Sansui Eight help - hum on one channel
- Thread starter KeithD
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- sansui eight
That's a bit strange, are you sure that's correct? - if correct it's far from ideal, but not a show stopper.
Put them back in.
TR853 & TR855 next
Then TR857 & TR859 - I know you've tested these before - but please test them again.
That's a bit strange, are you sure that's correct? - if correct it's far from ideal, but not a show stopper.
Put them back in.
Oops
typo! Should be:TR807: hfe=108, Vf=591
TR808 hfe=85, 631
TR853 & TR855 next
Then TR857 & TR859 - I know you've tested these before - but please test them again.
I only tested these for short, so I will get hfe & Vf with the new meter.
Yep, all good. (mark this board - so you know which of the two F-1278's you tested the transistors in
).So, we now know that all (but the input diff pair), of the Left channel transistors test OK. (but that doesn't necessarily mean they ARE ok
). Next thing, with the 4 main power rail fuses, F001, F002, F003, F004 removed, On DBT check ALL the power supply voltages.
That is:- B1, B2, B3, B4, B5, B6/7, B8, B9, B10.
And also please check the voltage across each of the two ZD801's on the F-1278 boards. (should be roughly 6.2V)
Yes please, have driver boards plugged in - but remember, no driver board fuses.
You could check TR801 & TR801 - they looked ok to me from their voltages (something I couldn't be sure of/was simpler to test, with the others).
OK, here are measurements on DBT with F001-4 pulled. All measurements in Volts, values from schematic in parenthesis.
B1: -7.0
B2: +7.9
B3: +46.0 (+46.5)
B4:-48.0 (-46.5)
B5: 0 (+14)
B6/7: +23.1 (+25)
B8: 0
B9: -40.7
B10: +41.3 (+40)
ZD801: L: +7.7, R: +7.7
Pretty obvious B5 has trouble, and ZD's are high.
B1: -7.0
B2: +7.9
B3: +46.0 (+46.5)
B4:-48.0 (-46.5)
B5: 0 (+14)
B6/7: +23.1 (+25)
B8: 0
B9: -40.7
B10: +41.3 (+40)
ZD801: L: +7.7, R: +7.7
Pretty obvious B5 has trouble, and ZD's are high.
B1: -7.0 = ok
B2: +7.9 = ok
B3: +46.0 (+46.5) = ok
B4:-48.0 (-46.5) = might be an issue here, 2SA486, or capacitors.
B5: 0 (+14) = problem, 2SD223 or ZD001.
B6/7: +23.1 (+25) = ok
B8: 0 = big problem - are you sure? we had B8 previously? - please re-check.
B9: -40.7 = ok
B10: +41.3 (+40) = ok
B4 - leave this for the time being
B5 - needs to be put right. 2SD223 = KSC2690AYS, ZB-1-14 = 14V, 1W zener.
B8 - please recheck
B2: +7.9 = ok
B3: +46.0 (+46.5) = ok
B4:-48.0 (-46.5) = might be an issue here, 2SA486, or capacitors.
B5: 0 (+14) = problem, 2SD223 or ZD001.
B6/7: +23.1 (+25) = ok
B8: 0 = big problem - are you sure? we had B8 previously? - please re-check.
B9: -40.7 = ok
B10: +41.3 (+40) = ok
I think this is OK after some thought the zener type "ZB-1-8" I think means an 8V zener, weird voltage but ok.
B4 - leave this for the time being
B5 - needs to be put right. 2SD223 = KSC2690AYS, ZB-1-14 = 14V, 1W zener.
B8 - please recheck
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That might suggest the series pass transistor is damaged.
OK, this is good.
Left Channel Bias = VR803 (ensure the 'slider' of the trimmer is set towards R817)
Left Channel DC Offset = VR804 & VR801 (sliders to track middle)
(Left channel Diff amp current = VR802, ensure the slider is set in the middle of the track)
So, I think we are ready to try inserting the 2 fuses for the Left channel only, still on DBT of course, and try adjusting DC offset (first) - NO speakers connected. Meter set to volts, Black meter probe to chassis ground, and Red meter probe to point 'S' or '8H' (same thing). Try adjusting VR804 for lowest voltage.
Please try to ensure that your meter connections are secure.
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OK, couple of things. SM uses both VR801 and VR804 for DC offset. Says to set both to midpoint, then adjust VR801 first. Just want to be clear we are not doing that, just leaving VR801 at midpoint and only using VR804?
I don't see anything marked "R" on the board, I assume I can measure 8H at the leg of C809 that is connected to H. Looks like any number of points would work - R838, R833, R837, R831, etc.? All connected to H.
