Stuck on a Sansui AU-5500

[Netlist]

Output has a 21VDC on BOTH channels.
The amp is playing, a sinewave on the input reproduces a sinewave on the output but clips at 5VPP.
What I did so far:
Disconnected plug 9/10/11/12 from the pre and control section
Power supply is a nice +/-38V
At R22 I measure +30.5VDC instead of +9VDC.
At collector TR06 I have +19.5VDC
The absolute weird thing is that both channels are effected.

I’ll scan the whole manual (It’s a UA-6600) for the one(s) who can help me solve this problem.

/Hugo

[Tube_Dude]

Originally posted by Netlist
who can help me solve this problem.

/Hugo

Hi Hugo

Inspect if the 9,1 volts Zener near R22 as the conection to ground...this conection to ground must be commum to both chanels...and if is missing you don't have 9 volts at this point...

Regards

[anatech]

Hi Netlist,
Look for the ground connection at the pcb or the other end. Tube_Dude has hit this on the head as far as I'm concerned.
An open zener solder connection would also do this. If this is the case, replace the zener and put a higher wattage unit in.
-Chris

[anatech]

Can't see the other end, but you have two ground connections. Zeners are okay I think. The common connection at the other end must be the fault area.

[Netlist]

Spot on!!
In disassembling the unit there were two ground leads detached.

You guys rock.

So if you like the manual, let me know.

/Hugo

[DigitalJunkie]

Ohh Ohh,I could really use the manual/schematic.

I've got a sick Sansui AU-5500 here.At first it plays fine,but after it gets warm,there will be a bit of a crackling in the left channel,then it will click the protection relay.I'm suspecting a thermal problem,maybe one of the old transistors has finally given out. (The problem is,some of them are impossible to find/get!) or maybe the bias circuit is acting up..
I still havn't figured this one out yet..I suppose I should get it up on the bench and see whats up one of these days.
Any info,or tips would be great.I'd love to get this old beast singing again!

Edit.. I looked at the PDF posted above,and it looks like the same circuit,but the P/N's of the transistors are all different. (Same PCB number..I guess they used that PCB in alot of thier amps,and just stuffed them with different parts..)

[Netlist]

Originally posted by DigitalJunkie
At first it plays fine,but after it gets warm,there will be a bit of a crackling in the left channel,then it will click the protection relay.

When you look at the schematic in my first post, you'll see the diff pair transistors Q1/Q3 and Q2/Q4 for the other channel. Together with C27 33µF/50V in the protection circuit these are most likely the culprit.
The crackling comes almost certainly from the diff pair. You can see my handwriting, it's Dutch but describes the prob. I replaced them with BC560. Watch the pinout, it's different.

/Hugo - so happy now, will scan upon anyone’s request

[Tube_Dude]

Originally posted by Netlist

/Hugo - so happy now, will scan upon anyone’s request

Hi Hugo

Some day when i need a schematic i will remember you!!

Cheers

[DigitalJunkie]

I think I may have found a/the problem with the AU5500 I have here.
The bias pots seem to be dirty and intermittent.I turned them back a fourth a bit,and set them to approx. where the originaly were..and it seems to work okay now.I'm gonna dig through my box and see if I have any suitable replacements.

My question is,What should the idle bias be in the output,and how do I adjust the front end pots? What do they do? (adjust the symmetry of the front end?)

[carlmart]

Originally posted by DigitalJunkie
I think I may have found a/the problem with the AU5500 I have here.
The bias pots seem to be dirty and intermittent.I turned them back a fourth a bit,and set them to approx. where the originaly were..and it seems to work okay now.I'm gonna dig through my box and see if I have any suitable replacements.

My question is,What should the idle bias be in the output,and how do I adjust the front end pots? What do they do? (adjust the symmetry of the front end?)

Change the pots onto sealed multi-turn types.

The input pots probably zero the DC offset.

There should be a bias idle value in the Sansui manual. I would stick to it first and then do a listening improvements check for higher and lower values.



Carlos