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HP 1340A Display Repair -2

Posted on December 29th, by Shaun Merrigan in Electronics, Oscilloscopes, Test Equipment, Troubleshooting. Comments Off on 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:

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.

A2Q2 replaced
A2Q2 Replaced
A2Q2 OEM installation from the second Display Unit

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.

Replaced A2Q2 and A2R13
A2R13 Replaced
A1R81 Replaced

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.

Repaired HP1340A Display

Repair complete, thanks for reading.

Shaun Merrigan

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