Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

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Craig 01
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Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Monday 23rd May 2011, 18:07

Hi all,
I am looking at broadbanding a GE A3-5825A and do have a few new old stock Phillips BB910 varactors and am asking here if these would be suitable for the task of extending the range on the vco and if so,what range extension can i reasonably expect ?.

Is there a better,wider capacitance range varactor that is easily/readily obtainable if the BB910 is not suitable?.

It is needed to have coverage from 26.965 to above 28.000 as far as is reliably possible.

Are there any other changes required in the VCO to get the coverage ?.

What other changes will be required/necessary for the Receiver and Transmitter to broadband the mentioned radio with board PTBMO58COX ?.

I have fitted a mod that replaces the 2 digit display with a 3 digit display and provides n codes from ch 1to ch 251,i know the radio cant be expected to cover all 251 channels but would like to try to get as much as is possible.

Hopefully one or more of you out there has good experience with these old chassis and knows what exactly is required and can take the time to help me.
For those that can help it will be greatly appreciated.

Thanks,
Craig.

lbcomms
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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Wednesday 25th May 2011, 22:56

You can get 2 to 3 megs out of these fairly easily. Remove it from the radio and remove the epoxy (careful!) on the PIN side. Don't remove the can.

Cut the tracks on the 2 places indicated. Add the 2 capacitors as shown - SMT ones work best, but the really tiny leaded ones will fit if you are careful.

Add your varactor to the place shown.

Put back into radio and reset the VCO tuning voltage so it covers the whole range (approx 2.5 Mhz with the values shown).

Image

We did dozens of these back in the 90's...

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Wednesday 25th May 2011, 23:18

It is needed to have coverage from 26.965 to above 28.000 as far as is reliably possible.
What other changes will be required/necessary for the Receiver and Transmitter to broadband the mentioned radio with board PTBMO58COX ?.
They will cover from 26.965 to 28.235 OK. Don't go above this too much or you will start causing interference to 10M ham, marine, etc.
Wipe out a marine emergency and people will come looking for you :shock:

Technical explanation - there is a mixing product in the 40 Mhz area that the tuned circuits need to remove, and will cause spurs once the local oscillator gets to about 39 Mhz (this is at about 28.3 Mhz, i.e. 39.000 - 10.695). A simple L-C filter can't be expected to remove an unwanted signal 1Mhz away. If you want to cover 10 metres (28.3 - 29 Mhz), you'll need to swap / switch crystals to do it cleanly. This will raise the 40Mhz spur to 42/43 Mhz, which the LC filter will be able to remove.

Have fun...

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Thursday 26th May 2011, 10:23

Hi lbcomms,

As is usual i have found, you are a valuable source of information and technical expertise willing to share your knowledge and help those of us of a lesser education and experience.
I searched for hours/days looking for a working mod for this vco with no joy.
I already had the epoxy removed and varactor fitted,although it did not work to expectation.
I will fit as per diagram the cap values given and retune.

Many thanks,
Craig

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Friday 27th May 2011, 0:37

it did not work to expectation
In what way? lack of range, unable to tune, etc?

Assuming you have a half decent frequency counter and a proper 50 ohm dummy load, the best way to mod / faultfind these is to temporarily disconnect the PLL.
Isolate pins 5 and 6 (make sure they ARE isolated or you'll blow the IC) and wire a pot to the VCO circuitry.

Center pin of pot > VCO tuning voltage test point
Left pin of pot > pin 1 of the PLL02A IC
Right pin of pot > pin 16 of the PLL02A IC

Turn the pot to halfway, turn on radio, adjust VCO slug for 27.6 Mhz (WARNING: don't do this with an antenna connected!)
You should be able to use the pot to tune the radio from the bottom to the top (typically 26.8 to 28.2) of its range.
Adjust / mod the VCO + mixers etc until it covers the area you want.

Once you have it covering your wanted channel range, remove ther pot and restore the connections to pins 5 and 6 of the PLL02A.

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Friday 27th May 2011, 3:50

The vco with only the varactor BB910 added had lack of range/unable to tune,intermittent operation,it was about a week or so ago i tried it and that is what prompted me to post here in case the varactor i was using was not suitable and there was something else that needed doing such as addition/removal of caps and their values.
The vco i used with the bb910 fitted might have had a separate problem and i'm pretty sure it did,i have resoldered all connections on vco board/block as the solder joints on the pins were suspect.It was a spare i pulled from the parts box to try.
I will take a guess and say this is probably a likely failure point on these vco's.
I have ordered the smt cap values shown on the diagram and when they arrive i will try it again.
Yes i have a decent frequency counter and dummy load so when vco is back in as per diagram given(except i will be using the bb910 varactor) i will let you know what it is getting.
What would be a good pot value? i'm thinking no less than 10k as it will have the 5v feed across it to ground.
I am not that good to know what values to change to on the vco + mixers etc to shift/increase the range if needed.Perhaps you could help me there again if necessary. :)
Thanks

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Saturday 28th May 2011, 12:08

had lack of range/unable to tune,intermittent operation
Two things cause this - bad original varactor (common) or <resistance> leaky 4.7uF electrolytic (rare).
What would be a good pot value? i'm thinking no less than 10k as it will have the 5v feed across it to ground
It's feeding a reverse biased diode (no current on the wiper terminal) so the value is not critical, anything from 1K to 100K will do.

