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

September 1, 2014 Leave a comment

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.  I’m still looking for another 1402A, or a 1404A – if you have one for sale or know where I might find one, please let me know.

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Bose 901 Series IV Equalizer #202245

August 29, 2014 Leave a comment

A bit different from the run of Series I and Series II that I usually see, I got to fix up this Bose 901 Series IV Active Equalizer which was sent to me from Austin, TX. The Series IV was sold from 1978-1983 and its construction reflects this.

The PCB on this model is single-ended, so can be completely removed from the chassis.

This unit came to me with one channel dead. The standard service for the Bose 901 Series IV Active Equalizer includes replacing the electrolytic capacitors and op-amp chips by default for best performance. Other parts are replaced as needed, but Bose used stable resistors and high quality, stable, true film bypass capacitors in this device so there’s no benefit to a wholesale parts replacement like on the Series I/Series II.

The 901 Series IV is the first of the active equalizers to use op-amp chips instead of discrete transistors. In this case they’re LF353N op-amps; Bose used the LF335N, TL072, or 4458 op-amps interchangeably and the schematic and service manual allow for any of those chips. Very nicely, they’re all socketed – these early op-amps weren’t that reliable and often developed noise and distortion over the years.

The electrolytic capacitors in this equalizer were about a decade better condition than the ones on the S1/S2, but they were universally bad.

I use a variety of manufacturers for premium replacement parts depending on what’s in stock at any given time; in this case it’s getting all Nichicon Gold capacitors. Every single one of the originals had high ESR and was changing in value.

These were definitely causing some issues – most likely, noise and distortion, although since they would have degraded over time it might not have been obvious. This capacitor, though, was the specific one which caused the channel to go out. It’s measuring only 157 pF, nearly open.

In contrast, a good new cap:

And some residue on the board:

I replaced the op-amps with RC4558s.

Then it was on to bench testing!

There we go! With it checking out on the scope, now I moved on to a frequency response and live listening tests. The curve came out textbook:

The Series III/IV share an interchangeable curve and it is very different from the Series I/II curve.

And back together!

Fully serviced, this should last a long time to come. The Bose 901 Series III/IV speakers have foam surrounds which can degrade, so that’s worth watching out for, but this equalizer is back to top shape.

Active Equalizer Repair

Other Projects

Check it out at Rain City Audio

Vintage Engineering is Beautiful

August 27, 2014 2 comments
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 1 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


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