Newbie with tricky SX-790 needs advice

brainbanger

Member
Hi guys,
just a short intro: I am from Berlin, Germany and in my mid-50s. The latter might be the reason why I started digging into vintage audio receivers and it developed into being my main hobby now. I started with a SX-535 and found this forum with tons of hints and tips and was able to fix the unit. I am very pleased with its sound but nevertheless went on with a SX-790, which now is the reason for posting here for the first time. Enough said, what's my problem?

I managed to find and fix a number of dead joints, a shorted Q20 (did replace Q25 and Q19 as well), a dead STK-0050 and one of its .22 ohm resistors, cleaned and deoxited the unit, cross-checked the important DC readings and - the thing died 10 minutes w/o being challenged (IPod via AUX on low volume). :scratch2:

After re-checking everything and reading all relevant posts here I ordered a new pair of Darlingtons (the just implanted ones are dead) and transistors Q11 -Q16 (just in case). Everything else showed no abnormal readings or shortages (incl. diodes, various resistors, etc.). Just finished the soldering job and the unit works ok - BUT: the DC readings make me crazy! Where there should be around 39 volts, I have +47.3 an -47.3! Pin 38 measures 12.5 Volts?!? So what is going on here?

Note: It is a Euro version with 240VAC. Everything else looks good to me, DC from the transformer reads around 40 Volts and nothing overheats.

Can someone please guide or advise me what to do? I am meanwhile used to soldering and metering, SX-780 SM and scheme are with me.

Many thanks in advance,
Andreas
 
what voltages are at pins 0 and 1 ?
also how many volts at vcc2 ?
welcome to ak ...
i looked for the manual but couldn't see it ..if you have a link could you please post it ?
max voltage for the stk is 53v by the way .. but should be as you stated closer to 39v ..
and did you check those 0.22 resistors again ?
and could you measure voltages across each 0.22 ohm resistor ..
 
What pins are you referring to?
Board PS? Or Amp? Bias or Offset?
I have a little experience but never worked on the 780-790 set.
Can the 790 SM be used in place of the 780?
 
Wow, that's what I call quick replies! :thmbsp:

@petehall347,
which pins do you mean, 0 and 1 at the modules or on the board (which one)? VCC2 is where ? Can't find it on the SM, sorry.
Resistors have been checked ok, will measure the voltages first thing tomorrow (quite late over here) and post them asap.

@zebulon1,
SX-790 ist the black dialplate version of SX-780, the interior is the same. Ah, and I am still a newbie to electronics so bare with me and let me Google what bias and offset mean :scratch2: I measured with the DMM on the Amp-board. A number of voltages from the SM do not match, i.e. R1 which is 26.4v at pin 12 and 50.4v at pin 13...

I will post the usual suspect's voltage / readings tomorrow. Maybe I am too worried only...
 
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yes pins 0 and 1 ..these are the input pins ..
the other voltages are the ones marked on the schematic +39.5 and -39.5
 
The ones marked with 39.5/-39.5 in the schematic measure 47.3 / -47.3 in my unit. Thats my observation in general, nearly all voltages are 15-20% higher than shown in the schematic. :sigh:

I will measure 0 and 1 tomorrow, but tend to remember that they've been at 40v or so. Will verify and then post.
 
0 and 1 should be +- 1.4v ...if they are high its normally a bad stk ..
get the voltages for all pins and note whether plus + or minus-
before anything check your line voltage and voltage selector is set for your voltage ..i suspect its running on too high voltage ..
 
Line voltage is 233VAC, voltage selector is at 240V. When changed to 220V, voltage readings go even higher in the whole unit, so I stay with 240V.

Left channel module readings:
1 = -1.17
2 = -40.1
3 = -0.01
8 = -3.7
9 = +40.1
0 = +1.14

Right channel module readings:
1 = -1.57
2 = -40.1
3 = -3.7
8 = -5.3
9 = +40.1
0 = +0.77

btw: I've got two modules with different numbers in the upper right corner: Left channel is "87ET", right channel is "24ET". The blown modules I exchanged were both "24ET".

Matches also my listening experience: the right channel actually sounds 'muffed' and is at a lower volume than the left one. (tested with Phono, FM and AUX inputs)
 
q16 may have a problem ..be sure its in correct way round and tests good . if you could get voltages for q16 b c and e it should help .
i do have some doubt about quality of the stk packs .
could you also get voltages for dc offset ..
 
