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Old 24th Nov 2020, 4:35 pm   #1
GreasedMonkey
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Default Line drive stage questions.

I am currently trying to save the PCB from my set and have it in the hands of a very good lab technician, but have some questions.

1- The line inductor (L201) was a bit of a bodge after the original part got smashed at my own ham-fistedness. While I am in the lab I intend to make another. So I know total inductance wants to be around 4.7mH. But it is made up of 2 windings with the junction of the 2 windings going off to the 11V rail. My question is, is this the center point? ie. should each winding be 2.35mH on the former?

2 - On the back of the PCB there is a capacitor strapped across the line drive transformer (T201) between the junction of R243 ant the primary winding and the ground side of the secondary. this looks to have been there for quite some tie, but I can see no mention of it in any of the documentation / schematics I have. Could this have been fitted from new, or is this likely a previous 'repair'? If so, what would be its purpose? some kind of filtering I guess.

Thanks in advance.
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Old 25th Nov 2020, 9:41 am   #2
kalee20
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Default Re: Line drive stage questions.

The added capacitor could have been fitted to alter the drive waveform for the line O/P transistor.

Rather unhelpfully, there are no dots added to the drive transformer - we don't know whether the BD160 is driven 'on' by the BC337, with base current limited by R243, or whether the BC337 being 'on' establishes current in the inductance of the drive transformer and when it turns 'off' this current is diverted into the BD160 forming drive current.

If the latter, then the added capacitor will give a sharp spike of reverse base current to the BD160, assisting rapid turn-off.
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Old 25th Nov 2020, 10:49 am   #3
murphyv310
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Default Re: Line drive stage questions.

What is the value of the "added" capacitor?
This could well be a factory modification and didn't make it to the printed diagram. Some manufacturers told no one about mods or circuit changes others did. My advice is to leave it. It may be there purely for tuning or wave shaping, it may have been added due to a different type of driver transformer that had different parameters so the cap was needed to compensate, again a manufacturer mod , we'd get better idea though with a value.
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Old 25th Nov 2020, 6:34 pm   #4
GreasedMonkey
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Default Re: Line drive stage questions.

Well he has been over the board and done what he can. As the line inductor was out when he did it, he has replaced this with a socket so the inductor can now be simply plugged in. He has replaced several tracks, found 2 suspect solder joints ad several paces where there was no continuity between the two sides of the PCB and there should have been. His opinion is that the set has been in a corrosive atmosphere at some point in its life.

However, I am a little confused.
If line frequency is 15 and a bit KHz and capacitance in parallel with the inductor is 22nF then I make the required inductance to be 4.7mH. But I decided to measure the one that came out to double check the maths, this measures only 1.413mH total inductance. I forget the exact numbers as I left the bit of paper in the lab, but I made that to be a frequency of a little over 9KHz, yet the set was displaying a picture (not a very stable one, but it was a picture). Am I missing something fundamental here?

As the inductor is now ‘pluginable’ I left the original as it was, and made one at 4.7mH with the tap for the 11V rail at mid point, one at 4.7mH with the ratio of the two coils the same as the original, and finally one at 1.413mH with the 11V tap at mid point. Let’s see if any of these gives a good stable picture.

Oh, also I checked the capacitor fitted, it is confirmed 22nF as per the schematic.
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