I did replace the zener, I haven't replaced D3.
Pioneer SX-780 Voltage Regulator Trouble
- Thread starter CaseyWKC
- Start date
Are you sure you have a 14 volt zener in there? Can you read the part number on it?
I don't know if it matters here but the KSC1845 is not a generally accepted sub for the 2SC954A. It is an accepted sub for the 2SC945 (not "A")
The generally accepted sub for the 2SC945A is a KSC2383.
Do you have the original 2SC945A you could put back
I can't make out the part number I will pull it to confirm in a moment, but I am 99% sure it is the appropriate 14v. I have the original here as well.
diode part # is tzx 14
I was wrong, it wasn't an 1845 I put in a 945.
I replaced it with a KSC2383 anyways to no change.
I am getting a reading of 0 v on the emitter of Q22 2SA904A.
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Spent the morning going over joints again and taking some more measurements. Double checked all of these so maybe they'll shed some light on what is going on.
Input side of R305 measures 24v
Q22 - 2SA904A replaced by KSA992 :
E 0V
C -24.9v
B -.598v
Q21 - 2S945A replaced by KSC2383
E 8.2v
C 24.4v
B 8.74v
Q17 - 2SC1384
E -14.3v
C -18.7v
B 15v
Q18 - 2SA850
E -14.4v
C -19v
B -15v
Zener D4 (replaced with new zener)
Cathode 8.2v
Anode 0v
D3
Cathode 8.76v
Anode 8.21v
Should I have subbed a ksa1013 for 904A instead of the 992?
Thanks everybody.
Input side of R305 measures 24v
Q22 - 2SA904A replaced by KSA992 :
E 0V
C -24.9v
B -.598v
Q21 - 2S945A replaced by KSC2383
E 8.2v
C 24.4v
B 8.74v
Q17 - 2SC1384
E -14.3v
C -18.7v
B 15v
Q18 - 2SA850
E -14.4v
C -19v
B -15v
Zener D4 (replaced with new zener)
Cathode 8.2v
Anode 0v
D3
Cathode 8.76v
Anode 8.21v
Should I have subbed a ksa1013 for 904A instead of the 992?
Thanks everybody.
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I keep going back to the cathode voltage on D4. It should be standing at +14 volts.
In a previous post you said that the Emitter of Q19 has +25 volts, so plenty of voltage for that Zener to sit at 14 volts.
Have you checked the value of R316? If R316 is the correct value then kind of a flyer here but pull Q25 and then look at the power supply voltages again.
The cathode voltage of D4 is the reference voltage at the base of Q25. Q25 is the +13 volt supply regulator. Maybe Q25 is leaky/flaky dragging down the voltage?
With Q25 out the tuner will not have power and the protection circuit will not have power.
In a previous post you said that the Emitter of Q19 has +25 volts, so plenty of voltage for that Zener to sit at 14 volts.
Have you checked the value of R316? If R316 is the correct value then kind of a flyer here but pull Q25 and then look at the power supply voltages again.
The cathode voltage of D4 is the reference voltage at the base of Q25. Q25 is the +13 volt supply regulator. Maybe Q25 is leaky/flaky dragging down the voltage?
With Q25 out the tuner will not have power and the protection circuit will not have power.
I think that is the root of the problem. Originally the thread started with the complaint of the +21 and -23 volt supplies being low.
Those supplies rely on the + and - 39.5 supplies being correct.
Do you know if the 2SK34's can cause these kind of issues? I'm not 100% clear on what they do.
I think that is the root of the problem. Originally the thread started with the complaint of the +21 and -23 volt supplies being low.
Those supplies rely on the + and - 39.5 supplies being correct.
Do you know if the 2SK34's can cause these kind of issues? I'm not 100% clear on what they do.
Last reported voltages on the + and - regulators. Base voltages are low. Collector voltages a tad low but it is on a DBT.
Q19/2SD325R
B 24v
C 47v
E 25v
Q20/2SB536A
B -24v
C -50v
E -25v
The 2SK34s are functioning as current sources to tighten the regulation of the zener reference. With the gate tied to source like they are, they are supplying (sinking/sourcing) full IDSS. If they don't work, the regulator won't regulate, there won't be enough current flow through Q19/20. More precisely, if they aren't conducting there won't be enough current. If they were shorted, Q19/20 would be full on, giving the opposite problem.
My first step here would be to determine if the low voltages are due to:
1) Something at the rectifiers.
2) Overloading of the power supply (shorted component somewhere downstream)
3) Actual loss of regulation.
The first one should be easy to eliminate. Check the voltage at the rectifier side of R317/318.
