sx-828

slayer44

Well-Known Member
Hey guys...just pulled an 828 from the ashes. I totally rewired the underside and rebuilt the power supply board. Powers up and plays through AUX input. Switching to phono 1 and 2 doesn't trip the relay (haven't hooked up TT yet cuz still in reassembly stage) but switching to AM/FM causes protection mode. Any quick ideas about this or do I have to diagnose the tuner board? any help would be appreciated.
 
Mine is pretty sensitive to dirty contacts everywhere. I'd give it a deoxit session first and see if that cleans anything up.
 
No quick ideas. We have to eliminate suspects.

This model runs the tuner output through the head amp, so it's almost impossible for a tuner DC offset to make it all the way through the signal chain to the power amp. As a sanity check I would first short the pre-out/main-in jumpers to ground, to eliminate all main-in DC offset sources before the power amp, then see if the relay still opened on FM. I expect it to. If it doesn't, you have multiple problems, one of which is leaky coupling caps at the power amp input. In either case, I'd be looking for a power supply interaction, and figuring that the amp is just barely out of protection in Phono and just barely in protection in FM. I'd check the amp outputs for DC offset. The SX-828 protection circuit also monitors output transistor emitter current, so if I didn't see DC offset, I'd look there next.

HTH,
Dave
 
OK, but first, watch the power supply voltages when it goes into protection, perhaps the tuner is zorching the power supply in some way. There are a lot of ways it could happen because of the multiple dropping resistors used for small voltage changes.

First and foremost, WHAT is the board number of your protection board: W28-008 or AWM-025?

WHEN it is in protection, get us some voltage readings to see what's annoying the protection circuit:
D5, D6, D7 each: anode to cathode (the 0.6v one is the active input)

Watch the main amp pin 22 to pin 2 protection's -13v voltage, it comes from the amp board, not the power supply. May not seem relevant, but it's quick to rule out.
 
Hello...thanks for the input! I ran some tests last night. put a DVM on my outputs from the power board. All voltages correct except pin #2 and #4 which could explain some things. I have 26V coming out of pin 2 when the manual shows 12V. And pin 4 is low at 1.6V to 3V (fluctuates). Does this mean the PCB is bad. I changed the transistors out from another unused board and no change in voltages. What could be left? all resistors check out with correct results. All I can think of is recapping the whole board and buying new transistors. I think the voltage problem is causing all of my lights to not work either. I will try out your suggestions tonight and let you know. Thanks so much for your help! I will let you know what I find. BTW, does anybody use the QL-600 or QL-600a with this unit? I would like to try it, but I don't have an opinion of sound yet. Thanks.
 
When the problem is not confined to one channel, check the power supply first. PS ain't happy -> ain't nobody happy.
 
Grrrr:

First and foremost, WHAT is the board number of your protection board: W28-008 or AWM-025?

Why haven't you answered the most important question I asked.

PLUS it would be NICE if you described EVERYTHING that was happening to it.

I think the voltage problem is causing all of my lights to not work either.

NO LIGHTS is a very important symptom. IF you have the AWM-025 protection board. The W28-008 doesn't DO that.

The AWM-025 protection board senses the AC to the lamps USING pin 12 and a FUSE from the BRIGHT lights circuit to determine whether or not the unit has been turned on or off. Turned off has to be fast to beat the power supply decay time and beat the weird stuff that happens when they do.

Why do I need to explain WHY I need the answers to HELP you?

Sounds like "totally rewired the underside" has a wire tapped in the wrong place. Again, referencing back to the pin 12 and the AC from the lamps.

Hang a voltmeter on pin 12 of the protect circuit, look for AC volts, about 8, when the unit is working into/with speakers, then switch to fm and/or am, and see if the 8 VAC goes away.
If so, re-wiring error....
 
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Sorry guys

Hey there...I've been away on vacation and couldn't answer your question. Sorry. My protection board is the AWM-025. Now, here is the sequence of events. I purchased a used junk 828 locally and switched the protection board to my unit. The boards had the same number.
 
Was the reason you swapped the protection boards the problems in the original post or a result of the problems in original post???
 
I will do what you suggested Markthefixer and get the results posted. I appreciate the help but don't always get to the computer as often as I would like. As far as wiring, I am a systematic guy. I take photos, draw my own pinout sketches AND label wires with numbers so I don't get it wrong. I will get back to you. I changed the protection board because at the beginning I had nothing. the relay wouldn't kick in. No lights, no hum, zero. After changing the prot. board and replacing the zener diode (burned and melted) on the power board, I finally got something, amplification through the aux input. And I do have phono output also, I checked this morning.
 
