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AU-999 initial powerup, after restoration

F-1228-1 resistors, measured on the board. All looked ok except:

R619, R620: measured 47k, SM says 470k
R603: measured 187k, SM says 220k
R607: was hard to get a reading on, fluctuated from 160k-460k, SM says 1meg. Probably affected by the circuit will remove & test.

R619,R620 might be a mistake from the factory? Or did they change this?

Thanks!
 
Hey this could be important, as those KSA992 & SC1845 I installed in 1228 had matched hFE's of 436-439. Which is surely way higher than what was in there. I might have to calculate/change something to compensate.
 
Well, maybe the whole point of those emitter resistors is to minimize any effect of transistor hFE/beta on current: they talk about a class-A circuit, but the same principle may apply?

"Then for a common emitter class-A amplifier circuit it is necessary to use a biasing circuit that will stabilize the operating Q-point making the DC collector current, IC independent of beta. The influence of β on the value of the emitter current can be reduced by the addition of an Emitter Resistance, RE in the emitter leg to provide stabilisation."

So possibly I don't need to worry about it.
 
Moving on: R601-604 are 220k 10% resistors in the SM. I measured ( in circuit )

601: 220k
602: 220k
603: 187k
604: 203k

So 603 is out of spec, 604 is barely within spec. I'll replace both 603/604 with new 220k.

And check out R607, which might the the root cause of everything.
 
It sounds like a ground loop, could it be you turntable and its ground wire?
Go with the marks on your resistors
Is the problem on both channels?
Check the Hex nuts on your pots and switches, do you have a scope?
I don't think gain is an issue here, at least IME, with that board and subs.
 
bolero said:
F-1228-1 resistors, measured on the board. All looked ok except:

R619, R620: measured 47k, SM says 470k
R603: measured 187k, SM says 220k
R607: was hard to get a reading on, fluctuated from 160k-460k, SM says 1meg. Probably affected by the circuit will remove & test.

R619,R620 might be a mistake from the factory? Or did they change this?

Thanks!
Click to expand...

Hi,
Assume you are measuring in circuit? R619 (R620) In circuit R619 (R620) has R611 39K and R623 10K (R612 and R624) effectively in parallel - this will result in the readings you are getting if measuring in circuit. When measuring high resistances in circuit the effects of other components connected at either end of the resistance being measured needs to be taken into account (or one leg lifted).

Hope this helps

Cheers
John
 
Monicker1 said:
I wish an old salt would help you. I didn't see any NF-C in the pictures in the SM. But it is alluded to on page 38. It looks like a post design fix, or even a latter day repair. But you did well to find what you could about it in previous AK threads. You were not the first to be wondering about it.

For those of us frustrated Sui owners who are watching their old equipment limp towards the final stretch of their repatched lifetimes.

View attachment 2734696

It's your thread and I'm thinking you're a good sport.
Click to expand...

The NF-C (my thinking here) isn’t a post design fix or latter day repair. It is clearly shown on the schematic of the 999, and depicted in the service manual. The omission of a part number in the service manual isn’t particularly significant, Sansui (like many manufacturers) had errors and omissions in their service manuals - and this could have been covered by a notification bulletin sent to service centres that we don’t have access to 50 years on. Also, people forget that the service manuals were meant for service people and not the general public - the level of information provided assumes a level of experience/qualification of the reader.

Cheers
John
 
skippy124 said:
..... When measuring high resistances in circuit the effects of other components connected at either end of the resistance being measured needs to be taken into account (or one leg lifted).

Hope this helps

Cheers
John
Click to expand...

Haha, yes! I replaced them all & then discovered that. Oh well, best way to learn is to do it the hard way first, sometimes.

I used 1% 1meg resistors for R607,608 ( they were 1 meg & 1.2 meg ) and matched up some new 220k for R601-604

Hipocrates, I sprayed contact cleaner with lubricant into all the switches, and the rear VR's. Worked them around to clean them.