5V/1K (i.e. the worst case for 5V current drain across the outer terminals) is only 5mA, well within the (approx 100mA) capacity of the 5V zener regulator.

Cheers

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Friday 3rd Jun 2011, 22:11

Ok,was able to get from 26.965 to 28.695 after adjusting mixer coils before vco quit operating on transmit/receive at 28.705.It would transmit at 28.705 then when tx button released there would be no receive no tx until i came back down into low 27 mhz where vco became operational again and unit would again receive and transmit and i could then go back up in frequency to where it would drop out as previously mentioned.

That was with pll control via n code.I havnt tried substituting in the pot as yet.
I also was measuring from the base of Q7 pre driver with the counter,driver and final were out of circuit so no idea of power output although i expect it will be very low.

The vco was good to 19.265 at 19.275 it would drop out after tx.When stepping up channels to 19.285 it would drop out on receive.

Increasing vco test point voltage made no difference to range although at stock voltage was low at 0.69v at 28.695mhz.

The bb910 varactor according to data sheet is useable to .5 volt.

Is this the best i'll get without changing values on vco and mixers apart from what i have added as per above diagram and the bb910 ?.

What needs to be done on vco and mixers to increase range above what i'm getting ?

The Vco at present is covering 1.73 mhz starting at 17.555 mhz.A fair bit short of the mentioned 2.5 mhz approx the mod should give.

Maybe the bb910 varactor has not as wide a capacitance range as the "Maxi Tune diode" in the diagram and i cant find anywhere what the specs of the "Maxi Tune diode" were.

Cheers,
Craig

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Sunday 5th Jun 2011, 0:26

Well with a bit more playing around with vco,before i realised that my main problem lay in the 21mhz injection into the vco mixer,i'm not sure what to do there to increase the signal could only go so far with the adjustment with tuning coil T3 and signal is low, perhaps changing cap coupling values ? but really dont know what to change or to what value.

I did add a second varactor bb910 in parallel with the one already fitted to vco which left the core hanging out the top of coil/vco a bit so cleaned up a brocken slug from the parts box trimming about a third off the slug from cracked end of core and winding it back in and found as a bonus i now have additional tuning range with core sitting about 3 mm below top of coil former of vco .

I'm now getting to channel 212 or 29.125 mhz or 19.750 mhz on the vco with 2.16 mhz range before the vco mixer looses output due to weak 21 mhz injection.
I havnt checked tx at predriver from tx mixer but not that worried about that yet as i dont intend transmitting that far up.

This is the first time i have modded a radio for extra coverage so quite a few head aches along the way trying to get my head around things.

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Sunday 5th Jun 2011, 3:09

before vco quit operating on transmit/receive at 28.705
Like I said before, don't attempt to go that high with the circuit as-is, you'll throw out spurious (unwanted mixing products). Read the earlier post for the explanation.

If you need to go higher (for example, to cover the 10m ham band) you need to change the 10.0525 crystal to keep it clean.
Formula: fChange = xChange * 4. For example, changing the 10.3125 will raise the thing by 1.04Mhz, giving you 28.005 on channel 1 instead of 26.965.
This will let you go cleanly up to about 29.5Mhz.

before the vco mixer looses output due to weak 21 mhz injection
Not quite, you are asking the mixer to something it can't do. On rx and tx, you'll get issues like 20khz steps and the radio getting to a point and quitting, until you either tune it back down or turn it off and back on again. On tx, a spectrum analyser screen will show fog and grass insead of nice sharp vertical lines :shock:
What needs to be done on vco and mixers to increase range above what i'm getting
...
I did add a second varactor bb910 in parallel
The second one won't help much - the actual capacitance isn't so important, what gives you the range is the ratio between Cmin and Cmax.

If you want more range, change the values on my diagram: 4.7pF cap to 2.2pF and 47pF cap to 100pF.
This will give more range but will also increase phase noise (increased "splatter" on rx, poor audio on SSB transmit are the 2 main symptoms of this).

It's a compromise to get it right - the more channels you cover, the worse the performance of the radio becomes.
By design, some chassis are better at this - the Cybernet tops out at 2Mhz without major work, the Uniden 858 and 8719 radios can double this.

