SX-880 with protection issues

[dustmaker]

D28 should be a single 14v zener, on the power supply board. Sometimes you can add another diode in series with a zener to increase the voltage, but it must be forward biased. Those don't look like they are connected that way. Sometimes you can use two lower voltage zeners to make a higher voltage. What is the dc voltage on the plus side of C410, sb +14vdc. That should be the same point as the cathode of D28.

+ side of C410 is measuring 13.8 volts

 

[merlynski]

+ side of C410 is measuring 13.8 volts

Close enough I think. Carefully measure DCV across each of the D28 combined diodes to see what is actually in there. Do you have a 14v zener?
Is pin 1 of Q35 (PA3004) still about +13.5vdc?
What is the DCV at the cathode of D35, sb about +56.4vdc?
Did you (pull and) test Q37 to see if it really had failed again? It is a KSC2690A in a TO-126 package, isn't it?
Did you replace the relay (or somebody before you)? It is a 48v relay, not a 24v relay?
Is R421 a 620Ω resistor (blue, red, brown)?
It almost seems like too much current is flowing in the relay/Q37 circuit . . .
While Q37 is out, connect the collector pad to ground to pull in the relay, then measure DCV on the R421 side of its coil pin, sb about +46.3vdc according to the schematic.
If the right side is not working, recheck DC offset and bias for both channels.

 

[dustmaker]

Carefully measure DCV across each of the D28 combined diodes to see what is actually in there. D28 A=0 C14. D29 A=14 C14.6.
Do you have a 14v zener? A spare. No - I have 13v zener - TZX13C but no 14v
Is pin 1 of Q35 (PA3004) still about +13.5vdc? Yes - measuring 13.54v
What is the DCV at the cathode of D35, sb about +56.4vdc? 56.6v
Did you (pull and) test Q37 to see if it really had failed again? It is a KSC2690A in a TO-126 package, isn't it? Yes - KSC2690A. I tested it and it still functions. Here are the readings on 6 way -/+. E-C= 0, E-B=555 C-E=0, C-B=560mv, B-E=0, B-C=0. I pulled a new C2690A and got almost identical readings
Did you replace the relay (or somebody before you)? It is a 48v relay, not a 24v relay? I don’t know if some one else did, but I didn’t replace the relay. The solder looks to be original (for what it’s worth). Pic attached. Relay says ASR-032 but manual says ASR-020
Is R421 a 620Ω resistor (blue, red, brown)? Yes - I had to pull it out as it was wrapped in a sleeve. I also measured it as 624Ω
It almost seems like too much current is flowing in the relay/Q37 circuit . . .
While Q37 is out, connect the collector pad to ground to pull in the relay, then measure DCV on the R421 side of its coil pin, sb about +46.3vdc according to the schematic. 46.4vdc. Relay pulls in nicely
If the right side is not working, recheck DC offset and bias for both channels. I rechecked DC offset for both channels and they were within +3mvdc. I adjusted down to zero easily

 

[dustmaker]

live 120v, Unit warmed up. All transistors are in. Nothing connected to speaker terminals and speaker select buttons are off. Unit is on AUX with no input signal, balance control is neutral

I went through every transistor and documented the voltages at the pins. The biggest takeaway is voltages are lower in almost every instance 5% - 10%. For example, Q1,Q2, pin 4 s/b -24.5. Mine is measuring -22.4. Pin 5 s/b-23.5, mine is -21.1. Pin 8 s/b +24.7. Mine is +22.9
Q9 B s/b40.3, mine is39.1
The only one running higher is Q35 pin 6 s/b-7v but I measure -12.3v and pin 8 s/b 7.4v but I measure 8.6. This lower voltage is pretty consistent throughout the unit

Looking at B-E difference, I am at 0.6 volts everywhere except the following:
Q7 & Q8 B-E is 0.5 v
Q13 is 0.5v
Q27 is 0.7v
Q10 is 1.3v B=39, C=1.19, E=37.7
Q28 is 0.4v B=14.3, C=40.8, E=13.9
Q36 is 0.15v B=32.65, C=0, E=32.8
Q37 is 0.7v
Q33 and Q34 are both near zero for B and for E, but I believe this is ok

