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.

$_12

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.

$_57

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.

$_57

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!)

$_57

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.

$_57

$_57

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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Marantz 2245 Stereophonic Receiver Rebuild

Posted on September 19, 2014 by retrovoltage

An iconic piece of kit from the golden age of hi fi just came through my shop: the Marantz 2245 Stereophonic Receiver. This fantastic receiver is in pretty good cosmetic shape, but its owner reported that it started acting up – it’d take a little while for the signal to come through, the controls were scratchy, and it was heavily balanced to one side. Time for some service! This receiver was made between 1971 and 1976, so it’s about 40 years old on its original set of components.

There’s a shop sticker on the back, but I couldn’t see anything obviously that was re-worked, so that might be the shop it was bought from when new. Underneath the cover, it’s pretty spacious. Most of the boards can be unscrewed and swiveled out of the way to work on them without even removing them from the chassis.

There are two massive main electrolytic capacitors, 9000 uF each at 55V.

Power supply:

AM board:

Phono preamp board:

Tone amplifier board:

FM detector board:

First thing’s first, the capacitor replacement. I went slowly, working one board at a time and powering up after each. There were no surprises along the way, and the receiver’s design made it quite easy to work on.

There’s some errata on the AM board. One capacitor’s polarity silkscreen is backwards, and one lacks a silkscreen entirely.

The replacement main filters are much smaller than the originals. They’re being run slightly de-rated; I believe these have a minimum 20,000 hour working life and should last much longer than that as de-rated.

The smaller caps will help the air flow in the receiver a bit, too. The only challenging part was getting the final amplifier boards out of the case to rebuild.

With those checks completed, it was ready for finish work! First was the master power supply adjustment. With the multimeter connected between J802 and J803, I adjusted the power supply trimmer for 35V.

Then it was time to adjust the DC offset on the speakers for 0.0 mV. One channel was pretty significantly off, measuring 43.1 mV; the other measured -6.3 mV. Both reset to 0 without any issue.

Then it was time to adjust the bias. There’s an error in the service manual here as well – if you followed the instructions, you’d never get it as it tells you to measure between two points which don’t vary and adjust to the wrong value. The correct adjustment is 10 mV.

With the receiver’s power supply and amplifier fixed up, I cleaned the controls. They were pretty easy to get to, open style controls. Very easy to clean with control cleaner and lubricant, but easy to get gunked up over the years. To ensure each control was properly cleaned, I turned the amplifier up with no signal and worked each control and switch back and forth making sure the crackles and rumbles which were previously present were gone.

Then it was onto a full AM and FM alignment on the receiver as well. The dial was significantly off on FM, and AM reception was very poor.

With this receiver there’s a series of adjustments to make. The AM alignment is pretty straightforward, substantially similar to an AM alignment on any other kind of radio receiver. The FM alignment, however, was an assortment of about 12 different adjustments targeting “minimum distortion”. Fortunately, that’s where my THD analyzer comes in handy:

The analyzer has a built in low-noise signal source, which I fed into the external modulation input of my AM/FM Stereo Analyzer, which fed the signal to the Marantz’s 300 Ohm FM antenna input over a length of twinlead.

With the FM adjusted for minimum distortion (best alignment), it was time to characterize the amplifier’s power output. Marantz specified 45 W/channel from 20 Hz – 20 KHz at 0.3% THD. Upon measurement, the actual receiver produced 0.101% THD 0-45W, and kept on going up to about 65W although at 20% THD. I’d never recommend overdriving the amplifier like that, but it does have some headroom. I’m going to go ahead and say it’s running better now than it did when it was brand new!

If you have a stereo receiver that’s on its original components and it’s just starting to act up, don’t wait to have it serviced until it goes out entirely – proactive tune-ups are much cheaper than repairing a catastrophic failure, and you’ll be able to enjoy your stereo for many years to come, just like the owner will enjoy this Marantz.

