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!

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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!

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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.

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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.

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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.

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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!

 

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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!

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Steampunk USB Vacuum Tube Flash Drives

Posted on September 26, 2014 by retrovoltage

I ran across these very cool steampunk-inspired USB flash drives drives built around a vacuum tube while looking for some tube shields. They look like they’re really well built and are a clever use of an old vacuum tube. I have a jar of dead 7- and 9-pin miniature tubes that I give away for art projects, and this is a perfect example of how electronic waste can be recycled into something cool.

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They come in 8-64GB sizes and 4 color choices. Looks like they’d make a good gift, too. They’re for sale from eBay vendor slavatech, and ship for free worldwide.

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NordeMende Globetraveler 7/601 Rebuild

Posted on September 23, 2014 by retrovoltage

This rare and valuable example of late 1960s German engineering came through my shop recently, the NordeMende Globetraveler 7/601. Marketed as the Globetrotter in Europe, RCA already marketed a radio called the Globetrotter in the United States and so the NordeMende was re-branded Globetraveler when sold over here. While it came in working on its original components, there were a few issues. First, the alignment was a bit off. As a battery radio, the dial lamps are only on when the lamp button is pressed; that switch was stuck with the lamps always on. The bass and treble controls were a bit gummed up, too, and it sounded muddy.

Since this is a rare, intricate and valuable piece it deserved a full overhaul, complete with a precision alignment of every band – and there’s a lot of them: FM, AM, Longwave, and 11 distinct spread shortwave bands. Talk about a lot of tweaking!

Those problems would’ve been extremely tough to fix, but fortunately the owner had a donor parts set in even worse shape he was able to send along as well. Between the two poorly working radios, there’s enough to make one good one!

These early PCBs are extremely fragile and even with lab-grade rework tools can still be damaged. Fortunately, this set used all axial electrolytic capacitors, so I was able to clip the leads and use the old leads as mounting parts. A few locations I was able to de-solder and re-solder directly to the PCB, but most were axially mounted.

You can see in the lower left corner of the MAWIM capacitor that it’s started to leak a bit.

Capacitors replaced, the audio quality cleaned up very nicely. Then, on to tackling the other issues. After unsuccessfully trying to clean the switch with several different methods, the only way to swap them out was to disassemble the entire front dial.

The controls in question are the lower two on the right:

I carefully removed first the lower, then the upper, keeping track of the lead arrangements.

I installed a new resistor to replace the one whose leads I’d cut.

Then it was time to reinstall the new controls in reverse:

Now all the functions work perfectly! I used some rubbing alcohol to clean a few switch contacts which were acting up but otherwise it was in good shape and ready to be aligned.

First up is a bias adjustment:

Followed by 8 AM adjustments:

Longwave adjustments:

Shortwave bias adjustment:

Followed by no fewer than 34 separate adjustments for each shortwave band. I used a new precision signal generator for the alignment:

Using the scope and generator together, it was easy to check the alignment out. I’m not going to post every band here, but due to the sharp tuning, it was very important to get these just right.

Repeat 3 dozen times, and the alignment is complete! Very, very fortunately none of the trimmers or coils were damaged so it was long but uneventful. Then, with it working perfectly, back in the case:

With all new parts and a precision alignment on all bands, this should play well for a long time. It’s pretty sensitive, too, although shortwave stations are pretty tough to pick up these days so I’m unsure how the real-world performance will be. It sounds great, and pulls in a ton of AM and FM stations though!

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Test Equipment Round-Up – September 20th, 2014

Posted on September 20, 2014 by retrovoltage

I always keep an eye on eBay for plug-ins for my HP 143 oscilloscope and even if they don’t turn up, it’s always interesting to see what else is in the category. Some of these are pretty interesting – there’s definitely a lot of project oscilloscopes, but you can get a bargain if you know what to look for. Some of these are definitely pretty rare and unusual, too. Here’s a few that I thought were worth sharing:

There’s an HP differential scope from just a couple years after my 143, the HP 175A oscilloscope. It has a built-in differential vertical input, optional x-y mode and delayed sweep capabilities. It’s priced a little high but some of that’s the vintage condition.

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There’s an odd looking HP 1980B Oscilloscope Measurement System, loaded with two dual-channel plug-ins and a dual time base. It screams 1980s  with the membrane keys instead of knobs – although I think it would be very annoying to actually use like that. Plug-in scopes are always a great choice but given I just have to assume plug-ins for this one would be very difficult to come by.

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There’s an example of HP’s very first oscilloscope out there, too: the HP 120A. This one is in pretty good cosmetic condition although it’s missing a knob. Great for a collector of test gear, although priced a bit high in unknown condition in my opinion.

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The HP 183B is a pretty intimidating looking piece of gear with a 4-channel, 200 MHz vertical amplifier. Looks like it has some functional issues, but it’s got a very cool looking front panel. (Test equipment user interfaces are surprisingly interesting!)

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There are plenty of the older HP 141T storage mainframes that come fitted with plug-ins. For some reason they mostly seem to come configured as spectrum analyzers, but that’s also a useful thing to have for a shop. The most common ones are from 0.1-110 MHz and there are a few others. Often in good working condition. One that’s there currently comes with a tracking generator, too.

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There were a lot of interesting plug-ins for that series. I’m looking for vertical amplifier plug-ins, but they made a variety of spectrum plug-ins, sampling plug-ins, and even a TDR. I can’t think of any use for that, but it might have some use if you’re designing antennas or transmission lines.

$_57

I am still looking for 1400-series vertical amplifier plug-ins, but hopefully someone finds something useful or interesting! I’ll continue posting more round-ups of cool, hard to find and unique test gear that pops up from time to time, too.

 

 

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