Bush DAC10 - MW Frame Aerial

[Ian - G4JQT]

The exact wire gauge isn't important, although it's probably better that it's not so wide all the turns don't sit side-by-side.

I think the front ends of the DAC90A and the DAC10 are almost identical apart from the switching, and use the same frame aerials.

If it's any help, this is what a DAC90a MW coil looks like.

Arthur

[AC/HL]

As this is no longer a wanted thread I'll move it to Vintage Radio.

[David G4EBT]

In answer to question 1: I can only explain what I'd do if I was rewinding the coil.

I'd poke a screwdriver through the centre of the spool then clamp the screwdriver is a vice so that the spool is free to rotate on the screwdriver shaft. I'd have several pieces of masking tape to hand, each about 5 cms long, so that if you have to stop winding for any reason, you can temporarily secure the turns by sticking a piece of tape over them onto the former.

I'd then solder the wire to the solder tag at the start of the winding, noting the comments in the earlier post by 'Shuttle' about the direction of the winding (anti-clockwise) because it must be in phase with the LW coil, as both coils are in series on long wave. Then just hold the former in your hands, rotating it while keeping the wire taut with your fingers so that the turns stay on the former.

As I said, if you need a rest, pop a piece of masking tap on.

In answer to question 2: I think that slightly thinner wire might be all to the good as it will ensure that the windings will fit the former, which might possibly not have been the case had the wire been slightly thicker. The resistance of 22 turns might be slightly higher with thinner wire, but I think only by a fraction of an Ohm with so few turns. The resistance is less important than the inductance, which is more a function of the diameter of the coil and the closeness of the turns, which are side by side anyway - not spaced.

When you've completed the coil, before you solder it to the end of the winding tag, you might wish to stick a piece of masking tape on the turns so your handiwork doesn't spring off the former, and check the inductance, (I think you have a 'multi-tester'), which isn't stated in the service data. I measure the one on the DAC90A that I'm working on (spasmodically just now!) and it measured 3.2 Ohms and 181uH (0.18mH). Before you solder the end of the coil to its tag, a few drops of superglue at intervals around the former will keep the turns in place.

I don't think the inductance is that critical because at the alignment stage, you have to adjust the trimmers for band coverage and for the pre-set stations.

Hope that helps.

Good luck with it!

[ms660]

For proper tracking the inductance is important, it needs to be as close to the original as possible.

Lawrence.

[DonaldStott]

Thanks to everyone for their helpful responses.

I'll have a go over the next couple of days and report back ...!

[DonaldStott]

Ok - first attempt at rewinding the MW wiring on this Frame Aerial - not the neatest of jobs but not bad for a complete novice.

I'm reading 5.7ohms 0.21mH - how does that look?

Some of the wires do overlap so to what extent does that matter?

[ms660]

Originally Posted by DonaldStott

I'm reading 5.7ohms 0.21mH - how does that look?

Bung the inductance result along with the approx min and max parallel capacitance values into this calculator for a ball park figure for frequency coverage:

http://www.1728.org/resfreq.htm

Lawrence.

[DonaldStott]

Thanks for that Lawrence - looks like a very useful tool if only I knew how to about calculating the approx min and max parallel capacitance values? I know that for capacitors in parallel you simply add the individual capacitances - but which ones??

Complicated as the Bush DAC10 has that Tuning Unit and Pre-Set switches!

[Al (astral highway)]

Originally Posted by DonaldStott

if only I knew how to about calculating the approx min and max parallel capacitance values? I know that for capacitors in parallel you simply add the individual capacitances

Hi Donald, good skills.

If this is the main tuning capacitor, work with 0 min to 500 pF max for a generic AM capacitor. That's both sets ganged together.

I don't have set specifics but that's your average twin-gang air-vaned capacitor. I think 365pF or so for the large section IIRC.

Your frame aerial is fine. -- I've just popped in some values for illustration.

Your 0.21mH tunes the lower frequency end of the medium wave band (550 KHz -- which is also approx the same no of metres) with 400pF. It tunes the higher frequency end (200m or 1.5MHz) with about 50pF . As I say I'm not familiar with the exact design but this is generic info. Most sets have padder capacitors to get these lower values of Pifs bang on - does yours?

[ms660]

Originally Posted by DonaldStott

Thanks for that Lawrence - looks like a very useful tool if only I knew how to about calculating the approx min and max parallel capacitance values? I know that for capacitors in parallel you simply add the individual capacitances - but which ones??

Complicated as the Bush DAC10 has that Tuning Unit and Pre-Set switches!