Q1,Q2 are showing -35mvdc to -160mvdc on pins 1,3,6,7 instead of zero. Q3,Q4 pins 2,3,6 likewise are showing some milivolts instead of zero.

in summary, almost all of my reading are different than expected, but at the same time, except for the things mentioned, there isn’t any major smoking gun

 

[merlynski]

My schematic shows -7v on pin 6 of Q35 also, but that has to be an error on the schematic, I have edited mine. Pin 6 is the -13v 'supply' input of the PA3004. If you look at the voltage divider of R423 and R424, with -41.5v on that supply (Q27 Emitter) it calculates to -13v.
All the regulated voltages on that power supply, both positive and negative, are derived from the zener reference voltage of D28, 14v. If D28 is a bit lower than 14vdc all the regulated voltages will be proportionally lower.
The dc voltage on pin 8 of Q35 comes from inside Q35. As you have the custom replacement installed it may show a bit different. That may have a slight effect on the pull in delay, but is not significant as long as the delay is inside the nominal 3 - 7 second window.
What dcv do you have on the emitter of Q27? Sb -41.5vdc.
What dcv do you have on the emitter of Q30? Sb +41.5vdc.
The collector voltage of Q10 is close to correct (+1.2vdc), a BE voltage of 1.3v is high.
What dcv do you have on the collector of Q12? Sb -1.2vdc. If it is wrong there may be a problem with the right channel bias.
Measure dcv on pin 65. That is the output of the right channel power amp before the relay, it should be close to 0vdc, and the dc offset there should be adjustable by VR6.

 

[dustmaker]

My schematic shows -7v on pin 6 of Q35 also, but that has to be an error on the schematic, I have edited mine. Pin 6 is the -13v 'supply' input of the PA3004. If you look at the voltage divider of R423 and R424, with -41.5v on that supply (Q27 Emitter) it calculates to -13v.
All the regulated voltages on that power supply, both positive and negative, are derived from the zener reference voltage of D28, 14v. If D28 is a bit lower than 14vdc all the regulated voltages will be proportionally lower.
The dc voltage on pin 8 of Q35 comes from inside Q35. As you have the custom replacement installed it may show a bit different. That may have a slight effect on the pull in delay, but is not significant as long as the delay is inside the nominal 3 - 7 second window.

What dcv do you have on the emitter of Q27? Sb -41.5vdc. Q27 E=-39.3….C=-42.1, B=-40.0
What dcv do you have on the emitter of Q30? Sb +41.5vdc. Q30 E=40.2…..C=41.0, B=40.8
The collector voltage of Q10 is close to correct (+1.2vdc), a BE voltage of 1.3v is high.
What dcv do you have on the collector of Q12? Sb -1.2vdc. If it is wrong there may be a problem with the right channel bias. I’m close enough, I think. Q12 C=-1.195……E=-38.3, B-37.7
Measure dcv on pin 65. That is the output of the right channel power amp before the relay, it should be close to 0vdc, and the dc offset there should be adjustable by VR6. Yes it is 0vdc

edited to correct a +/- sign

 

[merlynski]

Well, that all looks good. try some music . . .

 

[dustmaker]

I put it on FM and it plays and surprisingly, both channels are working again even though I never replaced any thing. I’m going to leave it on for a while to heat up and see if one channel dies when it gets hot.

however, the total power is still low. When it was working I verified the left and right channel power meters. I only had to tweak them a hair to get them perfect. Now with volume at full, I am only showing about 28 watts.

the other thing I noticed is the relay is pulling in instantly, there is zero delay. Flipping the on/off switch covers the sound of the relay pulling in and I only hear it clicking off when I flip the switch off.

 

[merlynski]

Sounds like a protection issue again.
Measure dcv on the collector of Q37, it should be about +0.2vdc if functioning correctly in saturation.
If that is correct try grounding the base of Q37, that should not hurt anything, but if Q37 is working the relay should drop.

For the low power issue:
Verify input to Aux is 150mVAC RMS at 1kHz Sine Wave.
Function to Aux. All Tone controls centered and/or Off. Tape Monitor to Source. Tape Duplicate to Off. Mode to Stereo. Low Filter Off. Balance Centered. Loudness Off.
Is anything plugged into the Tape jacks? Verify signal at Tape 1 REC L & R is 150mV RMS.

