Vintage Amp/Receiver Onkyo Dynamic Four repair - 15Vdc offset on both channels.

[carbono.silicio]

Hello all,

I've been restoring two 1970 Onkyo Dynamic Four Amp/Receiver. One of them it has a few issues with the FM tunner that often fails to stay tunned (AM ok), but after replacing all Electrolitic caps on the Tunner, pre-amp board and main amplifier board, i've noticed that on both channels i get +- 15Vdc reading on the multimeter.

I've isolated the main amp board from the others, changed the transistors except the power ones's, checked all resistors, diodes, caps, tracks but still i get +/-15Vdc offset.

The other Onkyo hasn't been touched yeat, but i also get +/-18Vdc off set at speaker output's.

Test was done without a load, Aux input selected, Volume at a minimum, Balanced and Bass/Treble also at mid.

My question maybe foolish but here it is:

Is there any specific situation in old Audio amplifiers where this DC offset voltage is normal?

Thanks in advance.

[floobydust]

Seeing DC voltage at the loudspeaker outputs, with no load, is normal here. The amplifier uses output coupling capacitors C511/C611 which have small leakage current and with no load you will read a voltage at the loudspeaker/headphone outputs. If you put a small load like 1k-10k ohms across the speaker outputs, the offset voltage should go down to zero unless the caps have failed.
If the amplifier is healthy, the output (transistor Q505/Q605 collector heatsink ) should be at 1/2 B+ for around 7V.
You are measuring +15V which is not good, there are further problems.

[carbono.silicio]

Thank you for your kind repply.

Yes, there's 15Vdc at Q505/Q605 collectors, but B+Hi is 34Vdc and B+Mid is 18Vdc, how do you get the 7V?

Seeing DC voltage at the loudspeaker outputs, with no load, is normal here. The amplifier uses output coupling capacitors C511/C611 which have small leakage current and with no load you will read a voltage at the loudspeaker/headphone outputs. If you put a small load like 1k-10k ohms across the speaker outputs, the offset voltage should go down to zero unless the caps have failed.
If the amplifier is healthy, the output (transistor Q505/Q605 collector heatsink ) should be at 1/2 B+ for around 7V.
You are measuring +15V which is not good, there are further problems.

[floobydust]

It looked like a 1, not 3 for 34V B+ on the schematic.
So everything looks OK if you have 17V at the output before the coupling cap. I think their leakage and charging current will give a misleading multimeter voltage reading when there is no load. Try connect a resistor or loudspeaker load.

[carbono.silicio]

Yes I did what you suggested and connected a 4,7K resistor to the speaker output and the voltage dropped to a few mV and stayed like that even after turning the amplifier off and on again. Interesting.

I've always heard stories of doom and gloom about the DC Offset in speaker outputs but I guess there's a lot more to learn about it from my part.

So everything seems ok now with the Amp, I can now focus on the FM tuner malfunction.

[floobydust]

[...] I've always heard stories of doom and gloom about the DC Offset in speaker outputs but I guess there's a lot more to learn about it from my part. [...]

Careful - DC voltage at the loudspeaker terminals indicates a problem with modern power amplifiers. If it is over say 1V, then there is a serious problem with the amplifier and it needs repair. Many AV receivers fail stuck in PROTECT mode with shorted output transistors.
BUT your amplifier is from the 1970's where they used output coupling capacitors, which block any DC from reaching the loudspeaker. So blown output transistors cannot harm the loudspeaker and there is no need for a protect circuit/relay. Small chip amps are also like this.

The FM front end and IF is not easy to troubleshoot without sophisticated test equipment.
I would use a multimeter and measure each transistor's DC voltages, and the AGC and AFC voltages to see if a transistor has failed. You will also hear noise on the radio as you probe around - unless you are ahead of a bad stage.

[carbono.silicio]

Understood!


In FM when trying to tune a station sometimes it fixes nicely but a few seconds letter it's off, and continues on and off... It's sort of an intermittent tuning. 

I've tested all transistors in the tuner board and they seem ok, but on the schematic there's an indication of 9V e 12V on the top left of the page and i have no 12V, 9V ok though.

