the value of old test and repair equipment

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