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Old 11th Oct 2012, 1:06 pm   #1
ITAM805
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Default DIY osc coil

Hi all

my search for a Wearite PA1 has proven fruitless thus far and I need one such to bring my 63' Watkins Copicat MK2 back to life.

I tried rewinding the the o/c coil but it kept snapping at the slightest tug. So I wondered if I could construct something out of modern off the shelf coils? In the above thread, Ed Dinning says the main coil was 2200uH so I wondered would something like these Murata coils, placed back to back work? I'm guessing the impedance of these would be too low for a valve circuit?

In which case can someone point me towards a suitable candidate.

Many thanks

.

Last edited by ITAM805; 11th Oct 2012 at 1:11 pm.
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Old 11th Oct 2012, 1:16 pm   #2
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Default Re: DIY osc coil

If a coil is 2200uH it's 2200uH and its impedance is defined by that alone except for a slight contribution from winding resistance and stray capacity. Unless Q is critical, lower winding resistance is simply better and stray capacity would hopefully have been considered in the original design and said design made adequately tolerant of it. If the new coil is lower resistance than the original and you're concerned, pad it up to the old figure with additional series R.
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Old 11th Oct 2012, 4:17 pm   #3
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Default Re: DIY osc coil

Quote:
Originally Posted by Herald1360 View Post
If a coil is 2200uH it's 2200uH and its impedance is defined by that alone except for a slight contribution from winding resistance and stray capacity. Unless Q is critical, lower winding resistance is simply better and stray capacity would hopefully have been considered in the original design and said design made adequately tolerant of it. If the new coil is lower resistance than the original and you're concerned, pad it up to the old figure with additional series R.
Thanks Chris

what I'm asking is, do you think 2 of these connected appropriately in circuit, back to back will oscillate, seeing as they are ferrite cored, compared to the original coil?

cheers

.
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Old 11th Oct 2012, 4:33 pm   #4
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Default Re: DIY osc coil

Do they need to be inductively coupled?
 
Old 11th Oct 2012, 4:40 pm   #5
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Default Re: DIY osc coil

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Originally Posted by merlinmaxwell View Post
Do they need to be inductively coupled?
Yes, see the pic of the original in the link in post 1

Schematic here the osc circuit is top right

Last edited by ITAM805; 11th Oct 2012 at 4:46 pm.
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Old 11th Oct 2012, 5:12 pm   #6
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Default Re: DIY osc coil

Could you not find an old VCR that could become a donor for a more modern replacement?
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Old 11th Oct 2012, 7:24 pm   #7
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Default Re: DIY osc coil

Hi Do you need something with a 0.9A rating? If not there are a variety of small chokes in RS/ Farnell and Maplin. No need for it to be precisely 2.2mH, within 20% should do and you can then adjust the tuning C if you need to set it precisely.

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Old 11th Oct 2012, 8:14 pm   #8
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Default Re: DIY osc coil

Quote:
Originally Posted by Ed_Dinning View Post
Hi Do you need something with a 0.9A rating? If not there are a variety of small chokes in RS/ Farnell and Maplin. No need for it to be precisely 2.2mH, within 20% should do and you can then adjust the tuning C if you need to set it precisely.

Ed
Ed, I chose the Murata's not for the current rating but for their physical attributes, that they would close couple inductively. Can you provide a link to what you consider suitable alternatives from Maplins etc, please.

many thanks

.
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Old 11th Oct 2012, 10:42 pm   #9
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Default Re: DIY osc coil

A pair of pot cores. Something like N28 material is good at 50kHz, RM8 size should be big enough. Al = 400 gap means you get 400nH for a 1-turn winding and L increases as the square of turns. Put 74 turns on each for 2.2mH.

You now have two 2200uH inductors. Put an added winding of 10 turns on each and connect them together, but add a small inductor in one connection. fiddle with the small inductor value to vary the coupling, swap the connections over to get the feedback phase right.

David
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Old 12th Oct 2012, 1:46 pm   #10
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Default Re: DIY osc coil

Many thanks for the info and detail David

Last night I decided to experiment with some junk box stuff and found two 100uH chokes. These I sellotaped together, stuck a 68R in the HT and a 2000pF across the primary (anode) coil, lit the blue touch paper and retired.... Somewhat to my amazement it works! The 100uH inductance suggested it was humming away in the LW/MW and I found a whistle at 600khz on my portable!

Anyway, this makes me wonder if I applied the same hi-tech method to a pair of 2200uH chokes that it might well give me the desired result?
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Old 12th Oct 2012, 6:53 pm   #11
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Default Re: DIY osc coil

The original uses two coils to provide an inverted signal to the grid.
You can make an oscillator without phase inversion by feeding back to the cathode. The tuning capacitor can be replaced with two capacitors in series with the junction going to the cathode. Earth the grid to AC and put in a cathode resistor (220R ?).
In this way the original coil could be used since the secondary is redundant.

What thinkest thou?
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Old 13th Oct 2012, 11:29 am   #12
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Default Re: DIY osc coil

Quote:
Originally Posted by TrevorG3VLF View Post
The original uses two coils to provide an inverted signal to the grid.
You can make an oscillator without phase inversion by feeding back to the cathode. The tuning capacitor can be replaced with two capacitors in series with the junction going to the cathode. Earth the grid to AC and put in a cathode resistor (220R ?).
In this way the original coil could be used since the secondary is redundant.

What thinkest thou?
Hi Trevor

that sounds interesting? I'm not clear though where the caps are connected, so could you kindly scribble out a diagram please.

many thanks
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Old 13th Oct 2012, 6:50 pm   #13
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Default Re: DIY osc coil

This is what I was thinking. It might need some component optimisation.
The primary must be in good condition.
You could rewind the secondary (for the original circuit) with plain wire since the current will be very low.
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Old 13th Oct 2012, 7:16 pm   #14
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Default Re: DIY osc coil

Stage 1 is getting it oscillating
Stage 2 is getting it oscillating on the right frequency
Stage 3 is getting it oscillating at the right amplitude

You just need plenty enough to make the erase head work, but the record head bias is a bit more critical. There's tons of stuff written on oscillators and what frequency they go off on, and how noisy they are and how much they drift and all the different circuits and all their names. There's very little written on what amplitude they run at.

David
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Old 13th Oct 2012, 7:59 pm   #15
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Default Re: DIY osc coil

Many thanks Trevor, that's an interesting circuit for sure!

Thanks David. the MK2 Copicat uses a permanent magnet for tape erasure. As for "how much" bias, I'll use the old chestnut of 1db overdrop at 1khz - by way of varying the value of the feed capacitor.

.
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Old 14th Oct 2012, 9:06 am   #16
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Default Re: DIY osc coil

1dB drop is an effective way. Should work quite well enough.

Permanent magnet.... Ugh! I suppose they wanted every head position they could make to cram in more delayed channels.

David
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Old 17th Oct 2012, 5:35 pm   #17
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Default Re: DIY osc coil

So... I lashed up the 2 new 2.2nH chokes and it started oscillating on switch on - that's good! I checked the frequency and it was 63khz - that's good too! But after about 15mins it suddenly stopped oscillating and I noticed the 2k2 1/2W in the HT smoothing was smoking - that's not good

I checked the HT on the anode on the 'NOS' 6BR8A and it was 160v, not the 250v I was getting when it was working ok.

If I remove the HT from the triode section of the 6BR8A, the HT voltage goes up to over 300v. So I threw in a 2N4007 in place of the original rectifier and replaced the only other components in the bias circuit, 2 caps but result is the same, a very hot 1/2W resistor and low anode volts. So is it likely to be my new/old valve gone faulty?

.
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