Steve
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Post by Steve on Apr 23, 2020 12:25:40 GMT 12
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Steve
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Post by Steve on Apr 23, 2020 13:54:21 GMT 12
Kempy, where did your reply go? anyway, I fixed it for you
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Post by Kempy on Apr 23, 2020 14:13:29 GMT 12
Hah, yea, I'm sitting here trying to work out the difference between different manufacturers and their 40Kpps laser galvo's and I needed a distraction...
Up next, 2 hours training on camera optics.
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Steve
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Post by Steve on Apr 26, 2020 11:07:23 GMT 12
First video is up - not a repair one, more of a rambling history lesson* www.youtube.com/watch?v=BVrzP_ORZhk*note: this is not a real history lesson. May contain traces of nuts. Not waterproof. Requires adult supervision.
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redxm
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Post by redxm on Apr 26, 2020 15:20:13 GMT 12
Some nice designs there
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Steve
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Post by Steve on May 2, 2020 17:44:39 GMT 12
Second spoiler alert
I've got the next video mostly done and ready to upload, which takes around 24 hours - and I'm working on the one following that now, which includes the spoilers below so don't look if you don't want to know yet. I was ready to upload mine when David Tiptons new one came out and I had to stop for an hour to watch that - and after that, my one seemed rather ho-hum. Damn he's good at making videos.
Anyway, I'll get over my feelings of inadequacy and post it regardless
Ok, spoilers below:
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There was some crazing where the original paint was still quite solid - this is under one of the IF cans. Its sanded out ok, but its odd. I'm going to put it down to me not cleaning the surface properly before painting it. It was well sanded with 400-grit wet and dry before I sprayed so the paint should have keyed in ok - and its a lacquer based primer so it should have gone on ok over the original lacquer on the chassis. I almost stopped and stripped the whole lot off to start again - but I gave it a wet sand and it all seemed to have stuck quite well. So I thought I'd give it a go - I can always strip the whole thing back again if it goes horribly wrong...
And here is the first coat of 'Pacific Gold'. No obvious crazing, everything looks good(ish). The only crazed thing was me because its blowing 40 bastards here at the moment and I have to do this outside, so I was in the most sheltered spot I could find and spraying between gusts. Not ideal, but it needed doing so went for it. It seems to have taken ok apart from the few spots where I got too close. That caused some darker spots which will hopefully even out after a wet sand and respray tomorrow.
This set is a learning curve. NSC is a bit mind-bending.
Editing this post to prefix this next bit with "I THINK IT IS" - because I'm not 100% sure... The more I look at it the less I think I understand what is going on - because I mistakenly saw the lead out of the primary of the 2nd IF being connected to the PI filter which its not I don't think. So this also messes up how I think the audio is extracted...
And its a reflex set! [AT LEAST I THINK IT IS] One that John may have missed in his research on the Columbus model 4 Clock Radio from Radio Corp NZ. In an article he wrote for the NZVRS bulletin he notes the model 4 as "their first and only example of Reflexing" (referring to RCNZ). I was quite surprised when I started looking through the circuit with the intention of trying to understand how it worked - only to find the 2B7 in the wrong place... At least I have a better understanding of reflexing now !!! - if not NSC... maybe
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Steve
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Post by Steve on May 2, 2020 17:49:43 GMT 12
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Steve
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Post by Steve on May 2, 2020 18:10:29 GMT 12
Question for those who know: On the output of the 2nd IF there is an RC pi-filter with 2 x 100pF caps and a 500k resistor.
In later sets this is typically a blue 4-legged 47k resistor and 2 x 100pF cap combo (is this called a triplet? I have heard a name for it, I just don't remember what it is). My understanding is that this filter arrangement is to block the IF carrier from getting to the audio stages, which it should do quite effectively even with the low 175kc/s IF in this set.
Later version of this arrangement as seen in a Columbus 525
I did some maths (so its bound to be wrong) with the values in the schematic - 100pf x 2, and a 500k resistor - and this makes me think the audio will be rolled off around 3.5kHz somewhere? At 100k which most other RCNZ sets of that era used, certainly from the following year, the rolloff is much higher at around 16kHz. And with the modern all-in-one device at around 33kHz.
Now, to be fair I was using an RC low-pass filter calculator for this, not a pi-filter calculator... the only one of those I could find was a little more confusing to use since it was a 2nd order calculator (two consecutive RC's) and so I used it with the 1M plate resistance of a 2B7 plugged into the first R position and got results I couldn't really understand
Can anyone clarify this bit of the circuit? Am I right thinking 500k is too high, or is there something else I'm missing here?
