the value of old test and repair equipment

Joseph Zatarski jzatar2 at
Tue Aug 9 14:56:55 CDT 2016

>I really like my old test gear and yes, it just seems right to be restoring
>vintage computers with vintage instruments.  HP scopes, logic analyzers,
>DVMs; function generator; Tek scope, frequency counter; as well as just
>'sundry'.  But I did break down and buy a DDS frequency generator to work
>on my VHF/UHF ham gear.
>On Sat, Jul 30, 2016 at 6:07 PM, drlegendre . <drlegendre at <>> wrote:
>>* In fact, the value of old test gear varies tremendously..
*>>>>* Vacuum tube testers of certain makes & models are near the top
of the food
*>>* chain, with clean, working examples pulling $1500+ (USD) on a very regular
*>>* basis.
*>>>>* There's also a strong following for much 'classic' audio
analysis gear (HD
*>>* meters, ID meters, spectrum analyzers, etc.) some very fine multi-meters
*>>* and anything really hi-end like General Radio, Breull & Kejjr, HP, and so
*>>* forth.
*>>>>* Some very early examples from the 1910s to 30's also pull good
value simply
*>>* for visual appeal. Much of this gear is resplendent with embossed, enameled
*>>* panels, sculpted Bakelite knobs, large meter movements and an overall Art
*>>* Deco styling.
*>>>>* Seen a nice Supreme Diagnometer recently? Or any of the 40s-70s
era English
*>>* made tube testers, like the AVO? Hickok also made a series of bench VTVMs
*>>* with massive chromed meters, designed to be large enough that they can be
*>>* read from many feet away.. those are beautiful for display, and guess what
*>>* - they work great, too!
*>>>>* On Thu, Jul 28, 2016 at 5:24 PM, Dale H. Cook <radiotest at <>> wrote:
*>>>>* > At 03:52 PM 7/28/2016, Electronics Plus wrote:
*>>* >
*>>* > >... does as-is old test and repair equip that won't be particularly
*>>* cheap
*>>* > have interest to you guys?
*>>* >
*>>* > It depends entirely on the make and model of equipment. I always have a
*>>* > laundry list of stuff I am looking for - one of the reasons why I bring
*>>* my
*>>* > tablet to meets.
*>>* >
*>>* > Dale H. Cook, GR / HP Collector, Roanoke/Lynchburg, VA
*>>* >
*>>* >
*>>* >
>Ian S. King, MSIS, MSCS, Ph.D. Candidate
>The Information School <>
>Dissertation: "Why the Conversation Mattered: Constructing a Sociotechnical
>Narrative Through a Design Lens
>Archivist, Voices From the Rwanda Tribunal <>
>Value Sensitive Design Research Lab <>
>University of Washington
>There is an old Vulcan saying: "Only Nixon could go to China."

All this talk of older test equipment reminds me of the HP 4261A LCR
bridge I repaired a while back, last winter I think.

My dad found the 4261A in the garbage years ago, and it seemed to work
fine, until one day he powered it up and the display showed garbage.
He decided to open it up, and noticed some uncovered windowed EPROMs.
Knowing that EPROMs sometimes flip bits in their old age, we decided
that was the first place we would look. We were also able to locate
the full HP service manual in PDF form for the instrument which helped
tremendously. In typical HP fashion, it had full theory of operation,
schematics, state diagrams, etc.

Now, I have an EPROM burner that does your typical JEDEC pinout parts,
27 series and such. The issue is that these were Intel i1702A's from
the early 70's I think. Not only are 1702's a totally different
pinout, but they run on 14V (a +5V, and a -9V rail, with no connected
ground, this is how intel got TTL levels on a MOS chip at the time).
The 4261A has a total of 4 1702's, two of which form a finite state
machine which controls the instrument, while the other two perform
display decoding.

I had to pull out my dad's DeVry Console 80, which has adjustable
positive and negative supplies, and I manually clocked out the data
and compared the contents to a dump I found online. I started with the
state machine EPROMs, and compared the data. I did find a few
discrepancies, but there was too much difference to have been bit rot.
Given the sudden nature of the issue, I would have expected one, at
most a couple bit flips, or something much more drastic (like total
chip failure). Upon reading through the state diagrams in the HP
manual, I noticed that there was a change noted in the state diagram
between certain minor revisions of the 4261A. I looked at what the
changes were, and deduced that my ROMs were in fact correct for the
serial number prefix.

At a dead end with the EPROMs, I decided to see if the state machine
was even running at all. I used a DVM in DC mode, and measured perfect
TTL ones and zeroes on all the state number outputs, which means those
outputs weren't changing: the state machine was stuck. I wrote down
the state it was stuck in and referred to the state diagram. I noticed
something interesting. The state machine in the 4261A is able to
evaluate simple conditions and control flow based on those. The state
path to get to the state that the FSM was stuck on meant the FSM was
always taking one of the conditional paths (always true, or always
false, I don't remember which). At that point, I started looking into
the condition circuitry, tracing out the path, checking IC's as I
worked my way back, until I made it back to 1/2 of a 7474 which had a
set input that was stuck active (low). This pin went to a pullup
resistor, and nothing else in our unit (certain options used this pin,
but not ours). We desoldered the IC, and sure enough, that pin was
shorted to ground internal to the chip. We replaced it with a 74LS74,
and the 4261A has been working great ever since, even with the
original 40 year old 1702's.

Also, on the topic of interesting HP products, and perhaps my personal
favorite so far, is the HP dynamic signal analyzer 35670A. This
instrument can perform all sorts of cool measurements. It can produce
a test signal, and measure two different points in the circuit being
measured. The measurement input channels give you a complex number
phasor of the measured signal, which means you can do all sorts of
cool measurements of networks, especially since you can do complex
number math with the equation support of the instrument. The signal
generator will perform sweeps too, of course. This was very useful
determining whether the speaker crossovers my dad built were working
as intended (actually they weren't, and this instrument helped us
uncover a problem). We also used this to do inductor and capacitor
characterization. There are all sorts of applications this instrument
is good for.

Joe Zatarski

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