1.8mm slabs of porcelain under TO220 devices.(This is continuation of Ampeg BA600-115, can of worms.I've got mains back on it and is in working order , now to try with highpower.)This is my educated guess as to what happened at manufacture as absent butdistinctive 20mm long bolt not found around here. The 4 output devicesshould be held to the heatsink via a cross-bar over all 4 of them and 2bolts thru it into the heatsink. The insulators are ceramic and the 2central ones, butted up together , do not clear the retaining bolts on theirouter edges. After graunching the thread the assembler decided to leave itout and trust to luck one off-centre bolt would hold.Coming to reassembly I found a 3mm screw for replacement but it would nothold, as tapped thread is stripped, so I tapped that one hole out to 4mm.Tried grinding a clearance notch on the side of one pad but got nowhere asceramic.Not only that but I had no trouble holding the slab in fingers while tryingto grind a slot , it was barely getting warm - good thermal insulator orsmall grinding wheel not generating heat as not cutting into the material ?So ,lateral thinking, decided to grind down the screw thread where itinterferes with the ceramic , on one bolt.I wonder if this was a one off at manufacture or a whole batch like this,all that was needed was shift a tapped hole 1mm, plenty of room there forthat. Or one ceramic pad 1 or 2 mm narrower.ps anyone use "chain mail glove" for small part in-hand grinding? At 80GBP andupwards unlikely , firstly on cost, second need fingers for gripping items.As an experiment wound some kevlar (ex fibre optic leads) around finger ofold glove and held in place with hot melt string. Simulatedgrabbing/skittering of Dremmel and grinding disc into this on a piece ofdowel. It did not penetrate to the dowel, I expected the kevlar to snag andstall the motor but cut clean through.

N_Cook Inscribed thus:I would check that ! Its more likely that they are Beryllium_oxide ! The dust from grinding or abrading Beryllium_oxide is highly toxic !-- Best Regards: Baron.

Porcelain thermally conductive insulators ?

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josephkk 15 years ago

The last time i saw a price they were over US $1 each, but that was over

30 years ago and for military use. Between the now rabid regulation and the expected cost i would be rather surprised to find them in commercial instrument amplifiers. Of course with Chinese manufacture such bets may be knocked into a cocked hat.

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Baron 15 years ago

josephkk Inscribed thus:

You mentioned "military use", the original packing had "Marconi" on the label ! I've had them for more than 20 years. They came as part of an auction lot. If I recall, a subcontracting company that went bust when Marconi was taken over.

Best Regards: Baron.

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N_Cook 15 years ago

The last time i saw a price they were over US $1 each, but that was over

30 years ago and for military use. Between the now rabid regulation and the expected cost i would be rather surprised to find them in commercial instrument amplifiers. Of course with Chinese manufacture such bets may be knocked into a cocked hat. ++++

I'm assuming boards soldered up in China but assembled into the cab in USA. So these insulators would likely be fitted in the USA. They obviously were not made for the job, wrong cross-section area, unnecessarily thick, hole not used - perhaps they just had them laying around. That grey silipad could have been used with FET legs preformed to accommodate.

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Porcelain thermally conductive insulators ?

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