 

Posted in Audio, Electronics, Hi-Fi, Projects, Stereo, Vintage | Tagged , , , , , , , , , , , | 7 Comments

Getting Started With Vintage Radio and Electronics: Basic Tools for the Hobbyist

Posted on September 18, 2014 by retrovoltage

This page is outdated, but I’m keeping it up for SEO reasons. Here’s the current version:

Getting Started with Radio and Electronics Repair: Basic Tools
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New Toy: LogiMetrics 921A RF Signal Generator – Refurbished by KISS Electronics

Posted on September 17, 2014 by retrovoltage

I just had an unexpected run-in with Murphy’s Law and found myself needing a modulated signal generator in short order. Bad news when my modulated signal generators decided to die right when I went to perform an alignment! My Leader Standard Signal Generator has developed a fault in the power supply somewhere which trips the crowbar, and my EICO seems to not want to produce any output.

Lacking the time to troubleshoot either, I purchased a “new” signal generator: the LogiMetrics 921A RF Signal Generator, refurbished by KISS-Electronics in Cornelius, OR via their eBay Store.  I generally take care of my own test equipment, but for something like this where I needed it done quickly and done right, it was worth it to spend a little extra for a professionally refurbished one.  The owner is a fellow amateur radio enthusiast as well which was encouraging. KISS-Electronics specializes in vacuum tube and transistor test equipment from the ’60s and ’70s, which definitely describes the 921A:

You might also notice the upgraded digital frequency read-out. More on that in a bit. This is a very cool generator with big analog panel meters, showing the calibrated output voltage (1 uV ~ 3V in 1-3-10 steps) and the modulation percentage.

This generator will produce an unmodulated sine, or a 400 Hz or 1 kHz tone from an internal oscillator; there’s also a panel jack for an external modulation input. There’s also the frequency analog output, which produces a voltage proportional to the frequency of the output, and the Freq Shift input which can be used as a remote frequency vernier or to produce an FSK/NBFM signal. It’s “ham radio FM”, not broadcast FM, but it might come in handy if I need to align a communications receiver at some point.

The output ranges from 50 kHz to 80 MHz in 7 bands. The replacement counter is bright and easy to read, and there’s dual-speed tuning plus a fine tuning adjustment near the frequency read-out. It’s pretty stable once warmed up, but it does take about an hour to stabilize.

Inside, it’s a well built machine. There are shop notes written on the cover to the left: it looks like this one was reconditioned on 1/5/2014 and sat around for a few months before coming to live with me. The tuning mechanism is under the shield, with the tuning knob and band switch passing through the plate.

One cool upgrade is the N3ZI Compact Counter, which replaced the Nixie Tube frequency counter this was originally fitted with. The original counter was good enough for what it was, but nixie tubes require a high voltage power supply along with a variety of high voltage logic chips that haven’t been made for decades and really never appeared in any other kind of equipment; they also tend to go bad over time and go bad due to other failures in the high voltage circuitry. All of that means that it’s generally not worth the trouble to fix up a nixie tube display. Swapping the nixie tube counter out for a modern counter saves time, money, power and weight. The shop calibrated the counter against their GPSDO for accuracy and I checked it against my bench counter and it’s spot on.

Underneath the cover, there’s a board, and the power supply for the N3ZI compact counter. Underneath on the bottom of the unit, there’s a few more pieces of gear. The HV power supply components have been removed, since there’s no need for an HV supply with the digital counter.

Quality work. A bunch of discrete transistors, and a few early op-amp ICs in Ceramic DIP packages, along with plenty of adjustments. It looks like it was aligned perfectly as the internal counter and my external counter perfectly tracked each other through the whole tuning range. Hopefully these close-up board photos of the LogiMetrics 921A circuit boards will be helpful to someone – according to various forum posts, the service manual is a bit hard to locate.

