HP 1340A Display Repair -2
This is Part 2 of my HP 1340A Display unit repair process. Recall that one unit had a ready to fail HV pass transistor and two dodgy resistors. The other unit had two dodgy resistors.
- Heat damaged A1R81 (both units)
- Heat damaged A2R13 (both units)
- Loose mounting hardware and corroded wiring on A2Q2 (one unit)
Part 1 of the repair is here: https://shaunmerrigan.info/?p=1377
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.
Repair complete, thanks for reading.