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Vintage Engineering is Beautiful

August 27, 2014 2 comments

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Categories: Collections, Photos, Vintage Tags:

Bose 901 Series II Active Equalizer #67402 Repair

August 25, 2014 Leave a comment

This equalizer got a full overhaul with new 1% precision resistors, electrolytic capacitors, audiophile-grade output capacitors, transistors, gold-plated RCA jacks, 4% silver solder for all connections, a thorough switch cleaning, and computerized frequency response testing and a listening burn-in to verify proper operation. It sounds just fantastic with a very dynamic and life-like presence that really draws you into the experience of the source material, just like the Bose 901 should.

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Check it out at Rain City Audio

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

August 21, 2014 Leave a comment

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


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:






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



Vintage Engineering is Beautiful

August 19, 2014 Leave a comment
Categories: Photos, Uncategorized, Vintage

1930s Vintage Detrola 335 Radio Repair

August 18, 2014 2 comments

Repairing this Detrola 335 was a major detective project with only a partial schematic after it was hacked on was a major project, but this radio cleaned up nicely and came back to life after tracking down all the faults, including replaced components, undocumented modifications, and a failed thermal switch. After adding an eye tube per the owner’s request, this fantastic art deco style tabletop radio is going to play beautiful music for many years to come. Click through for many more photos and schematic snips of the repair process!

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Check it out at Rain City Audio

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

August 14, 2014 2 comments

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.

New Toy: Tektronix 575 Transistor-Curve Tracer (Restoration Project)

August 11, 2014 1 comment

I picked up a new shop toy just the other day, a Tektronix 575 Transistor-Curve Tracer. This is a piece of lab gear I’ll use to match transistors for their operating characteristics, and can also be used to analyze and test other kinds of components too. This component tester is a tube-based device from the late ’50s, but tests BJTs, and with some adapters can test tubes and JFETs so I should be able to get some good use out of it.

This came out of a garage where it was stored for several years, so it’ll need a full overhaul, but these are pretty tough to find in any condition. When they’re repaired, they look like so, and produce the curve families shown on the display from the Tek enthusiasts wiki:

The test panel includes two sets of sockets, and a switch to alternate between them, perfect for matching pairs of transistors. I’m excited to get to work on this, although it won’t be for several months at least! I’d really love to find a Tektronix 570 curve tracer, though. It’s a dedicated machine for sweeping tubes.

There’s some trade-offs. The 575 can test both sides of a dual triode at once; the 570 would need have its test sockets re-wired for each, but can more flexibly accommodate multiple sets of tubes, and tubes other than triodes. The Tek 570 was the only professional tube curve tracer ever commercialized, though, so it might not be possible to find one easily or affordably – especially when it’d mostly be for collector value that I’d want both, not a lack of functionality.

The 575 can perform the same sweep, although lacks the heater and screen supplies. Adapters to provide the missing heater voltage for testing triodes on the 575 aren’t that tough to make, although this example tests a fixed pin-out only.

Here’s another which could accommodate changing pin configurations, although is set up only with a single socket.

I’m lucky that I happen to have an external tube power supply already, the Oregon Electronics A3 Regulated Power Supply that I refurbished.

That’s a pretty hefty piece of equipment; it provides a 6.3V filament at 5A, and a variable 0-300 and 400V fixed regulated output, helped along by neon gas regulators:

With this supply, I’ll be able to supply a high quality heater supply to most tubes where it’d be appropriate to match characteristics – octal and loctal dual triodes like the 6SN7 and 7N7, and miniature tubes like the 6AV6 or with a tapped heater like the 12AX7. The Oregon Electronics power supply also has a stiff, clean supply rail for the screen voltage and a meter to measure its output. Since I work on radios from many different age ranges, I’d need to build an adapter box with octal, loctal, 7-pin and 9-pin miniature sockets, but also the 6-pin and 7-pin pre-octal sockets. If I’m building a breakout box, it’s not much more effort to add a couple more sockets to take care of the edge cases like testing a #19 or 6A6 tub. I have a tube tester lying around with a bad meter-read switch that won’t be useful in this application, so I might use the chassis to make the filament supply and socket box.



The Tek 575 can deliver up up to 400V at 0.5A, 1A at 200V, or 20A at 20V; it can be paired with a Tektronix 175 High-Current Adapter which offers up to 200A at 0-20V, or up to 40A at 100V. From what I gather these are even more uncommon, although do turn up from time to time.

This should be a fun project, I’m excited for when I can get a chance to do a restoration.

If you’re around Seattle and do have a 570 or a 175 for sale, or have a replacement for the base step generator repetitive/single family switch, I’d be interested in hearing about it.


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