You can never have too many oscilloscopes; this GoldStar OS-904RD 40MHz dual-trace analog oscilloscope is a recent gift. There’ll be a post about it separately at some point, but I used it to check out the output waveforms on the LogiMetrics. It produces beautiful, clean, accurate RF envelopes. Here’s 430.2 KHz modulated by 1 KHz at 50% modulation:

It is a bit large, so I’ve had to rearrange my bench to make room, but it’s going to be great. It certainly will beat the EICO in any task, hands down, and has a greater frequency range than my Sencore AM/FM Stereo Analyzer which only works on the U.S. AM and FM broadcast bands. I’m going to get a lot of use out of this while aligning shortwave and amateur receivers.

I hear these are likely to drift a bit, and mine does seem to go about 200 Hz low as it warms up. The counter in the photos is showing 430.2 KHz, when the generator was set to 430 KHz and re-tuned over about an hour, so it’s definitely shifted some, but like any piece of precision equipment some time to warm up and stabilize is key. The seller already has another one for sale, and it’s the slightly enhanced model which I believe has an external 10 MHz timebase input on the back – great if you have a precision external timebase lying around.

KISS-Electronics eBay Store

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New Toy: Keithley 2015 THD Multimeter

Posted on September 12, 2014 by retrovoltage

I picked up a new piece of gear for the shop recently, the Keithley 2015 THD Multimeter.

It was a fantastic deal – a refurbished and calibrated unit for about 10% of the MSRP. These have been manufactured for about 20 years now and are still being sold for around $4000 and have about all the features you could ask for in a basic lab bench meter: 6.5 digit precision, 2- and 4-wire resistance measurements, direct readout in dB/dBm, temperature display from an external thermocouple, and most interestingly to me: a low-noise audio source 20Hz-20KHz, frequency counter, and distortion analyzer.

This meter will come in very handy in power amplifier repair, but also for performing FM alignments – it’s not always easy to check the alignment with a scope, but with this meter, you can watch the distortion decrease as the circuit comes into alignment. Dave at the EEVBlog has a teardown review of this exact multimeter posted last year. His teardown was instrumental in deciding to get this particular model over several others I was considering.

It’s not going to be used every day, but it’s going to be indispensable when it does come out.

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Parts of a Vintage Radio – Middle Tube Era

Posted on September 8, 2014 by retrovoltage

Photos with parts diagrams of a vintage tube radio – the Crosley 10-139, from about the middle of the tube era. Click each photo for full size.

 

Posted in Commentary, Photos, Radios and Tubes, Resources, Vintage | Tagged , , , , , , , , , , , , , | 3 Comments

HP 1405A Plug-In Mystery: Which Is It?

Posted on September 1, 2014 by retrovoltage

I’m fixing up an HP 143A Wide-Screen Mainframe Oscilloscope, which has a very big selection of plug-ins to provide a variety of functions. The oscilloscope has symmetrical vertical and horizontal circuits, so it can be used as an X-Y amplifier if suitable vertical amplifier plug-ins are installed in each bay. I’m looking to buy 1400-series vertical amplifier plugins like the 1402A, 1404A and 1405A and came across an interesting bit of a mystery while doing research.

HP’s General Purpose Plug-in Oscilloscopes catalog for the 140 system, dated March 15th 1971, lists the 1405A dual trace amplifier plug-in as a DC-5MHz plugin with 5 mV/div sensitivity, more or less a down-rated version of the 1402A plug-in.

In my research I did find a photo of an HP 140B oscilloscope mainframe loaded with an HP 1405A amplifier plug-in…but in this photo, it’s a double height dedicated X-Y amplifier.

That’s definitely a compatible mainframe, shown right in the plug-in guide on the bottom (with the 140A on top, the only difference being a circular vs. square display area.)

This panel clearly fits in the double height 140-system plugin opening, and has the same style of knobs and controls so there’s no question it’s period correct. I’ve been unable to find any mentions of it on the web or at the HP Memory Project.

