caesar56
Active Member
There's only 9mm shaft units left... and those won't go in...
Looks like it's drilling time.
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AU919 in trouble.
- Thread starter caesar56
- Start date
There's only 9mm shaft units left... and those won't go in...
Looks like it's drilling time.
caesar56
Active Member
Sorry for the late reply, i've been lost with the forum changes.
I'm still patiently awaiting for the parts to arrive, i've ordered an extra pot from taiwan alpha with *apparently* the right dimentions. If none fit or end up not liking 'em ill send you an email.
Thanks a lot.
caesar56
Active Member
Good idea - no sense getting yourself burned out - gotta have some playtime every now and again
Hey Jhon, i'm almost done with the amp, currently double checking my job and placing it all back togheter.
However, after some thinkering, I was wondering if i'm correct assuming that the toroidal tranformer has dual windings, one for each channel, thus I could place a mix of outputs (half original/half on-semis) feed it mono audio and use this to try and spot differences by ear. Can this cause any harm given the difference in bandwidth?
Regarding the transformer and windings this is not an issue - what you suggest wouldn't be a problem.
caesar56
Active Member
Regarding hearing the difference...
It sounds different, period. Also interesting.
The amp is working, yes. I have to work on a gound loop that happens exclusively when i touch the volume control. Odd.
Question Jhon, how hot... is too hot for this amp? -Outputs sinks are too hot to the fingers atm. Even the metal body of the amp is hot.... so is the secondary transformer.
Yes i did all the adjustments mentioned in the service manual... one for offset, one for bias, one for the eq board as well. Only missed the EQ phono THD one (no o'scope, no cigar)
Ideas? uhmmm, oscillation impossible to tell w/o the scope right?
I like it's sound, demn yes.. it's full and separation is amazing.
Thanks a lot for your help, again and again.
It does get quite warm in service, but what you describe sounds a bit too hot. As 'too hot to touch' is always too hot with a class Ab amplifier. With both of the ones I reconditioned the heatsink was certainly warm but never too hot to touch. And left on for several hours the case would be just pleasantly warm (not hot) to the touch. Are you sure you measured and set the BIAS correctly - and after a ½ hour initial warm up? - ventilation ok? However every person's definition of hot, or at least, their tolerance to heat is different, so it is difficult to be sure here.
Interesting you mention a 'ground loop' (increased buzz/hum?) when you touch the volume control, as this could be a sign of oscillation. Sansui warn you to be careful with the interconnection of the components in your system and also grounding, and I believe Conrad (AK member ConradH) has mentioned the sensitivity of this amplifier in this regard. Can I suggest you minimise the configuration, (1 set of speakers and one input) or try letting the amp sit with nothing connected to it (not even speakers) as see how warm it gets then?
Interesting you mention a 'ground loop' (increased buzz/hum?) when you touch the volume control, as this could be a sign of oscillation. Sansui warn you to be careful with the interconnection of the components in your system and also grounding, and I believe Conrad (AK member ConradH) has mentioned the sensitivity of this amplifier in this regard. Can I suggest you minimise the configuration, (1 set of speakers and one input) or try letting the amp sit with nothing connected to it (not even speakers) as see how warm it gets then?
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caesar56
Active Member
Aaaight, found part of the problem. Open trimmer @2835 board. Placed 200Ohm in place of 330Ohm, not good. Voltage would swing as much as 50mv.
I'll buy 300 ohm trimmers tomorrow and see what happens.
Will report back,
Regards, J.C
For the F-2835 trimmer, the schematic says 220Ω, and I think I fitted 200Ω - the voltage swing you see at the output of the F-2835 MM Head Amp is normal - they all do it (even mine ) - not the reason why yours is getting so hot (by your description). The MM Head amp gets hot BTW, very hot heatsink - hotter than the main heatsink.
) - not the reason why yours is getting so hot (by your description). The MM Head amp gets hot BTW, very hot heatsink - hotter than the main heatsink.
caesar56
Active Member
For the F-2835 trimmer, the schematic says 220Ω, and I think I fitted 200Ω - the voltage swing you see at the output of the F-2835 MM Head Amp is normal - they all do it (even mine
Excuse the ignorance Jhon.
Mine had 330ohm trimmer there. I couldn't source these locally so i had to order them, gonna take at least 15 days. In the meantime however, i did try adjusting with the ones i pulled from the G-9000.
The thing is, the voultage would rise slowly but steadily until about +/-150 mv and bounce between 0 and X mv. Really, a random behaviour (manual calls for something between 0 and 50 mv when adjusting) is this normal?
I left the amp idle for a couple of hours, it got hot but not as hot.
Completely normal - this is expected for the MM Head Amp.
So the main amp heatsink was cooler after adjustment of Bias?
caesar56
Active Member
Hi there John, after being busy for a while i gave it another shot once i had some free time.
Burrowed an O'scope from a friend, rigged it up and i found 2 issues with the amp.
1st) When i touch the volume pot, a 60Hz looking hum is introduced to the amp. It is audioble, as i mentioned before. It only happens if i touch the volume knob, the other knobs are safe (is this oscillation?)
2nd) At about 40W of power, the negative rail shuts down giving an odd wave shape. Positive side hits 100W and then, at about 120W, protection kicks in. This is on both channels (common). I'm curretly thinking this issue is originated in the power supply, main suspect are that matched differential pair in the F-2844 board (2SA979 & Co). I don't know what to replace those with. Any heads up are welcome.
Thanks a lot again.
C.
When you say "negative rail shuts down giving an odd wave shape" which negative rail do you mean? - guessing it is the regulated supply, as you don't mention anything about it being limited to one channel?
I shouldn't think it would be too unusual for the protection to engage at 120W - the amp is rated for 100Wpc, and if clipping at that point, the waveform produced might trigger protection more easily.
As for the sensitive volume knob, grounding issue? - I know the effect you mean, possibly the pot itself has oxidised contacts and is thus effectively lifting a ground?
Anyone else with any ideas?
I shouldn't think it would be too unusual for the protection to engage at 120W - the amp is rated for 100Wpc, and if clipping at that point, the waveform produced might trigger protection more easily.
As for the sensitive volume knob, grounding issue? - I know the effect you mean, possibly the pot itself has oxidised contacts and is thus effectively lifting a ground?
Anyone else with any ideas?
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caesar56
Active Member
I didn't take a picture at the moment however here's a poor drawing of the situation by the truly yours....
Issue is common to both channels.
I was using a sinusoidal wave generator and an old hitachi 35Mhz scope. Measurements were taken at the 200W dummy load end. One channel at a time.
The volume pot issue is a mistery, it is a new alps blue velvet 100k dual log pot that came from japan. (that I bought as suggested)
Issue is common to both channels.
I was using a sinusoidal wave generator and an old hitachi 35Mhz scope. Measurements were taken at the 200W dummy load end. One channel at a time.
The volume pot issue is a mistery, it is a new alps blue velvet 100k dual log pot that came from japan. (that I bought as suggested)
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