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Maplin 100w mosfet amplifier built from 1980 kit
Maplin 100w mosfet amplifier built from 1980 kit
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Old 5th June 2014, 12:21 AM   #1
nigelwright7557 is offline nigelwright7557  United Kingdom
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Default Maplin 100w mosfet amplifier built from 1980 kit

I have bought a Maplin 100WRMS mosfet amplifier off ebay that hadn't been built, it was still in kit form. As rare as rocking horse droppings !
So I built it up tonight and it worked first time after all these years.
Sounds very good.
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Last edited by nigelwright7557; 5th June 2014 at 12:24 AM.
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Old 5th June 2014, 08:01 AM   #2
Wildwater is offline Wildwater  United Kingdom
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Positive karma - I hope.
This is exactly the board and components (even to the the "staining" on the o/p Hitachi TRs) that you very kindly helped me be confident enough to power up last November.
Mine continues to do excellent duty driving a pair of same vintage Castle Conways thus making my Television sound much better than it did.
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Old 5th June 2014, 09:25 AM   #3
AndrewT is offline AndrewT  Scotland
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The flange of the To3 looks quite thick.
Can you tell if the flange is aluminium or steel?

Does the contact face of the flange have an insert?
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Old 5th June 2014, 11:02 AM   #4
Ian Finch is online now Ian Finch  Australia
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Probably only steel could be as thin as the cover and still be welded consistently to the baseplate. It certainly is steel, even being a little rusty, hard and magnetic so you can lift them even with a weak fridge-magnet.... I have a 2SK135 and a similar few vintage Hitachi TO3 transistors that have just as thick a base (3mm) that is light and soft enough to be scraped like aluminium. I'd guess they are an aluminium alloy suited to the type of weld used.

Perhaps the "insert" on the base is just an indentation mark of the bottom die of the welder. That can be any size or none at all depending on the die and whether it's for attaching a heat spreader, or the cover.
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Last edited by Ian Finch; 5th June 2014 at 11:20 AM.
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Old 5th June 2014, 04:17 PM   #5
manso is offline manso
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Trip down memory lane.

I built more than 20 of these kits for me and friends when I was a teenager. Each one worked flawlessly. 6 modules were employed as Pro amps and used to power the speakers in the school hall where school dance parties were held. Never did one fail, that is upto the time I left highschool. Thinking of which, they probably still there.
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Old 5th June 2014, 05:36 PM   #6
Vostro is offline Vostro  South Africa
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I had to fix a amp for a church about 10yrs ago.
Turns out it was a DIY amp, based on the Maplin MOSFET amp.
The two modules were attached to the same heatsinks, with the one module mounted upside down.
One channel still worked, turns out it was the upside down module that was faulty.
I measured the output fet to be a short.
When I took out the fet, I saw that one pin was soldered to the case.
I cleaned of the solder and put it back in, working still today.
Turns out that the module got so hot that the solder on the pin ran down the leg, shorting it to the case, very hardy amps.

This was also the first discrete power amp kit I built while at school.

Last edited by Vostro; 5th June 2014 at 05:38 PM.
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Old 5th June 2014, 05:40 PM   #7
voltwide is offline voltwide  Ireland
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These Hitachi MOSFETs were the best I got into my fingers ever. In the eighties I built several 5kW Monsters each with 36pairs of 2SK135/2SJ50
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Old 5th June 2014, 09:46 PM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
The flange of the To3 looks quite thick.
Can you tell if the flange is aluminium or steel?

Does the contact face of the flange have an insert?
The top metal has oxidised just slightly since the 1980's so I would guess it is aluminium. The base looks aluminium too.

The transistors are just standard TO3.
The heatsink had quite big holes drilled in it for the isolation bushes.
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Old 5th June 2014, 10:56 PM   #9
sreten is offline sreten  United Kingdom
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Hi,

You need to uprate the series resistors to the Mosfets or
it will inevitably die at some point, I recall 100R to 270R.
Fine at low volume, but it dies being being hammered.

rgds, sreten.

YMMV, but I've had bad experiences driving said amplifier
hard. The flange in the kit I had I recall being aluminium.
The devices being steel T03's.

Last edited by sreten; 5th June 2014 at 11:17 PM.
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Old 5th June 2014, 10:58 PM   #10
nigelwright7557 is offline nigelwright7557  United Kingdom
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I am currently working on a update to the Maplin mosfet amplifier circuit.
I have added a Vbe multiplier, added 390R gate resistors and changed the mosfets to irfp240/9240 with source resistors.
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