Originally posted in March, 2018

I purchased a couple of HP 1340A XYZ Displays which I thought would be useful as troubleshooting aids and as graphical displays of CRT technology (oscilloscope clocks, and the like). Upon receiving them, they both worked, but careful visual inspection showed some potentially serious problems. I will document the repair process here.

Visual inspection of the units revealed the following problems:

• Heat damaged A1R81 (both units)
• Heat damaged A2R13 (both units)
• Loose mounting hardware and corroded wiring on A2Q2 (one unit)

A2Q2 is mounted directly to the case for heatsink purposes. The hardware was loose, therefore the transistor effectively had no heatsink. I suspect that it had been running hot for quite some time because the PVC insulation had degraded releasing hydrogen chloride which immediately formed hydrochloric acid on contact with moisture in the air. This corroded the copper wires. Note that this transistor is part of the +165v regulation circuit, so a failure would most certainly damage other components.

Transitor A2Q2 is HP Part Number 1854-0330. It is an NPN Power transistor which is still readily available surplus for a reasonable price. I purchased several of these against future need and replaced the failed one. I used teflon insulation rather than PVC on the transistor leads and used some heatsink compound under the transistor for good thermal conductivity to the case.

Next, I replaced A2R13, originally 56 Ohm, 2W Metal Oxide with a 56 ohm 3W Metal Oxide part. Similarly, A1R81 was replaced with an 8200 ohm, 3W part. I took care to mount these two resistors well off the circuit board to allow for good airflow . A2R13 runs hot in normal operation (about 50C or so.

After replacing these components, I adjusted the +165vdc level as described in the manual. It was running at about 172vdc before the adjustment. The voltages on the LVPS were as follows using an HP 1253b:

• +15v: 15.239vdc, 30mv ripple
• -15v: -15.070vdc, 24mv ripple
• -7.5v: -7,491vdc, 25mv ripple
• +3.5v: 3.577vdc, 3mv ripple
• +165v: 164.88vdc, 300mv ripple

I then ran the unit for several hours using a Dutchtronix Scopeclock board to test the display.