Have a look at the export (120 - 200 channel european) versions of the Cybernet and Uniden radios - the Cybernets switch in a bunch of crystals for the "banks", the Unidens use binary adders. This is not an accident - they did it this way to keep them clean.
i cant find anywhere what the specs of the "Maxi Tune diode" were
They were made by ACS in the states, but went out of production in the mid 2000's sometime.
Over a 10V control range, they do 3.3 to 50pF, a 15:1 range, making them excellent for VCOs and tuned mixers.

They were sold locally by Hatadi Electronics here at Bayview (NSW) - we bought all of their stock of these (and many other spares / components they sold) when they went out of the radio business around 2003.

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Sunday 5th Jun 2011, 16:30

Thanks lbcomms,

I will heed your advice and set it up for 120 channels to 28.2 maximim leaving the cap values in vco as they are per diagram and leave the second varactor in to save removing from board again.

Unless,you recommend removing the second parallel varactor.

Cheers,
Craig

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Monday 6th Jun 2011, 21:12

Unless you recommend removing the second parallel varactor
This won't make any difference to ther range - in an ideal word. In the real world it might help a tiny bit...

First of all, from the datasheet for the BB190:

Image

The PLL02A operates over a PD range of 0.5 to 5 volts - this is shown highlighed in pink.
Look to the left, that tells you what values of capacitance are attainable.

0.5V: 38pF
5.0V: 16pF
38/16 = 2.375:1 ratio. This is what sets the usable range of a VCO.

Now, add a second one in parallel. This will add the capacitances, in this case they are identical so we'll double them:

0.5V: 76pF
5.0V: 32pF
76 / 32 = 2.375:1 ratio

The lock range ratio is identical! The number of channels is the same, only the position of the ferrite slug is different :D

Cheers

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Wednesday 8th Jun 2011, 14:23

Thanks for the heads up Sue that the varactor capacitance value has no effect to the circuit apart from core slug position.

I had a dig in my varactor parts and checking against datasheets found i have Toshiba 1SV149 varactors with low voltage operation and large capacitance ratio,perhaps i should have used 1 of them instead.

Therefore if i can adjust core slug for a start voltage on ch1 for about 4v i should be good to go.

Toshiba 1SV149 specs

1v-8v operation range

1v: 500pf
5v: 50pf

500 / 50 = 10: 1 ratio

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by lbcomms » Friday 10th Jun 2011, 0:13

Toshiba 1SV149 specs
1v: 500pf
500pF is wayyyy to high, the radio will want to go down to about the 40 meter ham band :)
You might be able to "unwind" the coil to make it go back up, but that will cause other problems.
Try to get some suitable varactors first. The BB809 will give you about a 4:1 range, not quite as good as a maxitune but it should give you at least 2 megs coverage.
1v: 500pf
5v: 50pf
500 / 50 = 10: 1 ratio
Not quite - when you go that high, you need to take into account the 47pF blocking capacitor. This is effectively in series with the varactor capacitance.
<if you don't know the maths for caps in series, google it>
50pF in series with the 47pF = 24pF
500pF in series with the 47pF = 42pF
42/24 =1.75:1 ratio

The reason I never mentioned the blocking capacitor before is because with a smaller varactor, it can be ignored:
5pF in series with the 47pF = 4.5pF
50pF in series with the 47pF = 24pF
24/4.5 = 5:1 ratio

By using a more suitable varactor, we have gone from 1.75:1 up to more than 5:1 total.

Search Ebay for BB809 - there are 2 sellers, one in the UK (cheap) and one in the USA (very expensive).
You should be also able to get them or something similar from the usual sources such as Mouser and Digikey.

If you can't get them, I may be able to give you a couple if you are in no hurry.
We archived (boxed) a lot of this stuff when, due to changing business conditions, we went from a fixed-location repair service to mainly doing mobile / callout type work - therefore I can't get at the old spare parts easily.

Have fun

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Friday 10th Jun 2011, 1:35

G'day Sue, thanks very much for your offer although wont need to take you up on it as i now have some bb809 varicaps on their way :) .
Thanks also for taking time again to set me straight,so to speak :) and explain in a way thats easy to understand.

Next on the list will be getting the chassis to have a relatively even spread of useable rf output over the newly acquired range from standard as i expect the GE A3-5825A to have a bit of power drop off even after careful tuning of the coils.
The receiver is still reasonably sensitive over the current 120 channels after re-alignment.

I pulled a 1969 from one of the parts chassis i have and although the junctions checked out ok it was a dud,it only passed the rf from the new 2166 i fitted,of which i still have a few new.So,decided to order some of those eleflow hfr-20w replacements to try as i didnt want to go through the parts chassis checking and trying unknown 1969's soldering / unsoldering from the board.Figured it better to fit a new supposedly good,compatible working replacement.

Cheers

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Re: Help Please,GE A3-5825A Cybernet VCO ZZ-Z017 range extension

Post by Craig 01 » Saturday 9th Jul 2011, 1:05

Update:
The old expanded GE is up and running with hfr-20w very well .

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