Redifon R551. ARU11B Synthesiser
 

Evening all,

Im working my way through the R551 that was obtained through this forum - AN interesting beast to work on.

Other than a very annoying intermittent connection somewhere in the main chassis of the receiver, the radio works well.

The same cannot be said for the ARU11B synthesiser.

I am working though the faultfinding section, and have come across the "divide by N" section.

I have found a problem there, in that there is a specific time period to be measured, but in the case of my unit, it is a long way off.

The BR 340 manual I have gives timings to test this loop of around 1017ms, my unit is achieving timings of around 740ms - This is obviously wrong, and may explain why the output frequency is LF of the target of 600-700kHz (currently sitting around 470kHz, and front panel switches have no influence.

I have a copy of the Redifon manual in my dropbox - it is too big to go here.

I attach a snippet of what I am up to - any suggestions from the experts :)

Re: Redifon R551. ARU11B Synthesiser
 

Hi Sean,
It's been a while since I worked on the ARU11B but for what it's worth......
The loop is obviously open as the front panel switches have no effect on the ouput.
A quick check of the VCO is to short pin2 to pin3, removing the tuning voltage. The output at the rear panel should be just below 600kHz. If this is Ok then the VCO, divide-by-8 and output filter are probably OK. If not investigate further.
A quick check of the Reference divider is to look at the waveform on TP27. This should be a 1:1 square wave at 800Hz. If not investigate further. Check this using the 100kHz from the R551 and also the input from the clarifier.
The DC Amplifier is a little more difficult to check. Remove the wire connection to pin3, use 2 x 1k resistors between the 5v rail and -ve to give a potential of 2.5v. Apply this to pin3 and check that a similar voltage appears at the ouput pin4 and that the output frequency is about 5.3/5.4MHz.
For the divide-by-N stages I have always found it more useful to use the test method described in para.7.5.4 section 11-14 as this gives a better indication of where the fault lies if one exists. Note that the rotary switch contacts programming the divider should be viewed with suspicion - the current drawn by the J/K inputs on the divider chips is tiny and may not be enough to wet the contacts of the switches if any tarnish is present - ther switches are 40+ years old after all!
Good Luck