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Old 10th Mar 2023, 1:18 pm   #21
ms660
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Default Re: Mr Carlson does an AR88/CR88

There was a problem with traditional sources of Mica supplies during WW2....India for instance.

Lawrence.
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Old 18th Mar 2023, 2:52 pm   #22
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Default Re: Mr Carlson does an AR88/CR88

In the latest instalment he chopped open one of those caps and it was mica, not paper, inside. He did say it was unusual for mica to fail like that and probably due to thermal cycling from the nearby resistors
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Old 18th Mar 2023, 3:16 pm   #23
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Default Re: Mr Carlson does an AR88/CR88

I've chopped open various failed ones from AR88 and always found paper. Maybe these were self-selected by failing. Also the CR88 was a later set, likely made post-war and maybe trade routes had reopened. There may also be a matter of higher value ones versus lower value ones.

CR88s are rare in the UK, AR88s are common. So beware of these capacitors, not all are what they seem. Also look at the various AR88 restoration texts and notice the numbers of faulty domino Micamolds. They are also in the RF sub-chassis coil box.

So anyone restoring an AR88 still needs to distrust and test these things. If they're part of a tuned circuit, they should be mica, but for simple DC blocks and decouplers plastic film should be fine. Note that the 'that' capacitors are a parallel pair of dominoes and are known to be unreliable.

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Old 18th Mar 2023, 3:30 pm   #24
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Default Re: Mr Carlson does an AR88/CR88

Fully agree. Each to their own, but I always replace 'that' capacitor, the HT smoothing caps and any other high-voltage bypass/decoupling caps as a matter of course no matter what type they are or seem to be. With components of that age it's not worth the risk of leaving them
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Old 26th Mar 2023, 11:43 am   #25
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Default Re: Mr Carlson does an AR88/CR88

Episode 7 is out now and i think he has got he message regarding the paper Capacitors.

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Old 27th Mar 2023, 10:24 am   #26
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Default Re: Mr Carlson does an AR88/CR88

It's made for TV as it were, I watched some of the saga then

Lawrence.
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Old 27th Mar 2023, 3:21 pm   #27
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Default Re: Mr Carlson does an AR88/CR88

He does drone on and is a bit too far up his own doodad about his inventions, but there are occasional gems in his output. He also gets some interesting bits of kit to work on.

From all that test gear, he can't live far from the old surplus outlets near el camino real.

He suffered from excessive set-dressing before his recent lab move. There is a limit to the amount of duplicated test equipment needed on a bench.

It's worth keeping an eye on him and Dave Jones for when something interesting comes up.

I was a bit surprised at how he wasn't aware of all the standard AR88 stuff.

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Old 27th Mar 2023, 5:06 pm   #28
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Default Re: Mr Carlson does an AR88/CR88

Quote:
Originally Posted by Radio Wrangler View Post
He does drone on and is a bit too far up his own doodad about his inventions
Just a little.

Quote:
Originally Posted by Radio Wrangler View Post
I was a bit surprised at how he wasn't aware of all the standard AR88 stuff
All factored to keep folks following.



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Old 28th Mar 2023, 5:45 am   #29
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Default Re: Mr Carlson does an AR88/CR88

Quote:
Originally Posted by Radio Wrangler View Post
From all that test gear, he can't live far from the old surplus outlets near el camino real.
David
He lives near Vancouver, Canada. He's been collecting (hoarding?) this equipment for a while. His focus appears to be a general audience rather than crusty old gits that apprenticed with Marconi.
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Old 28th Mar 2023, 6:53 pm   #30
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Default Re: Mr Carlson does an AR88/CR88

I'm sort of both sad and glad I didn't discover said surplus outlets when I lived on California Avenue (off El Camino Real) 20 odd years ago. I seem to remember luggage weight problems as it was (and those were the days when 32kg hold bags were allowed).
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Old 28th Mar 2023, 7:55 pm   #31
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Default Re: Mr Carlson does an AR88/CR88

I did, and still haven't got over the mental anguish. Only now after 30-some years has it subsided to a general regret.

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Old 7th May 2023, 9:15 pm   #32
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Default Re: Mr Carlson does an AR88/CR88

More video posted. This time IF alignment in fair detail. I'd have done the narrowest bandwidth as a first stab to find out where the crystal needs to be, then I'd use that frequency rather than a perfect 455kHz number for the IF centre frequency, and then work through the formal procedure with some confidence that when doing the crystal-assisted bandwidths, the centre doesn't jump. This way you can get all 5 selectivity settings with the same centre. Otherwise, if you peak up with one bandwidth and then select narrower, you don't go off tune. The IF doesn't have to be a perfect 455kHz, but it does have to perfectly agree with the crystal. Crystals may have been selected back in the day, but they aren't so available, and they have aged over the years.

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Old 8th May 2023, 12:34 pm   #33
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Default Re: Mr Carlson does an AR88/CR88

I watched it last night and fell asleep, missing most of it. No disrespect to Carlson, I'd had a VERY long day and was tired. I'll try to watch it again properly later.
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Old 1st Jun 2023, 10:25 pm   #34
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Default Re: Mr Carlson does an AR88/CR88

He's finished the CR88 now. If mine turns out anything like his then I'll be happy
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Old 7th Jun 2023, 7:54 pm   #35
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Default Re: Mr Carlson does an AR88/CR88

I was interested to watch his approach to IF alignment using the SA+TG. When the manual says it is very difficult, it isn't telling lies ... I'd be really curious to know what the "Ganging oscillator No 2" was like, but even with modern test gear it's not an easy job.

Given a set in bad alignment I found a Dishal method alignment of the coupled IFTs was the best way to get them into a reasonable starting point then follow the sweeping method in the CR88 manual.

Getting the No 1 and 2 selectivity working was ok, but then trying to get the No 3 to be wide enough puzzled me. It also found a problem in my sweeping setup see attached screenshots which show response over a 4kHz region in selectivity modes 3,4,5 (vertical scale unchanged between sweep).

I'm pretty sure there is spurious output +/-1kHz off the synthesizer frequency. Will have to look further when I have an SA to hand. Good to see the AR88 crystal filter testing out the HP.
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Old 12th Jun 2023, 9:45 am   #36
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Default Re: Mr Carlson does an AR88/CR88

If anyone wants a good laugh I found the cause of the triple peaked IF sweeps. The signal generator now has a working AM modulation indicator fitted (I'd been using AM with 1kHz tone in the previous steps of the alignment procedure and hadn't realised it was still on in the sweep due to a defective latching 7-segment display (it read 0% modulation when it was in fact 80%). Trust HP to use latching 7-segment displays (1990-0330) apparently to avoid rfi from a multiplexed display. Fortunately the same as the one in the 8640B so was able to borrow one that worked!

Still more work to do on the symmetry in position 3!
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Old 12th Jun 2023, 10:46 am   #37
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Default Re: Mr Carlson does an AR88/CR88

Spectrum analysers are amazingly sensitive compared to simple diode detetors, so you can afford to use coupling methods with lots of loss.

Two probes in the form of 50 ohm coax with a 50 Ohm terminator on the end and a series 100k resistor to the pin work well. You can inject and sample a signal with litle stray C, allowing individual resonators to be twiddled. Do one resonator at a time with its neighbours shorted for the Dishal method.

Shorting those resonators leaves the coupling mechanism intact and so its pulling effect is properly accounted for.

David
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