1st think in manual tuning mode not preset tuning mode, it's the maximum capacitance that's mainly of interest as it's not adjustable to any great extent, that will determine the lowest frequency the RF tuned circuit will tune down to, the value for the tuning capacitor fully meshed is given as 528pF in the manual and the value of the trimmer is given as 4 to 40pF, supposing the trimmer is set to approx. half it's maximum capacitance say 20pF, that gives a value across the coil of 548pF adding on to that the valves input capacitance of approx 4pF gives 552pF, that with your 210uH inductor means that already it will tune way down below what it's supposed to, in fact it's almost tuning down to the IF of 465kHz....not good....and that's not taking into account stray C due to the wiring and the self capacitance of the coil itself etc.

Lawrence.

[DonaldStott]

Originally Posted by astral highway

I think 365pF or so for the large section IIRC.

Your frame aerial is fine. -- I've just popped in some values for illustration.

Thanks but as Lawrence has pointed out " ...the value for the tuning capacitor fully meshed is given as 528pF in the manual and the value of the trimmer is given as 4 to 40pF"

If I input 552pF with my 0.21mH inductor into the formula I get 467.46kHz - as Lawrence says much too low as the IF for the DAC10 is listed as 465 kc/s.

I note from the Bush DAC10 Service Instructions that the MW range is 191-560 metres (1,570-535 kc/s) so if we reverse the input parameters for the formula to give us the inductance as the frequencies and capacitance are known:-

1570kHz & 552pF gives us 0.018617mH

535kHz & 552pF gives us 0.16032mH

As fewer turns of wire in the coil results in less inductance I can experiment with this until I'm within shouting distance of the required figures for the MW aerial!

I noted that some of the wires do overlap so to what extent does that have an effect on the inductance - I am assuming it's quite insignificant with a single wire and only 22 turns?

[Al (astral highway)]

Yep, mine was a generic value to give you a bunk up as you didn’t know what to enter at the time.

A pF or two isn’t much of a deal at these frequencies , it arises in stray capacitance all over the place.

Basically you’re in a very good place for a first attempt and need to lower the inductance a bit by taking off a winding or so. Remember inductance is proportionate to the square of the turns.

You can’t achieve the level of precision you suggest with your inductor. So maybe to nearest tenth of a mH is more than good enough. Your units are mH so eg 18.6mH and 16mH, not as you have expressed it.

The windings are already overlapping, you say, so as you’re taking turns off, you’ll soon find out the impact ��

[David G4EBT]

Originally Posted by DonaldStott

Thanks but as Lawrence has pointed out " ...the value for the tuning capacitor fully meshed is given as 528pF in the manual and the value of the trimmer is given as 4 to 40pF"

If I input 552pF with my 0.21mH inductor into the formula I get 467.46kHz - as Lawrence says much too low as the IF for the DAC10 is listed as 465 kc/s.

I note from the Bush DAC10 Service Instructions that the MW range is 191-560 metres (1,570-535 kc/s) so if we reverse the input parameters for the formula to give us the inductance as the frequencies and capacitance are known:-

1570kHz & 552pF gives us 0.018617mH

535kHz & 552pF gives us 0.16032mH

As I said in my earlier post, the one on the DAC90A that I'm working on measured 3.2 Ohms and 181uH (0.18mH).

As to the tuning capacitor, the manual for DAC90A states that the tuning cap gangs are each 528 pF. (a rather precise figure) - the same as that stated in the DAC10 Data Sheet. I removed mine, checked it before cleaning and it measured 570pF per gang. I cleaned it it in my ultrasonic bath, then lubricated it. Both section then fact measure 550 pF, so when I see values stated in the Service Data on sets that had quite long production runs, I suspect that some changes arose along the way. Given how narrow the air gaps between the plates are, I can only assume that dirt on the vanes resulted in the gangs measuring 570pF before cleaning.

Good luck in tweaking the coil.

[DonaldStott]

Some late night tweaking to remove three turns of wire from my MW winding.

I'm now reading 5.3ohms and 0.17mH.

Using that inductance and a capacitance of 552pF in the link provided by Lawrence we get a frequency of 519.55kHz - that seems to be going in the right direction?

Maybe remove one more turn ...??

[PJL]

The inductance is determined by the diameter of your coil and the number of turns so if you have counted the turns correctly I would not fiddle around trying to get the measured inductance and resistance correct as your meter is not going to be 100% accurate and the meter leads have both resistance and inductance.

[DonaldStott]

Thanks PJL for your advice which seems eminently sensible - time to stop tweaking and get back to rebuilding the other parts of this Bush DAC10 that need attention!

[ms660]

When you do the final turns adjustments with the coil in circuit don't forget to check that the IF, the cursor and the oscillator alignments are correct first, always remember when doing/checking the oscillator/RF alignment to make sure that the cursor alignment is correct before starting.

Lawrence.

[Davewantsone]

Do not get over concerned with the theory of getting mathematically the correct inductance. The MW coil will be close to LW coil and the metal work of the radio chassis will all help to affect the inductance.

[DonaldStott]

For those still interested, the adventure continues from Post#24 here More BUSH DAC10 Woes!