HP 8711(A): modifications, tweaks, refurbishing

Introduction

I recently acquired a fairly well preserved HP 8711A network analyser. As sold, this is an economy model, offering only scalar measurements, although vector information can be acquired through the HP-IB bus.

What makes this unit interesting, is that the hardware is the same as the never released 8712A, which has vector capabilities. Some changes in the bootrom allow the user to “convert” the unit, making it behave like a full fledged VNA.

Conversion

Credits for this rom modification go to Massimo Porzio (IK1IZA). As far as I know, he is the one who took the time to figure this out. I also found a lot of information on the site of Michal Lewczuk (SP2XDM) (Polish site).

Before doing anything, a backup of the correction constants was made to disk.

Unit boot screen as received:

Boot screen as received

I used the TL866 USB High Performance Programmer which can be bought online for around €40. Download the software while you wait for the package to arrive, download speed reminded me of the late nineties…

The bootrom with attached label (on the A1 CPU board, removal of the CRT/PSU module is necessary):

Bootrom The bootrom

Steps:

Set IC to P28F010 (DIP 32 package)
Read the contents of the EEPROM
Change the range 0x1FFC1 - 0x1FFFEF with 0xFF
See pictures below for what the end result should look like
Write the modified content back to the EEPROM

Contents of the ROM at address 0x1FFA0 prior to modifications:

Eeprom before modification

After modifications:

Eeprom after modification

New bootscreen (SRL and fault location enabled as well):

Boot screen after patch

Smith chart as proof of vector capabilities:

Smith chart showing vector capabilities

CRT realignment and brightness setting

Some adjustments on the CRT were done:

When looking from a normal viewing angle with the unit placed on the desk, the alignment of the options on the right of the screen seems “off” when compared to the location of soft buttons next to them.
The screen was rather dim.
Sizing of the display compared to the cut-out in the front panel could be enlarged.

I marked the locations of the front panel cut-out and the top and bottom buttons on the CRT:

CRT with markings

After disassembling the CRT/PSU module and attaching the PSU to the back, followed by some creative cabling, the unit was powered on while leaving access to the alignment potmeters:

Alignment of the CRT

The positions of the potmeters before adjustment were marked:

Potmeters for alignment

After adjusting the height/width and brightness, the screen looked much better.

Power supply patch

According to a ECN from HP, units with serials between A00000 and 3325A00941 need a 3W, 680Ω resistor between pin 9 and 16 on J5 in the power supply.

Before:

Power supply before modification

Resistor added and pcb cleaned:

Power supply after modification

Speaker “modification”

The A models of this network analyser had an issue where noise would get into the audio circuit, causing a high pitched, squeeling noise. When used in a silent environment this was really bothersome, so I first tried replacing the speaker by a version with a slightly different resonant frequency.

This didn’t help enough, so I ended up with this ugly (but fully working) solution:

Speaker, with tape

Cleaning and restoring the unit

A lot of time was spend cleaning and restoring the unit, as to give it as much of its original appearance as possible:

The inside of the case was cleaned, connectors on the backplane (which are notorious for causing problems with this model) were cleaned using a combination of MEK, IPA, soda, warm water, patience and lots of love.
Front panel and keypad were cleaned, and the yellowing of the plastic due to age and UV was reversed through the use of high concentration hydrogen peroxide. Dents, holes and cuts were fixed.
The vinyl was cleaned, restored and treated.
Internals of the disk drive were cleaned.