The bass ctrl switch is feeling rough now, like there's something grinding in there. And before I cleaned them I noticed it seemed to engage even though it was off. working the switch fixed it, which is why I decided to clean them all.

I have a scope, have never used it. Willing to learn though: will watch some tuts on youtube. Must be easier to see what's going on with the amp, if you can trace & watch a signal.

Also I'll try a different TT once I fire it up again, it's idling to check bias & make sure I didn't screw anything else up

Thanks!
 
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Ok got it, thanks. Yes all the front panel ctrls are mounted snug. The ground problem with headphones earlier occurred whenever I touched the chassis, or knob, other metal part too.

But with the TT & speakers attached it doesn't do it; It actually could be the same hum problem I'm hunting down now, it's just quieter with the TT hooked up.

I have the TT plugged into the 999 power outlets.
 
Wow, it's even worse now!

Definitely an electrical zapping/sputtering noise. But only on phono. CD/aux is dead quiet.

I noticed a while back ( before the resto ) the orange power light flickered randomly a little bit. It still does this: but just now when I happened to plug the tuner power cord up to the switched outlets on the 999 rear, the tuner dial lights were flickering in sync to the 999 power light fluctuations. I am wondering if this could be related?

I moved the tuner power to the non-switched 999 outlets & the tuner flickering stopped. 999 power light still flickers.

With those new 12,000 uF filter caps, I'd be amazed if the 999 was hurting for power?
 
Going to try & clean that bass ctrl switch, maybe something fell in there. It's really rough turning it now.

I saw reference to a pair of diodes D601,602 ( IN60 type ) in another thread, that have to do with grounding the phono. They're mounted on the selector switch. Will check those out too.
 
According to the SM, D601 & D602 are mounted under the chassis right behind ( or on? ) the slot for 1228-1. Pic below. But I can't find them anywhere, including on sw18

999_D601_D602.PNG

Can anyone else confirm whether you have these diodes on your AU-999? On the schem it looks like they ground the phono LR & through sw18.

Thanks!
 
I've just been examining things, for the most part. But I fired it up again under DBT with an 8ohm dummy load, and the power transformer is making more audible noise: it sounds exactly like the noise I'm chasing down in the phono channels.

I wonder if the PT is arcing inside, it sure sounds like it. Possibly my mis-wiring of the new 12k filter caps stressed it? Since only 1 cap was wired up properly.

I checked voltage at the rectifier & got +30v, -30v off the tabs. Which is correct. I'm wary of running the amp anymore until I verify the PT isn't about to die.
 
Actually that's not right: DC should be +/- 44v after the rectifier. It should be 30v AC going into it.

I took it off the DBT & got +/- 36v DC after the rectifier. So something is up with the PT I think.
 
Ok let's start again.

1. Do not connect stuff on the AC outlets of the amp, 50 years old Bakelite and corroded stuff can create problems.
2. Secondary voltages ( purple and blue wires) it should be around 30V.
3. Check your rectifier.
4.B+ B- voltages are?
5. Check grounds. Sansui wiring sucks, so look for any broken one.
 
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Thanks!

PT sounds like frying bacon, if I power it up

AC purple wires measure 45-50v at the rectifier tabs

The other AC blue wires both go to board F-1230, correct?

DC voltages are 34v on the rectifier tabs

Rectifier wiring looks ok, I'll unsolder it & measure PT secondaries again.

I checked the voltage config on the back of the AU999, it's set to 117v which is where it should be. Fuse is good.
 
I think the PT is failing, and I have been thinking it was a ground issue in the phono section. It started as minor background hum after I got the amp running, and has been getting gradually worse, as the PT noise has increased.

when I got the filter caps sorted out, I was getting +/- 44v DC off the rectifier & 30v AC going into it.

So I don't even want to power it up, if it's feeding too much AC into the amp now.

I'll unhook the PT leads & test it unattached from the amp, that should resolve it
 
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