Sansui AU-555A Dead channel

[argonrepublic]

I have amp with bad power in amplifier section.
I am certain it is not in the preamp stage.
I have have heard "caps" and I have heard "transistor"
I am away from home and was hoping to order parts for a weekend project.
Could somebody suggest what to replace on this amp?
I want to to order my components asap and have them waiting for me.
I'd rather solder it all in then to underestimate what I may need.
Thanks

[Ian Finch]

This is a very old amplifier. You describe the fault(s) as 'bad power" which could refer to anything but let's assume you mean bad sound - perhaps distortion? Is it low power, lacking bass, treble etc? What sort of bad power do you mean?

In the case of old gear, capacitors eventually dry out and fail and this leads to semiconductor failure too. Faults will likely be as you already suspect but there could be collateral damage to resistors, PCB traces, connectors even. Corrosion can be a factor too so assume nothing without checking the obvious physical evidence and re-joining any connectors, if used for parts or leads.

I don't have a schematic to refer to but it will be unlikely that you will find original replacement power semis anywhere local and there is little point replacing them until the electrolytic type capacitors are replaced. When you have done this, power up with a current limiting device in the power line, such as a conventional lightbulb or variac or current limiting power suppy to prevent further damage in case other parts are taken out when the circuit operation is restored with good caps. It is easy enough to re-cap, allowing that you can use higher voltage ratings and say 50% higher capacitance ratings if necessary to obtain parts that both fit with hole spacings and the space available plus even mounting clamps if they are used. Replace them all in an old beast like this and use reasonable quality parts by Panasonic, Elna, Nippon Chemicon, Rubycon etc. Don't try to fit special grades until you find what is wrong and sort that first.

Keep us updated with progress and good luck

[RJM1]

Schematic available here.
http://akdatabase.org/AKview/albums/...0Schematic.pdf

[Ian Finch]

Thanks indeed, RJM1.
Like the contemporary Sansuis, it's a Quasi- complementary, single rail power source design and output is via a big cap (C819, 820). Check for DC on the +ve output connector, as a priority - it should quickly settle to zero volts via R843,4 if it is OK. Using ground as a reference point for the -ve probe, check the power supply voltage at fuses F001,2 and also measure voltage on the +ve terminal of C819,820. Take care not to short close spaced tracks or connections. It should be close to half the supply voltage and can be a very helpful indicator in isolating output stage faults.

Note that speakers and inputs should be disconnected for this test. It has a preamp disconnect even - how good is that?

[sakis]

there you go :

vintage amplifier repair/upgrade manual

[argonrepublic]

I should have clarified that the dead channel is not completely dead but is weak and distorted.
I swapped channels VIA the jumpers and the right channel continues to be the bad one.
I was hoping there would be an easy solution like "order this, that and this".
I am thinking it is best to just replace the caps with good ones and solder in a handful of outputs.

[sakis]

your approach is bad ...

outputs are on-off devices either work or not

1-RTFM
2-change as many caps possible starting from the small ones on the amp section
3-clean all pots switches and trimmers in the best possible way


Additional information
Even if there was a solution like : ""change R128 and all will be fine"" then
A) usually forum members are tech guys do not like ""fast food solutions"" and see most repairs as an opportunity for a small upgrade also

B) that any way will not apply in your case since the amplifier is quite old and suffers from a number of problems eventhough only one take the amplifier down ...

As a closing argument please notice that repairs is about being methodical and follow procedure which in both you seem to fail .

Kind regards
sakis

[RJM1]

I would do some more troubleshooting before you order parts. The simplest thing you could do is plug in a set of headphones and see if the distortion goes away. If it does then it means that everything is ok up to the driver transistors. Then you would have an open emitter resistor and probably a shorted output transistor (R837,839 TR809,811 in one channel R838,840, TR810,812 in the other channel. That's the first thing that I would look at since you say one channel is distorted.
You didn't say which channel was distorted so I included the location numbers for both channels.
You only have to check the distorted channel. At least you have one channel that works so you can make resistance and voltage checks between the two channels to find out where the problem is.

[argonrepublic]

Yes it is didtorted even in the Headphones.
I am not sure whether the previous comment was an insult or constructive critisism.
I have no education and very little experience with this stuff. I am an automotive technician and have experience only with 12VDC circuits. I would really love to understand electronics but every time I attemp to learn it, I get overwhelmed. Does that mean I have small brain?

[sakis]

No it was a bit of arrogance to people that look for fast food solution and/or do not appreciate the simple help given . Please do not take this as a personal attack or an insult .
Take it as a helpful advice cause this was the real intention . To read the manual will take just a few minutes and to perform the advice given will take for an amateur a couple of days and may be 10-15 usd .

In between many people will run to your aid including me ... but please ...don't expect the amplifier to fix itself ! you have to do a few things like you ve been told so far .

Kind regards
sakis