AR88D IF1 IF2 Schematic v Reality

[Garglon]

I’m restoring an AR88, found a bit of a mess at the socket of the IF1 tube and have tidied that up (probably caused by a resistor replacement as the one there was not quite the right value).

noticed that the resistor on IF1 went from pin 3 of the K/G3 cathode pin to ground via pin 2 on the actual radio - on the schematic it is shown going from pin 5 (K1) to pin 2 and on to ground. It’s clear that pin 5 on the base of IF1 has never been used.

So I looked at the base of IF2 and the resistor here does match the schematic, going from pin 5 (K1) across the socket to pin 2 and on to ground. It’s clear that on IF2 it’s pin 3 that’s never been used.

Would there have been a reason that they wired that resistor in a different way than the schematic at IF1?

These valves have two connections to the cathode, K and K/G3, so pin 3 is KG3 and pin 5 is K - the K3G side is tied to Grid 3 but so is K I guess, just not quite as direct.

I’m tempted to fit the resistor as per the schematic just for OCD and correctness, but I won’t if there’s a good reason that it was wired this way - any help gratefully received.

Thanks in advance,

James

[turretslug]

I would be inclined to fit a wire link between valve socket pins 3 and 5, and take the cathode resistor to 0V by whatever route is easiest/neatest. Taking the cathode resistor to tag 2 combines the valve's cathode current with its heater current on their way to chassis- not "nice", but they evidently "got away with it" on this short link. Whilst the AR88 was a highly-specified receiver with the latest developments at the time, there's a lot about its construction that's somewhat "domestic" and cost-pared in nature, including the avoidance of tag-strips/boards (apart from the audio/bias board) and terminal posts, they used spare valve socket and bathtub capacitor tags wherever they could. I suspect that lots of them were built in a hurry using employees and lines from domestic radio production, so the AR88 doesn't have the almost pernickety build thoroughness of some military stuff. My OCD would lead me to run pin 2 and parallelled pins 3,5 to chassis separately (I currently have an AR88LF undergoing square-one rebuild when I get a bit of time every now and then) but RCA was run by famously/notoriously hard-headed businessmen rather than perfectionists. IF1's cathode current is used to drive the signal strength meter, so someone may have altered things to drive a non-original meter.

There was a case here a while back where someone found that the pin 3 to pin 5 connection had failed internally on a 6SG7 and reading around elsewhere it seems that this isn't unknown. In the RF stages, where it matters, both pin 3 and pin 5 are separately connected to chassis but a time-and-motion person might have figured that doing this in the IF stages as well might have taken another minute per set!

Colin

[ms660]

Originally Posted by Garglon

noticed that the resistor on IF1 went from pin 3 of the K/G3 cathode pin to ground via pin 2 on the actual radio - on the schematic it is shown going from pin 5 (K1) to pin 2 and on to ground. It’s clear that pin 5 on the base of IF1 has never been used.

Post#1:

https://www.vintage-radio.net/forum/...d.php?t=150717

Lawrence.

[Garglon]

Hi Colin

Thank you so much for replying to my first post here, very comprehensive and much appreciated.

Your comment about the meter feed is interesting, when I sorted out the slightly messy wiring at the base of IF1 I did notice a tiny short bit of modern wire there - could have been a feed to a meter, although mine doesn’t have one on the front glass like some do, it just has the make and model.

Amazingly I was watching a You Tube clip of someone showing the inside of an AR88 this morning and it’s close up enough to see the different resistor wiring on IF1 and IF2 and it’s the same as mine in that the resistor is wired differently on each base, although on this one it seems to go from pin 3 to pin one - again to ground but not on the exact same pin as the cathode current on pin 2 as you mentioned.

So my current plan is to put the resistor between pin 3 and pin 1, but that may change by lunchtime!

Out of interest, what would you class as one of the very best made military receivers/transceivers?

Regards,

James
MW0ZAP

[Garglon]

Thanks Lawrence, very helpful, James

[ms660]

Originally Posted by Garglon

Thanks Lawrence, very helpful, James

No problem, the schematic in the original AR88 manual also shows Pin 5 being used and not Pin 3.

Lawrence.

[turretslug]

Originally Posted by Garglon

Out of interest, what would you class as one of the very best made military receivers/transceivers?

It's a good question, there are folk here who are a great deal better versed in military sets than me, I would characterise my vintage radio interests as spanning the swankier domestic broadcast sets to the more straightforward commercial comms receivers but I get the impression that the British industry in particular latterly took quite a few cues from captured German equipment. As an example the Murphy B40, first appearing towards the end of WW2, featured turret band selection and modular assemblies built on light alloy castings, both characteristic of German military kit and contrasting strongly with the predecessor Marconi CR100, another set that smacks of being designed for inexpensive mass production by consumer production lines. The B40 is about the only set I've ever come across, domestic or professional, whose RF section I would describe as "easy" to overhaul, the turret construction and overall layout being big contributors. Against it, the over-engineered and wear-prone tuning drive with its bevel gears and long drive chain.

My comments about the AR88 were in no way putting a downer on it, in fact I think it is an excellent example of the US strength in precision mass production and value engineering, making something "only as good as it needs to be". I believe that its apparent origin as a high-grade commercial receiver was a point in favour for wartime use in that it was expected to have excellent performance but at a commercial, rather than military price. When looking over military electronics, particularly Cold War stuff, I've often thought, "yes, this is mechanical confectionery, but is this degree of over-engineering strictly necessary or even helpful?".

[Garglon]

Thanks again Colin