[wasedadoc]

What exactly happens with the FM tuner?  Does it stop receiving anything or does it go "off-tune"? Meaning if you turn the tuning knob do you get reception again?  If yes, investigate the AFC circuit (Automatic Frequency Control) that feeds back a dc signal from the FM detector to the local oscillator.

[carbono.silicio]

It tunes but looses it suddenly an then tunes again, loses, tunes, and so on... I don't have to move the dial to tune it again. It happens in all stations and only FM.

AM and TAPE are fine.

[carbono.silicio]

Hello all,

I made a video trying to FM tune it. What's weird is that sometimes it randomly tunes a station and produces nice and clean sound, but it only lasts a coupe of minutes.


https://youtu.be/WqrWxo4WyEM


According with schematics it should have 12V on the AGC pin but there's only 1V. On the other twin Onkyo i have here to start recaping, FM works ok and there's also 1V and not 12V.

[floobydust]

AGC voltage more likely "1.2V" the schematic has a fuzzy decimal point.
What is the signal strength meter doing? The FM Stereo lamp is always on dim? It sorta sounds like there is a noisy transistor in the IF amp that fools the AGC into thinking there is a strong signal present - for a while. Wild guess it looks like an old transistor has failed in the IF or less likely an electrolytic capacitor has shorted maybe C201, C209.
I would apply a little heat to say Q200, Q201 to flush out a dud transistor, or as I previously said measure the DCV on a few transistors and see what is going on.

[carbono.silicio]

Yes, you're right, probably is 1,2V

Yes the FM Stereo lamp is always on, dimmed but on. Capacitors are not in fault, they were all replaced by new ones.

Yes, tomorrow i'l start with the transistors. I'll post voltages and replace one by one, testing them in and out circuit.

AGC voltage more likely "1.2V" the schematic has a fuzzy decimal point.
What is the signal strength meter doing? The FM Stereo lamp is always on dim? It sorta sounds like there is a noisy transistor in the IF amp that fools the AGC into thinking there is a strong signal present - for a while. Wild guess it looks like an old transistor has failed in the IF or less likely an electrolytic capacitor has shorted maybe C201, C209.
I would apply a little heat to say Q200, Q201 to flush out a dud transistor, or as I previously said measure the DCV on a few transistors and see what is going on.

[carbono.silicio]

Before making some transistor replacement tests, here's the voltage readings on both Onkyo.

[floobydust]

Most of the voltages look higher on the repair unit, so I would first make sure the two zener regulated rails are ok. D900 for 18V and D207 for 9V.
Another important value is the B-E voltage, try subtracting to get that number in your chart.
The low E-B voltage for Q102 yet higher current seems odd, I would start with Q102.

[carbono.silicio]

The zener regulated 18V and 9V are ok.

Replaced Q200, Q201 and Q202 with exactly the same type luckily i had here in stock, trying one by one, but no change has happen,
but then Q103, Q102, Q101 and Q100 i took them from the good Onkyo and replaced them on the bad one and it's much better but still has some noise and fails, less often though.

I wonder if Q001 and Q002 inside the FM front end chassis are in need to be replaced also!!?

The Q100/1/2/3 taken out do not test bad on the multimeter in diode mode but for sure something is not right with them.

[carbono.silicio]

Yes, Q102 (2SC772) is not within diode drop specs.

[carbono.silicio]

So, here's what happened:

FM is finally tuning great.

By experimenting installing Q100, Q101, Q102 and Q103 taken from the good Onkyo, initially it tuned much better but still with some noise and fails but somehow it all faded and now the sound is clear.

The "bad" transistors are not bad after all, they were installed in the donor good Onkyo and surprise! it works very well.

One think new is that the AFC switch behaves differently with a clear difference between On and OFF while tuning stations. Before there were no audible difference.

My conclusion: Maybe bad solder joints!? very weird because all pins were inspected and resoldered before.

Anyway, the receiver is now tuned and i'm letting him play for a few hours and ear if there's any change.

Thank you, floobydust and wasedadoc for yout kind help.