I'll verify that the 790 is the black dialplate version of the 780, as i have one. USE THE 780 MANUALS. HOWEVER, The manual that is out there is for the US Model (KU). There is supposed to be an additional manual for the KC(Canadian 120V, Export 110-240V, and US Military 110-240V) but I've never seen it.

I'd look at the power supply if all your voltages are 10-15% higher.

Here's the S/SG Schematic.
 

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supply is looking ok now at +- 40v ish at pins 9 and 2 ..
i forgot to ask again for voltage across the 0.22 ohm resistors ..red lead one side black the other ..expect around 10 to 20 mv ..
 
New developments - back to zero...

The unit worked ok last night for about 3-4 hours. Low volume FM listening, with the mentioned weaker right channel (barely to notice when in stereo mode and balance is centered). Switched it off and went to sleep.

This morning I switched it on, relay clicked and FM played. Nothing changed from yesterday. Then I accidently kicked the small table the unit is placed on and - off goes the music!!!

Relay does not click any more, dial lamps are still on but the green 'source'-LED is now off. :tears:

Supply is live +39.7/-39.7, modules measured the same results as posted. Differences observed so far: Pin 38 should read 13V, was 12.5 and is now only 3.7V! The readings at Q25 changed as well: B=+3.9, C=+47.2, E=+3.2. That's way off from what it should be, right?

Relay in was measured 51.3V, but Pin 3 at the PA3004 circuit is 0.0, so there will be no 'click'.

The four 0.22ohm resistors in front of the STKs are still working but mV readings differ: left channel both at 2.7mV, right channel both 0.0mV...

Seems like an opened can of worms. What should I tackle first and how?
 
Clean the FUNCTION SWITCH (AM/FM/AUX/PHONO), and double check the solder joints on the POWER SUPPLY DIODES, and the VOLTAGE REULATORS(Q19,20,22) and Q21, Q25(2sd712).

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Huu, it proofs to be that can of worms, some more wiered things came to light.

I deoxited all pots and Switches yesterday for the second time, so (hopefully) no need to do that again?

Diodes D10-D13 and D6-D8 tested and working, joints look good, too.

I replaced the usual suspects Q19, Q20 and Q25 already before the Trouble started. Q25 got an improved heatsink as well. Those are ok incl. solder joints.

Then I checked Q21 and Q22. Q21 should be 2SC945A but I found a C548C instead? It was fitted somewhat strange, as its layout of CBE needs to fit in BCE order. Anyway: this guy IS dead and is not even recognized as a transistor by my MTester (I use it to inspect and rate every part I take out from the unit - very handy). Q22 is tested ok, maybe I replace it as well as it is desoldered.

But now for the 'new worm': When testing D3 and D4 I saw that the part supposed to be D4 is no Zener-diode!?! Looks like a standard black diode to me and the measures are irritating as well. No short for the DMM diode-test but similar readings for cathode/anode and vice versa on Ohms. Someone worked on this unit before and I already spotted a socketed protection relay, which is not standard as well. But why did he use a wrong diode?

Questions now: D3 Looks good but I need to replace D4 with something 'Zener-like'. Also I Need to get a 2SC945A replacement. Proposals for both parts welcome (I am already a Mouser customer...).

What else to look for? Should I replace the damaged parts and fire the box up? (yeah, I know: DBT is key, will have to put one together before going on...).

I am still bothered why this thing went dead when I bumped the table leg? Looks more like a shorted pin to me? Is it possible that there is a grounding issue somewhere?
 
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d4 looks like a 14v zener .
that replacement transistor is no use there as its 36v with 40v on it ..
looks like you are best to check everything is the correct part and installed correctly .
and be sure to check q12 q14 q16 .
 
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D4 is ok 'as is'? The solder joints seem to be original. What about the readings (460/560 ohm)? Is it normal for a Zener to be working in both directions? I'll replace it anyway, just curious.

Q12/14/16 were replaced together with Q11/13/15. New parts used:

Q11/12: 2SC1845
Q13/14: 2SD669AL
Q14/15: 2SB649AL

I will revisit the whole unit and try to find out what was worked on before...
DBT will be available before I turn the unit on next time. :D
 
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