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Old 29th Mar 2023, 10:22 pm   #281
Chris55000
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Default Re: Help needed with - Racal Dana 9081

You do have to be careful of the minimum capacitance as well (highest rev. bias) as the maximum (lowest reverse bias) or you'll find you can't get enough fine tuning range if you use parallel varicaps that add too much minimum capacitance!

Chris Williams
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Old 30th Mar 2023, 12:21 pm   #282
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Default Re: Help needed with - Racal Dana 9081

Been looking through my diodes.

I have found 1 each glass MV1622 , MV1642,
2 x MBD701, 3 x MV2115, both to92
Several each plastic, small tabs at end, BB105B & BB105G

Let me know if any are of use and send pm with details and i will post foc.
Rob
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Old 30th Mar 2023, 4:32 pm   #283
Chris55000
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Default Re: Help needed with - Racal Dana 9081

The MV1622 probably would be ok with a 1.0pF or 1.5p capacitor added across it it has a slightly lower capacitance to the MV1624!

The remaining ones offered are too high in capacitance and tuning ratio!

(Obviously the OP can check and decide for himself of course!)

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Old 30th Mar 2023, 5:07 pm   #284
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Default Re: Help needed with - Racal Dana 9081

Unfortunately, the diode that is needed is an MV1634 which, annoyingly, has significantly higher capacitance than the MV1622 and a significantly lower capacitance than the MV1642. As Chris has said, the other diodes are all well away from the needed capacitance. Thank you for looking, Rob.

I've made some measurements of the capacitance swing available from various diodes (normal rectifiers) and some LEDs that I have here ,using some better test gear than my previous rough and ready measurement. Three 1N4007 diodes in parallel are very close to the same capacitance and capacitance swing as the original diode. I suspect that the Q factor will be lower, but I can't measure that easily. I only tried a couple of LEDs and neither showed any promise for this application.

I tried the fine tuning facility on my 9082. That gives a total swing at 300MHz of 300kHz. Scale that down to 300kHz, that would represent a swing of 300HZ, very much less than was being measured on your 9081.

Before fitting a replacement diode, try a 22pF capacitor in place of D3. The fine frequency control won't work but it will be useful to see if the rapidly changing reading on the counter stops. If it doesn't, there will be some more fault finding to do to sort that out.

Paula
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Old 30th Mar 2023, 6:23 pm   #285
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Default Re: Help needed with - Racal Dana 9081

I have some unknown varicap diodes that are 17 pF at 0 volts and 10 pF at 4.6 volts. I don't know the minimum C as I measured these some years back.
I reckon they might be as low as 2pF minimum at a pure guess.
I can pop a few in the post if you message me.
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Old 30th Mar 2023, 8:31 pm   #286
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Default Re: Help needed with - Racal Dana 9081

Hi all
Firstly a big thank you for looking into the diode situation for me, I have made some more progress and attach some pics which may show something as well.
Paula suggested trying a few 1N type diodes I checked and I had some 1N4007's so dropped one of those in, have only just seen the follow up posts as typing this now.

So I replaced C11, no improvement
Attempted to replace D3 - original broke replaced with 1N4007 and unit still the same with cycling display.
Changed C29 all still the same.
Put scope on T6 and got the following as shown in the pic 2v/div

Pic 2 - this is the trace with coarse and Fine set at min.

Now I increase the coarse control, get to about 10pm and then it goes to a single line at about 4.8v as shown in Pic 1

Continue twisting coarse towards max then you get Pic 3 very bright more than one trace again.

Then right at max the display the frequency display changes from 76.000 to 0.000, but only right at max , turn coarse display backwards the minutest bit and the frequency display reappears. So looks like I have a rogue signal, when packing away before typing this I noticed that the coarse and fine control is a little loose so I will try and tighten it and see if that makes any difference.
Regards
Mark
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Old 30th Mar 2023, 9:07 pm   #287
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Default Re: Help needed with - Racal Dana 9081

Hello Mark,

If those waveforms are at TP6, the emitter of Q3, they are nothing like they should be. Some of that will be because the capacitance of the single 1N4007 is too low. Three in parallel will sort the capacitance out, or try an 18pF capacitor in parallel with the single diode. You won't get the full tuning range, but it will allow some testing.

The frequency at TP6 should be in the region of 5.3MHz, so you will need to set your timebase to 1us per division or faster, if your oscilloscope can do that. Then you should be able to see the something approaching a distorted sine wave. There will be around 5 cycles per division at the 1us setting. Make sure your oscilloscope probe is in the X10 position for the test.

I doubt that the controls being slightly loose will be the cause of the trouble this time.

Paula
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Old 31st Mar 2023, 7:44 am   #288
3pinplug
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Default Re: Help needed with - Racal Dana 9081

Hi Paula.

Will add the extra 2 diodes as suggested and then retake the scope measurements, will post results this evening.

