Sansui AU-6900 Woes .......

[camgilbert]

Thanks, i now have a component tester and did get the change in the c location corrected.

 

[camgilbert]

So at R41 im getting a big V drop (using DBT) -24V to -5V.

I have checked for shorts on components before and after with no luck.

Any thoughts on where to look for issue?

Thanks

Cam

 

[petehall347]

c23
tr03
or r41 itself gone high ohms .

 

[camgilbert]

Hello all,

I continue to be stuck on this one component after check voltages.

R41 and R42 share the same trace but the drop across 41 continues to be 10V which causes the components after it to heat up.

I have checked all components on the voltage drop side if the circuit and they all test out ok. (Using my component tester)

Any thoughts on what to try?

Thanks

 

[slimecity]

Im not as skilled as some of the guys commenting here but i'd be checking the trannies that are after R41. A short in one of those trannies could be causing the voltage drop? I cant quite read the schema - but this resistor connects to the bases of, is it, TR08 and TR09?

 

[Hyperion]

The first thing to check is the orientation & voltage rating of C23 the 100µF, 50V right next to R41 on the schematic diagram, It is unlikely but the capacitor itself could be faulty. Then it's anybody's guess, could be a solder splash? a shorted or a wrongly orientated transistor or diode? make no assumptions, check your work and any replacement components for all these things.

However, you have one good channel? - compare voltage readings for the same places on each channel. This should show a pattern which may get you closer to the real issue, in theory (at least), the more wildly out the voltage reading, the closer to the fault you are likely to be.

 

[petehall347]

are the voltages really positive ? i guess not .
get tro4 voltages first .
if you could mark all transistor voltages it will make it easier to track the fault down .

 

[sregor]

The question is - where is the current going. (Voltage drop across the R41 means current is going through it. Normally it would end up going through R31, but not here (I think...) My guess it is somehow going to + supply through output or driver transistors. First check voltages around TR11 and TR15, then TR15 and TR17 (the Tabbed transistors mounted between the outputs), Shen checking, the numbers next to the decimal point are important, as we are interested in the difference between points, as well as there value. (23.3 and 23.5 would be very different from 23.2 and 23.7)
. One thing to be careful about, is that the tab transistors and the outputs have the collectors being the tab or case, and the connection is through the mounting screw - so have to be snug, and no insulating shoulder. Is output (emitter resistors - white blocks) still in the +20 range.

 

[camgilbert]

OK.

Here are the measurements across TR15 thru TR18

TR17 (b,c,e)

-44.56, -49.75, -44.05

TR15

-42.75, 49.5, -43.5

TR18

46.2, -49.60, 46.22

TR16

47.20, 49.7, 46.57

The collector being measured from the Phillips head to the chassis ground.

Emitter resistors measured from lead to chassis ground are around 46-47V or -46-47V.

NOTE: I put the original transistors for TR15 thru TR18 back in which seemed to have solved the large Voltage drop across R41 (it is now about a 2V drop)

 

[camgilbert]

Well for the first time in months, I finally got the relay to click in then out again.

I turned both vari resistors counter clockwise as far as they would go.

In that position, I powered the unit on, the relay clicked on then clicked off again.

Here's the interesting part, while adjusting VR01 i can change the voltage across the emitter resistors 59,69,61,67.

I cannot do the same thing with VR02! Adjusting it through its full range does nothing to the V across 60,68,62,70.

I have spare 470 ohm varistors so I did replace VR02 but no change.

Hopefully this is progress and not a step back!

Thanks

Cam

 

[petehall347]

what are the base voltages for these ? and do they adjust with the bias trimmers ?
tr11
tr12
tr13
tr14

 

[petehall347]

you said you took voltages for collectors at screw heads . they should be isolated . take voltages from the tab .
also make sure the tabs are isolated from the heat sink .

 

[sregor]

you said you took voltages for collectors at screw heads . they should be isolated . take voltages from the tab .
also make sure the tabs are isolated from the heat sink .

On the drivers (TR15,16,17,18) (also on the outputs) I believe the electrical connection is through the screw, so isolators will make it not work. However, checking tabs is the better way to test.

 

[sregor]

OK.

Here are the measurements across TR15 thru TR18

TR17 (b,c,e)

-44.56, -49.75, -44.05

TR15

-42.75, 49.5, -43.5

TR18

46.2, -49.60, 46.22

TR16

47.20, 49.7, 46.57

The collector being measured from the Phillips head to the chassis ground.

