Category: Electronics Projects

Electronic projects that I work on in my spare time.

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Nakamichi 620 Amplifier Repair

After the restora­tion of the Nakamichi 620 ampli­fi­er last year, it had been work­ing great for 6 plus months. Then one day I turned it on and there was no sound out­put. Luckily the speak­er pro­tec­tion cir­cuit did it’s job and no speak­ers were dam­aged. The right chan­nel had it’s out­put short­ed to B+ (53 volts).

Nakamichi 620 ampli­fi­er with new mod­ules at 120 watts out­put power
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Power Designs TP330 Power Supply

I decid­ed to take a gam­ble on pur­chas­ing a Power Designs Inc. TP330 triple out­put pow­er sup­ply. It was a bit rough look­ing with a bunch of stick­ers on the front pan­el, but no scrape marks from past stick­er removal vis­i­ble. The sell­er stat­ed that the bot­tom ana­log meter was non-func­tion­al, but the out­put ter­mi­nals mea­sured volt­age that changed with the volt­age knob.

Power Designs TP330 pow­er sup­ply as received
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Nakamichi BlackBox Series Refresh

I was gen­er­ous­ly donat­ed three Nakamichi BlackBox series units, by the same per­son who donat­ed the “System One” rack from a pre­vi­ous post.
The units were in great con­di­tion, and more than like­ly could have been used as-is. They also sur­pris­ing­ly came with their orig­i­nal own­ers man­u­als, which usu­al­ly get mis­placed or lost over the years.

Nakamichi BlackBox mod­ules PS-100 BA-150 EC-100
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Nakamichi 610 Control Preamplifier Lamp Replacement

I did not take any pic­tures of the ini­tial refur­bish­ment of the Nakamichi 610 con­trol pre­am­pli­fi­er as it was main­ly clean­ing and capac­i­tor replace­ment. But as soon as I thought the work on all the com­po­nents of the System One rack had been com­plet­ed, one of the dB meter lamps failed in the 610 preamp.

Nakamichi 610 con­trol pre­am­pli­fi­er after restoration
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Nakamichi 620 Amplifier Repair

I was kind­ly donat­ed a Nakamichi “System One” rack of audio equip­ment recent­ly. Overall the equip­ment was in good shape, but had been in a build­ing fire and suf­fered from sig­nif­i­cant smoke dam­age. There was not any detectable heat dam­age and only minor water dam­age.
After suc­cess­ful­ly repair­ing the 610 FM Tuner / Preamp I decide to tack­le the mod­el 620 audio pow­er ampli­fi­er next.

Nakamichi 620 Audio pow­er ampli­fi­er installed in rack
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Monroe 430 Calculator repair and cleanup

I man­aged to snag a Monroe 430 cal­cu­la­tor that was in good con­di­tion and a rea­son­able price. The Monroe 430 is 14 dig­it four-func­tion cal­cu­la­tor with mem­o­ry, per­cent, and a few oth­er extras. I have worked on a mod­el 425 and 420 in the past so I kind of knew what I was get­ting into, espe­cial­ly the lack of documentation.

Monroe 430 cal­cu­la­tor with 14 dig­its displayed
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Monroe 650 Calculator repair and cleanup

Welp, anoth­er cal­cu­la­tor tale of woe. This time it is a Monroe 650 cal­cu­la­tor found on eBay. The 650 is the top-end mod­el of Monroe’s 600 series cal­cu­la­tors. It is a 16 dig­it Nixie tube dis­play cal­cu­la­tor with addi­tion, sub­trac­tion, mul­ti­pli­ca­tion, divi­sion, square root func­tion, and two mem­o­ries. It oper­ates in fixed dec­i­mal or float­ing point modes, and has 5 chang­ing indi­ca­tors spaced every incre­ment of the base-10 expo­nent of three from the dec­i­mal point.
The price was right and includ­ed free shipping.

Monroe 650 cal­cu­la­tor after cleanup and repairs
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Burroughs C 3207 Vintage Calculator

Well I just fin­ished work­ing on the APF Mark 1 cal­cu­la­tor so I was­n’t look­ing for anoth­er cal­cu­la­tor project right away, but I stum­bled upon this Burroughs C 3207 cal­cu­la­tor with sev­er­al issues for a price that was too good to pass up. And it’s a vin­tage 14 dig­it Nixie tube cal­cu­la­tor Built in late 1969.

Burroughs C 3207 Nixie tube cal­cu­la­tor after clean­ing and repairs
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