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Old 19th Jul 2015, 11:42 am   #61
turretslug
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Default Re: RCAF AR88LF (GR-17) Project

There are a few detail differences in the internal PSU and AF O/P stages between 'D. 'LF and CR91 receivers but nothing that would have significant bearing on 6V external PSU requirements- I would be surprised if a 'D receiver couldn't be used with an MI-22215 psu, or an 'LF with an MI-8319 unit. As the 'D receiver was made (largely) at Camden, US and the 'LF in Montreal, Canada, maybe the different numbers just reflect plant cataloguing.

I think that the CR91 was the post-war equivalent spec. of the AR88LF, but built at Camden, so maybe the code for the associated PSU reflects Camden, rather than Montreal house-coding. Could the -A suffix reflect the substitution of a component that had been found to be wanting in the original version, perhaps? Or maybe improved LF filtering to suit the receiver coverage. Manufacturers like to re-assure users of improvement in this sort of manner.
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Old 24th Jul 2015, 9:14 pm   #62
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Default Re: RCAF AR88LF (GR-17) Project

Turretslug

You could be onto something there. I have only seen a handful of the PSU's in various states over the years. Interestingly, all were manufactured by Radiart, which I understood to be based out of Clevland, Ohio in the USA. All were MI-8319-A units and one had a square orange Signal Corps acceptance stamp on it.

I have now discovered that Radiart also had a plant in Montreal, Canada, so I suspect they were likely the sole manufacturer for all of RCA's AR88 line in both countries. Quite possible, therefore, the different model numbers might just be a product of the two different plants.


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Old 15th Nov 2015, 5:45 am   #63
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Default Re: RCAF AR88LF (GR-17) Project

Been sidetracked for a while with some parts chasing for my 19-Sets and a C42, in the process of which I remembered I need to locate the two adjustment tools normally supplied with the AR88 series of receivers. Both of these tools are missing from my set, but the Allan Wrench for the knobs is still alive and well, safely stored in it's retaining clip, for some strange reason. I guess it's something to be able to say a third of the tools for my AR88LF have survived, but it seems a stretch!

Does anyone have these two adjustment tools they could post pictures of? I think one of them is double ended for setting the plunger type trimmers. Both are apparently stored in the fuse clips on the tuning cover, under the main RF cover, but I also have no idea which tool goes where.

David
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Old 15th Nov 2015, 6:21 am   #64
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Default Re: RCAF AR88LF (GR-17) Project

I'm away from my home computer at the moment, but the pictures you want already exist on this website. Someone made the same request and I photographed the tuning tools from a second AR88 I'd bought as a spares donor for my first. I think it was 2012, in the summer when I bought that receiver.

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Old 15th Nov 2015, 6:36 am   #65
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Default Re: RCAF AR88LF (GR-17) Project

Thread "A short success on an RCA AR88D" started by Gezza123, post number 18

29th July 2013.

https://www.vintage-radio.net/forum/...5&postcount=18

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Old 15th Nov 2015, 8:57 pm   #66
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Default Re: RCAF AR88LF (GR-17) Project

There are enough AR88s still around with enough missing tools that there's almost a market for someone to tool up to make replicas.... The small hollow-ground screwdriver for core adjustment isn't too difficult to substitute with something suitable but alignment without the piston trimmer adjuster can be a bit fiddly and tedious.

Does anyone know why their absence is so frequent? Perhaps the intercept usage receivers (a service that would have been considered vital but the set would have been used by operators, rather than skilled radio technicians) had them removed and held centrally to deter "tweakage"/ensure calibration consistency, the same might have applied to post-war military service sets. The scale accuracy is that good when carefully aligned that maybe it was felt that they shouldn't be aligned by other than a central workshop with standardised signal generator.
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Old 15th Nov 2015, 9:33 pm   #67
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Default Re: RCAF AR88LF (GR-17) Project

Maybe the rumour of a mineshaft backfilled with AR88s was close but not on the money... maybe it was only the AR88 tuning tools?

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Old 15th Nov 2015, 9:58 pm   #68
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Default Re: RCAF AR88LF (GR-17) Project

Quote:
Originally Posted by Alistair D View Post
Regarding the output transformer failure, there is a mod that helps the valve and the transformer. It is described at the bottom of page 1.
http://www.qsl.net/w/w2vtm//88/Getti...he%20AR-88.pdf
Al,

I read the article with interest looking for this mod. I couldn't see it. I can see reference to the 0.01uF going dead short - but once that has happened, presumably the output transformer has already gone open circuit anyway.

