Marantz 2270 Stereophonic Receiver Overhaul

Posted on October 26, 2014 by retrovoltage

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!

Posted in Commentary, Photos, Vintage | Tagged , , , , , , | 1 Comment

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

Posted on October 24, 2014 by retrovoltage

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!

IMG_20141011_143900

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.

31RvI39XypL

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!

Posted in Commentary, DIY, Tips and Tricks, Tools | Tagged , , , , , , , , , | 1 Comment

Toshiba 11H-540F Tabletop Transistor Radio Overhaul

Posted on October 20, 2014 by retrovoltage

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!

Posted in Audio, Electronics, Vintage | Tagged , , , , | 5 Comments

HP 143A Wide Screen Oscilloscope Mainframe Restoration – Part 2

Posted on October 15, 2014 by retrovoltage

Part 1

I’m continuing to work on getting my HP 143A ocsilloscope mainframe fixed up as a dual-trace X-Y display.

A lot of the time has been spent on reading and researching, but I’ve picked up an important tool that will help get me there: a Heathkit Oscilloscope calibrator.

This is a simple but important little device which will help me calibrate several of my oscilloscope projects. It produces a precise DC output level from 1 mV – 100V in multiples of 10, along with precise crystal-controlled frequencies for calibrating a timebase, etc This one was a kit, but it was put together well and is in great cosmetic shape for its age.

The heart of the calibrator is a 4.000 MHz crystal oscillator. This reference is passed through a series of frequency dividers and multipliers to obtain the calibrated reference frequencies.

I hooked it up to my scope to test. The frequencies were spot on with my scope’s internal counter (4-digit, so 1.000 MHz). I didn’t check against my more precise counter but this will certainly be good enough – I’m not an NIST cal lab (and don’t want to be!) so a “bench cal” will be just fine for these old scope repair projects.

All the functions check out. I’ll probably replace the 4 electrolytic capacitors as a preemptive measure just to ensure it doesn’t develop problems while I’m using it just to be sure. Otherwise, though, it’s going to be very useful for the 143A, and also my HP 130C oscilloscope project as well.

I’m still looking for another HP 1402A dual-trace amplifier, since this mainframe needs two identical amplifier modules to be used as an X-Y display. If you have one, please let me know!

Posted in Electronics, Gadgets, Test Equipment, Vintage | Tagged , , , , , , | 1 Comment

Harmony H500 Solid State Guitar Amplifier Repair

Posted on October 10, 2014 by retrovoltage

And now for something a little different, this time it’s a vintage guitar amplifier: the Harmony H500.

The owner sent it to the shop out of the cabinet to be serviced since it quit putting out any sound. It’s a pretty straightforward circuit with 3 transistors and a handful of passive components and tone controls to adjust the sound.

Harmony H500

Inside, it shows clear evidence of having been worked on before, and not quite up to standard. The caps were replaced with more or less random values of capacitance and voltage ratings; it was originally fitted with a 40 uF, 30 uF and 2 uF cap.

Since the capacitors on something this age always need to be replaced, that was the first step. Afterwards, though, no amplification on the input meant the transistors were dead. I replaced them with new 2N3391s and she fired right up.

This little amp delivers about 1.5W of power output from 8.2W at the wall. Not too bad!

Posted in Audio, Electronics, Projects, Vintage | Tagged , , , , , , , , | 1 Comment

DIY Replacements for Vintage Couplets and Printed Electric Circuits

Posted on October 9, 2014 by retrovoltage

Starting in the ’50s, they’d figured out how to start miniaturizing components. “Couplets” and “printed electric circuits” started showing up in equipment around then. These components were combinations of several resistors and capacitors in a single package.

flatpack

Like most components from that era, they do start to go bad over time – and you can’t exactly run down to the local Radio Shack and buy a Centralab PC-191 or a Sprague TC-24 anymore.

PC-191

So, sometimes it’s necessary to roll your own.

IMG_20141008_192619

Those two boards replace a set of tone control circuits in the EICO HF-81 amplifier. A little bit bigger when made with discrete components, but not too bad! These will be much more reliable, too.

Posted in DIY, Electronics, Ideas, Vintage | Tagged , , , , , , , , , , | Leave a comment

Phase Linear 2000 Pre-Amp Overhaul

Posted on October 6, 2014 by retrovoltage

I had a Phase Linear 2000 on the bench for a repair recently. It was running okay, but had some pops and clicks and the controls weren’t all that smooth. Time for an overhaul before anything worse cropped up!