I don't see anything marked "R" on the board, I assume I can measure 8H at the leg of C809 that is connected to H. Looks like any number of points would work - R838, R833, R837, R831, etc.? All connected to H.
stereofun
Super Member
One note on the trimmers, if you haven't done yet, then spray a tiny bit of de-ox on them followed by several turns back and forth. also put probes on either outer leg and center leg (ohm), just to see that they are more or less smoothly varying their resistance. Then proceed as john says. 50 year old trimmers are really a weak link here and can mess up your readings and even in some cases be the very culprit, just trying to make sure they are sound and well.
Yep, I forgot about VR801
- I have amended my post above to include it - + VR804 & 801 sliders to mid-track position.Voltmeter Red probe to junction of R837 // R838 would do. (Left channel)
And Tom is absolutely right - oxidised trimmers can cause failure of these adjustments leading one to think there are bigger problems - trimmer replacement is very much on the cards here.
OK, so I'll be following the SM procedure on the left side of page 15. Says to set both VR to middle, then adjust VR801 to zero. If not possible, then adjust VR804 (step 6 in procedure).
Thanks for the reminder on the trimmers, Tom. Have already Deoxit the trimmers and worked them back and forth. They do turn more smoothly now. Will check resistance of both legs. When I get this working I'll be making a BOM of things to replace and the trimmers will be on there.
Thanks for the reminder on the trimmers, Tom. Have already Deoxit the trimmers and worked them back and forth. They do turn more smoothly now. Will check resistance of both legs. When I get this working I'll be making a BOM of things to replace and the trimmers will be on there.
I did check that these trimmers move smoothly in resistance as they are moved. Looked to be the case, I got 0.5 ohm on the low end and 3.5 kohm on the high (clockwise). Movement seemed continuous.
Getting +35.5V. Moving TR801 or TR804 makes little difference. This is what I had on the speaker terminals earlier before checking all the transistors. Since I did not change components, I guess this is no surprise?
Getting +35.5V. Moving TR801 or TR804 makes little difference. This is what I had on the speaker terminals earlier before checking all the transistors. Since I did not change components, I guess this is no surprise?
Well, assuming you are actually measuring at the junction of R837 & R838 - you have an open resistor or two - or wrong values, or a cracked trace / dry joint, or something similar. OR the trimmers are so badly oxidised that they are the cause of the issue.
(assume you mean VR801 & VR804 here).
The positive side of the amplifier is turned on and it shouldn't be, not that much anyway! That is TR807, TR853, TR857, because of too much positive voltage at the base of TR807. It could also be because negative voltage is missing from some parts of the circuitry (i.e. wrong voltage at the base of TR808), thus preventing a balance to be achieved at the output.
What components if any, have been changed on this driver board?
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Yes, measured voltage at leg R838 connected to H.
Only thing replaced on this driver board is R837, that's the one that burned up after putting in the 2SC1079s on that side without being on DBT.
Yes, was moving VR801 and VR804 (sorry for the typo, need to be more careful.)
As I mentioned, I checked trimmers VR801 and VR804 with DMM for resistance, and they both showed smooth change in resistance from about 0.5 ohm to about 3.5 kohm. I had already Deoxited all trimmers on both boards.
Since H is connected to speaker outputs, it makes sense to me that I'm getting the same voltage as I was on the speaker outputs after replacing R837. As I mentioned in post #35, I had checked both speaker outputs (L/R) and both were reading +37.7V. So, seems like something is bad on both driver boards, or something common to both - such as something on the PS feeding both boards? Visually, I don't see any burned up/melted resistors on either driver board.
Looking at the PS voltages, a few are higher than spec. Shouldn't these all be slightly lower than spec when on DBT?
Only thing replaced on this driver board is R837, that's the one that burned up after putting in the 2SC1079s on that side without being on DBT.
Yes, was moving VR801 and VR804 (sorry for the typo, need to be more careful.)
As I mentioned, I checked trimmers VR801 and VR804 with DMM for resistance, and they both showed smooth change in resistance from about 0.5 ohm to about 3.5 kohm. I had already Deoxited all trimmers on both boards.
Since H is connected to speaker outputs, it makes sense to me that I'm getting the same voltage as I was on the speaker outputs after replacing R837. As I mentioned in post #35, I had checked both speaker outputs (L/R) and both were reading +37.7V. So, seems like something is bad on both driver boards, or something common to both - such as something on the PS feeding both boards? Visually, I don't see any burned up/melted resistors on either driver board.
Looking at the PS voltages, a few are higher than spec. Shouldn't these all be slightly lower than spec when on DBT?
Yes, you might expect that on DBT - but with an unknown fault condition affecting both channels, required current draw is not correct so some voltages may be higher than in fault free running mode. This is with the exception of the B4 voltage which seems to be a little too high, suggesting a damaged associated series pass transistor in the PSU. This higher voltage would slightly improve the present incorrect DC offset situation.
And R837 was replaced with a 150Ω - correct? - if it was a higher/wrong value it would cause havoc - please double check this - with your meter.
What are the base and emitter voltages for TR807 - can you measure those for me?