Thanks all again.
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Old 31st Mar 2023, 8:44 pm   #289
3pinplug
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Default Re: Help needed with - Racal Dana 9081

Added the two extra diodes but display still scrolling, made no diferernce to original fault as when fitted with original diode.
taken with probe set t0 X10
Pic 1
Amp / Div 50mv
Time / div 1us

Pic 2
Amp/div 50mv
Time / div 50us

Also put frequency counter on TP6 and measured 4.505mhz coarse min and 4.75 mhz coarse max figures are approx as digits still scrolling.
If there is an IC feeding the caps / diode should we consider changing that?
Regards
Mark
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Old 31st Mar 2023, 10:01 pm   #290
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Default Re: Help needed with - Racal Dana 9081

Hello Mark,

Your first picture shows that crystal is oscillating, with a slightly higher than 5MHz signal, which is good. The second one shows something completely different, a pulse waveform at about 500kHz, possibly with something of a higher frequency as well.

The crystal is capable of oscillating at somewhere near to the right frequency, so you need to check around the oscillator circuit to see what is going on.

The metal cased 2N2369 transistors are a help here, as the case is connected to the collector of the transistor. Set the tuning to wherever it needs to be to reproduce photo 1. Next measure the waveform on the collector (case) of TR4. You should see a waveform that is about 15 times greater in amplitude than you saw at TP6, so you will need to adjust the input attenuator of the oscilloscope to suit. If there is still a lot of RF seen, you will need to change C19, which should be removing the RF. In fact, it is probably worth changing it anyway if it is the original component.

Another thing you could try is to lift one end of C18. That will break the ALC loop. The voltage on the collector of Q5 should rise to at least 8V. It should be lower when operating with C18 in place.

Paula
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Old 31st Mar 2023, 11:30 pm   #291
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Default Re: Help needed with - Racal Dana 9081

I've found another varicap that's a pretty close match to your MV1632 (I misread it as MV1622 so looked at the wrong types my apologies!) the BB910 appears to be a close match to your original, with a capacitance of 18p at 4 V (the MV1632 is 20 2pF @ 4 V), and the BB910 is cheap and plentiful :

https://www.ebay.co.uk/itm/184138986...mis&media=COPY

. . .If you try one of these with a 1.02.0p capacitor across it you've got something virtually identical to your MV1632 me thinks!

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Old 31st Mar 2023, 11:55 pm   #292
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Default Re: Help needed with - Racal Dana 9081

Quote:
Originally Posted by 3pinplug View Post
Added the two extra diodes but display still scrolling, made no diferernce to original fault as when fitted with original diode.
taken with probe set t0 X10
Pic 1
Amp / Div 50mv
Time / div 1us

Pic 2
Amp/div 50mv
Time / div 50us

Also put frequency counter on TP6 and measured 4.505mhz coarse min and 4.75 mhz coarse max figures are approx as digits still scrolling.
If there is an IC feeding the caps / diode should we consider changing that?
Regards
Mark
Have you checked the voltage at FP1 and FP2?

FP1 Should be a variable voltage depending on setting of Fine and course tune knob. Should be a smooth linear transition from both knobs at minimum to both knobs at maximum. What is min and Max voltage?

FP2 should be a stable voltage for any setting of the fine and course knobs.
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Old 1st Apr 2023, 7:16 am   #293
3pinplug
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Default Re: Help needed with - Racal Dana 9081

Hi all

Chris - Thanks for looking at the component in more detail, have just ordered 2 of those and will fit once they arrive.

Paula - will change the cap and run the additional tests and post results this evening.

Peter - can you confirm where FP 1 & 2 are located please as i cant see them on the logic or phase det board. Will then run the tests and post this evening.

Many thanks
Regards
Mark
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Old 1st Apr 2023, 3:21 pm   #294
Chris55000
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Default Re: Help needed with - Racal Dana 9081

I think "FP1" and "FP2" are typos for "TP1" and "TP2!"

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Old 1st Apr 2023, 7:44 pm   #295
PJL
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Default Re: Help needed with - Racal Dana 9081

FP2=pin 6 on the logic assembly (Power to fine tune control)
FP1=pin 1 on the logic assembly (Output from fine tune control)
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Old 2nd Apr 2023, 9:10 am   #296
3pinplug
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Default Re: Help needed with - Racal Dana 9081

Good news!!!
It was either C17 or C25 (10uf tants) on logic board causing the rolling display as when i changed these the display no longer scrolled it is now working perfectly - result!👍
I tested the 2 tants on my component tester and 1 was completely dead the other was ok.
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Old 2nd Apr 2023, 3:59 pm   #297
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Default Re: Help needed with - Racal Dana 9081

Hello Mark,

That was a marathon but well done for keeping at it to resurrect this 9081!

Paula
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Old 2nd Apr 2023, 4:31 pm   #298
3pinplug
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Default Re: Help needed with - Racal Dana 9081

I only did the soldering - I had the easy bit!

It was the perseverance of yourself Paula, Peter, Chris and everyone else who kindly helped - just want to say a big thankyou to you all for getting me to this point. There were times there when i thought it was a parts item but your methodical approach allowed it to live on once again.

Thanks all!
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Old 2nd Apr 2023, 5:10 pm   #299
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Default Re: Help needed with - Racal Dana 9081

Well done!
As I constantly remark on here "beware the Tants"
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Old 2nd Apr 2023, 6:17 pm   #300
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Default Re: Help needed with - Racal Dana 9081

Well done to everyone, a great and well deserved result at long last!
My seldom used 9082 has been switched on maybe 4 times this weekend, from a few minutes to a couple of hours and worked fine every time. Last used probably 2 years ago?
Keeping everything crossed here!
Rob
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