AU-317 - bias problem causing blown outputs?

[audiogymp]

Hi all

I'm repairing a 317 that went pair shaped a few weeks ago. I'm learning as I go but I've got into new territory and I'd appreciate a little help narrowing the search for what might be the problem.

I replaced output transistors TR17 and TR19 and driver transistor TR15.

It now seems to be operational but I attempted to set the bias and noticed something strange.

The right channel (that did not require any of the transistor replacements) is fine. The bias on the left channel won't settle down.

I had it running on a DBT, which glows dimly at idle. The bias on the left/problem channel responds to the trim pot and can be adjusted to the specified 13mV with it. It fluctuates a little (+ 1-2 mV) and sits there like that for a while. Then is starts to move around more - up to 2-300mV. Once the relay tripped but otherwise the only noticeable effect was the bulb growing brighter when the voltage goes up.

It seems to play while this is happening. The affected channel might be a little quieter but I'm not sure.

On these big drifts in bias voltage, you can't get it back down with the trim pot. But once it settles, you can dial it back to 13mV until it starts to move around again.

Given the way the bias voltage effects the current drawn, I'm inclined to think that it may be connected with whatever took out the channel I repaired.

The manual is too large to upload but it is available on the following link.

http://www.hifi-manuals.com/Download/Sansui/AU-317-Mk2-service-manual

I'd very much appreciate any thoughts on where to look for the problem.

Thanks, Jon

 

[skippy124]

Hi Jon

Is it a 317 or 317 mk2 that you are repairing?

 

[audiogymp]

Sorry for the obvious omission - 317 mk2.

Cheers.

 

[skippy124]

You have checked/replaced the fuse resistors?

 

[audiogymp]

I hope this isn't a breach of etiquette - I thought the old thread would be cold so I started a new one.

In short, I checked the fuse resistors and they seemed to be w/in about 5% of spec.

This is from the earlier thread ( http://audiokarma.org/forums/index....hooting-help-appreciated.689486/#post-9209282 ) where I posted where I was at with repairing the outputs etc.

If anyone has time to share some thoughts, it would be much appreciated.

The 317 mkII stopped working. Cutting to the chase, I opened it up and did some checks. The following thread was most helpful.

http://audiokarma.org/forums/index.php?threads/sansui-au-317ii-troubleshooting-help-requested.412472

But my checks showed that more components appear to have failed on my amp, so it would be great if someone with the sort of knowledge shared on that thread would let me know how much further I need to go before replacing components.

Before the amp blew, I had inadvertently connected it to 4 Ohm speakers (back panel warns against anything less than 8 Ohm). I assumed that this would have been the cause and that, with luck, the damage might be confined to the output transistors. However, there appears to be a number of related components that have failed.

The immediate sign of failure was blown fuses (the ones supplying the output stage). I checked the output transistors to find that TR17 and TR19 are conductive C-E. The thread above contained the following advice:

I would check the following before refitting the outputs -

TR13, TR14, TR15, TR16 ( these are the driver transistors for the outputs). Check them in circuit using the same testing principles as with the outputs, they should measure the same for both channels (TR13/TR15 in one channel, TR14 and TR16 the other channel).

R47, R48, R53, R54 shoud be 82 ohm

R49, R50, R51, R52, should be 220 ohm

R33, R34, R43, R44, should be 680 ohm

R55, R56, R61, R62, should be 6.8 ohm

The above are "fuse" resistors and are known to drift high in value over time.

Also check R37, R38, R59 and R60< should measure almost short circuit as they are 0.33 ohm.

And while you are at it, also measure R17 an R18 on the Control Amp F2752 board, these are also fuse resistors (not marked as such in the schematic) These can be measured from the solder side of the board (refer to the board layouts in the service manual).​

I did the checks described (components in place).

I found that TR13 and TR15 were conductive C-E (~150 and 3 Ohms, respectively), whereas their counterparts on the other channel measured as an open circuit.

The related resistors R49 and R51 were about 130 Ohms and 3.5 Ohm, respectively, whereas their counterparts on the other channel measured at the correct 220 Ohms.