Cheers
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Post by Richard on May 3, 2020 11:07:06 GMT 12
maybe its a printing mistake and should be .05 rather than .5? or maybe they are trying to keep that part of the circuit "High impedance"so as to try and stop the .1uf cap coupling the vol pot from saturating and causing distortion? Also thats a strange setup with the directly coupled 57's
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Steve
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Post by Steve on May 3, 2020 11:35:11 GMT 12
And now I'm convinced I was wrong about this set being reflexed The Radiotron Designers Handbook has a section on reflexing which shows the audio feeding back through the valve, which this does not. The more I stare at this the more I want to go eat icecream...
I'd love a bit of feedback on this. I've actually pulled the next video because I've realised my explanation I made of how the 2B7 was being used was based on (a) me misreading the circuit diagram (seeing a joining dot where there wasn't one) and (b) me misunderstanding how the audio is detected and passed on to the volume control. Sorry to the thousands of screaming fans who were waiting with baited breath but I need to fix that first...
I've also redrawn the actual layout of this circuit from my other chassis
The tone control is in parallel with the volume cct, the screen supply to the NSC valve would surely have burnt out very quickly wired this way and the second 100pF cap in the PI filter out of the 2nd IF is nowhere to be seen.
My guess now is that they used this unusual valve arrangement to save on valves because they needed one for audio phase inversion, so put the 2B7 in the IF position to get the audio out at that point. Could they have used a 58 for the IF, then a 2B7 then the 2 57's? Can anyone else see any other reason why they might have done it this way?
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Steve
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Post by Steve on May 3, 2020 11:51:40 GMT 12
maybe its a printing mistake and should be .05 rather than .5? or maybe they are trying to keep that part of the circuit "High impedance"so as to try and stop the .1uf cap coupling the vol pot from saturating and causing distortion? There also a sledge hammer IF bypass cap of .1uF? from the plate of the 57 to ground, which would roll off the Audio. Also thats a strange setup with the directly coupled 57's
Its definitely a 500k, the other chassis has it fitted and it looks pretty original. That one measures around 890k though, so its just a touch of tolerance
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Post by Richard on May 3, 2020 12:50:17 GMT 12
I didnt look too carefully at the circuit either, what does that first 57 tube do? it looks like the audio is filtered out at the grid and the anode, its seems to be adjusting the dc voltage only on the suppressor grid of the 2nd 57 tube. also, the second 100pf cap in your IF filter isnt required as the .1uf in series with the vol pot to earth effectively does the same job to filter out IF
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Steve
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Post by Steve on May 3, 2020 23:25:16 GMT 12
Hi Richard I have no idea what NSC does although I have some theories which I talk about in the video, which I'll hopefully re-finish and re-upload tomorrow - I have two 107 sets, neither is working yet... well, one works, but its been seriously hacked into a standard 6V radio with the NSC valve completely bypassed. The other one is missing parts. So I'm rebuilding one back to factory to see what it does. I'll document that as extensively as I can once its going. Also, I take back the reflexing statement. I was wrong - I thought I knew what reflexing was... which, it turns out, I did not. It might come as a surprise, but it seems John Stokes was right, the model 4 probably IS the only reflexing set from RCNZ
Its taken a lot of staring and pondering and redrawing of the circuit into a more conventional layout, then more staring... MUCH more pondering... and a little common sense. The 2B7 is acting as a normal IF amplifier, but rather than have detection occur in the first valve after the second IF, its in this one just before the second IF - but in both methods the audio is still detected on the output of the second IF, so it theoretically doesn't matter where it is (practically it probably does) - but in this circuit, the audio is needed for the next stage, so they're simply detecting the audio here. It looks very foreign and backwards which is possibly why it took me so long to get it straight in my head - but I understand now - so NOBODY explain it any other way, even if I'm completely wrong!
I think I will redraw this circuit in a more conventional way to help me, if nobody else...
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Post by Richard on May 4, 2020 16:10:17 GMT 12
The first 57 tube looks like it shifts the bias of the second 57 tube? depending on the detected agc voltage, as the Audio appears to be filtered out by the .01 uf cap on the grid and the .1uf cap on the Anode of the first 57 tube . Maybe NSC= Noise Suppressor Control ?
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Steve
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Post by Steve on May 4, 2020 16:40:17 GMT 12
Good guess Now while you're on a roll, how does it work?
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