HP 8711A: modifications, tweaks, refurbishing

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).

Conversion

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.

Credits for this rom modifcation 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).

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

Unit boot screen as received:

20160927_173814

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):

20161006_211100

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

eeprom-before

After modifications:

eeprom

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

20161114_171152

Smith chart as proof of vector capabilities:

20161114_171232

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 cutout in the front panel could be enlarged.

I marked the locations of the front panel cutout and the top and bottom buttons on the CRT:

20161031_091501

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:

20161031_094911

The positions of the potmeters before adjustment were marked:

20161031_093910

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:

20161107_212030

Resistor added and pcb cleaned:

20161107_212732

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:

20161031_103828

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.

Documenting this is worth a separate post.

Advertisements