Personally, I’d rather have a matched pair of 1402A or 1404A vertical amplifiers, which would give me dual- or quad-trace X-Y capabilities instead of the single-trace X-Y with the pictured mystery amplifier, but that’s a very interesting piece of equipment.

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Wanted: Vintage HP 1400-Series Vertical Amplifier Plug-Ins for Oscilloscope

Posted on August 21, 2014 by retrovoltage

I’m looking for plugins for my HP 143A Wide-Screen Mainframe Oscilloscope which I’m in the process of restoring. I’d love to run it as a 15″ high speed X-Y display, and to do so needs matched vertical amplifier plugins in each bay. If you have any of the following HP plug-ins from the late 1960s/early 1970s available and wouldn’t mind parting with some of them, please write and let me know!

Plugins 1

Chart

I’m primarily interested in the 1402A dual-trace vertical amplifier, as I already have one, but will consider purchasing any of the other 1400-series amplifier and time-base plugins.

This mainframe is interesting because, with the correct set of plugins installed, it could feasibly be a dual- or even quad-trace X-Y display; most X-Y displays are limited to a single trace.

These are some of the other amplifiers and their specifications:

1402A

1404A

1405A

1406A

1408A

If you have any vintage HP 1400-series plug-ins for sale, or know where to find some, let me know!

 

 

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New Toy: HP 143 Wide-Screen Oscilloscope Mainframe Restoration – [Part 1]

Posted on August 14, 2014 by retrovoltage

At the risk of becoming a test equipment collector, I am picking up a few pieces of vintage test equipment that need some work, but will be very useful pieces of shop gear as well as interesting pieces of history lately. I’m needing a high speed analog X-Y monitor for some more complex alignments, and this old HP 143A oscilloscope came up on the local Craigslist for only a little more than the cost of the gas to drive and pick it up.

This is a really old school, transitional piece of gear. The CRT and mainframe assembly boasts all-transistor design, but the plug-in modules themselves are a mixture of tube, transitional nuvistor, transistors depending on the circuit position and the module’s capability.

They’re fairly rare, too, from what I can figure. A high school student in a family of engineers has one; a gentleman who did happen to be local had one earlier this year, and one configured with spectrum analyzer plugins is for sale reportedly removed from a Department of Defense HF monitoring station – a true cold war relic.

These wide-screen mainframes were a big ticket item back in the day, found in the HP scope catalog:

Adjusted for inflation, that’s very nearly a $10,000 CRT display – and don’t forget the options!

With what I’d consider the “base package”, add another $3,700 for the vertical amplifier and $3,100 for the timebase, you’re up to $17,000 for the instrument as shown below:

And the price only goes up from there! The mainframe supports some very high sensitivity (50 uV/div), differential, and high speed (GHz+) plugins – not to mention a frequency response generator, TDR, and spectrum analyzer that only go up get more expensive.

I’m lucky – all the knobs and switches are present. Those frequently get lost.

The HP Archive has good information about the scope including the operation manual and schematics. The plugins slide out easily:

Inside the timebase plugin, lots going on. Transistors and nuvistors together on this double-sided etched and silkscreened board:

Tubes on the other side of the enclosure, right next to some TO-92 package transistors.

Nuvistors were a very late-game evolution of the vacuum tube, and arguably held off the full transistor revolution for a few years.

Vintage really is beautiful.

This mainframe powers on successfully but doesn’t show a meaningful trace; it’s got some minor damage inside but should come back together nicely.

The vertical amplifier is pretty interesting inside, too:

This one has a mix of transistors, noval miniature tubes, and compactrons.

I’m looking forward to getting this one going again, but doubt I’ll be able to work on it before the end of the year.

Because this mainframe has identical vertical and horizontal amplifiers, the way to make it into an X-Y display is to use “vertical” amplifiers in both bays. If you have an HP 1402A plug-in for sale (or really, any other 1400-series HP plug-in), I’d love to hear about it.

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Close-Up of a Magic Eye

Posted on August 7, 2014 by retrovoltage

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