Rath Amp Retro 50

[gaioshin]

Hi all,

I've recently picked up one of these weirdly coloured rare(r) solid state amps. Unfortunately the output is all hum now and I've just opened it up to find a bunch of zeners and a couple of resistors at the power amp stage that have been fried.

Annoyingly, the amp doesn't appear to have any markings on the board as to which resistors and diode numbers they are.

I've contacted Solton but am not hopeful they'll even have any information at all on this.

Does anybody have a copy of the schematic of the amp, or have decent suggestions as to how to go about troubleshooting it?



Thanks,



gaioshin

[drewl]

As with troubleshooting any solid state amp,
Hum? Is there DC on the speaker output?
Can you pull the output drivers and check if they are good?
There should be a + and - power supply, with voltage on the large filter caps
Are the power supplies there?
Is there large ripple on the power supplies?

[Jazz P Bass]

Mosfet Output Stage

How do you know that the diodes are zeners?
If they are zeners it may be an output section similar to an Ampeg BA-115
Mosfets.

[gaioshin]

Thanks for the advice guys, and sorry for the late reply.


drewl - I am still very new to this and will try and follow your steps. With reference to the questions:-

There is some DC output on the speaker, but the hum does vary with adjustment of the master volume, and it's not particularly loud - which may mean that I am barking up the wrong tree looking at the power amp section? Irritatingly, I am surrounded by fluorescent lighting, cordless 'phones, etc. so I'm not ruling out the possibility that those things are also adding to the puzzle.

The preamp section appears to have quite a few TL072s plugged in, so my next job is to see if any of those have died. I've just spotted an LM3886T bolted under a big heatsink here - is that what you mean by the driver? I'll check the filter caps after those I think?



Jazz P Bass - I know they are Zeners simply by the number markings on them ("C16PH"), and I haven't seen that many components that look like those diodes (other than one or two switching diodes). I've included a pic below as to what I initially saw. I swapped them out for new ones, but still no good (also checked the resistors but they appeared fine despite the slightly toasted appearance).







Will update as I get a chance to keep checking each bit, step by step.

[Jazz P Bass]

The zener circuits (diodes, resistors & capacitors) are your +16 & -16 Vdc supply for the opamps.

[g1]

The LM3886 is the power amp chip.
You said the original burnt resistors and zeners were in the power amp section but I'm thinking they are in the power supply section. Like JazzP said, they are for the +/-16V supply.
Are they the ones in the picture? If so, they usually run hot, so the board may turn brown but that doesn't mean the components are bad. Often the solder joints will go bad from the heat though. There appears to be one missing, did you replace it? I would guess it would be the same as the other one, which appears to be 33 or 330 ohm ?
Check for + and - 16V at pins 4 and 8 of the op amps.

[biggestgerbil]

Thanks for the advice guys, and sorry for the late reply.


drewl - I am still very new to this and will try and follow your steps. With reference to the questions:-

There is some DC output on the speaker, but the hum does vary with adjustment of the master volume, and it's not particularly loud - which may mean that I am barking up the wrong tree looking at the power amp section? Irritatingly, I am surrounded by fluorescent lighting, cordless 'phones, etc. so I'm not ruling out the possibility that those things are also adding to the puzzle.

The preamp section appears to have quite a few TL072s plugged in, so my next job is to see if any of those have died. I've just spotted an LM3886T bolted under a big heatsink here - is that what you mean by the driver? I'll check the filter caps after those I think?



Jazz P Bass - I know they are Zeners simply by the number markings on them ("C16PH"), and I haven't seen that many components that look like those diodes (other than one or two switching diodes). I've included a pic below as to what I initially saw. I swapped them out for new ones, but still no good (also checked the resistors but they appeared fine despite the slightly toasted appearance).

[ATTACH=CONFIG]24624[/ATTACH]


[ATTACH=CONFIG]24623[/ATTACH]


Will update as I get a chance to keep checking each bit, step by step.

My Retro 50 had exactly the same fault. My repair guy said that the negative supply resistor was missing from the diode circut. The power supply circut is running too hot and he said that I may require an upgrade of the diodes and resistors in the future. It is now repaired for the time being.

I must say that it is the most fantastic sounding solid state amp I have ever heard.

Regards

BG

[gaioshin]

My Retro 50 had exactly the same fault. My repair guy said that the negative supply resistor was missing from the diode circut. The power supply circut is running too hot and he said that I may require an upgrade of the diodes and resistors in the future. It is now repaired for the time being.

I must say that it is the most fantastic sounding solid state amp I have ever heard.

Regards

BG

Super job - thanks for the info



gaioshin