[R-390] Carbon Comp Resistors in RA.17's, RA.117's and R-390's

[Bob Camp]

Hi

This also relates to the resistors in the R-390 carbon composition 
resistors in the R-390 so it is worth repeating here.

Carbon comps came in three flavors 20%, 10% and 5%. A lot of people 
have commented that the 5% tolerance was more a wish than a 
specification. A 10% 1K resistor that reads 912 ohms is still in 
specification.

In order to check a carbon composition resistor for value there is a 
bake out procedure you have to go through. I don' t remember it exactly 
but 48 hours at 125 C sounds about right. The value of the resistor was 
measured something like 24 hours after it returned to room temperature.

On the R-390 most of the carbon composition resistors are 20% tolerance 
parts. They tend to drift up in value with age. As far as anybody can 
tell the radios work every bit as well with resistors that are 30% high 
as they did with resistors that are on value.

Every time we swap out a part on these radios there is a risk of 
damaging something else on the radio. I would only swap out parts that 
I am sure are a problem. I'm not suggesting that you keep the BBOD's in 
the radio, or that you do a ten hour test of every part you swap out. 
I'm only suggesting that you be reasonably sure the reward from 
replacing the part is worth the risk of damage.

	Enjoy!

		Bob  Camp
		KB8TQ

On Jul 25, 2004, at 12:09 PM, Jordan Arndt wrote:

> Can anyone give me any tips or info on the reliability of components 
> in the
> older RACAL receivers...I'd like to know what someone else may have 
> found
> regarding the Paper Caps , resistors etc....Does anyone know an e-mail 
> address
> for Robin Filby, who is supposed to be one of  the vintage RACAL 
> experts..?
> The RA.117 that I'm working on has a date of 1965 and a Canadian 
> Marconi
> 'Depot date' of 22/10/1975....
>
> The previous user had dissassembled the ENTIRE radio right down to the
> modules, and I really could use some discussion with some who has some
> experience on these fine receivers...I'm breaking it down section by 
> section ,
> and re-capping and replacing out of spec resistors...So far, what I've 
> found
> is the resistor values are lower than spec..
> ie., a 1k resistor measures 912 ohms... a 15k resistor reads 14.2k....I
> realize that the heat of summer isn't the best time to be starting 
> this, but
> given the extent of the work, and a 1 module per week in my 'spare 
> time' ,
> it'll still be a couple of months before I attempt to re-apply power 
> to the
> reworked receiver...
>
> Interesting to note, the reason the previous user did the rework in 
> the forst
> place was that he had no 6688 tubes, so he 'modded' the radio to use 
> 6EH7
> tubes...it's also filled with 6AU6WA tubes in place of all the 
> 5749/6BA6
> tubes...and he tried to use 1/2 of a 6AL5 dudl diode to replace the 
> EA76
> subminiature AGC rectifier....The radio did work to some extent , 
> otherwise I
> wouldn't have bothered...
>
> The RA.17 is one of my favorite 'Band cruisers' , and I'm sure this 
> will be
> the case once the buttons go back on this RA.117....
> I will be taking detailed pictures and notes of the entire process, 
> just for
> posterities sake, and to document what is easily the broadest 'rework' 
> I have
> ever attempted...
>
> Here's to summer and to Vacuum tubes...!  73 de Jordan....
>
>
> Jordan Arndt wrote:
>
>>> I have just acquired a Racal RA.117 receiver....
>>> I would like to know if anyone who has one in good original 
>>> condition ,
>>> that could take some clear quality pictures of some detail I need in 
>>> the
>>> IF section , at least to start with...
>>> The previous "user" replaced the 6688 tubes with 6EH7 tubes, redid a
>>> bunch of the wiring, and generally hacked up one of my favorite all 
>>> time
>>> BA's...
>>> I got it 'fairly' cheap, and I would much prefer to get it back in 
>>> shape
>>> and working, as opposed to parting it out or selling it as is...
>>>
>>> I have the online manual, but the pictures, and even the schematic
>>> aren't of much help , as they are not at all clear enough for 
>>> detailed
>>> viewing...
>>>
>>> 73 de Jordan....
>
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