Sansui AU-D11 ii repair attempt

Hyperion said:
The voltages don't appear to be all that wrong, - could possibly be explained by damaged voltage selector, or wrong mains voltage selected (or wrongly wired after the previous experimentation) - and you said it delivered 60watts output.

But perhaps the voltage issue is more subtle and needs further investigation? - nobody knows exactly 'what' and 'why' the wiring was changed - so we're all guessing.
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Amp is wired correctly for 120 volts ac. Also after further thought I think the primary are using pin connectors so likely that wasn’t the issue either (mixing primary and secondary).

This is why it’s great to have a working unit when I have one in for repair.

Keep the suggestions coming. I’ll keep testing the amp stages next. Until someone has a suggestion on the power supply.
 
ouch, at least we know why the 42/52V wires were shorted

Can you test those windings to see if they arc inside the PT under load? I assume you tested the external wires & there's no short anymore

If they got hot enough to melt wire insulation outside the transformer who knows what happened inside
 
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Hyperion said:
Just for information, the AU-D11 II is an extremely fine amplifier, I own one and will never be parted from it. It would be a great shame if it were not repaired or parted out. As an opportunity to own one of the 'greats' will have been missed. :(
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Never had a project I couldn’t fix so that is my motto too. Is yours anywhere you can test that secondary voltage I’m having an issue with? It’s accessible right on the switch on top.
Thanks!
 
Edit: I have only tested the filter caps and they do not have ac leakage in circuit so that tells me they are still good for testing purposes.

Thanks for all the help! This unit deserves to sing again!
 
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restorer-john said:
Totally correct, I missed one of the pictures and read something completely different. The Pure Focus got me. (what were they thinking?)
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To be fair, I had to look twice! I'm very familiar with the "HITACON" branding from the top, but had forgotten about the "Pure Focus" text on the side.
 
Hyperion said:
Your voltage readings from the DC output of the high current windings don't seem to be that far off +/- 34V (measured) vs +/-36.5V (schematic).

Getting half the power suggests a ground fault maybe? - how are you measuring the power output? - to measure the power output from one channel you need a 2 channel oscilloscope, with one probe connected to each speaker terminal one Red, one Black, and the probe ground connections going to chassis - with the oscilloscope set so that you can read the sum of the output in Volts RMS or Volts peak-to-peak, and then calculate the output power from those readings.

Those burnt wires are a bit of a worry, you need to make sure you haven't missed some more damaged wire.

The overall situation suggests that whoever had it before didn't know what they were doing and didn't realise that with this design BOTH the speaker terminals for each channel are effectively 'hot' and neither should be grounded under any circumstances.

As you are discovering, these amplifiers are a handful for someone unfamiliar with the design. Each channel consists of a pair of amplifiers connected in a push-pull arrangement (so actually 4 amplifiers in total). The Push being the 'Hot' amp, and the Pull being the 'Cold' amp making up one channel. At the moment I don't suspect that the transformer is faulty, but I don't know how much damage the incorrect wiring caused.
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Rereading this post. Is it possible this has to be connected or tested differently to an hp 8903 than say a Sansui au919?
 
pjrengineer said:
Rereading this post. Is it possible this has to be connected or tested differently to an hp 8903 than say a Sansui au919?
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Yes, you cannot connect the negative speaker terminal of the AU-D11 II to ground - if you do, you will seriously damage the amplifier. The AU-919 is an earlier design non X-Balanced model - so it is permissible to ground the negative speaker terminal for this model.

Both speaker terminals are to be regarded as 'Hot' - hence the absolute requirement to use a means of measurement (as described), where grounding either terminal does not happen.
 
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It might be an idea to try running the power amp off an external transformer. I don't have many that I could donate to the cause however. I have one from a Technics SA-600 receiver, but the AC output is given on the schematic as 37.8V, which would make 75.6, which is too high for the 52V you need.

I also have a NAD7240PE receiver transformer. I don't have a service manual here for it, but I do have one for a 3240PE amp which is the same. There are no AC voltages given on the schematic, but the rectified DC voltage is given as 43V. Dividing this by 1.414 gives 30.41VAC, which would double up to 60.82VAC which is still too high for what you need. It does have a 4/8 ohm switch, but not voltages are given for the 4 ohm tap on the transformer.
 
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Thanks for the updated feedback. Let me get back into testing the power output again. I just wish someone had an audio analyzer they knew how to test with this type of amp and could explain that. I’m using an 8 ohm test load and I never ground the input to the audio analyzer so I don’t think that is my issue completely but I will test it with the scope as you suggested later and report back.
 
leesonic said:
It might be an idea to try running the power amp off an external transformer. I don't have many that I could donate to the cause however. I have one from a Technics SA-600 receiver, but the AC output is given on the schematic as 37.8V, which would make 75.6, which is too high for the 52V you need.

I also have a NAD7240PE receiver. I don't have a service manual here for it, but I do have one for a 3240PE amp which is the same. There are no AC voltages given on the schematic, but the rectified DC voltage is given as 43V. Dividing this by 1.414 gives 30.41VAC, which would double up to 60.82VAC which is still too high for what you need. It does have a 4/8 ohm switch, but not voltages are given for the 4 ohm tap on the transformer.
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So Sansui potentially fudged the service manual voltages here? That is what I was trying to determine earlier but I have little design theory knowledge so this helps.

Edit this maybe useful for future Sansui tweakers like me diagnosing.

Also, I’m going to retest it with a scope to confirm total power output hot and cold per H. What is wrong with my idea of two variacs wired to the input of the power supply one per channel.

I hope that it’s just a matter of testing the amp wrong due to the x balanced design. I just would like to run some distortion numbers before I call it fixed (hey I fixed the wiring and the switch after all).
 
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After testing it with the scope as hyperion suggested and a fluke multimeter and measuring across the terminals i get roughly 130ish watts per channel prior to distortion setting in. I guess theres no throwing in the towel on this one, the service is complete. All i had to do was replace the wiring that was melted and fix the miswiring of the switch, and repair the switch. In addition to the usual cleaning of the controls and verifying operation. Now the unit is performing well and I'm returning it back to my friend. Thanks for the sanity check friends, i really needed your support and you came through.
 
This things in excellent condition!
 

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bolero said:
Congrats!

So did you finally end up getting the service manual's 42V & 52V out of the power transformer, where those melted wires originated?
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Schematic is wrong with respect to voltage. My voltages were correct originally. Caused me some grief.

Melted wires were due to the 4 ohm switch wired wrong. Fixed those too.

Will it present another problem in the future? Hope not. It’s meeting power output on both of the switch settings now and wasn’t before.
 
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