except I am short one wedge bulb.