From the late ’50s, I had this nice example of an EICO HF-81 amplifier come through my shop. These are nice little stereo amplifiers with a built-in phono pre-amp and RIAA equalization and feature about 14W of power output per channel from a pair of push-pull EL84s each at about 1% distortion.
EICO was a popular brand of electronics kit maker back in the day. They made a lot of hobbyist or entry level shop test equipment (oscilloscopes, generators and testers) but had a handful of hi-fi stereo kits as hi-fi listening grew more and more popular. The fact they were assembled as kits makes them pretty straightforward to service, too – the layout is clean and there’s a lot of room on the chassis. It had been worked on a few times in the past – several coupling capacitors were replaced with early film caps in what looks to be the late ’60s/early ’70s. Just recently, the owner reported it was giving noise from one channel and took it to another local shop where the technician replaced a 50 uF capacitor (orange, bottom left) with a 100 uF cap that looks to me like it was probably an old-stock capacitor (blue, bottom left.) They’d clipped the old cap out of circuit but left its shell intact; not exactly what I’d call a best practice. More problems started cropping up shortly thereafter as the rest of the original capacitors degraded, so it came to me for an overhaul. After replacing the parts, there was still noise, distortion, and lower volume on one channel seen on the oscilloscope screen in yellow. Tracing through the circuit, it turns out the CRL couplets in the tone network were failing.
I fabricated new PCBs with the same values to replace the damaged parts and mounted them to the shield.
That fixed the problem right up! After some final tweaks and a burn-in test, she’s ready to go home. Lots of parts out of this one:
It’s going to sound great for years to come!
as I use this while repairing Bose equalizers and stereo receivers, but it comes in other sizes. A 
might be a better choice if you don’t see yourself using it often, or if you’ll only use it as a soldering iron shield. It’s available in smaller, narrower sizes also: 
RETROVOLTAGE 
Marantz 2270 Stereophonic Receiver Overhaul
I just had a great stereo receiver through the shop for repairs, the Marantz 2270. It’s the big brother to the Marantz 2245 I had through the shop a little while ago – mostly the same front-end circuitry, but with the addition on a multipath indicator to the FM tuner, and of course different final amplifiers.
This is a very early example, serial #3593 from the beginning of the production run. It was designed in the USA and manufactured in Japan.
These receivers were really built incredibly well. They’re easy to work on, for the most part, and delivers a lot of power with very low distortion: the 2270 is rated for 70W per channel into an 8 ohm load < 0.3% distortion; the FM section is very sensitive and selective, too.
This receiver came into the shop working “okay”. There was a lot of distortion in the FM, and overall it sounded muddy and a bit thin. The original capacitors were going south but fortunately nothing had catastrophically failed so it would clean up very nicely. I started off replacing the regulated power supply capacitors and adjusting the power supply. After each board repair I like to power it back on and verify each subsystem, so this made sure nothing would go wrong while doing the rest of the receiver. The board-mounted electrolytic capacitors were replaced with top of the line Nichicon Fine Gold audiophile capacitors; the chassis mount capacitors with slightly up-rated Kemet caps.
There’s only one cap on the phono pre-amp board:
Next, onto the tone board where I cleaned and lubricated all the controls:
The FM discriminator on this one had 6 more electrolytic capacitors, and an old school TO-5 style IC – actually the only IC in the entire unit.
The AM board in this receiver is mounted above the FM stereo demodulator.
Underneath, the stereo demultiplexer board got the same treatment:
This receiver had a major repair at some point in the past. One final board was original; one was from a much later 2270. Most likely something failed back in the ’70s and it was replaced under warranty.
Each final amplifier only has two electrolytic capacitors to replace:
Finally, it was time to check it all out! I started with adjusting the bias and DC offset to ensure the amplifiers would perform up to spec for the rest:
, then proceeded through the RF alignments:
The FM receiver was definitely out of spec. The dial alignment was correct but the distortion spec was very bad.
After 11 adjustments for FM, the distortion vanished:
I measured the power and distortion of the amplifier through the line input, too. This receiver was able to produce a bit over its rating, with 96W output at 0.25% THD. Not bad at all!
Ready to go!
This receiver is fantastic. It’s cleaned up perfectly and now it’s performing better than new. Should last for a long time, too!
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