This is a pretty fancy pre-amp made to be used with the Phase Linear 4000 power amplifier. It has a variable level output to be paired with a fixed input amplifier.

After fixing it up, I found this one to have a neutral, tight, almost laid-back sound without any extra embellishment. It was very nice – and part of that is owing to the very simple design.

The underside of the single-sided board Quality construction for the era. The bank of switches is an open design which gets gummed up easily but control cleaner easily solved that problem.

Of course, a pre-amp like this deserves nothing but the best: Nichicon Fine Gold series electrolytic capacitors.

The matching power amplifier will come up for service soon, so stay tuned! This Phase Linear is good as new and another piece of gear from the height of hi-fi is saved.

Posted in Audio, Electronics, Hi-Fi, Projects, Stereo, Vintage | Tagged , , , , , , , , , | Leave a comment

What do you get from a $25 multimeter?

Posted on October 4, 2014 by retrovoltage

Fred Clift wrote over on his blog about why it’s worthwhile to pick a more expensive multimeter. He’s comparing a $200 Fluke to a $6 Harbor Freight multimeter, which really is a no-brainer. I’m not sure the Harbor Freight model would be safe to use on any of the higher range measurements with those whispy, poorly insulated test leads.

I use a few mid-grade meters for basic bench measurements and was interested to see how they compare with the Fluke 87V and the Cen-Tech from Harbor Freight.

I use the Mastech MS8268 meter which I recommended in my “Getting Started with Basic Tools” post.  It’s durable, rugged, and costs only $25. Really, it’s about what I expected to find. As you go down in quality you lose accuracy, but the Mastech meter has all the same functions as the Fluke – just not quite as tight.

The Fluke does have some cool features that Fred picks out. It will tell you min/max/average over an interval, faster continuity beeps to quickly check a strip of connections, and a higher voltage continuity test that can actually light up an LED. Otherwise, though, the Mastech has the same basic capabilities including 200 kHz frequency measurements. I’ve been able to use that in a pinch as a signal tracer – set the meter’s range to measure frequency, and probe the stages of an audio amplifier. Meter shows the tone? Move on to the next stage.

A lot of applications don’t need 0.05% accuracy on a DC voltage reading, so I stand by my recommendation of the Mastech as a “good enough” meter for most hobby work and vintage equipment repairs. There’s definitely situations where it’s worth it to spend a bit more, though.

Posted in Commentary, DIY, Electronics, Gadgets, Test Equipment | Tagged , , , , , , | Leave a comment

Wherever you go – you’re headed for FUN with Motorola Portables

Posted on October 1, 2014 by retrovoltage

motorolaportable

No matter where you’re going, you’re headed for more Fun with a Motorola Portable! They play anywhere – on their own batteries or on house current. You’ll like the dependable performance that comes from the ruggedly built chassis and shock-resistant tubes. And you’ll be proud of the beautiful styling of the sturdy, weather-resistant cases. Make your choice the choice of millions…Motorola!

 

Posted in Collections, Photos, Radios and Tubes, Vintage | Tagged , , , , , , , , | Leave a comment

New Toy: HP 130C Oscilloscope

Posted on September 29, 2014 by retrovoltage

I’ve been looking for a nice balanced input oscilloscope to use as a bench audio scope for a bit, and I do enjoy some vintage test equipment…so I recently ended up with an HP 130C oscilloscope.

This early ’60s tube scope from HP is the third revision of their first oscilloscope line, the 130 series, which began with the 130A. The 130C has a much more serviceable construction and a variety of improvements, including a no parallax CRT for more accurate readings. It’s often regarded as one of the finest oscilloscopes HP has ever produced for the care that went into its design. It’s not the most full-featured by any means, but it was top of the line in its day and still offers extremely sensitive inputs for the smallest signal measurements.

This one was being used at HP Microwave Semiconductor in California until 1982 when it was put into storage.

Being in use into the ’80s, most of the paper capacitors which would have failed have been replaced. There’s only one 0.1 uF capacitor in place which needs to be swapped out, and the electrolytic capacitors, and it will be ready for a first power-up.

This one will be a great audio scope. It has a built-in X-Y mode and very, very sensitive – 200 uV/div, which is 20x more sensitive than any of my other oscilloscopes for detecting the smallest amounts of distortion and high frequency noise on an audio signal. There’s a working condition HP 130C on eBay right now which displays a trace for an example of what they look like. It’s a very classy instrument!

Should be a fun project. I’ll post here when it’s ready to go!

Posted in Electronics, Test Equipment, Toys, Vintage | Tagged , , , , , , , , , , | 1 Comment