Hello again (being stuck at home sure frees up time to work on other things 😀)
I decided to get some work done on the sony TA-F770ES that had been sitting for a while. The symptoms were that the led's on the front were not working and tone controls had no effect.
Turns out someone unplugged the led for the volume knob that is actually crucial for the rest of the circuit to work ok. I plugged it back in witch fixed the led problem and got the amp out of direct source mode thus giving me access to the tone controls. That's where things got a little more complicated and it also explains why the led was unplugged in the first place (this forces the amp to be in source direct mode all the time). With the tone controls activated and the volume up to about 10W into 8ohms a pretty nasty oscillation appears when turning the treble down, if I go up in power output the oscillations kicks in faster and at full power I even get oscillation when treble is at 0. Turning treble up doesn't produce oscillation until I get very close to max power. It is present on both channels although it doesn't always appear exactly at the same time (volume level).
Here's the schematic for one channel :
I decided to get some work done on the sony TA-F770ES that had been sitting for a while. The symptoms were that the led's on the front were not working and tone controls had no effect.
Turns out someone unplugged the led for the volume knob that is actually crucial for the rest of the circuit to work ok. I plugged it back in witch fixed the led problem and got the amp out of direct source mode thus giving me access to the tone controls. That's where things got a little more complicated and it also explains why the led was unplugged in the first place (this forces the amp to be in source direct mode all the time). With the tone controls activated and the volume up to about 10W into 8ohms a pretty nasty oscillation appears when turning the treble down, if I go up in power output the oscillations kicks in faster and at full power I even get oscillation when treble is at 0. Turning treble up doesn't produce oscillation until I get very close to max power. It is present on both channels although it doesn't always appear exactly at the same time (volume level).
Here's the schematic for one channel :
Some more info that might help, touching both input or output caps makes the oscillation worse (especially the input cap). Oscillation is present also before the input cap but disappears when in source direct mode. I measure 0.5V at non inverted input and 2.6v at output on both channel when the manual indicates 0v on both. Also oscillation only appears on negative half of the sine wave.
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Hello,
Should have mentioned that caps in that section are new, they were fine but I thought I'd give it a try.
Should have mentioned that caps in that section are new, they were fine but I thought I'd give it a try.
Hi Patrick101,
I'll verify tomorrow. I did check continuity from chassis to the different ground points on the tone control board and they seemed fine but I didn't trace them back to the volume board. I had a big oscillation problem at first with the treble all the way up even with no signal and that turned out to be one of the ground connections for the faceplate chassis. The sub chassis isn't metal and after some fiddling the screws didn't quite tighten the ground lugs down so I drilled a hole directly in the metal chassis and connected it there with a bolt which fixed that issue.
I'll verify tomorrow. I did check continuity from chassis to the different ground points on the tone control board and they seemed fine but I didn't trace them back to the volume board. I had a big oscillation problem at first with the treble all the way up even with no signal and that turned out to be one of the ground connections for the faceplate chassis. The sub chassis isn't metal and after some fiddling the screws didn't quite tighten the ground lugs down so I drilled a hole directly in the metal chassis and connected it there with a bolt which fixed that issue.
it's funny, I must receive a Sony TA-F770ES in the coming days for a source selection problem.
it works well but loses a voice or the other or both from time to time.
if you need values on a working amp, you can send me a mp
it works well but loses a voice or the other or both from time to time.
if you need values on a working amp, you can send me a mp
I would suggest checking power ground continuity at SCN14.
Could be a degraded opamp, too. If you can't find M5220L I imagine NJM4580L would make a good substitute.
Could be a degraded opamp, too. If you can't find M5220L I imagine NJM4580L would make a good substitute.
I'll check the ground connections tomorrow (didn't have time today) and maybe take you up on that offer if it doesn't turn out to be that thanks 😉.
Looks like a perfect candidate indeed 😀
Finally found some time this morning.
Power ground return at scn14 and ground line between tone control board and volume board both check out. I noticed that it seems to be power dependant in that way that up to a certain point I can make the oscillation disappear by slightly decreasing the power output on the other channel with the balance knob (works for either one or the other channel). I checked the power supply rail to the tone control op amp and they stay perfectly stable.
Power ground return at scn14 and ground line between tone control board and volume board both check out. I noticed that it seems to be power dependant in that way that up to a certain point I can make the oscillation disappear by slightly decreasing the power output on the other channel with the balance knob (works for either one or the other channel). I checked the power supply rail to the tone control op amp and they stay perfectly stable.
I still suspect a ground problem because shorting a cap connected between the ground from the control circuit and chassis limits oscillation somewhat. Also connecting earth from scope probe to chassis (this amp has a 2 prong cable) changes the shape of the oscillation. I don't know if I mentioned this but without a load the amp doesn't oscillate even with volume at max.
This may be a valuable clue. Where could there be a shared ground return between the output and the preamp section? I don't think there would be too many options - I mean we're talking a rather high-current ground return path, that shouldn't be too long or complex. Gotta be somewhere between the output terminals and the filter caps.
If only it was that simple... The schematic isn't clear at all about ground connections and they don't seem to correspond with what's actually in the amp. The only thing I see are the four heavy wires coming from the speaker terminal board back to the main amp not even passing close to the preamp. One thing I can see is that the relay command that also goes to the volume board and tone control section runs to the output relays that are on the speaker terminal board. I can try disconnecting those and scoping before the relays and see if that changes something.
Doesn't change anything 🙁.
I pick up the oscillation absolutely everywhere, ground, power and signal 😕
I pick up the oscillation absolutely everywhere, ground, power and signal 😕
Well replacing the IC didn't work. I'm still convinced it's a ground issue because shorting ground on the control circuit decreases oscillation somewhat. I'm not ready to give up but I'm starting to be out of ideas.
I'll just keep posting updates hoping this might trigger something 😀.
Some more observations :
-When ground of scope is connected to the amp chassis and I put my finger on the connector between power amp driver board and the main board with the power transistors, the oscillation almost completely disappears on that channel and gets less pronounced on the other. If I do this without the ground of scope connected things get way worse.
-I think it might be a power supply problem because the supply for the tone control circuit is shared with the one for the phono equalizer circuit and when I inject a signal into the phono input I get oscillation whether we're in direct mode or not. When turning on the amp without my dim bulb there is some buzzing sound while the caps charge that isn't there when I use the dim bulb (less current ?) so I though that maybe one of the diodes in the bridge rectifiers for the negative power rail was bad but I tested them and they read fine in diode mode.
Is it possible a diode might read ok but develop some sort of default with high currents ? Also could a cap be at fault here (even though I measured both rails when the oscillation occur and I see no drop nor ripple on any of them) ?
Some more observations :
-When ground of scope is connected to the amp chassis and I put my finger on the connector between power amp driver board and the main board with the power transistors, the oscillation almost completely disappears on that channel and gets less pronounced on the other. If I do this without the ground of scope connected things get way worse.
-I think it might be a power supply problem because the supply for the tone control circuit is shared with the one for the phono equalizer circuit and when I inject a signal into the phono input I get oscillation whether we're in direct mode or not. When turning on the amp without my dim bulb there is some buzzing sound while the caps charge that isn't there when I use the dim bulb (less current ?) so I though that maybe one of the diodes in the bridge rectifiers for the negative power rail was bad but I tested them and they read fine in diode mode.
Is it possible a diode might read ok but develop some sort of default with high currents ? Also could a cap be at fault here (even though I measured both rails when the oscillation occur and I see no drop nor ripple on any of them) ?