Reverse engineering a John Widgery Sondex/Radford SS amplifier.

[duncan3x3]

Hi everyone,

some years ago I purchased an Amadeus Gold designed and manufactured by sondex ltd in the uk. It was blown up at one of the childrens parties and got boxed in the workshop years ago for later repair. My grandaughter just had her 16th so I thought it fun to resurrect the amp her mum had blown up.

I am not an audiophile by any stretch but I think I can recognise quality. I do have very good practical skills and passable theoretical knowledge, although linear circuits have not previously been my main interest, I thought this would be fun to repair and I would benefit from learning. Maybe somebody would like to see the design, and maybe advise me as to how I might be able to set it up as the designer intended.

In the first Instance I will try to post a raw pic of the amp itself.


lets see what happens...................... lets see if I can upload a pic


...... first lesson... do not try to view thumbnail in preview mode firefox will not let you go back and the whole post is lost!!!!!


cheers

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[Mooly]

99% of faults will be failed output transistors (on the heatsink) and probably the driver transistors too. Low value resistors can get taken out too when devices fail.

If you had a circuit we could advise much better but it looks a classic and straightforward design.

Too late in the day to "get heavy" but a start would be checks on the output devices to see if any read short circuit.

If and when it gets powered up the advice is always use a bulb tester to save damage and mishaps.

Two threads here will give you insight into what is involved,
Arcam Alpha 3 Left Channel broken...

Help repairing Pioneer M3

[duncan3x3]

hi Mooly,

Actually I too am a little tired today....

History is that I replaced the TIP3*c pairs and two of the bd140 drivers, a couple of resistors were out of 1% range so they were replaced and have replaced some pots that were also degraded.

The amp is working but I would like to understand a little better what is going on .

Classic design but layout with letraset and the resist pen (those were the days)

First off I mapped part of the the board layout straight into cadsoft and then reverse created a schematic and have been fiddling with the nets to make sense of what you correctly say is fairly classic and straightforward design. ( not my department though )

I am having difficulty exporting cadsoft eagle layout and schematic into a form that I can post. It is difficult to make the feedback loop make readable sense in the schematic.

thanks for interest, will read your links

cheers

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[davidsrsb]

The "style" of that looks very like my old Cambridge C70, the capacitors, switches, pcb silkscreen etc

[Mooly]

Any non standard files can be put in a zipped folder and attached. (I don't use Eagle )

One thing that stands out in the picture of the amp is the (apparent) lack of thermal sensing of the output stage which in 99% of amps is a transistor in thermal contact with the heatsink, what is called the Vbe multiplier. Thermal stability and possibility of thermal runaway is a real danger.

[duncan3x3]

Thanks David, I believe that there does seem to be similarity between some of the designs that I have come across under "vintage" especially the stock components, the remote selector for example, seen as the blue ribbon that has a steel insert which slides. A first glance it reminded me of a grounding strap. I did have to disassemble the switch assembly and clean the contacts which had tarnished/oxidised, as two of the inputs were inactive. I will have to service the selector for the preamp output at a later time. I chose not to put it in the sonic tank but did it by hand with cotton buds and finally cleaned up with isoprop and safe low residue solvent. I try to be as careful as possible with all solvents, a slow and inevitable destruction to any component is the dreaded WD40.


hi Mooly,

I was hoping to be able to post in a format that was easy and convenient for the forum to read, I do not mind spending a little time getting them converted. On that subject I have tried to trace the current owners of the design and have emailed Woodside Electronics who I believe inherited all designs from sondex and Radford(mainly tube amp bods), I have not had any replies, I was respectfully asking permission to publish the design. I can only assume that nobody holds the rights, but I would have preferred as a courtesy.

I have two silhouetted photo images taken of the front and back (mirrored) of the pcb which I used to copy the layout.

Thanks for explanation of the Vbe multiplier module, I replaced both BD140 with as best matching hfe as possible the BD139Y had a BC182 mated back to back, again I remounted balanced replacements taking care to mate the surfaces correctly, added a bit of thermal compound and rubber booted. I am assuming this might be the thermal protection module to which you referred.


cheers


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[Mooly]

I think I see what you refer to re the BD's and the BC182 in contact. Very unusual.

If you have a schematic in Eagle then something like Windows snipping tool (Vista and W7) would make a .jpg from whatever is displaying on screen.

[duncan3x3]

here are the silhouette photos of pcb.

still working on exporting images from cadsoft.



cheers


..

[Mooly]

It should be fairly straightforward to sketch the circuit itself, certainly for the main power amps.

Looking at the outline, the power transistor emitters appear to go to the rails so its going to be a CFP (complementary feedback pair) configuration.

[duncan3x3]

Hopefully this is partially readable, I am concerned that I do not seem to be able to get the file small enough to maintain readability, The reason all the components are well spaced is that I keep rearranging to try to make sense of the design..

anyway here is a possible interpretation of the power amp section.

also included specification from the original manual

cheers


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