Award-Session “The Punk” 100W transistor amp

On my travels when collecting up odd and repairable guitars, basses, keyboards and amps, one of the rarities encountered was a variant of the Award-Session Sessionette amplifier – namely an adapted version of the 100W bass amp they did. The combo houses a 12″ Celestion G12-75T, rated at 100W, to match the fact that the combo itself is about 90W RMS output. Like the majority of Award-Session amps, it is a solid state unit.

When I spoke to the chap who was selling the unit, he told me that he “switched it on, and it started smoking”. Naturally, my assumption was that he left it to cook and simmer first before switching it off and may have even reheated it too. There’s a fine line between it annoying and saddening me, but at the same time, it should prove to be an interesting project. When I got back and decided to open up the unit, I noticed a couple of things:-

1) The preamp unit, speaker, toroidal transformer, as well as all wires and connectors looked fine

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2) The power amp board had at least one component that had died, leading to a small fire and possibly an exploded capacitor

The latter of the two meant that there was potentially acid goop and cack all over it. Upon disassembly, this is what I had seen…

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Even after an inital wipe down with some rubbing alcohol, it was blatant that it was pretty much impossible to decipher some of the component values. Coupled with the fact that some of the PCB tracks had been destroyed on the underside, this caused some major issues.

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Just when things didn’t look bleak enough, I noticed that I couldn’t actually get a replacement board, as they are now effectively explicitly out of production.

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Still, I decided to email Award-Session anyhow to see if there were any schematics, references, clues, support that can be either provided to me whether free or purchasable. Very kindly, they came back with the schematic for the bipolar power module that at least covered some of the component details. Alongside the available SB-100 schematic, I should hopefully be able to make some sense of the power amp section that has died. That way, alongside component replacement, I should be able to point-to-point wire across the damaged track sections.

I set about trying to recouncile the diagrams with the circuit board, when it became apparent the standalone bipolar power module and the rest of the power amp were on the same board, whereas the diagram referred to specific parts.

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I set about making notes of all the component values on the board, as well as cleaning off as much of the soot and remains of the incinerated resistors using ethanol as best as I could. Although it was tempting to use slightly more invasive chemicals, I couldn’t really risk removing any of the existing component markings on the board.

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On the rear of the board, the already damaged tracks did not take cleaning all that well, which further reinforced the fact that I would need to do some point-to-point wiring in place of those former connections.

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A little bit more cleaning was done after the remaining components had been removed. I left the MOSFETs in place as they did not appear to be impacted by the burnout (visually). I am always warying that subjecting the MOSFETs to potentially unnecessarily desoldering is an unnecessary risk, plus I wasn’t entirely sure whether replacements from RS / Mouser could be sourced for them easily.

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My sense of colour isn’t brilliant and although I don’t need glasses, my eyesight is far from perfect. Rather than junking the majority of parts pulled from the board, I labelled and taped them to a piece of paper in the event I misread any of the resistor colours or capacitor codes. The black tray there were for totally fried, unidentifiable parts, as well as electrolytic capacitors that I could easily read the values from.

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To be continued…

 

 

2 Responses

  1. Graham
    Graham October 24, 2014 at 3:02 pm | | Reply

    Hello. Did you know the SG75 – PM120 module is currently available again from A-S (Oct 2014)? I’ve just ordered one. All the same your descriptions and photographs are very helpful as the SB100 amp I’m working on had a replacement power module from Maplin also faulty and your pics help me understand what the original connections were. Regards Graham

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