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SX-828, won't come out of protection.

fucanay

fucanay

AK Subscriber
Subscriber
I picked up a SX-828 recently and it's in great physical shape. Electronically, not so much. It will not come out of protection. Since there was no reason to not have the caps updated, I've recapped the power supply and the Main Amp, both read close to the same as they did before the recap. I've measured the voltages again and I'll list them below. If anyone could give me a better idea of what to check next or to make sense of what I've already checked, it would be greatly appreciated.

All measurements in Volts unless mV is specified.
Power Supply (AWR-010B)
1. 0
2. 12.6
3. 0
4. 4.6
5. 31.8
6. 33.4
7. 35.5
8. -.2
9. 44.1
10. -.2
11. 0
12. 0
13. 0
14. 41.8
15. -41-8
16. 0
17. -37.5
18. 0
19. -34
20. -33.7
21. This is negative, but drifts all over the place.
22. This is negative, but drifts all over the place.
23. 0
24. 47.2

Protection Board (AWM-025C)
1. -34
2. -13.2
3. This is negative, but drifts all over the place.
4. 0
5. -34
6. -34
7. This is negative, but drifts all over the place.
8. This is negative, but drifts all over the place.
9. 35.7
10. 35.7
11. 35.7
12. 0

Main Amp Board (AWH-010D)
1. -22.4mV
2. -17.2
7. This is negative, but drifts all over the place.
8. -34
9. 42.1
10. 42.1
11. This is negative, but drifts all over the place.
12. -33.5
17. This is negative, but drifts all over the place.
18. -33.5
19. -41.9
20. -41.9
21. 42
22. -13.2
23. -36.2

It seems based on the amp board, pin 2 being -17.2 that there is something wrong with the Right channel. As for what that is, I am not knowledgeable enough to know. Once again, any help would be appreciated.

Cheers,

Matt
 
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pin 8 amp board is first thing i went for . pin 1 protect confirms it ..right channel has railed .pin 7 protect is because of pin 8 voltage .
check right channel transistors and resistors .
guessing open transistor or high resistor ..just a guess ....
 
Nothing visually looks bad on the boards, meaning nothing looks burnt. I'd prefer not to have to pull each resistor to check them and I'll have to see what transistors are in there. I've confirmed it has the 484's and some other marked A726, but there are some others I can't see without removing them.
 
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I think I got something confused before and got my left and right mixed up. Pins 1, 5, and 6 on the Protection board are Left Channel.

Upon researching numbers and confirming it on the Main Amp Board, these are what are installed right now:

2 x 2SC1080 (Q13/Q14)
2 x 2SC680 (Q15/Q16)
2 x 2SA484 (Q11/Q12)
2 x 2SC484 (Q9/Q10)
4 x 2SC1124 (Q5/Q6/Q7/Q8)
4 x 2SA726 (Q1/Q2/Q3/Q4)

The odd numbers being the right and the even numbers being the left. I decided to measure the base voltage on all of the small ones. I did this by testing to ground, but I'm not sure that's right.

Right
Q1 = -2.8
Q3 = -2.5
Q5 = -35.5
Q7 = -35.5
Q9 = negative, drifts all over the place.
Q11 =negative, drifts all over the place.

Left
Q2 = -34
Q4 = -32.2
Q6 = -38
Q8 = -36
Q10 = -36.1
Q12 = -34

Somehow, at midnight, this seems to point to Q2 and Q4 to me. Unfortunately, I don't have those here right now. But is swapping out all of the 2SA726's the logical next step?

Also, what does the PSU pin 17 to Main Amp pin 23 do? That is very low at -36.2 when it's supposed to be -45. All other voltages have been higher while not under a load, this one is lower. It looks like it connects to the collectors of Q1-4 and the emitters of Q5-8. Has to be related, right?
 
2. -17.2 main amp board ..this could be coming from the preamp .
pull the pre-main jumpers and see if voltage at pin 2 drops .
 