The other resistors all measured within ~5% of correct value (noting that the 0.33 Ohm resistors are hard to accurately measure) save for R17, which was 75.8 Ohms - about 10% over the correct value of 68 Ohms.

At this stage I would prefer to replace what I need to. These are

1. Output transistors TR17 and TR 19
2. Driver transistors TR13 and TR15
3. Resistors R49 and R51

I was planning on leaving R17 alone for the time being.​

Turned out, once I took out the fried output trannies, R49, R51 and TR13 measured all right, so I only replaced TR17, 19 and 15.

The fuse resistors seemed to measure OK. Could they still be the cause of the problem

Jon

 

[Hyperion]

The fuse resistors seemed to measure OK. Could they still be the cause of the problem

I would get rid of ALL the fuse resistors, they are more trouble than they are worth.

Have you replaced the trimmers? - all of them - this is important in my opinion.

 

[audiogymp]

Thanks, Hyperion.

As for the fuse resistors - do you know if the fuse resistors identified in the thread I'm working from (i.e. R55, R56, R61, R62 on the main amp board and R17 an R18 on the Control Amp F2752 board) are all of the fuse resistors I need to be concerned with. No big deal replacing these.

As for the trim pots - can I just go to the local electronic parts shop and get trim pots of the right value or do I need to be more selective. Also, the DC offset didn't appear to be an issue. I know you said ALL the trim pots but could I get away with leaving these alone?

Cheers, Jon

 

[skippy124]

Hi Jon,

Those that are noted are the only fuse resistors. As there is are coarse and fine DC offset pots, then it is not critical to replace these, but changing the bias pot to a multi turn variety make adjustments much easier, and potentially more stable over time. It may be that the bias transistor on the problem channel is the cause (some freezer spray can be a good diagnostic tool in tracking down drifting bias)

Cheers

John

 

[audiogymp]

Gentlemen

Many thanks for the helpful advice. The amp is now running again.

Bias voltage settled down after replacing the fuse resistors and the trim pots. Some drift remained but seemed to diminish with time after the repair: was not affected by cooling transistors in the vicinity but did seem to respond to mechanical/movement effects. I observed it over a couple of days with speakers and meter connected and it seemed to be good enough to put the cover back on.

I replaced all the other resistors in the check list I was working from but that didn't seem to make any difference after the fuse resistors and trim pots. The amp was also set up for 220V and I changed the transformer leads for 240V use.

John (Skippy), I notice you have a fine array of Sansui amps. If you (and I'm not intending to exclude others) have time, I'd appreciate your thoughts on the following things.

If ever an amp needed a mute control, the 317 mk II seems to be it. I seem to get most of my usual range of listening volume out of the first 10% (i.e. the first three of the 30-odd) indexed notches of the dial. Is this normal?

People have views about the various series. Some people say the AU-x17 series is less warm and more revealing than older amps like the AU-xxx and AU x0x0 series. But when I compare the AU-317 to my AU-5050 or AU-555A, I don't really hear this. The 317 has more muscle and the 555A and 5050 are warmer sounding but my impressions are that the 555A and 5050 have something (musicality or depth of detail for want of a better descriptions) that the 317 has less of. This is obviously very subjective but do you think I'm listening to a 317 that is representative of the model or series?

Finally, if I had the opportunity to land a 717, would it be a big step up from the 317? If the comparisons I described reflect the different series (or at least my tastes for them), should I be looking in a different direction? (I'm also assuming that the 717 can be swapped for use at 240V, which has been the case for all the other Sansui amps I have opened).

Cheers, Jon

 

[Hyperion]

The AU-717 will be a very noticeable step up from the AU-317 - the overall sound is similar, but much, much more refined with the AU-717.

However the AU-317 is a very pleasing amplifier, especially after reconditioning.

 

[39cross]

f ever an amp needed a mute control, the 317 mk II seems to be it. I seem to get most of my usual range of listening volume out of the first 10% (i.e. the first three of the 30-odd) indexed notches of the dial. Is this normal?

I think this is typical, I have the same issue with my AU-317 (the first one, not the mk II). I've wondered if this is possible to change but haven't discovered any advice yet. Maybe a voltage divider of some sort? If it's possible, I am confident our Sansui experts here will know.