Almost finished a Receiver repair, then disaster strikes. What to do?

[Donguri]

Just read previous posts again and realized I've might got this wrong, discrepancy in Voltage drop over resistors doesn't mean that the resistor itself is bad, just an indication of something drawing more current ahead from the resistor, correct?
Maybe one of the old Q506 / Q505 2SA844´s are bad, even though they are measuring identical voltages...??

 

[elnaldo]

Voltages at +B5 and -B5 are clamped to +- 15V by the 15 V zener diodes D17 D18. Any voltage higher than 15 V will be dropped at R12 R13 R11 R10. But, the zener diodes need some current to work, so the voltage reaching R10 R11 must be higher than 15V, probably at least 18V to work as expected.

A 40W bulb is pretty small. If the unit is not shorted, I think you need to try a larger bulb and then full voltage to check for proper operation.

 

[Donguri]

I see, when I measured the voltage drop across those R10 / R11 resistors a couple of days ago with a 60W bulb the voltage that reached them was around 20V but the discrepancy was still present, but will get on it tomorrow and try, will have some more time to troubleshoot.

 

[Donguri]

You were completely correct! Went full voltage today and it definitely did even things out, so all that troubleshooting was for nothing but at least learned a lot in the process.
I´m on a 240V line here in Sweden so that might be why voltages are a bit high.

Results:
+ / - B1: 35V (SM states 32V)
+ / - B2: 35V

-B5 C13: -15.29V (SM -15V)
+B5 C12: +14.91V
Guess this is acceptable?

However the small mV in the Pre amp is still present, should I still pay attention to this or just move along to the Main amp?
Could the any of Caps in the pre amp be the cause? Haven't checked the Q502/504/505/506 out of circuit.
I did measure ohm on most of the resistors in the pre amp but found nothing too obvious.

 

[elnaldo]

A few mV offset from 0V could be something as simple as a corroded screw not making perfect contact between a board and chassis. Or, a small voltage between the board and the point you are taking as ground reference. I'd check how each board is connected to the PS ground, and how PS ground is connected to chassis.

14.9 or 15.2 V are within the zener tolerances, but it could be also a small offset caused by the mentioned issue, voltage differences between ground points supposed to be firmly connected and at the same potential.

 

[Donguri]

I'd check how each board is connected to the PS ground, and how PS ground is connected to chassis.

Ok will try to check that to the best of my ability.

Since I disconnected the Main amp and can't test the sound I did a observation with a little 1 channel oscilloscope at C515 / C516 Pre amp output with a 1Khz sine wave, it seems like both channels produces some jumping/pulsing in the waveform, maybe the 1845´s are to weak..

I want to try to calculate the mA in the circuit at the transistors to see if the 1845´s 50mA is enough, have to try to read up on how to do that, it is a bit complicated for me atm from what I've found.

 

[elnaldo]

I' d assemble everything and test the amp again. With DBT first, and if it's not shorted, give full voltage and a test signal without a load first.

 

[gadget73]

see if the output of the preamp is still doing the goofy pulsing with it at full voltage before putting those coupling caps back in.


measure the voltage drop across the emitter resistor at each transistor. Volts / ohms = current. If its greater than 0.05, then you need a transistor that can deal with more than 50ma.

 

[Donguri]

Got straight on it.

see if the output of the preamp is still doing the goofy pulsing with it at full voltage before putting those coupling caps back in.

If you mean the waveform jumping around, just tried on full voltage 1khz signal at those output capacitors C515/516.
It does jump around but I believe it might be more me not understanding how to operate a scope properly yet, and it is a cheap scope as well.
With a 2V signal feed and volume turned on low it does jump around but as soon as I reach 9 o'clock on volume and the waveform becomes bigger it stabilizes and is more or less consistent.
Does this sound ok?

