Racal Dana 9081 - +12V Supply Problems

[PETERg0rsq]

Great to see so much progress, but of course the more you fix, the more you see that needs fixing!

Very normal

With the up/down switch i think tp12/13 only give the direction information!

TP10 is the trigger for the pulses.

You should see the up/down switch pulses on TP5, as well as the divided pulses from the encoder.

Watch out for logic levels on pins 17 and 15, as these come from the EPROM to controle the band edges (sends pulses to the oposit tunning direction when the output reaches a pre-set band edge), worth disconnecting during initial fault finding, as the EPROM can have issues which can confuse matters at this end!

The pulses seen on TP5 then are used to trigger the correct number of tunning pulses in the channel spacing control. This is a complex logic arrangement, with every gate seeming to feedback into another, which can send you in circles!!! Careful fault finding, better with a logic analyser, can make sence of it though!

Dont forget that there are different pulse numbers generated depending upon the range selected, the step size, and the tunning rate! There is a drawing in the manual that trys to indicate this.

This is where a storage scope, or logic analyser can help.
I generaly measure the pulses for each range and step size, and again it then becomes more obvious where problems lie.

These pulses should be seen on pin 18 of digital board for each pulse of the encoder in slow tunning, or operation of the step switch. In fast mode you will get much more pulses for each step of the encoder.

As far as changing all the ic's, I dont like doing any more work than is necessary

Peter

[AC/HL]

One post moved to a new thread:
https://www.vintage-radio.net/forum/...d.php?t=100349

[Gerry G3OGK]

I also have had the usual tantalum capacitor failures with a +12 v rail short circuit to deck.
While tracing out the wiring, I found that my chassis does not have the mains switched by the left hand on/standby switch, but in my case the +12v is switched with the mains always on. I have never seen this in any circuit diagrams that I have seen, although I do have a full 9081 + 9082 technical manual. Mine 9081 has a serial number 1393 so is, I assume, an early one. I wonder if anyone else has such a variant, especially as in my clumsiness when un-lacing a harness I detached 2 wires which I think are from the on/standby switch but can see no tell-tale small bits of wire sticking out of any soldered joint. If anyone has the 12 volt switched like this, I could do with asking about the connections to this switch.
If it helps anyone else, with the +112 volt rail grounded, on my machine it showed just a figure 7 in the display at least significant digit position, un alterable by any front panel controls.