Monthly Archives: April 2016

Matchmaking

I recapped an old Dell power supply, as it has some useful voltages (-12, +12, +5, +3.3) and was more than enough powerful (160W) for most of the projects I do. Added a small load to the +5 rail and the voltages stabilised, that’s the light bulb.. spent a bit of time and built up matching circuit on Veroboard.

messy

Initial testing was good, so tidied things up a little.

board

Set about measuring transistors to see the Vbe of each. Matching at 72F to within 2mV.

measuring

Did 100 of them, they are surprisingly close (all within about 5mV) so I have more matched and/or similar pairs than I thought! That’s good, means all the transistors installed can be identical.

McIntosh recap

Along with the parts for the Marantz, I bought a set of parts to make a transistor matching circuit. I bought 100 of each NPN and PNP silicon to match up accurately, as that’s what the spec calls for on the amplifier boards.

recap preamp

I recapped the preamp boards first. All ultra-low impedance capacitors, as per the original spec, though these are aluminum rather than tantalum.

regulator

Redid the regulator/phase-shift board.

compare

As a comparison, I put the power amp boards side by side, the left one done, the right one yet to be.

power amps done

As if by magic, the power amp boards are recapped. I bought some very tall and thin caps to restuff the multi-cap (4 capacitors in one can) because it growls a bit on power-up. I want to keep the appearance of the original can because it’s a prominent feature of the top of the case when you look at the chassis. It’s polished aluminum. That’s up next. I’m going to save up and get the main filter caps at a later date because they are rather expensive ($25 each).

AM Stereo

Back in 2013 I picked up as part of a package deal a Marantz SR840 receiver. I got it for nothing because “it didn’t work” and “if you feel like you want to fix it, go for it”.

sr840

Sure thing! The above picture is from after a thorough clean. It was grimy and horrible. Powered up but didn’t do much more than blink lights and show it was tuning to a radio station.
I’d had a bit of a dig about, found by bypassing the preamp and feeding the power amp it would make fairly acceptable noises. The preamp did not put out but the radio board had output. It was very one-sided when probed with my oscilloscope, and sounded distorted.
I tried to poke about for voltages but without a schematic I hit a dead end. I hunted about for a service manual but found none. It got shelved.

I was thinking about it the other day, and decided to go on the search again for a service manual. In 2014 someone put up a good scanned copy for sale at Analog Alley for $16. Fifteen minutes of work later I determined the cause- no -15V on the preamplifier.

schematic

I ordered some bits and bobs. There’s a small regulator chip for the +15 rail, I’m guessing there was no -15V integrated regulator available at the time so a PNP transistor, bleed resistor, couple of capacitors and a Zener diode formed a -15V regulated supply.

R443

I managed to make one of the resistors smoke up accidentally on the radio board when I disconnected the -15V supply to see if it would come up with no load.

new resistor

Replaced that, bought some new parts.

broken bits

Removed the power regulator circuit and replaced it with new. The 100uF capacitor had gone short circuit and had taken the Zener and transistor with it. The resistor was a bit high due to the current draw so was replaced also.

new

All the new bits in on the board. The entire chassis is packed quite tightly.

alive

Net result was to be presented with stereo FM! It worked for about fifteen minutes then overheated a capacitor in the power supply… the display and LED’s went all funny then it shut off. It came back up a short while later after it had cooled off, so it just needs all the electrolytic caps replacing. It’s kinda worth it for the stereo AM.. the tuner on it is actually pretty good. It’s being buttoned up for now, but I’m happy in the knowledge there’s nothing drastically wrong with it.

1866 for 6100

I bought a new set of #1866 bulbs for the McIntosh, as most of them had burned that illuminated the front panel.

panel off

The old foam surround going around the buttons had dried out and was turning to dust.

vacuum

Too the vacuum cleaner to it, it all just disintegrated and fell off.

peel

That left just the original sticky backing, which peeled off.

foam

A new length of foam that I’d bought for things like this came into play.

new foam

Created a new surround for the buttons following the original pattern.

hot

The original bulbs run quite hot. The speaker ones were not even brightness, so I fought for half an hour getting them out.

nice

All buttoned up, new bulbs fitted. I think once I get a new face for it, the incandescents will be replaced by LED equivalents. This is mostly just a hold-over because it was annoying me not being lit up.