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Vintage Engineering is Beautiful

October 30, 2014 Leave a comment


EL84 output tubes in an EICO HF-81 integrated stereo amplifier.

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Sometimes you just have to build it yourself.

October 28, 2014 Leave a comment

When a vintage component network goes bad and you can’t exactly go down to the corner store and buy a Sprague TC-24 anymore, sometimes you just have to build a new replacement yourself.


Marantz 2270 Stereophonic Receiver Overhaul

October 26, 2014 Leave a comment

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!

Rain City Audio Stereo Repair

Use Copper Shielding Tape to Protect Dial Strings from Hot Tools [Tips and Tricks]

October 24, 2014 1 comment

Dial strings are, quite frankly, about the worst things in the world. They’re usually a complex and finicky mechanical system parked right in the middle of an otherwise straightforward electrical project, and if a string breaks good luck getting it back together again in all but the simplest of dial string arrangements. And they break easily. On older, tube gear the dial strings have often worn out and snapped with age and friction. On newer gear, the dial strings have often been snagged on the case at some point during a previous repair attempt, or even worse, they were accidentally nicked with the soldering iron and burnt or melted apart.

That’s a problem which has happened to me quite a few times, even with a steady hand and the best of intentions. After spending many hours re-stringing the dial on a 1970 Toshiba tabletop transistor radio after my soldering iron caused it to snap where the string passed very near the amplifier PCB, I was inspired to come up with a solution that’s a bit more reliable than “just be more careful”: copper shielding tape!


Copper shielding tape is an extremely important part of an electronics test bench. It’s very handy to shield a sensitive part of a circuit from electrical interference, but also, it can shield from thermal interference as well! In this case, because the copper foil is a thin piece of metal with a high melting point, the soldering iron brushing up against the foil won’t damage the string under it, and won’t heat it up nearly enough to cause damage to anything underneath for a short tap. This is the perfect solution to the problem of dial strings snapping when trying to solder too close to them. Copper shielding tape can be soldered, so it’s perfect to provide some protection against an errant soldering iron. A small 2″ section wrapped around itself with only a small section of the adhesive removed to form a cylinder was all it took.

Copper shielding tape is extremely useful to have around. It comes in a variety of styles, but I’d recommend one that’s about 2″ wide and has a conductive adhesive so it can act as a shield without soldering as long as it’s touching a metal chassis somewhere.


I stock a full roll of 2″ x 55 Yards ($56.95) as I use this while repairing Bose equalizers and stereo receivers, but it comes in other sizes. A five-foot section ($15.95) 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: 1″ x 5′ ($10.99),  0.75″ x 18′ ($8.28), and 0.25″ x 18′ ($5.05) but these smaller and narrower sizes are really more appropriate for actually shielding seams, etc. than trying to protect a dial string.

If you try this out yourself, let me know how it goes!

Vintage Engineering is Beautiful

October 22, 2014 1 comment

1972 Marantz 2270 AM Radio Module – Rebuilt with Nichicon Fine Gold audiophile grade capacitors.

Toshiba 11H-540F Tabletop Transistor Radio Overhaul

October 20, 2014 4 comments

I recently got to work on a little tabletop transistor radio from the late ’60s, maybe 1970 at the latest, from Toshiba: the 11H-540F. Not the catchiest name, but it was near the top of its model line-up featuring 11 transistors, AM and FM, and a line input. Audio power output about 1.4W into a 4 Ohm speaker.

It’s the transistor-age equivalent of a personal radio from the tube era. Solid middle of the road performance and a decently stylish package. Its owner was reporting that it wasn’t sounding that great, and she got an electric shock when hooking up an iPod to the back. It also had a lot of noise when first turned on, and the band switch was dirty and wouldn’t stay in one position. Basically it just wasn’t working well at this point.

I pulled the chassis out to get started. Definitely time for a rebuild. This 500 uF 6V capacitor had cracked and started leaking out the top.

The power supply board, with one pass transistor and a set of rectifier diodes. Replaced the 500 uF 15V capacitors with new 470 uF 16V models.

To work on the rest of the boards, the dial face has to come off:

In progress replacing components. I’d power up every few capacitors just to check, since these very early PCBs can develop cracked traces very easily. By checking regularly, I’d know if a trace broke with the last component I installed. Fortunately, none did!

Ultimately, everything ended up getting replaced.

A shot of control cleaner into the band switch and into the volume control cleaned up the scratchiness and intermittent connection. With all new caps, the static and noise on turn-on was completely gone, too.

It’s back to like new condition. No more shocks, no noise and crackling, no randomly cutting out and needing to fiddle with the switch. Just a warm vintage sound. This radio is rated for 1.4W max into its internal 4 Ohm speaker, and it managed this at around 2.5% THD. That’s after adjustment, but the nature of the distortion, the frequency response, and the speaker setup meant that it actually was still a pleasant sounding and non-fatiguing audio source. Perfect for background music in an office, for instance. It looks pretty sharp, too, with the atomic design above the dial and a nice wood cabinet and reddish grill cloth.

Here’s to many more years of happy listening!

Rain City Audio Transistor Radio Repair

Esteemed Bose® Model 901 Active Equalizer – Serial #230 Repair

October 17, 2014 2 comments

Cross-posted from Rain City Audio:

I had the distinct pleasure of working on one of the earliest  Bose® Model 901 Active Equalizers from Series I’s first production run. Serial #230, in fact! It’s made entirely by hand and shows traces of hand re-work as the design was being tweaked – really a piece of history!

This particular equalizer was a gift from Dr. Amir Bose himself to its current and only owner as an undergrad on a factory tour in the early ’70s, and after a lifetime of enjoyment it was time for a rebuild.

Incredibly, this one still has the “Acoustic Suspension Loudspeaker” decal in the center of the treble contour knob.

The owner reported it was having several issues, and opening it up one problem was obvious: the bass contour inductors had broken free of their mounts and were rattling around inside.

It was fairly similar to the later Series 1 Early boards I’d serviced, but had different style inductors and a few extra jumper wires.

It did have discrete Zener diodes installed instead of the reverse-connected transistors. And as for the transistors – the early production models used the 2N3393, and on this one they were so worn out they were only delivering about 20% of their normal gain tested out of circuit. That’s definitely no good!

It cleaned up very nicely, though:

This equalizer is restored back to full performance for its owner to enjoy for another few decades. With all new Nichicon Fine Gold electrolytic capacitors, 1% resistors, new miniature wirewound inductors, rectifier diodes, transistors, Zeners, and a thorough switch cleaning this one is good as new and will sound great for a long time to come.

Rain City Audio Bose® Active Equalizer Repair


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