sansui r50-low o/p one channel

[keithgreenhalgh]

I have a sansui r50 reciever I got for $35. The guy said it works fine but one channel is ok at low volume but when you turn it up its gets distorted and looses volume.Looks like single o/p transistor.both sides have +43 on one side and -43 on the other.I thought all transistor amps had a capacitor between the output transistor and the speaker but I get zero ohm resistance to the o/p transistors on both channels.Any ideas or is it worth fixing.

[nigelwright7557]

Originally posted by keithgreenhalgh
I have a sansui r50 reciever I got for $35. The guy said it works fine but one channel is ok at low volume but when you turn it up its gets distorted and looses volume.Looks like single o/p transistor.both sides have +43 on one side and -43 on the other.I thought all transistor amps had a capacitor between the output transistor and the speaker but I get zero ohm resistance to the o/p transistors on both channels.Any ideas or is it worth fixing.

You have maybe already found the problem in that one of the output capacitors has gone short circuit ?

[keithgreenhalgh]

Originally posted by nigelwright7557



You have maybe already found the problem in that one of the output capacitors has gone short circuit ?

but it measures that way on both sides and one side works fine.Do all transistors amp have output capacitors and if yes could the other side work ok with a shorted capacitor? Does anyone have a schmetic for this amp?

[nigelwright7557]

Originally posted by keithgreenhalgh


but it measures that way on both sides and one side works fine.Do all transistors amp have output capacitors and if yes could the other side work ok with a shorted capacitor? Does anyone have a schmetic for this amp?

It depends on the amplifier type.

Class A usually have a capacitor on the output.

Class B or AB dont.

If there wasa shorted capacitor the amplifier would probably overheat as the speaker would be drawing a lot of current even with no signal present.

[keithgreenhalgh]

On the channel thats bad it looks like the p.c. got hot and burnt.there are 2 resistors(1 or 2 watt). I could measure any d.c. current with the speaker hooked up orthough I looked at the woofer to see if it was centered but the current may have not been enough to move the speaker.

[nigelwright7557]

Originally posted by keithgreenhalgh
On the channel thats bad it looks like the p.c. got hot and burnt.there are 2 resistors(1 or 2 watt). I could measure any d.c. current with the speaker hooked up orthough I looked at the woofer to see if it was centered but the current may have not been enough to move the speaker.

You could measure the DC voltage across the speaker with no sound input to the amplifier but the amp switched on. Thats would prove if the output capacitors are OK.

I usually find output transistors are a good thing to check.
See if any have gone short circuit across C-E.

[keithgreenhalgh]

output voltage to speakers=.03v on both channels.I don't know if it has an output capacitor or not but if it does I think it may be the problem. If you turn it up loud with no speaker and then connect a speaker ,it works for a second or so then the volume goes down and it starts to distort.You can do this time and time again with the same result,it sounds like a capacitor discharging or something.

[ilimzn]

OK hoooold it right there, before this goes too far out of line!

First of all, the R50 doesn't have a 'single transistor output', if it did, the 50W rated output would need heatsinks as big as the whole receiver.
It has one pair of complementary transistors per channel, under the metal piece attached to the heatsink (have you checked?!).
It is a DC coupled amplifier so has no output capacitors, a simple look inside could have told you that, as there are only two large caps, suggesting filter caps for each power rail (+ and - 43V) - did you forget these were needed?
The power amp in the R50 is connected to the putput connectors through the speaker switch on the front panel, there is no protection whatsoever, so if it goes bang, so do your speakers. The only fuse is the mains fuse on the transformer, which generally does not go open even if the output transistors turn into smoke and evaporated plastic (ask yourself how I know that...).
That being said, it does work quite well when it works. The reason you may have problems could be multifoled - for starters, check all the solder joints. Also, all the pots and switches will need cleaning. The R50 is particulairly problematic here because the tone control pots are connected into the feedback loop of the power amplifier. If there is a problem in the power amp section, and there could be, it is likely that one of the pair of output transistors is dead. They usually go short circuit, and burn out one side of the double emitter resistor (IIRC it's a 2x0.33 ohm part). At this point, the driver transistor for that output transistor becomes the 'output transistor' for the amp, but is incapable of providing anywhere near full output. If you continue trying it tends to burn the E-B safety resistor that connects it's emitter to the base of the dead output transistor. Once that happens, the amp loses the ability to reproduce half of the waveform and either distorts badly or oscilates, possibly burning out the speaker or remaining part of output stage. If that happens, the bias servo transistor (a small part leaning onto the heatsink with thermally conductive grease all over it) will go dead, and if you then just replace the output stage, it will go up in smoke when you forst switch on, if this bias servo transistor is not replaced as well (and the bias set per manufacturer recomendation).

[forr]

Hi keithgreenhalgh
I met a problem wich might be similar to yours in two Sansuis.
There were burnt resistors used to decouple the power supply of the low power stages. They were easy to identify as they were mounted higher than the others on the printed board. They may look ok, but the value can become wrong with time.

nigelwright7557
---Class A usually have a capacitor on the output.
Class B or AB dont.---
Sorry, no relation at all.

[nigelwright7557]

Originally posted by forr
Hi keithgreenhalgh
I met a problem wich might be similar to yours in two Sansuis.
There were burnt resistors used to decouple the power supply of the low power stages. They were easy to identify as they were mounted higher than the others on the printed board. They may look ok, but the value can become wrong with time.

nigelwright7557
---Class A usually have a capacitor on the output.
Class B or AB dont.---
Sorry, no relation at all.

A good trick on fixing amps is to remove the output stage altogether and feedback the output from the driver stage into the LTP. That way you can prove the driver stage without risking big amps in the amp. Only then would I risk a set of output transistors.

You clearly need to check all the output transistors before even considering switching the amp on.

Also check all other transistors i n the driver stage.