 

[dustmaker]

Sounds like a protection issue again.
Measure dcv on the collector of Q37, it should be about +0.2vdc if functioning correctly in saturation.
If that is correct try grounding the base of Q37, that should not hurt anything, but if Q37 is working the relay should drop.

. Q37 C=0vdc. Grounding the base does nothing. B=0.7vdc, C=0, E=0

For the low power issue:
Verify input to Aux is 150mVAC RMS at 1kHz Sine Wave.
Function to Aux. All Tone controls centered and/or Off. Tape Monitor to Source. Tape Duplicate to Off. Mode to Stereo. Low Filter Off. Balance Centered. Loudness Off.
Is anything plugged into the Tape jacks? Verify signal at Tape 1 REC L & R is 150mV RMS.

. I measure -3mvdc on left and 0mvdc on right

 

[dustmaker]

. Q37 C=0vdc. Grounding the base does nothing. B=0.7vdc, C=0, E=0
. I measure -3mvdc on left and 0mvdc on right

And the left channel is not working while on AUX. It does work on FM so this explains why it was and then it wasn’t. I had different inputs set.

 

[dustmaker]

And the left channel is not working while on AUX. It does work on FM so this explains why it was and then it wasn’t. I had different inputs set.

I had a bad solder connection. I am now reading 40mvdc at C on Q37. I followed this back to the relay and I am reading 40mvdc on one side of the coil and 45.2 volts on the other side. I calculate there are 0.016 amps feeding into the input side of the relay. (620Ω resistor, 55.2 volts - 45.2 volts = 10 voltage drop). 10/620Ω= 0.016 amps. Is this the correct amperage?

Following up on your instructions above, when I ground the base of Q37, the relay does drop.

 

[dustmaker]

I had a bad solder connection. I am now reading 40mvdc at C on Q37. I followed this back to the relay and I am reading 40mvdc on one side of the coil and 45.2 volts on the other side. I calculate there are 0.016 amps feeding into the input side of the relay. (620Ω resistor, 55.2 volts - 45.2 volts = 10 voltage drop). 10/620Ω= 0.016 amps. Is this the correct amperage?

Following up on your instructions above, when I ground the base of Q37, the relay does drop.

I’m looking at the schematics, the voltage drop is correct and the input voltage looks good, so I suspect my diode D35 is again bad. Am I on the right track?

 

[merlynski]

As long as the relay pulls in the current is OK. Your calculations are spot on.
If the relay is pulling in D35 is probably OK.
Grounding the base of Q37 dropping the relay says Q37 is good (and D35). That means the base current turning Q37 on is coming from the PA3004, unless there is another connection to that trace, like a solder bridge. Look closely along the trace from Q35 pin 3 to the base of Q37 for any solder bridges or other contamination.
Ground pin 8 of the PA3004, that will zero the charge on the timing cap and should also drop the relay. If it drops, remove the ground and listen for a delay before it pulls in again. If there is no delay try replacing C417 (delay timing cap), if it has not been already. There may be a problem on the PA3004 board.
FYI, I am being very careful about where I am asking you to ground certain points. Grounding the wrong points WILL destroy things, it is not a good random troubleshooting technique.

. I measure -3mvdc on left and 0mvdc on right

The intention here is to see if the 150mVAC 1kHz signal is making it through the signal path from the Aux input. Measure ACmV at the L & R REC jacks with the 150mV 1kHz signal coming into the Aux input. You still have the Y input on Aux?

We are working on two different problems at the same time, a protection circuit problem and a signal path problem. That complicates the process . .

 

[dustmaker]

As long as the relay pulls in the current is OK. Your calculations are spot on.
If the relay is pulling in D35 is probably OK.
Grounding the base of Q37 dropping the relay says Q37 is good (and D35). That means the base current turning Q37 on is coming from the PA3004, unless there is another connection to that trace, like a solder bridge. Look closely along the trace from Q35 pin 3 to the base of Q37 for any solder bridges or other contamination.

. Looks clean, no bridging.