Emitter resistors measured from lead to chassis ground are around 46-47V or -46-47V.

NOTE: I put the original transistors for TR15 thru TR18 back in which seemed to have solved the large Voltage drop across R41 (it is now about a 2V drop)

Well for the first time in months, I finally got the relay to click in then out again.

I turned both vari resistors counter clockwise as far as they would go.

In that position, I powered the unit on, the relay clicked on then clicked off again.

Here's the interesting part, while adjusting VR01 i can change the voltage across the emitter resistors 59,69,61,67.

I cannot do the same thing with VR02! Adjusting it through its full range does nothing to the V across 60,68,62,70.

I have spare 470 ohm varistors so I did replace VR02 but no change.

Hopefully this is progress and not a step back!

Thanks

Cam

Well, I believe you still have problems, It looks like you have + rail voltages on one channel out, and - rail voltages on the other. However, if you look at the protect circuit, they connect the two outputs through resistors. The problem is that one positive rail and one negative rail, the combination is near zero going into the protect circuit. They used different values for R75 and R76, but I'm guessing fiddling with pots just made one far enough off to cancel out the other. It does seem to indicate that the protect circuit is working.

 

[camgilbert]

Happy New Year to all!

Ok, its a new year and i'm going to take one last kick at the can on this AU-6900 before I part it out, make it a cat box, run it over with my car, put it back together carefully and just turn it on to stare at the LED or maybe use the TR plate to keep my coffee warm.

Current status: Relay will NOT click in. Trimming the Vr's for offset does nothing, voltages do not change.

All AC voltages thru fuses are as per schematic.

All measurements shown were against the chassis ground.

TR601,602,603 replaced.

Anyone see any Gremlins that I might have missed?

Thanks as always.

Cam

 

[camgilbert]

Hi,

For verification, can anyone confirm that I'm reading and measuring the power amp circuit correctly?

Many thanks in advance.

 

[petehall347]

Hi,

For verification, can anyone confirm that I'm reading and measuring the power amp circuit correctly?

Many thanks in advance.

View attachment 1097289

yes correct

 

[camgilbert]

Hey gang,

I'm hoping to get by this core issue that I cannot resolve so far.

In my Au-6900;

I have a large voltage drop across on R41. I'm measuring -21V on the R42 side of R41 and -4V on the R07 side of R41 (on DBT)

The other three resistors R42, R31, R32 have a small voltage drop across them (2V or less)

Staying on DBT for now as R41 really heats up off DBT.

I have tested most of the components in that part of the circuit, with pretty much the same result.

Any thoughts on where to look?

Also I have a newbie question, I know at least one of the 480ohm pots is problematic as just touching it forces the relay circuit to trigger on and off. Is it safe to install replacements at factory settings or does the pot need to be set to max or min resistance prior to installation?

Many thanks in advance.

 

[camgilbert]

Many thanks to all on helping with this project. I have learned so much about repairing these vintage units!

I have decided to end this project and just keep the parts for people who need them or maybe for a project I do in the future.

If my experience can ever help someone, as a new person to more complex repairs here's what I have learned:

1. NEVER, EVER, replace components on mass, as a process of elimination, or as a troubleshooting process. If you start down this path, there is very likely no road back. In my case I ended up putting many good original components back in and because the circuit board was old the soldering pads started to break.
2. Use and trust the DBT. This was the best advise from this group and I will use my DBT faithfully on future projects.
3. Be careful adjusting the bias current before or as part of resolving issues. This is a very sensitive setting and can cause all sorts of "changes" which made troubleshooting for me more difficult.
4. Read the experts responses carefully! In many cases, i read the first sentence in a response and started to work on that ignoring other information provided. (I really picked up on this when I went back over the entire thread as a post -mortem on my project.)
5. Be very careful about testing the correct orientation of transistors prior to insertion. I ended up buying (sadly too late) a cheap component tester which worked great.
6. Persistence is a double edged sword. While generally a good asset, sometimes we go beyond capabilities and experience and spend way too much time and money on a project. In my case, this project while not successful in reviving the amp, taught me all of above for the next project!!!!

Again, thanks to all that helped.

If anyone needs parts for an AU series Sansui Amp, I will be happy to oblige.

Cheers

Cam

 

[The Fuxtor]

it happens .... I just shelved a Luxman with some gremlin hiding on the amp board.... one day in the future , you can always come back and give it another round!!