Instead of dropping on pin 2, it went up to -37.8. I think I'll leave those disconnected for now.
 
Unfortunately, there is now some magic smoke. I can't tell where it came from exactly, but it was near where the protection unit and AF Unit are close together. No signs of burnt anything. Before I noticed that, I got through R30 and R32, didn't get to R18 before I shut it down.

R32 = On the side that is shared with R30, I got a reading that drifted between +10 and +20, on the side that goes to the base of Q10 I got the same -34 that I got before at the base of Q10.

R30 = On the side that is shared with R32, I got a reading that drifted between +10 and +20, on the side that goes to the collector of Q14 I got +42.

This thing might be a little over my head. I really wanted to see if I could get it to work before changing every last component in the thing.
 
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sorry for the delay getting back to you .
now the smoke has come out you need to put it back in . a rebuild is the best way as long as you do it right first time .
otherwise its test everything first and replace bad parts and test to see if then works .
 
Right, that makes sense. I've been putting a parts list together for the main amp. I'd like to go board by board, but don't want them to kill each other if one is still bad. I'm thinking power supply, main amp and protection all at once first. Will anything from the AF or head amp boards fry any of the rebuilt boards if they have problems as well?
 
logic says the power supply has to be good or all is lost .
a short further down the line can of course damage it . this is depending on how well it was designed though . as in current limiting resistors in the right places to minimise the damage .
it is never easy to diagnose things over the internet .
all i can say right now is shorted transistors seem to cause more smoke than anything else . i generally go over them in circuit with power off and if appear bad remove them and test out of circuit .
 
How do you test the transistors in circuit with the power off?

Also, where would you look for the smoked parts? If it smokes, it should be visible, right? What does a smoked transistor visibly show?

So far, I have completely recapped the power supply and protection boards and have new diodes and transistors for them. I will need to order the higher watt resistors because I don't have those in my parts supply. But I'll do those first. I have recapped the main amp board, but don't have the transistors or all of the resistors for it yet.
 
Ok, I tested all of them on the main amp board, in circuit with the power off, using the diode function. I followed a spreadsheet template I found in the transistor testing sticky and I've attached my results. Where it says climbs, the voltage continued to go up and I pulled the leads off. I feel like this isn't a perfect test because surrounding components may be effecting the readings. However, I did find a pair that don't match each other. Q15 and Q16 have reading that don't line up with each other. So with pin 8 having -34V, I would assume it is Q16 that is the culprit here. PLEASE, correct me if I'm wrong as I frequently am.
 

Attachments

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i need to decipher your readings . i just looked and dont really understand the methodology .
however i dont see any shorts there .
and yes in circuit testing with diode test can only see shorts . a transistor tester designed for in circuit testing is a better choice .
i get by with diode though even though i have an in circuit tester that just takes up too much time .
p.s you just caught me at near on 1am plus after too much beer so will check back in daylight ...
 
Sorry, I should have explained. Red column is positive lead and which pin it's touching. Black row is negative lead and which pin it's touching. The readings are where they intersect. I did this to ensure I got readings from all possible combinations. The gray boxes are obviously impossible as they are base to base, collector to collector and emitter to emitter.
 
I suppose I should just pull Q16 and test it out of circuit now, that's an easy one.

EDIT: I pulled Q14 and Q16, both tested fine.
 
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I'm still trying to find the original fault with the power off. I've checked so many things, but nothing looks bad on the main amp board.

To recap, the left channel is railed and the unit is in protection. There was some smoke while I was testing, but not from the amp board and it's location hasn't been determined. I have disconnected the jumpers that connected to preamp to the amp to isolate them, but the voltages on the main amp remain off. It is possible that this caused something up stream to smoke, but nothing is visible to the naked eye.

I have tested the transistors in circuit with the power off with the diode tester on my DMM and everything was symmetrical from left channel to right channel with the exception of Q15/Q16, but when those were tested out of circuit, they were fine.

Not sure where or what else to check. Ideas?
 
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