the value of old test and repair equipment

curiousmarc3 at curiousmarc3 at
Thu Aug 11 02:12:14 CDT 2016

For some reason the 7474's have a higher failure rate than other TTL IC's in HP equipment. I don't know if it's true in general for 1970's TTL. Signetics MSI chips (counters and stuff) seem to be prone to failure too. 

> 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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