Test Equipment Anonymous

At the Dunstable Downs Radio Club's rally I picked up a Hewlett Packard 8505A system including 8501A normalizer and 8503A 50Ω test set from one of the scrap dealers. He had a few more systems and partial systems but I'd also bought many, many resistors and the Panda was full so I left them. At the McMichael rally a *different* well known scrap dealer had the same units and for a tenner each, so I bought all four. That was three RF source/frequency control units and one display/processor. I already had a partially working base 8505A system, all three service manual volumes and one interconnect.

My plan was to take the best parts and build one working system, but it has gone awry. I've ended up with four RF source/frequency control units and one display/processor unit.

During my repair adventure I rebuilt two of the optical rotary pulse generators (we'd call them encoders now, I think) with LEDs instead of filament lamps and 74LS instead of 74 series. This took the power requirements for each encoder from 5V at 170mA to 5V at 17mA.

I also discovered what must be a design fault in the sweep generator, which destroys two 1.1kΩ resistors responsible for passing current through a stack of Zener diodes that form a -13V reference. Two machines had lost the reference completely with both resistors open-circuit and a further machine was heading that way, with the resistors a long way from 1.1kΩ

The strangest fault was in the 8505A that I already had, a LM339 quad comparator on the front panel had corroded, perhaps because a nearby wet tantalum had leaked on it. This caused the "overload" reference voltage to be absent, which made the warning annunciators for all three inputs show overload.

One unit had suffered a failure in its negative voltage regulator board, blowing a few fuses and another had lost the 20dB section of its attenuator. I consolidated these broken bits into the fifth unit, which also donated a few bits of case hardware.

Three of the four RF systems are "fully loaded" with the label generator card and the phase lock option but I only have two display/processors with the label generator option and one of those has a non-working HV supply.

Here's the stack of working bits (I don't know if the 8501A and 8503A work fully yet as I don't have interconnect cables). Yes that is the wrong bezel on the top one, I'm going to make a crude laser-printed bezel for it for now.

I really enjoy reading accounts of enthusiasm and dedication, and this shows both in abundance.

The fifth unit, which I've dismantled for spare parts, got given the "bad" attenuator which was missing control of its 20dB and the 40dB section was effectively "open circuit".

I dismantled the RF plugin with this attenuator today and tested the attenuator with (perhaps ironically) my nanoVNA H4. The 20dB step was working, but the 40dB section was still open circuit in both states though the solenoid was still moving the plunger.

The reason for the loss of the 20dB section appears to have been corrosion on the PCB caused by water getting into the unit, the water also dripped through the middle of the attenuator block itself and that's where the 40dB section is situated. I'd never dismantled one of these HP programmable attenuator blocks before but it was a lot less scary than I had imagined it would be. A big cleanup with IPA, removing a lot of obvious aluminium oxide followed by exercising the plungers (fnarr!) seems to have restored normal operation on all three sections.

I suspect that I could've got the fifth unit working in its entirety but I didn't feel it was necessary considering the damaged power supply and any related downstream damage. There are many interesting and useful parts in it, some of which I've already filed.

P.S. I didn't mangle the connector, it was already like that, also the hex bolts that hold it together do not all appear to be of the same type so it has been opened before by someone.

synx508 wrote: ↑Wed Jul 24, 2024 8:53 pm

Love the dymo label on the middle one...

Update!

I spotted an ebay listing for the cable that connects the 8501A storage normalizer to the 8505A, it was £40 but they're quite rare…

It arrived and after a bit of jiggling of the innards of the 8501A it is working. Wobbly video link below.

https://flickr.com/photos/synx508/54178653351/

I must say I admire your dedication. It doesn't make me more inclined to be drawn into the world of HP 85XX, but it is admirable.

Zenith wrote: ↑Mon Dec 02, 2024 8:48 pm I must say I admire your dedication. It doesn't make me more inclined to be drawn into the world of HP 85XX, but it is admirable.

They're strange systems, there's a mix of brilliant design and not so good design. This evening I've run into an issue with the labels, it seems the 8501A is not making labels. I've tried it from HP-IB, it can draw vectors into the graphics memory (page 6 and others) but it can't put text in there. Attempts to add text silently fail and that makes the label-bus cable that I've spent nearly two days making a bit pointless!

The service manual isn't readily downloadable unless I give Artek or similar some money. I may do that, the 8501A, like the rest of the system, has a lot of stuff inside. I've a feeling that it's similar in design to the "computer" in the 8505A, in that most of it is a cleverly designed state machine. There's a nanoprocessor in there, too, but I think that's just doing the HP-IB. There's a ROM, too, which BAMA has a copy of, so I've grabbed that. The ROM holds the character vector data, so perhaps mine has gone bad leading to no characters. I will dig the cheapo logic analyser out tomorrow and see if the ROM is being scanned for character data and producing no output.