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Hitachi Amplifier inside

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Finally getting around to posting this, a teardown of a rather odd Hitachi amplifier, circa 1980 design. Advertised as a 120W per channel class A power amplifier, and it idled at around 900W of heat when you turned it on. I picked it up as scrap at the scrapyard, and a lot went back as metal parts. 7kg of heatsink alone in the 2 castings, and around 5kg of steel in the chassis, though I still have the 2 potted power transformers, which are 7kg each.

So here we have the underside view, after removing a dozen screws and a steel bottom cover for the unit with the 4 rubber feet. Did I mention this thing is 30kg in weight, and the 4 handles, one on each corner, are NOT an afterthought. I got tired of kicking it in bare feet, and my toes suffering the Zeno Paradoz.

The mystery white plastic cases, all will be revealed later....

The power supply and protection board, with the odd thing being that the capacitors there are all bypassed by 1uF 200V polyester foil capacitors, all on long leads. The amplifier has a pair of power supplies for each channel, arranged on each side of the board. The capacitors and fuses are there for the driver stage power supply, which is around 70VDC, and it uses 1A diodes in the bridge rectifier, which has a fuse per AC input. The power stage uses 3A diodes, and is strangely enough not fused, relying instead on the 5A fuses on the tag board above, which are on the transformer primary.

Here is half the amplifier stage, still in the board. Mains switch with the arc suppression capacitor across the terminal.

Other half of the amplifier, with the output relay. Operates a few seconds after power on, then on this unit opens again after 20 seconds as the little red "protection" light slowly comes on. 4 pole change over, using all 4 poles in parallel to switch the speaker output from either shorting to itself or to the centre point of the output MOSFETS. There was a bridging module available that made it a 200W per channel class A amplifier.

Heatsink side with the 4 power MOSFETS per channel, 2 P type and 2 N type, and the dust collection.

Underside of the one amplifier board, with the obligatory bodge components and wires, and the very crispy board where it was lovingly toasted over the years. Those little caps are not supposed to be that brown...

Under the mystery white plastic rolls, a pair of film capacitors across the main reservoir capacitors, as extra bypass. They seemed to love adding bypass capacitors across the main electrolytics, though I doubt it made much difference.

One of the side panels with the 4 TO3 pattern holes, basically a 400W rated heatsink.

Under the power board, with the cooked area which is the protection circuitry.

One of the 2 power transformers, all nicely potted with resin in the steel box. Universal primary, 100VAC to 250VAC using assorted taps, and with a built in thermal fuse as well, though there is a monitor point for it which is used in 100-130VAC operation to connect the other half of the primary. 2 secondaries, which are all led out in the same tangle of wires.

The rating plate, with the Made in Japan proudly on it. Do not let the serial number fool you, I think they did not make too many of there at the price they originally sold for. 1001221 probably means it was number 1221 or the 21'st one in December.

Forgot the best to last, genuine Hitachi brand electrolytics, and Hitachi branded semiconductors throughout. 15000uF 80V, 4 in 2 steel boxes. 

hmmm, pitty seeing it all dirty and coroded

Well, it did come from a scrap yard.......


For sure, it is not in the worst possible condition.
Imagine: The former owner was a heavy smoker...

BTW: Is it possible to implement an autoscaling for pictures into the forums software?
Viewing 4000x3000 pics on a 1600x900 display makes no fun.  :(
Especially if the text is also stretched to this width.  >:(

Mr Eastwood:
A bit of a beast that is!  do you save the boards or just dump them?


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