Then there is another reference to fitting a cathode bias resistor in the output valve circuit - but that refers to saving the valve - not the transformer.

Am I missing the mod in question?


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Old 15th Nov 2015, 11:10 pm   #69
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Default Re: RCAF AR88LF (GR-17) Project

Slightly puzzled at the mention of 0.01uF at the O/P valve anode- certainly, there's a 3000pF tone corrector as standard and, as it's connected from anode to chassis, it's just ripe to kill the O/P transformer primary if it goes short. It's always possible that this capacitor value was subject to variation over production, though, or possibly it was a mod to introduce a slightly drastic top-cut for "communications" AF tailoring.

Regarding the discussed calibration inaccuracy on Band 6, there's a little murmur going on in the back of my head that several comms receivers adopted "osc. low" on their highest frequency band, as opposed to "osc. high" on their lower bands. A reason put forward for this was favourable mixer signal grid conductance due to heptode electron stream characteristics, reducing input circuit damping at high frequency. (It's entirely possible that I'm suffering memory mangling here!). If so, could it be that the AR88 should be aligned "osc. low" on Band 6, despite the manual? This would actually be a fairly trivial adjustment to make on a trial basis, as RF alignment would not need touching- apart from HF trimming if it was found that scale calibration did fall into line at the high end.
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Old 15th Nov 2015, 11:31 pm   #70
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Default Re: RCAF AR88LF (GR-17) Project

Turretslug, I merely read the SWM article without checking the AR-88 manual. The article says "Shorted .01 !,F by-pass condenser to the anode of the 6K6GT output valve."

I presume the capacitor in question is C119 from the output valve anode to ground. That's C119 on my diagram. As you say the parts list gives this as 2,700pF, so presumably its some other capacitor. I can't find the 0.01uF caps at the moment.

I made the point that the article makes little sense in the context of mods to avoid o/c output transformers. So perhaps there is one more things to explain.

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Old 15th Nov 2015, 11:36 pm   #71
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Default Re: RCAF AR88LF (GR-17) Project

While we are talking about o/c output transformers, the 2 x neon modification sounds like an excellent one - at least against transient voltages.

What I am less sure about is another problem I have heard of - but not investigated myself. That is the problem of switching the set to standby, and the HT rail then floating up to some excessively high value - over 500 volts I've heard. That is similarly going to put a big strain on the insulation of the output transformer. Unfortunately neons across the winding won't do much in this case.

A possible mod - again not one I have tried as yet - is to move C96 to put it in parallel with C97. This has the effect of changing the power supply filter from a capacitor input type - to an inductor input type. This arrangement should produce a much more stable HT rail voltage with changes of load, albeit at a slightly lower HT rail. This needs testing, as I suspect a bleed resistor will also be needed on the HT rail to keep a minimum HT current flowing - even in standby.

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Old 15th Nov 2015, 11:48 pm   #72
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Default Re: RCAF AR88LF (GR-17) Project

Sure thing- I hadn't seen that article linked to previously, but had a browse through it earlier this evening when I picked up on its mention of the capacitor value. As you say, C119 is implied.

I get the impression that AR88 production went through three fuzzily-defined phases- relatively low pre-/early-war volume with a few changes, a big ramp-up during the war with little alteration and medium-volume post-war production with quite a bit of detail tinkering, so I guess it's possible that "0.01uF" may have featured at some point. Or maybe the author mis-quoted, there are a few details that are slightly at odds with other AR88 articles- I often feel, as with so many other famous artefacts, that it's best to read many different articles on the matter and the truth will average out across them!
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Old 16th Nov 2015, 4:05 am   #73
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Default Re: RCAF AR88LF (GR-17) Project

David.

Thanks for the link to your photo post. I have now downloaded them for future reference.

Turrretslug.

From what I have learned from chatting with several veterans who used these receivers during the war and post war, your assumption is spot on. Seems that in any location where a number of these sets were in operation, the two adjustment tools were removed from the receiver upon setup to prevent operator tweaking. Servicing was done by a specialized unit other than the operators. In locations where only one or two sets were up and running, the tools tended to stay put and the operator was generally skilled enough to tweak the set, with available equipment on site, if needed.

