The Marconi 2955 and its variants are popular for amateur use but are now getting on for 40 years old.
We aim to try and keep these going where possible at a reasonable cost and are usually able to repair these with a few exceptions.
One of the common failures of the 2955 series is the reference oscillator. This is usually characterised by just the fan running when switched on and possibly the odd random LED lighting up, but otherwise the unit appears dead.
As well as providing the frequency reference for the RF tests, it also supplies the clock signal to the processor amongst other things.
During normal shut down of the instrument, a write inhibit signal is applied to the EEPROM, which contains the calibration data as soon as the supply voltage starts to drop. This prevents any spurious data being written to the EEPROM as the processor dies.
Unfortunately, when the reference oscillator fails, there is no drop in supply voltage, so the processor can and will very often overwrite the calibration data with random garbage.
The original oscillator can usually be repaired, but failing that, we do have an aftermarket ovened crystal oscillator kit, which can be fitted as replacement.
This then leaves the issue of the corrupted calibration. If the appropriate equipment is available, it is possible to do this manually via the keypad, but it is very time consuming (approx 3 working days) and involves many 'blind' key presses. This would render most 2955's beyond economical repair if paying for the labour.
Fortunately, we have written our own automatic calibration software, which, whilst it can take many hours to run if a lot of adjustment is required, it can largely be left to its own devices, so doesn't incur such a large labour charge.
We are not able to supply an official calibration certificate, so it may not be suitable if you're using your 2955 commercially, but the actual calibration is as good as the real thing and you can be assured that every one of the 350 or so calibration points has been adjusted where needed. We can also supply CSV files of the actual results of the software calibration upon request.
The software calibration will automatically adjust the power meter, signal generator levels and AM/FM mod levels at all calibration points, so a corrupted 2955 can be easily recovered.
Occasionally, other data in the EEPROM will get corrupted, which can cause various symptoms ranging from random characters on the screen, locking up and even apparent attenuator faults. In such cases, we will remove the corrupted EEPROM and overwrite it with a known good image before calibrating.
One of the other major well known failures of the 2955 is the keypad. We carry stocks of replacement key switches for all versions apart from the 2955B. These are available to purchase and fit yourself, or we can replace either the full set or just the faulty switches for you depending upon budget.