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

[sregor]

First a comment. DC can be check most easily at the emitter resistors - the large white blocks associated with each output resistor. Unless one is open, they will all be at the DC offset, or within 30 mV of the DC offset.

Best to use clipleads to measure. The ends of the resistors marked Lch and Rch are the output voltages. (should be near zero - if no0t go back to amp and trouble shoot) . Next check the 33 volts. If below 25, could be bad capacitor C603 (usually buzzing from relay or intermittent not pulling in). The rly+ should be 33 volts if the relay is not trying to energize , and should be 24 when energized. The rlly- will be 33 (same as rly+ if the relay is not energized. It should be less than a volt when relay energized. rlu+ and rly - can be measured off the diode d613.
The "V" where a couple of resistors and diodes come together is the (for lack of a better description) I call the trigger voltage. It should be around 1.8 volts. If there is a malfunction, it will be lower. If it is 1.8 (or higher, then the relay should be on. If not trouble shoot diodes d612 and transistor tr603. The collector of 603 is same as rly-. The emitter is ground.
V th is the voltage from the thermistor, which is mounted on the heat sink and connected on the back side of the board (You can see it on the third picture of post 8 in this thread - lower right of the red 2 dot.) . It has to be higher than 15 volts, or the "V" will be pulled down.
The funny reddish brown blobs are dual diodes. They can also cause problems.
Start with the L and R ch voltages, then the 33 volts, before going after the rest of the protection circuit. Hope this helps.

 

[sregor]

Picture (from wrong thread) shows DC offset in right channel. (16.5 to 17 volts). Have to take care of this before getting back to protection circuit (probably not needed.). Since voltages are not rail voltages, problem is not likely output transistors.

 

[camgilbert]

Hi guys,

Please see the measurements I took, voltages seem very high.
Virtually everything on the board has been replaced except select resistors, ZD's, misc diodes, bias pots and some small ceramic capacitors. Every transistor has been replaced.
L&R channel bias were set to approx 18mv as per manual.
Thanks.
Cam

 

[slimecity]

Please see the measurements I took, voltages seem very high.

Do you mean, "high compared to the voltages you should be getting, based on the schematic?"

 

[camgilbert]

No based on images in prior post.

He showed V of 33v and im getting 45.

 

[sregor]

The 33 volts is from the schematic - higher shouldn't prevent it from working, but it may be higher because relay not engaged. And it is nominal, affected by line voltage as well.
Have a question or 2: from the picture of the emitter resistors, you had 17 volts - but at the left and right voltage points you have 160 mv. One of these is wrong, or was corrected. If the 160 mV is correct, then you need to trouble shoot the protect circuit. You have one loop around both resistor ends, but they are not connected together at that end.
I don't know how accessible the pattern side of the PC is - the simplest way to trouble shoot is to unsolder the collectors (center leads) of TR601 and 602, then see if the "trigger" voltage comes up to 1.8 . About the only other possibilities are shorted diode d609 or d610 or leaky cap c609. Good luck.

 

[camgilbert]

Steve,(and all) again, thanks so much for your time, patience and assistance to help on this!

Let me re-test the emitter resistor voltages as I received and replaced a few components after those measurements were done.

Am I correct in measuring with the positive lead on the resistor lead and the negative to the chassis ground? (For all of those tests I showed in my last measurement)

As an example, the service manual shows testing the bias driver voltage by measuring with a positive lead on R60 and R59 and negative on R62 and R61.

How do I know whether to test using the neg on the chassis ground or on a specific component? (And if thats a stupid question please tell me so!!!)

Thanks

Cam

 

[sregor]

Except for the bias, the other measurements are made with reference to power supply ground, which should be same as chassis ground. The bias, as you read, is measured across the resistor. Sometimes measurements are more useful when directly across components, especially I'm cases where all the voltages are (numerically) close to one of the supply rails. But hopefully, I or whoever is helping will say so. Common sense and experience will eventually make this clearer.

 

[camgilbert]

Again, thanks to all.

I spent some time on taking the readings enclosed.

I also replaced c609 because it was low hanging fruit!

d609 and d610 were recently replaced.

As a reminder, protection circuit still not clicking off.

Hopefully we are getting closer here!

 

[camgilbert]

Also C604 has always confused me. It's electrolytic but there are no clear markings on the power board for polarity or on the schematic that I can see.

 

[petehall347]

should be a bipolar ..thats what bp means ... otherwise called non polar ... best get the correct one .

 

[camgilbert]

Thanks for that!

 

[sregor]

It won't turn on with the 5.6 and 7 volts offset from the two channels. The grounding question started a few thought processes. To me it looks like you're missing the ground to the upper part of the amplifier. With power off, check continuity from the various grounds. I would use the wire going between the two power supply capacitors as my starting point. Check there to the chassis, to the outer parts of the input RCA jacks, then to the amp board. If you look at the PCB pattern for the amp in the SM, there are two separate ground leads. The go to two of the connectors in the middle of the amp board. When I have some time, I'll try and draw up a picture.

 

[camgilbert]

Thanks Steve.

Ill check that out, for my interest, is the "formal" DC Offset measured between ground and those emitter resistors?

 

[camgilbert]

For reference here is a pic of the PS board

 

[camgilbert]

My apologies above is EQ board.

 

[camgilbert]

I tested continuity to most ground points. From PS Cap ground bar to:

All RCA Outer covers working.
Most chassis points working.
Worked through the black cable to the PA board then on to various components from there.

 

[sregor]

Mu concern would be the "green" ground area (drawing is from the circuit side - mirror image of component side)
The ground should be the shield of the cable coming into the connector nearest the outputs. If that is not a good ground, the output will "float" to a random voltage and protect circuit will not turn on relay. The ground corresponds to pins 7 and 9 on the amp board in the schematic, even though the schematic has 2 separate grounds, there should only be one. The other end should be at the pre/main connectors on the rear apron. The schematic is very confusing, so best practice is to trace the wire.

 

[camgilbert]

Steve, thanks so much for your efforts!

Ill check that out and let you know.

Cheers

 

[camgilbert]

Hi guys,

So I checked all potential ground issues to the two traces listed in your demarcation Steve. All grounding seemed correct. I traced each connection on the ground traces and all seemed fine.

The two ground points (on the component side of the PA board) appear to be; 2) The ground wire to the power LED, 1) the ground wire (as you specified) to the shield of the EQ board.

I tested continuity from the PA shield to the EQ board and it was fine.

The one thing about the measurement on the emitter resistors, when I turn the amp on its about 15V then eventually after 20 mins or so it slowly leaks to below 8V. (not sure if that's normal)

Sorry about all of the images, but I'm an IT guy and by nature document everything!