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

[3pinplug]

Hi Paula
Replacement IC finally arrived will get changing and report back!

[3pinplug]

SUCCESS!!!!

I have replaced both IC’s and now the channel spacing is correct to whatever setting I select on every band!!!
Here's my fully controllable frequency coverage now in the slow position:-
Band
1. 4.800 - 36.795
2. 30.000 - 67.595
3. 60.000 - 135.195
4. 120.000 - 270.395
5. 240.000 - 540.795
Channel spacing set to 5khz just to emphasise it's now working. Also at any point if I press the step up down switch I only have to press it once now for it to change the display and it works properly in both up and down and changes display correctly. The fine tune when selected via the int switch position is still making the led display just constantly cycle in a loop making the 3/4 digits towards the end of the number unreadable so that bit still not working by looks of it. But what a great bit of progress to fix the frequency spacing, thankyou ever so much Paula, Peter your knowledge on these is amazing and really appreciate your time in helping me get this far. Hopefully we have now crested the hill and are on the home run!! 👍

[CambridgeWorks]

Great news, a well deserved result.
I remember that if you set the frequency to minimum and then changed the channel spacing switch, you could somehow fool it into going lower than specified. I think on a 9082, the 1.5MHz would go down to a few 100 KHz?
Rob

[frsimen]

Hello Mark,

That's really good news. Let's hope that having got to the top of a hill, it doesn't reveal another one which was previously hidden!

The next problem is still on the logic board. This isn’t the fastest approach but it will help with the diagnosis of the fault.

With Fine Tune selected, check for:

+12V on terminal 8.
-6V on terminal 7

Pins 5 and 6 of IC1 should be 12V too. If they’re not, check L3 for continuity.

+2.7V approx on terminal 5.
+7.3V approx on terminal 6.

Terminal 1 should be adjustable over the range 2.7 to 7.3V by adjusting the coarse and fine tuning controls. The voltage should change smoothly as the controls are adjusted.

The approximate voltages depend on the voltage at the emitter of Q11. I’ve assumed 11.4V, that may be out by a few percent. If the voltages you measure at terminals 5 and 6 are much lower than expected, check Q11, C29 and R62.

There should be a 5MHz clock on terminal 3

Check at TP6 for a 5.3MHz signal. The actual frequency will depend on the setting of the coarse and fine tune controls. Check and note the range of frequencies that you obtain there. The output should be present over the full range of adjustment.

Move to the emitter of Q6. Use your oscilloscope and check that there is a signal present, the amplitude of which should be constant as the coarse and fine tuning controls are adjusted. Note the amplitude.

Move to TP7. Expect a 300kHz or so signal of at least 8V peak to peak.

DC voltage readings, based on 11.4V at Q11’s emitter.

Q4 base: 3.5V
Q4 emitter 2.9V
Q4 collector 9.5V
Q6 emitter 8.9V.

Paula

[3pinplug]

+12.35V on terminal 8.
-5.67V on terminal 7

Pins 5 and 6 of IC1:
+2.88V approx on terminal 5.
+6.59V approx on terminal 6.

Terminal 1 was adjustable over the range of 2.88v to 6.57v by adjusting the coarse and fine tuning controls.
Controls exact impact as follows:-
Coarse on own = 2.88v to 6.44v
Fine on own = 2.88 to 2.98v
Fine and coarse both at max = 6.57v

There is a 5MHz clock on terminal 3, adjustable by a little by fine & coarse, when both at max it is 5.3mhz.

Emitter of Q6 has a signal and it is not affected by the turning of the controls it remains constant.

Moved to TP7 and got 12v peak to peak.

DC voltage readings, based on 11.4V at Q11’s emitter.

Q4 base: 3.08v
Q4 emitter 2.41v
Q4 collector 7.61vV
Q6 emitter

I am struggling to positively identify the E,C,B of Q4 and Q6 from one side of the board to another therefore I am not 100% sure of the above values, i have tried shining a torch from one side through the board but still not getting them to match up / idenify them, is there a trick to finding these as I will repeat again tomorrow?

When you flick the switch to INT the display starts scrolling and looks to be droping whatever frequency is shown on the display by about 3 mhz, the fierst digit on the left doesn't start scrolling but the others do, not sure of this is our fault or not?

Regards

Mark

[frsimen]

Hello Mark,

The output at TP7 is a very good level, so there is no need to worry about the voltages around Q4 and Q6's emitter.

Everything you have measured looks good except for the voltage on IC1 pins 5 and 6. They should both be the same and 12.3V. It look like you may have had a poor contact for that measurement.

Can you measure the range of frequencies that are on TP7, as that is the most important thing here? You should be able to set the fine controls to give 300kHz and it should be reasonably stable. Expect a drift on the Hz/10s of Hz digits on your external frequency counter, that's normal but it shouldn't be worse than that and it should be a slow drift. The controls should allow the frequency to be changed by a little either side of 300kHz.

If that is what you see, move over to TP1. You should see the same waveform and frequency there. If you don't, change IC1.

Paula

[3pinplug]

Sorry i have misread the test requirements for terminal 5&6, as reported they are not IC1 pins 5&6 but posts 5&6 on the logic assy, i just carried on measuring “terminals” after the first two as thought they were those. Will check IC1 pins 5&6 and check for the 300khz and send in the right results.

[3pinplug]

Hi Paula
IC1 (Logic board) pins 5&6 are both the same at 12.39v.
TP7
Both coarse and fine set at min
Fine tune 290khz min changes to 287 at max?
Coarse 290khz min, move control to 11 o clock position get 300.7khz, continue to increase and it starts to drop down to 294khz max?
Certainly seems a bit weird i would have thought both from go from min khz to an increase at mak?
Regards
Mark

[frsimen]

Hello Mark,

That does sound a little unusual. Has C11 been replaced, if not try a new one. The only other obvious suspect is D3. On my 9082, the frequency can be adjusted both above and below 300kHz with the control.

Check what the waveform looks like at TP6 with your oscilloscope, to see if something odd happens at the same time as the frequency starts to go down.

Paula

[3pinplug]

Hi Paula
I changed C11 and then the second digit was more stable but the three after the decimal point was still scrolling. I then started to desolderD3 but whilst removing it, it broke in half, i looked in the manual and it said it was an MV1634. Initial looking round hasn’t identified one I hope its not an obsolete part. Would you know of a source for these or an alternative I could substitute it with?

[Cruisin Marine]

https://duckduckgo.com/?va=u&t=hi&q=MV1634&ia=web

[frsimen]

Hello Mark,

Thanks firstly to Cruisin Marine for the link to the datasheet for the MV1634.

Wire ended varicap diodes are getting a bit thin on the ground these days. Cricklewood Electronics stock two which are fairly similar, but not identical to the original D3. The BB109G and the BB139. They are quite expensive, though.

The capacitance needed is quite high, so there is another approach that you could try to see if it works. The 1N400x diodes will behave as varicap diodes quite successfully. The capacitance is in the right region too, so if you have a 1N4004, 5, 6 or 7, you could try one of those. Another option is to use a regular LED, although I've not measured the capacitance of one, they have been used successfully in VFOs. It neither work, then you'll need the proper varicap diode.

There is one other capacitor which could cause jittery tuning, C29, which is used to smooth out the supply to the VCXO. Worth changing if you haven't already done so.

If it were mine, I'd try the cheap 1N400x or LED approach first, before splashing out on the more expensive parts.

Paula

[Cruisin Marine]

You can parallel up diodes to achieve the required max. Capacitance, so bear that in mind. As long as the min capacitance is also achieved of course.