Help needed with - Racal Dana 9081

[PETERg0rsq]

You have an indeterminate logic level on IC6 (When you measure 4.13v on a 12v logic gate you know there is an issue that needs investigating).

Again ... are you sure the knobs are positioned correctly on the switch shafts. They can be secured in any position.

How are you checking this?

Also the switch drawing for S2/12 doesn't seem correct to me. To me it is shown in 256-512 position, else it is a mirror view of switch position. I will dive into the manual and check if it is correct or not.

[PJL]

IC6 Pin 11 looks very wrong! It is supposed to be the inverse of IC24 pin 13 (TP15), can you check the voltage on TP15.

The switching is very complex, Sz is biased to '0' by R34 (10K) BUT via S2/1R it can connect to RN when R27 (100K) connects it to SPECIAL CHANNEL SELECT pulled up by Q2 when Sc is 0. At best the potential divider created here seems likely to impact noise rejection.

What it does highlight is how critical it is to perform each test with the same settings as they interact in surprising ways.

[3pinplug]

Hi Paula
Have checked and i have a replacement IC6 in the batch of IC’s that were ordered early on, that was a really good call. Hopefully the replacement for IC15 will be here this coming week and the two new ic’s do the trick, will try and get IC6 changed tomorrow evening.
Regards
Mark

[3pinplug]

Hi Peter
Is there a specific way of confirming if the channel spacing knob is fitted in the correct position? I tried checking the channel spacing on the display against the setting at the knob some match and some don’t?

Paula,
Will add TP15 to the checks.

Thanks again to you borh for all your help with this.

[PETERg0rsq]

Originally Posted by 3pinplug

Hi Peter
Is there a specific way of confirming if the channel spacing knob is fitted in the correct position?

Look at the front panel switch diagram.

For each switch position tabulate the expected logic V The actual logic. Look for patterns that match.

If you loosen the nut behind the cap on the knob, the knob becomes free turning on the shaft, and can be locked anywhere.

The channel knob is the only switch without end-stops if I recall correctly so you cant use the end stops to position it.

Another tip...Dont use a meter to measure the logic levels in digital circuits. Use your oscilloscope.
The logic could be switching very fast or oscillating and a meter may give an RMS or averaged value. The 4.13V on IC6 pin 11; is it 4.13VDC, or a 12V pulsed output with a duty cycle of 1:3?

I tend to use my oscilloscope for most testing (digital and analogue), and the DMM's for when I need to make an accurate measurement.

Edit
Another way to check the position of the knob on the channel spacing switch is to check the pulses released by the logic board pin 18 match the specified number of pulses shown in the manual. A digital storage scope, logic analyser, or frequency counter with pulse counting will make this easy as you need to count up to 64 pulses for each operation of the step switch.

remember the speed switch MUST be in slow position for all testing...

[3pinplug]

Hi Paula , Peter

Few further results:-
IC6 pin 11 on scope shows 12v
TP15 shows 8.13vdc on dvm but 12v on scope?
I then went through the band selector from 5khz to 60hz and noted the stepsas follows:-
5hz shows 40khz steps so X8
10khz shows 40khz steps x4
20hz shows 40khz steps x2
6.25khz shows 50khz steps x8
12.5khz shows 50khz steps x4
25khz shows 50khz steps x 2
50khz shows 50khz steps x1
15khz shows 60khz steps x 4
30khz shows 60khz steps x1
60khz shows 60 khz steps x1

So each of the above on band 1 is
8,4,2, 8,4,2 the changes to 1,4,2, then goes to start again at 1,4,2 but maxs out at 60khz ch setting so stops at x1
Not sure if this is useful but as a pattern thought it may indicate channel spacing switch is fitted correctly?

[frsimen]

Hello Mark,

I think you have the step switch knob aligned correctly, as the switch between multiples of 5kHz and multiples of 6.25kHz is taking place at the right positions.

The odd multiples issue should be sorted out when IC15 is replaced as this has been shown to be faulty and governs the selection of the pulse multiples.

You could try different timebase settings on the oscilloscope to see if you there is a pulse waveform. If there is anything badly wrong there, the band edges will be wrong. You can check that the band limits on range 1 and range 2 are indicating as expected, despite the steps being a little odd. If they are correct, there isn't a problem.

Paula