Help Pioneer SA8800 low power and clipping

[Marckx]

Dear all, i need help for this problem (i am quite good in integrated circuits and tubes, not in so complex transistor circuits).
I recap the amplifier, clean everything and check the sound live and was quite good. Then i wish to adjust the bias and to do some measurements. I apply with signal generator 1 khz 150 mv to both channels 8 ohm resistance load and connected oscilloscope 2 channels on output. With as much as 8V rmrs the protection relay open the circuit. And are only about 8W output power (so many 8!!) and as low as 5v rmrs the negative leg starts to clip. Both channels are identical.
then i had a look at the bias. DC balance was good but slight adjust make it better. Idle current is very low 4ma (instead of the 25) and no action on trimmers can do anything to change this value (again on both channels) changed trimmers (just to be sure), same results (both trimmers involved have no real effect on the voltage). (obviously i followed instructions, no load, no input, etc.)
i do not know if the problems are related, but still i do not know really from were to start. Having same issue on both channels i cannot just see the differences. I checked the power supply and seems ok.
thanks for help, i attach the scheme.
P.S. without load i can get 27 V rmrs before clipping (equivalent to 91W)

 

[markthefixer]

[IMG alt="markthefixer"]https://audiokarma.org/forums/data/avatars/m/6/6768.jpg?1443894819[/IMG]

markthefixer​

On Hiatus, hospitalized and NOT doing Clutches​

does this not mean ANYTHING to you?

he sent a PM to me

Marckx:
Hi, sorry to bother you, but i really need some help on this https://audiokarma.org/forums/index.php?threads/help-pioneer-sa8800-low-power-and-clipping.1045685/

I have no data, my thinking is fuzzy.

icant even read it laying in the bed looking sideways, it all jumbles up


if I said "no soup for you" you'd be in a severe Hurt.

i'm NOT sayingn it... you have no idea how hard this was to do, nor how insanely iong it took.

 

[avionic]

Dear all, i need help for this problem (i am quite good in integrated circuits and tubes, not in so complex transistor circuits).
I recap the amplifier, clean everything and check the sound live and was quite good. Then i wish to adjust the bias and to do some measurements. I apply with signal generator 1 khz 150 mv to both channels 8 ohm resistance load and connected oscilloscope 2 channels on output. With as much as 8V rmrs the protection relay open the circuit. And are only about 8W output power (so many 8!!) and as low as 5v rmrs the negative leg starts to clip. Both channels are identical.
then i had a look at the bias. DC balance was good but slight adjust make it better. Idle current is very low 4ma (instead of the 25) and
changed trimmers (just to be sure), same results (both trimmers involved have no real effect on the voltage). (obviously i followed instructions, no load, no input, etc.)
i do not know if the problems are related, but still i do not know really from were to start. Having same issue on both channels i cannot just see the differences. I checked the power supply and seems ok.
thanks for help, i attach the scheme.
P.S. without load i can get 27 V rmrs before clipping (equivalent to 91W)

What are your rail voltages on the output collectors ? Looks like you should have around + / - 50 vdc.

Having same issue on both channels

Both channels is indicative of a power supply problem....

 

[Marckx]

[IMG alt="markthefixer"]https://audiokarma.org/forums/data/avatars/m/6/6768.jpg?1443894819[/IMG]

markthefixer​

On Hiatus, hospitalized and NOT doing Clutches​

does this not mean ANYTHING to you?

he sent a PM to me



I have no data, my thinking is fuzzy.

icant even read it laying in the bed looking sideways, it all jumbles up


if I said "no soup for you" you'd be in a severe Hurt.

i'm NOT sayingn it... you have no idea how hard this was to do, nor how insanely iong it took.

I am deeply sorry, i overlooked this. My apologies. i wish you the best for your recovery, did not mean to be unpolite. My fault.

 

[Marckx]

What are your rail voltages on the output collectors ? Looks like you should have around + / - 50 vdc.


Both channels is indicative of a power supply problem....

Hi, thanks for answering at no load +49 -49 . At full load before protection circuit cut +46,5 - 47 V. The protection circuit start to work at about 9w of power under load (no load, no intervention)

 

[avionic]

Check your other power supply voltages.. The + / - 48vdc that feeds the power amplifiers front end.

 

[Marckx]

Check your other power supply voltages.. The + / - 48vdc that feeds the power amplifiers front end.

All power supplies are ok excluding the couple of voltage drop under the 8-9w load. I had recapped and changed few transistor known to be prone to give troubles. I am also wondering why i cannot set bias voltage, maybe with a very low bias 4mV, the point of work of final transistors is not correct. Quite surprised to have it on both channels. The amplifier seems to work, i have it from not long ago, but at low power levels (normally i do not push) i cannot see the defect. So i do not know if it was like this before or only now.

 

[Marckx]

Check your other power supply voltages.. The + / - 48vdc that feeds the power amplifiers front end.

I connected the amply in the audio chain and it sound as before, very good till few watts, then starting to get not so smooth sound around 10w indicator (but is already quite loud for my room). I Can further up the volume, distortion increase. So i connected the tone generator and the probe of scope on output with real loudspeaker load and i was able to see the protection at same level of power output (9W) but i listen to level of noise and it was very loud, so i am sure i never had that level of volume before, so i am quite sure the amplifier was like this when i bought it. I can always see that, even if not clipping, as on resistive load, the negative leg has more distortion, on sawtooth the upper corner is a corner, the lower is a curv. Now i have to understand what happened to this amplifier.

 

[Marckx]

I check some voltages on board and i see
Q15 collector -6,85 (should be -2,7)
Q17 collector +3,15 (should be 2,7)
Q21 collector -4,7 (should be -1,2)
acting on bias voltage trimmers give no changes, the zener D7 is good

 

[Marckx]

Check your other power supply voltages.. The + / - 48vdc that feeds the power amplifiers front end.

Any hint? All power supplies seems good.