The other interesting thing I heard from a chap based in the south of England, somewhere near London, was that he was sent on loan to another site to help install and set up a dozen or so AR88s at some location. He had never done this before and was surprised to find that each of the receivers when unpacked, were complete in cabinets, and the first thing the team had to do was pull them out of the cabinets and rack mount them. The cabinets were stuffed back into the boxes and taken away. No idea what happened to them.

That struck me as a little odd, but when I looked carefully at my AR88LF Manual, I noticed several things: the weight info does not mention w/o cabinet, reference is made to overall external shielding for the set (which may explain the presence of the elusive cover plate over the rear terminals), and the Installation Section does, specifically, state the the receiver can either be table or rack mounted, and if the latter is employed, the receiver must be removed from it's cabinet.

I wonder if those cabinets and tools went back into stores at the get go to be sold off post war, or were simply tossed?


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Old 16th Nov 2015, 9:39 am   #74
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Default Re: RCAF AR88LF (GR-17) Project

It's said that most AR88s are without signal strength meters because most were assembled into a triple diversity rack, and one panel in that rack contained three S meters which were connected to the receivers.

The significant rarity of the original S-meter does fit this belief.

So most AR88s were divested of their cabinets as well as their tuning tools. Various wartime photos showing AR88s usually seem to have them in racks.

But the majority of AR88s I've seen on the surplus market have been in the original steel cabinet.

So I conclude that when AR88s were sold off in the 1950s, they were reunited with lots of stored cabinets, though not with the removed tuning tools!

David
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Old 16th Nov 2015, 9:57 am   #75
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Default Re: RCAF AR88LF (GR-17) Project

David,

Another possible reason for AR88s being without S-meters is that they were made that way. There was a field modification kit to fit them published by RCA - see enclosed.

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Old 16th Nov 2015, 10:14 am   #76
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Default Re: RCAF AR88LF (GR-17) Project

So they were either ordered that way because they were destined for diversity racks, or because limited supplies of meter movements were best deployed elsewhere and the receiver as a stand-alone set was still functional without it. My first AR88 has one and as a kid I didn't know that almost all of them lacked the meter.

My friend's father's AR88 had an ex RAF (yup radium paint) meter upside down behind the window, but I assumed that was a one-off for some reason. (He also had Western Electric 717As in the front end which was popular at that time)

I know about the field mod instructions. which does suggest they were ordered without meters.

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Old 3rd Dec 2015, 3:02 am   #77
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Default Re: RCAF AR88LF (GR-17) Project

Well, I have been able to find an lovely NOS Coil adjusting screw driver tool for my AR88LF. Quite an impressive bit of craftsmanship in the making of the thing, I must say. Just have to figure out where exactly on the Tuning Coil Cover this particular tool gets stored.

Two out of the three tools that are supposed to come with the receiver are now accounted for, with just the Trimmer tool with the hook and socket ends left to track down.

In the meantime, I have been ruminating over the great lengths RCA went to in shielding these receivers from potential interference. The cabinet for them is quite likely a part of their overall shielding efforts, and I started to wonder, what effects, if any, would be encountered when these receivers were removed from the cabinets and rack mounted, in potentially large numbers, in close proximity to each other.

Maybe I've just got too much time on my hands.


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Old 3rd Dec 2015, 7:53 am   #78
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Default Re: RCAF AR88LF (GR-17) Project

The shielding was good, and in WWII served a second purpose, Any leakage of the LO signal and especially out of the antenna connection could give away the location of a listening post, ship or whatever.

There was a story, I think printed in Brian Johnstone's book, that an unknown serviceman had been captured and interrogated about why allied attacks on U-boats had become so much more effective. The real answer was centimetric radar of course, but he told the interrogators that the aircraft were direction-finding and homing in on signlas radiated by the receivers ('Metox') which the surfaced U-boats were using to listen for the old UHF radar which they thought the allies were using.

That shielding and two tuned RF stages are quite a good block against LO leaks.

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Old 3rd Dec 2015, 8:06 am   #79
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Default Re: RCAF AR88LF (GR-17) Project

Quote:
Originally Posted by David Dunlop View Post
The cabinet for them is quite likely a part of their overall shielding efforts, and I started to wonder, what effects, if any, would be encountered when these receivers were removed from the cabinets and rack mounted, in potentially large numbers, in close proximity to each other.
you mean like this David?
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Old 3rd Dec 2015, 10:32 am   #80
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Default Re: RCAF AR88LF (GR-17) Project

THey had to rack them like that - with spacer panels to prevent them collapsing into a gravitational singularity.

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