Problem with Vintage Pioneer receiver SX-1000TW Output

[Jimkarl]

So it does. There is no explanation in the text for what the value in parentheses means. My guess is that it's the voltage with the amplifier driving a heavy load - for example, operating at the maximum rated output power of 50W into 8Ω per channel.

If you want, try to duplicate the reading. But be careful because something would have to be drawing some significant current out of the power supply to pull the voltage down that low, so watch out for components getting hot!

Well, the final part of the amplifier, involving the right-most three transistors in each channel on the circuit board as well as the output transistors, is a single block.

Once you've checked the power supply output voltages, you can test Q811/812 by checking for about 23V on their emitters, and if that voltage is right, you can check Q801/802 by confirming that the collector voltage is somewhere in the range 10~20V (I can't tell you an exact figure). You could also check that there is audio on the collector of Q801/802 but if the DC voltage is right, it's pretty likely that it will be amplifying properly.

The four 1N60 diodes are germanium types but this is not critical; they can be replaced with 1N914/1N4148 types.

I confirmed the power supply board voltages. With pins 5 & 7 disconnected from the main amp circuits, I see the following:
Pin 2 - 70ac/31.7 dc
Pin 3 - 70ac/31.7dc
Pin 4 - 0
pin 5 - 63.7
pin 6 - 33.2
pin 7 - 40
pin 8 - 13
pin 9 - 18.7ac
pin 10 - 19.3ac
pin 11 - 22.9

That was all under the bulb limiter connection. I briefly connected direct to ac and noted pin 5 was slightly over 1V higher - basically 65V. Went back to the limiter connection and saw 63.7 again. I didn't check the other pins when out of limiter circuit/connection, but I assume a minor value changes there as well and that the bulb has an impact on the voltage/current going at the unit. That's the whole point right? to defer some current draw to the bulb instead.

Given the two 31.7v feeds from the the transformer coming in on pin 2 & 3, when summed they match the 63.7 output on pin 5. The power supply board circuit schematic is about half way down this thread. Is there something in the circuit that should be boosting the DC voltage for pin 5 higher the sum of the two inputs on pin 2&3?

I'm left with 4 possibilities:
1) Some part of that power supply circuit that is supposed to boost that voltage has failed (correct me if I'm wrong, but I don't see anything in the circuit path to pin 5 that would boost voltage)
2) Some part of that circuit is chewing up current and bringing the input voltages down (anything to measure here to verify?)
3) Transformer has degraded and is failing to put out proper voltages (but only on those two rails). Possibly but seems remote. Maybe some how stressed because of the previous failure, but I'm not sure how a slightly lower voltage failure would occur. Short in the winding someplace?
4) the (64V) on the power supply block diagram note represents an unloaded (disconnected from amp board) output. But you're thinking that is a full load value instead.

 

[Jimkarl]

Once you've checked the power supply output voltages, you can test Q811/812 by checking for about 23V on their emitters, and if that voltage is right, you can check Q801/802 by confirming that the collector voltage is somewhere in the range 10~20V (I can't tell you an exact figure). You could also check that there is audio on the collector of Q801/802 but if the DC voltage is right, it's pretty likely that it will be amplifying properly.
.

For this, I assume you meant once I have all the parts for the whole board installed including the power resistors, since you didn't suggest a way to disconnect the latter half of the circuit. Just checking.

 

[KrisBlueNZ]

I confirmed the power supply board voltages. With pins 5 & 7 disconnected from the main amp circuits, I see the following:
Pin 2 - 70ac/31.7 dc; Pin 3 - 70ac/31.7dc; Pin 4 - 0; pin 5 - 63.7; pin 6 - 33.2; pin 7 - 40; pin 8 - 13; pin 9 - 18.7ac; pin 10 - 19.3ac; pin 11 - 22.9
That was all under the bulb limiter connection. I briefly connected direct to ac and noted pin 5 was slightly over 1V higher - basically 65V.

Ah, I see the reason. When you disconnect the wire from pin 7 of the power supply board, you disconnect the load (the amplifier output stage), but you also disconnect C12 and C13 (they're on the overview diagram - the first image in post #1 - in the bottom right area). These are the smoothing capacitors for the 82V rail. Without any smoothing capacitance, you will measure the mean (average) voltage of the rectified voltage from the transformer secondary.

To isolate the amplifier from the power supply, you should break the connection between C12/C13 and the amplifier circuitry - that is, break into the wire that runs from C12/C13 to Q1/Q2 collector and the two pins marked 6 on the amplifier board.

It's not a good idea to disconnect pin 5 of the power supply board from C12 and C13 because this could cause ripple on the 40V DC output on pin 7 of the power supply board, which is derived from the 82V rail by Q901.

You can disconnect pin 7 of the power supply board to disconnect the 40V rail from the amplifier if you want. It's just pin 5 where the designers have used this "trick" (read "trap").

Went back to the limiter connection and saw 63.7 again. I didn't check the other pins when out of limiter circuit/connection, but I assume a minor value changes there as well and that the bulb has an impact on the voltage/current going at the unit. That's the whole point right? to defer some current draw to the bulb instead.

To use the bulb to limit the current. The bulb is similar to a resistor. Current through it causes voltage to be dropped across it. If it's connected between a voltage source and a load, and the load has a fault and tries to draw too much current, or goes short circuit and tries to draw infinite current, the resistance of the bulb limits the maximum current that can flow - even if the bulb is forced to drop the entire power supply voltage, the current through it will be low enough that nothing will be damaged.

Given the two 31.7v feeds from the the transformer coming in on pin 2 & 3, when summed they match the 63.7 output on pin 5. The power supply board circuit schematic is about half way down this thread. Is there something in the circuit that should be boosting the DC voltage for pin 5 higher the sum of the two inputs on pin 2&3?

First, it is not meaningful to add the voltages on pins 2 and 3 together. I won't get into details because it would take a long time to explain. Do a web search for full wave rectification.

Re pin 5, yes, the smoothing capacitors "boost" the voltage. Smoothing capacitors charge up to the peak voltage of the full-wave-rectified sine wave if there is no load on them. This voltage seems to be around 86V. When a load is added, "ripple" appears on this voltage, and the mean voltage across the smoothing capacitors drops - to 82V.

Without any smoothing on pin 5, the voltage you measure will be less than the peak voltage. The mean voltage of a full-wave-rectified sinewave is about 64% of the peak voltage, which is about 55V. The reason you measure 64V instead of 55V could be the small amount of smoothing effect due to C901 via R902. Some web research will explain full-wave rectification, smoothing, and ripple in detail.

For this, I assume you meant once I have all the parts for the whole board installed including the power resistors, since you didn't suggest a way to disconnect the latter half of the circuit. Just checking.

Good question. Actually, if you want to test the first two stages before you've replaced the transistors in the output stage, just disconnect the +82V rail from the two pin sixes, and the collectors of Q1 and Q2. That will disable the output stage and allow you to test the voltages on the first two transistors. They are powered from the +40V rail.

 

[Jimkarl]

I knew about power filter caps and dc ripple to maintain a constant voltage but I missed the (now obvious) fact that they can boost the voltage too by taking the peak and averaging together. Another light bulb moment.

I had thought/hoped that 82V source could be disconnected from the second stage of the main amp, but didn't realize the intricacies of WHERE to cut the line. That backlash to the 40v line wasn't apparent to me if cut prior to the power caps. Now it is.

Good stuff. Looks like most of my parts don't show up for a couple of weeks, unlessthey overstated the delivery time, so you finally get a break from all my questions! Thanks.

 

[KrisBlueNZ]

LOL cool