QX-949 Power supply issue

[petehall347]

are all the negative power supply voltages correct and steady ?

 

[petehall347]

i think i missed the part about in 2 or 4 channel mode ..

 

[KeithD]

are all the negative power supply voltages correct and steady ?

All power supply voltages are within spec, except for -B3, which should be -42V and I'm getting -44.4V.

After more tests, I am only getting sound from two front channels on AUX and tape monitor settings.

No sound from all tuner settings.
No sound from either Phono setting.
No sound on rear outputs from a 4 channel source (only front has sound).
No sound at all from a two channel source with selector set to RM or SQ. Only get sound when selector is set to 2 Channel or CD-4.

Tuner has correct 13V power, -B4 has correct -13V.

This is a mystery.

 

[petehall347]

yes indeed it is strange ..
if all 4 channels have correct bias and offsets i first suggest a quick finger test on the inputs at the amp boards and see if you get a hum on the speakers . if so next troubleshoot phono board as easier than tuner board .. same finger test but this time on phono boards outputs with phono selected .
that should be enough for now .

 

[KeithD]

yes indeed it is strange ..
if all 4 channels have correct bias and offsets i first suggest a quick finger test on the inputs at the amp boards and see if you get a hum on the speakers . if so next troubleshoot phono board as easier than tuner board .. same finger test but this time on phono boards outputs with phono selected .
that should be enough for now .

I get hum on the front channels, but not rear.

 

[petehall347]

so must be .. or some other strange thing .. i remember seeing a switch for lower half of supply as shown on schematic for the other channels that are out .. not sure how the switch is routed now ..past my bedtime

 

[KeithD]

Ok, double checked this, and I do get hum on all four outputs of the power amp. Then checked control amp, and I do get hum on all four outputs of the control amp. However, when I touch the inputs of the control amp, I only get hum when touching the front inputs (pins 13 and 16). When I touch the rear inputs (pins 1 and 4), no hum. So, I assume this is a problem on the control amp board. I did recap that board and replace Q5-8 (2SA725) and Q9-12 (2SC1312). All was working after I did that about a week and a half ago. So, not sure what happened there to drop out both rear channels? Or, would this be something before the control amp (and not the control amp itself)? The next section before the control amp is the switch assembly, and that is a mess of wires. Further up are the quad signal processing boards. I suppose something could have gone bad on one of those???

 

[petehall347]

check to see if those inputs are shorted to ground or not .

 

[KeithD]

Found the problem. B3 connection to pin 3 on the power amp board, which powers the rear channels on that board was disconnected. This must have happened when I was closing things back up. Reconnected wire, and all is working. I can see why that would take out the rear channels, but why would that cause no sound from tuner or phono?

Now, back to the static problem...

Edit: sorry, meant to say control amp board, not power amp board where the B3 wire was disconnected.

 

[KeithD]

I am playing with the thought that the right rear channel static influences the other amps through the +30V or -42V supplies, most likely the latter since the -42V power supply has some output resistance of 1.5 ohms. Since you have replaced all transistors and el-caps, I can only think of a bad connection somewhere. What voltage do you measure on both sides of R33? Do you see fluctuating voltage there as well?

Back to tracking the static. At this point, the receiver will come out of protection only for a few seconds after turning on, then goes back into protection.

Measured voltages on either side of R33: 27.9V on side connected to Q7 base, 2.05V on other side. Both sides voltage is variable.

Also checked VR1 again. Pin that should be negative is measuring +0.022V. This side is connected to R13. Other side of R13 is measuring -2.22V and very steady. Measured R13 on the A power amp board, and it is also -2.22V steady, but other side connected to VR1 reads -0.05V. Does this indicate R13 is bad on the B board?

 

[petehall347]

yes ..its worth checking it to at least rule it out .
think you measured r31 and r33 .. or something is strange around there .. r33 might have drifted high

 

[Unjoy]

On R33, I think you have voltages swapped? Q7 base should be around 2V, the other side of R33 should be 19V. 27.9V is not ok. Check R33 and R31?

 

[KeithD]

Yes, sorry, I did type those in backwards. Measured again and about 2.2V on side connected to Q7 base, about 28.2 V on other side. R31 has 43.4 V on side connected to pins 11/12, 28.2 V on other side. OK, so that is not good, B3 voltage for some reason is now at 43.4V!

 

[petehall347]

regulator transistor shorted or something else on power board driving it hard . too late to start looking at schem right now .
need to go through the thread again in daylight as its groundhog day to me right now ..

 

[KeithD]

Here are voltage readings on Q1-4 on power amp:

E, C, B
Q1: 43.3, 45.5, 44.7
Q2: 44.7, 45.5, 45.4
Q3: 13, 45.3, 13.2
Q4: 13.5, 30.5, 14.1

Voltage on Q2 E/Q1 B is too high (should be ~33V). Voltage on Q2 B/Q3 C is also too high (should be ~33V). Will need to look at R4 and R5. Acting like they are both shorted since the voltage is not dropping to 33V from 45V at D2 & 3.

 

[KeithD]

Resistors OK. Had an idea that maybe amp board was causing problem. Disconnected wire between B3 and power amp, and B3 voltage was back at +30V. So, again, we see some fault on the power amp as the cause of multiple issues.

 

[petehall347]

yes . something there for sure .

 

[Unjoy]

Check for a short between pin 11/12 and 13, or between pin 15/16 and 14?

 

[KeithD]

When I connected B3 back to the power amps, voltage was back where it was supposed to be! That was very strange and no idea what caused it.

Pulled R13 and checked resistance, it is within spec at about 99k. Also checked R31 and 33, both in spec.

At this point, all supply voltages to the power amp board are as before. However, can not get VR1 on B board to get pin 5B (RR output) below about 0.2 V. I can adjust all other channel VRs to zero, but with lots of variability in the signal. At this point, the unit does not come out of protection.

Just wondering what would be helpful at this point to narrow down the fault? I continue to believe there is something failing on the RR channel of the B board, but what? I have replaced all e-caps and transistors. Only things left are resistors, ceramic caps, one diode (D1), and coil (L1).

 

[Unjoy]

Ok, so all transistors and e-caps replaced. You also ran the amp with Q13 and Q15 disconnected and still noticed the static in the remaining operational amps. It will not hurt to replace D1 (or swap with D3). Still, I would start with giving the board a good visual inspection. The higher voltages you measured could be caused by something not ok on that board.