Volts / ohms = current

Thanks for this info, am I calculating this correct?:
R513 0.773 minus 0.703 = 0.07 divided by 330 = 0,0002

At Q505/506 Emitters there is a Diode + a resistor connected, should voltage drop be calculated over just the resistor or Diode and Resistor combined?

 

[Skywatcher]

BTW, just to mention,
Wasn't sure if these Blue caps in C509/10 &C511/12 where generic ones or Low leakage?
Replaced them with Nichicon KT and FG But maybe they should be LL?
Any way to measure capacitor leakage in circuit?
View attachment 3332449

Replace every one of those blue electrolytic capacitors you find. They are well known to be leakers and corrode. I normally use Nichicon UKL (low leakage), but your suggestions will also work fine. Although I don't personally care for the "sound" from the FG series, you may find them very acceptable. Anything in place of those blue ones is an improvement.

 

[gadget73]

not sure what you're subtracting there but 0.77 volts across 330 ohms is 2.3 milliamps. Well within the capability of a 50ma transistor.

Just measure across the resistor directly and use that voltage. That will show you the current flow.


Do you have the ground lead on the scope connected to the amp you're working on? It will show you strange things if not.

 

[Donguri]

Replace every one of those blue electrolytic capacitors you find

Thanks for chiming in.
Indeed those blue ones measured a lot out of tolerance.
Been a bit unsure of using UKL in too many positions because of couple of comments sounding sterile, but actually don't have too much experience myself so might try it I don't like the sound.

Just measure across the resistor directly and use that voltage

Aaaah, was measuring each side to ground, Thank you for clearing that up.
So R513 was 0.070V over the resistor so this would be 0.070 / 330 = 0.00021?
So it is even lower, 0.21mA? Checks out anyway.

And in the case of Q505 measure over just the resistor as well not over diode?

Do you have the ground lead on the scope connected to the amp you're working on?

Yes I got it connected to chassis ground.
Tried to find something wrong with the ground points (bc of the -100mV) as recommended but visually nothing stood out.

 

[Donguri]

Connected the input caps to the Main amp and tried on a 60W bulb, no shorts!
So that problem is sorted.

But there is still a problem in the Main amp:
There is a voltage swing present on multiple transistors and at the Base of Bias transistor Q415 it starts with sometimes 4-5V, sometimes much higher, and trickling down to 0.4 - 1V, the other good Channel has just a 30 - 40mV on the Base from the start.
The new installed Bias Bourns trimpot doesn't affect the voltage at all..

Anyone see any potential culprit?
Measured with a 60W bulb:

 

[Donguri]

Drawing up the values on the schematic, to me it seems like Bias transistor Q415 is the "spider in the web"
Is it possible to remove this transistor out of circuit and measure the surrounding Transistors? (ON DBT)
Or will it endanger the surrounding components?

 

[Skywatcher]

Drawing up the values on the schematic, to me it seems like Bias transistor Q415 is the "spider in the web"
Is it possible to remove this transistor out of circuit and measure the surrounding Transistors? (ON DBT)
Or will it endanger the surrounding components?

No

 

[Donguri]

Ok, got it.
Will remove it and put a replacement in place instead.

 

[Donguri]

Received and installed new Bias transitor (a BC517-D74Z), no change.
Desoldered Driver Q417 and it tests good on the small tester.
Replaced the D407 MV12 Varistor with 2 x 1N4148, no change.
Voltages are still jumping/swings...
Is it jumping because the Output transistors are disconnected or shouldn't it matter?

Not sure how to proceed.

 

[Donguri]

Update on this unit, after installing the Output transistors the voltage swings measured in the Main amp disappeared.
So in conclusion the only problem was probably some bad soldering job by me in the Pre-amp that caused it...

If anyone tries to replace the MV12 Varistor with 2 x 1N4148: In this case the legs of the new ones was too short to reach the holes so what I did was to cut another leg from a diode and with one strain of a thin copper wire from an old chord, wire the legs together tight for support and solder them together, a bit tedious but seemed to work.

Thanks to everyone that contributed