Ground pin 8 of the PA3004, that will zero the charge on the timing cap and should also drop the relay. If it drops, remove the ground and listen for a delay before it pulls in again. If there is no delay try replacing C417 (delay timing cap), if it has not been already. There may be a problem on the PA3004 board.
FYI, I am being very careful about where I am asking you to ground certain points. Grounding the wrong points WILL destroy things, it is not a good random troubleshooting technique.

. I grounded pin 8 of Q35 and the relay did drop. There was then an appropriate delay for it to pull in.


[QUOTE="merlynski, post: 15201867, member: 255447"The intention here is to see if the 150mVAC 1kHz signal is making it through the signal path from the Aux input. Measure ACmV at the L & R REC jacks with the 150mV 1kHz signal coming into the Aux input. You still have the Y input on Aux?

We are working on two different problems at the same time, a protection circuit problem and a signal path problem. That complicates the process . .[/QUOTE]
AC…. I was thinking DC so I didn’t properly read your comment. I get 127mvAC on right channel and 122mv on left channel but as the unit warms up, both of those readings decrease to about 122 and 113. It bounces around a bit, not steady.
Yes, I am still using a “y” connector

 

[dustmaker]

As long as the relay pulls in the current is OK. Your calculations are spot on.
If the relay is pulling in D35 is probably OK.
Grounding the base of Q37 dropping the relay says Q37 is good (and D35). That means the base current turning Q37 on is coming from the PA3004, unless there is another connection to that trace, like a solder bridge. Look closely along the trace from Q35 pin 3 to the base of Q37 for any solder bridges or other contamination.
Ground pin 8 of the PA3004, that will zero the charge on the timing cap and should also drop the relay. If it drops, remove the ground and listen for a delay before it pulls in again. If there is no delay try replacing C417 (delay timing cap), if it has not been already. There may be a problem on the PA3004 board.
FYI, I am being very careful about where I am asking you to ground certain points. Grounding the wrong points WILL destroy things, it is not a good random troubleshooting technique.


The intention here is to see if the 150mVAC 1kHz signal is making it through the signal path from the Aux input. Measure ACmV at the L & R REC jacks with the 150mV 1kHz signal coming into the Aux input. You still have the Y input on Aux?

We are working on two different problems at the same time, a protection circuit problem and a signal path problem. That complicates the process . .

I did replace C417. It was the wrong value. 47μF instead of 33μF per schematics. However, after putting in the 33μF capacitor, I am right back to the beginning. Relay drops when I turn the volume up. I’m only showing about 5 watts on the meters when it drops (while on DBT). I did try it live and I blew out a 5A fuse. I have to check everything over again. The only thing I replaced was C417 and I did put it in the correct orientation. I did not test the capacitor befor I put it in.

 

[merlynski]

OK, signal path to the Tape 1 REC jacks looks good.
Are you getting an appropriate delay of the relay pulling in at power up now?

Measure the 0.47Ω resistors, R277, R279, R278, R280.

 

[dustmaker]

Here is an update before I proceed with your directions above. I just removed Q37 an hour ago. When I said above (yesterday) that the only thing I replaced was C417, well that was only sort of true because yeaterday I also pulled out Q37 as I was cleaning up the solder trace that had broken and when I put it back in, I think I put it in backwards. (I had difficulty getting it out this morning and when I did, I dropped it and I lost the orientation.). When I typed up my comments last night, I never thought of that transistor.

when I measure the collector pin of Q37, it is now showing as 44 vdc. I have the exact same reading on either side of the coil in the relay. I now no longer have the voltage drop I had In post 92 above.

 

[merlynski]

That means the PA3004 is not sending a pull in current through the base of Q37, so it is not turning on. With no collector current flowing there will be no voltage drop across the coil (same voltage at both ends), and the relay will not pull in.
You must have had Q37 installed correctly before because grounding the base dropped the relay, and grounding the timing cap dropped the relay.
You can test the Q37 installation by connecting a 100kΩ resistor from the base of Q37 (PA3004 pin 3) to PA3004 pin 1 (+13vdc supply). If Q37 is installed correctly (and no open traces) the relay should pull in.
KSC2690A is ECB when facing the front (writing) side toward you.
If it does work (pull in) the next step is to get the PA3004 and related circuits functioning correctly again. Then lets look at the signal path problems.

 

[mark long]

is anyone making the pa3004 clones for those of us that cannot make them ourselves?