Well I've not had any problems buying stuff from the EU, accidentally won a rack mount HP 180A scope last month from Germany
, import charges were just over €3, shipping was quite a bit more than the auction price. Well I've always wanted the rack width version & they never seem to appear in the UK, not helped by all the people that prefer to scrap stuff instead of selling it.
It's not in perfect condition, but is quite an early one built at the South Queensferry factory (U647-00149), the 1801A/1821A plug-ins are a little bit newer (must have got swapped at some point), being an early one most of the diagrams in the manual are for the later ones, thankfully the change sheets had all the required info.
Powering it up when it first arrived last week resulted in not much happening, the beam find button did produce a spot on the CRT, but initial testing was quickly interrupted by smoke emerging from the 1821A timebase section.
Yesterday I finally had some time to look at fixing it, first thing as always was to check the power supply rails (I removed the plug-ins), all were OK apart from the -12.6V rail, which was over -20V.
After checking the voltages around some of the transistors it became apparent the amplifier transistor Q412 was doing nothing, it was not turning Q409 off. After removing Q412 it was found to be completely open circuit. The manual gives part no. 1854-0071 and the part fitted was marked MPS3391, I replaced it with a 2N3704 (pinout is different to the old part) and the -12.6V was now working & could be adjusted.
The plastic cover for the fuses was missing, but they helpfully left the screw in place, I found a plastic draw divider that was good size to cover the fuses, it was drilled and a couple of plastic spacers fitted to get the height correct.
Many of the missing screws from the frame & one of the PCB's have been replaced and I cut a piece of thin plastic sheet to replace the missing piece that should be stuck to the metal cover for the HV section.
Now to look at what caused the smoke release from the 1821A timebase, resistor R309 was found to be burnt, but not open circuit.
Checking for shorts found the axial tantalum capacitor C308 next to it was shorted, this one was made by STC. I replaced both the resistor & capacitor before testing the plug-ins.
With the plug-ins refitted we now had a trace, well half a trace. None of the controls seemed to be able to get it to fill the screen.
I decided to try a signal to see if the vertical plug-in was working first, in the time it took to retrieve an oscillator the trace had filled the screen. The problem turned out to be the beam finder switch having dirty contacts (a common problem with HP scopes).
Both inputs were working, but it would probably benefit from some switch cleaner too.
Everyone knows HP scopes do not trigger and this one is no exception
, I cheated and used the external trigger input for the picture below.
It needs a couple more capacitors replacing in the 1821A before I think about further checking & probably adjustment/repair to cure the trigger problem. These capacitors are C327/328 and are wet tantalum
types (Sprague 109D), I will probably end up using a slightly higher value regular electrolytic to replace them due to cost.
David