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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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11th Oct 2012, 1:06 pm | #1 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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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. |
11th Oct 2012, 1:16 pm | #2 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
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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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11th Oct 2012, 4:17 pm | #3 | |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: DIY osc coil
Quote:
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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11th Oct 2012, 4:33 pm | #4 |
Guest
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Re: DIY osc coil
Do they need to be inductively coupled?
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11th Oct 2012, 5:12 pm | #6 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
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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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11th Oct 2012, 7:24 pm | #7 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
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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.
Ed |
11th Oct 2012, 8:14 pm | #8 | |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: DIY osc coil
Quote:
many thanks . |
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11th Oct 2012, 10:42 pm | #9 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
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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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12th Oct 2012, 1:46 pm | #10 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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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? |
12th Oct 2012, 6:53 pm | #11 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
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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? |
13th Oct 2012, 11:29 am | #12 | |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: DIY osc coil
Quote:
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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13th Oct 2012, 6:50 pm | #13 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
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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. |
13th Oct 2012, 7:16 pm | #14 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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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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13th Oct 2012, 7:59 pm | #15 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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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. . |
14th Oct 2012, 9:06 am | #16 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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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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17th Oct 2012, 5:35 pm | #17 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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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? . |