Ok, finally got around to getting voltages from my receiver. This is what I get. Unit powered on and playing tunes thru AUX, voltage at pin 2 on P.S. 27.34v (Climbs to 29.7v). Pin 4 reads 3.63v. When I switch to FM, pin 2 goes down to 10.75v. Pin 4 drops to 1.52v, and relay kicks off. Turn back to phono, and voltage immediately goes back to 29v (even before relay switches closed). As a test, I disconnected wire from pin 4, and ran same test as above. Relay stays on and voltage stays at 29v. Pin 4 stays at 3.63.
 
It sounds like my tuner board is causing, or ASW-017 (dimmer switch) is causing these issues. I checked my voltages at the lamp sockets too and only get about 1.8v at any time. Thanks for any help you can give me. FYI, this is a hobby of mine and not my regular job, so if I sound like a newb, well, I am.
 
Hold on, rebuilt the power supply board? What was replaced, and what were your output voltage readings on it when you finished? Melted zener diode? Well, there is only one.

Were you getting 35v at the regulator pin 7 and 12.5v at q3 emitter? What about pin 2?

uh oh:
AUX, voltage at pin 2 on P.S. 27.34v (Climbs to 29.7v)

THAT'S only supposed to be 12 volts

Your POWER SUPPLY has problems. major problems.

A melted zener is a SYMPTOM, you never pursued what melted the zener???

Just because you got sound didn't mean things were copacetic.

WHAT, EXACTLY, did you do to the power supply?
 
Hi Mark,
I got the unit this way. When it didn't power on, I opened it up and could see the 1850 diode was deformed right off. The correct ac was going in. I replaced the diode from a salvage board and the unit powered on correctly. I checked all the voltages out of the board after and the only two incorrect is pin 2 (29v) and pin 4 (3.6v). After perusing the schematic, I also determined that the transistor 2SD313 should reduce the voltage to twelve, nothing else in the circuit. So I am buying all new transistors today and will replace them. Does this sound like a decent plan?
 
I will test pin 7 tonight. My diagram doesn't show what should be coming out of pin 7, so thanks for the info. I am at work, so I have to do it later.
 
Okay...I am back. First and foremost, let me thank you for all your help!. I had to buy a new multimeter yesterday, so now I have some more accurate readings. Here are the voltages out of PS

P1 = gnd
P2 = 28 VDC (supposed to be 12)
P3 = 27.64 VDC (AUX not used)
P4 = 5.35 VDC (4.5VDC)
P5 = 25.63 VDC (32VDC)
P6 = 26.95 VDC (31VDC)
P7 = 28.54 VDC (35VDC)
P8 = 38.38 VAC
P9 =
P10 = 38.36 VAC
P11 = gnd
P12 = 32.00 VAC
P13 = 31.97 VAC
P14 = 42.8 VDC (44v)
P15 = -42.7 VDC (-44v)
P16 = gnd
P17 = -43.1 VDC (-45v)
P18 =
P19 =
P20 =
P21 =
P22 =
I am getting 29VDC at junction of r12 and q3 (2SD313) and the same at the emitter side which tells me the transistor is not reducing the voltage to 12v. I am having trouble finding these transistors on mouser. Does anyone have an equivalent or better for all three? (2SD234P, 2SC373 and 2SD313). Any help would be appreciated.
 
You should have asked us about replacements :D

That's our stock in trade (parts info, not parts)!!

2sd313, 2sd234 = mouser.com 512-KSC2073TU npn power transistor 4MHz
2sc373 mouser.com 512-KSC2383YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17

one is ridiculous overkill, the other doesn't have as high of a current rating (4a) as the original, BUT it (1.5a) will handle the load just fine.
Others MIGHT advocate the mje15032, but that is a 30MHZ part, and in a unit that old there might be instability problems DESPITE the 100pf neutralization cap in the circuit (inadequate for a transistor THAT fast?)

Q3 may be the only real problem, but those transistors are inexpensive - so why not deal a whole new hand for this regulator...
 
Mark,
Thanks for responding...I am terribly sorry for your loss. My sincere condolences go out to you and your family.
Thank you for the info. I was contemplating doing the complete board rebuild anyway. I just needed a little push. I did manage to get the receiver running good. i got the lights problem figured out. Turns out it was the dimmer switch! I pushed the button a few times and I would get a flicker occasionally. Did some continuity checks and found the switch was shorted between pin 1,2,and 3. I have never had a switch get shorted in my life, maybe an O.L., but not shorted. Anyway I had a replacement I bought from a lot on ebay and now the lights work. Go figure. The only thing left is the power from pin 2 and 4 then I should have FM and done! I ran it all day through AUX input and no issues. Thanks again for your help.
 
Also I had better order another diode (1S1850) since that was the cause of my initial errors. mouser can't find that one either. Is there a replacement?
 
Mark, just to be sure, I can use the 512-KSC2073TU npn power transistor for both 2sd313 and 2sd234? Just checking...
 
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