Two. After checking that fusible resistors R317/318 are in spec, check the voltage drop across them. The regulated supply sources/sinks about 50mA, so the drop across those resistors should be no more than 100-150mV. Off DBT. Since we're not working on the power amp or main rectifiers/filters, there's really no reason to be on DBT.
If it's an overload, then removing the loads would be the next step. If it's not an overload, then assuming all of the new parts are good and not installed backwards, and nothing else was missed, then suspicion could be leveled at Q23/24. One would think anything pulling down the P/S that far would have let out its payload of magic smoke by now.
Though it would be really odd for both to fail identically and simultaneously.
My first step here would be to determine if the low voltages are due to:
1) Something at the rectifiers.
2) Overloading of the power supply (shorted component somewhere downstream)
3) Actual loss of regulation.
The first one should be easy to eliminate. Check the voltage at the rectifier side of R317/318.
Two. After checking that fusible resistors R317/318 are in spec, check the voltage drop across them. The regulated supply sources/sinks about 50mA, so the drop across those resistors should be no more than 100-150mV. Off DBT. Since we're not working on the power amp or main rectifiers/filters, there's really no reason to be on DBT.
If it's an overload, then removing the loads would be the next step. If it's not an overload, then assuming all of the new parts are good and not installed backwards, and nothing else was missed, then suspicion could be leveled at Q23/24. One would think anything pulling down the P/S that far would have let out its payload of magic smoke by now.
Though it would be really odd for both to fail identically and simultaneously.
Thanks for the info on the 2SK34's I will have to think on that for a bit.
As you have probably seen I post a lot of schematics and pictures. For better or for worse I lean heavily to the visual learner side of things.
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No smoke reported so far. 
After a little bit of eyeball wondering around the schematic I see where without the 2SK34's the pass transistors would not be turned on at all.
The voltage for the zener reference and the rest of the regulating "guts" comes from the output side of the transistor.
So I guess the 2SK's sort of jump start the pass transistor and the rest of the circuit then takes over.
After a little bit of eyeball wondering around the schematic I see where without the 2SK34's the pass transistors would not be turned on at all.
The voltage for the zener reference and the rest of the regulating "guts" comes from the output side of the transistor.
So I guess the 2SK's sort of jump start the pass transistor and the rest of the circuit then takes over.
Ok. Had to think about that one
They "jump start" the regulators, and hold the zener current to a fixed value, yielding tighter regulation.No smoke reported so far.
After a little bit of eyeball wondering around the schematic I see where without the 2SK34's the pass transistors would not be turned on at all.
The voltage for the zener reference and the rest of the regulating "guts" comes from the output side of the transistor.
So I guess the 2SK's sort of jump start the pass transistor and the rest of the circuit then takes over.
You guys are great.
I checked rectifiers, I have my -41 and 40v and -53 and 50v. All test good. Voltage drop across R317 and R318 is .829 and 740mv, they are 22 ohm resistors.
No shorted components have been found yet, I replaced the 14v zener to ensure my replacement was correct as well.
I think it comes down to the SK34's or Q22.
The 0v on q22 emitter and -.598v on base
2sk34 measurements are
Q23
D 50v
G 25.7v
S 25.6v
Q24
D -26v
G -52v
S -53v
Off of DBT going forward.
R316 measures at the 5.1k ohms it should.
I pulled Q25 and lo and behold 14v is there at the zener. The tuner didn't have power before this anyways, I only get the dial light illumination when the unit is powered on, no other lights or response from any meters regardless of input selection source.
Q25 is a new MJE15032G.
What happens to the other power supply voltages with Q25 out? + and - 39.5 and the +21 and -23?
WIth Q25 out the +13 volt supply will be non-existent, It sounds like the tuner was dead before we pulled Q25?
The +13 volt supply also powers the protection circuit so it won't pull in the relay. Was it pulling in the relay before?
The power meters are after the relay, so no click no sound no meters.
Lets poke around that 13 volt supply. Leave Q25 out for now
There is a large 5 watt 220 ohm resistor clipped to the heatsink. Check the voltage on both ends of it. Edit: and also check the value of it.
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With Q25 pulled I get the following
Q19 2SD325R
B 40v
C 52.4v
E 40.3v
Q20 2SB536A
B -40.7v
C -52v
E -40v
I measure 53.5 v across the 5W 220ohm ceramic resistor mounted to the heatsink.
Measurements from where Q25 was pulled
B 14.26v
C 53.5v
E -.027v
I did not get relay before, no lights other than the 3 bulbs illuminating the dial meter.
Double checked all orientation of caps and they are correct.