From Hobby to Professional

HP8562A Spectrum Analyzer Repair: Fixing Video Offset / Display Issues

—

by

Alfred But
in

The Problem

Fig. 1 The problem: Displayed trace shifted up.

My HP8562A has been in excellent condition for many years until one day when I powered it up, the displayed trace was weird. The whole spectrum display was shifted up, as shown in Fig.1. Trying to display the built-in cal. signal further confirmed the problem. The photo below showed the 300MHz cal. signal, which level was incorrect, and the whole trace shifted up. On the lower right corner of the display, error message “ERR 560 LOG AMPL” appeared. (Fig.2)

Fig.2 The displayed spectrum shifted up. Amplitude reading incorrect.

Troubleshooting

The service manual suggested that the problem was with the Log Amp in the A4 assembly. I did some painstaking checks on the DC levels at various test points to find out that U408 was the culprit. This chip is a transistor array which in this design constitutes part of the 9th stage Log Amp. Actually the problem was an opened Q408B as indicated in the schematics (Fig.3). Fig.4 below shows the location of the A4 assembly. Fig 5 shows the disassembled A4 with the faulty chip removed (lower right corner).

Fig.3 Partial schematics of the A4 assembly.
Fig.4 Locations of the assemblies
Fig.5 The A4 assembly with the shielding covers opened.

The chip was marked with a HP proprietary number, but the part list indicated a CA3127E, which was a transistor array by RCA. This chip (and RCA) has been obsolete for a long time. Fortunately it is still available at eBay in small quantity with a “reasonable” price. I ordered a few pieces and had the damaged chip replaced. My HP8562A is now back to normal. See Fig.6 below.

Fig.5 Problem fixed. The Cal Signal is displayed correctly

Remarks

The HP8562A is relatively straight forward to disassemble. However you should study the figures in the service manual carefully before working on the assemblies. Several “long” screws with different length must be removed before the assemblies can be unfolded. Note their locations. During reassembling pay attention to use the screws with proper length on their designated locations. Avoid stressing the connecting coax and/or ribbon cables when you unfold the assemblies. Refer to Fig.7 and Fig.8.

Another precaution: Do not adjust the trim pots!

Fig.6 The assemblies unfolded
Fig.7 Assemblies unfolded (more)

Updates

29th April 2026: Add partial schematics of the A4 Assembly. Add more remarks.

Comments

2 responses to “HP8562A Spectrum Analyzer Repair: Fixing Video Offset / Display Issues”

  1. Alberto Chihuano Avatar
    Alberto Chihuano

    Well done Mate!

    Liked by 1 person

    Reply
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