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Old 29th Jun 2009, 3:10 pm   #1
tim_mills
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Default Racal RA17 MKII - Very hot filter coil L46

Hi,

I'm hoping somebody could point me in the right direction. I recently acquired a RA17 MKII (working). On internal inspection I found several of the resistors cracked. I replaced the suspect resistors and took the opportunity to replace about half of leaking capacitors with their modern day equivalents – may be not such a good idea!

I now find that when powered on the filter coil (L46) associated with the 1st VFO gets extremely hot. Of note: in this section the 470R resistor (R55) was replaced with two 1K resistors in parallel.

If somebody could enlighten a newbie on what he may have done wrong or where to look. It would be greatly appreciated.

All the best,
Tim
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Old 30th Jun 2009, 11:15 am   #2
Skywave
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Arrow Re: Racal RA17 MKII - Very hot filter coil L46

Hi Tim - and welcome to the Forum.

The RA-17 (& its variants) is a fine receiver. There are several members here who have had a good deal of experience with it, so you should be able to get it sorted.

First, you did the right thing in replacing defective caps. and resistors; quite a common need on these - although it does seem that the history of any particular sample of RA-17 significantly determines who badly these components have suffered over the years. Some of these receivers would have been used in hot, humid atmospheres & will have seen many hours of continuous use - and it gets qute hot beneath the chassis, anyway!

However, to move on to your Q. . . .

L46 and its associated caps., C193 (0.1 uF) & C104 (0.001uF) form a LPF in the 6.3 v.a.c. line feeding V3, V5, V7; V1, V2; V4, V6, V8. Now L46, being a choke that is wound with wire of a substantial gauge, will have a very low d.c. resistance. Consequently, if it's getting that hot on account of its resistance and the current flowing through it, then that current must be sizable indeed! Check for 6.3 v.a.c. at each end of this choke: if 6.3 v.a.c. is at both ends, then all is well with this heater line. If not, then C193 / C104 are suspect as S/C. You could have a S/C in any of the valves (unlikely) or a solder-splash or something that has shorted out the heater line.

As you say, L46 sits in its own screened compartment with R55: two 1k-ohm Rs in parallel: this is normal Racal build: each R is rated at 2 watts. It may just be that the radiated heat from these Rs is heating up L46 to the temp. that is making it seem like there is a fault associated with L46. (I've never had the need to look that hard! ) In addition to the voltage checks around L46, measure the voltage at each end of L43 (the HT choke): should be about 220-v.d.c. at each end.

Finally, if that lot fails to produce a conclusion, remove L46 from the chassis & inspect it for a cracked / broken core. I've never known this - but there is always a first time!`

Please let us know how you get on with this.

Al. / Skywave.
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Old 30th Jun 2009, 5:20 pm   #3
XTC
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Default Re: Racal RA17 MKII - Very hot filter coil L46

Quote:
Originally Posted by tim_mills View Post

I now find that when powered on the filter coil (L46) associated with the 1st VFO gets extremely hot. Of note: in this section the 470R resistor (R55) was replaced with two 1K resistors in parallel.
Firstly, what do you mean by 'extremely hot'?

Warmer than you expected.
Too hot to touch.
So hot the shellac insulation has been damaged.
Smoking and glowing.

As Al says, it's a substantial piece of copper wire and it would take a lot of current to heat it.

One possibility is to check the current flowing through it. It should be about an amp (3 x 300mA). If you disconnect L46 from the transformer, you can check the resistance to earth with the valves removed - it should be O/C, or you could supply it from a current limited PSU.

Quote:
Originally Posted by tim_mills View Post

I now find that when powered on...........
Secondly, are you saying this wasn't the case before you replaced components?

I suspect you've mis-wired something, or disturbed something, such as a heater lead with damaged insulation, which is now earthing, or have a solder bridge or bit of component lead shorting to earth. If L46 is getting hot because it's conducting a lot of current, be very careful about running the set for long like that. The mains transformers are well-specified, but you can kill any transformer with overload and replacements aren't that easy to find.

According to my cct diagram, R55 is supposed to be two 1K resistors in parallel. Sometimes you find sets which have one high wattage resistor instead of a pair in certain positions. Resistors in RA17s are notoriously troublesome. I think that for some reason, Racal sailed very close to the wind with resistor ratings.



Pete.
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Old 30th Jun 2009, 11:57 pm   #4
Skywave
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Question Re: Racal RA17 MKII - Very hot filter coil L46

Quote:
Originally Posted by XTC View Post
One possibility is to check the current flowing through it [L46]. It should be about an amp (3 x 300mA).
Hi Pete . . .

Sure about that? My cct. diagram shows V1 to V8 being fed from L46.
8 valves @ 0.3 A per valve . . . about 2.5 amps.

Incidentally, I once had an S/C on a heater line in a RA-17. The LV transformer secondary winding can sure provide some current - you should have seen the smoke! Made a right mess of the heater wiring

Al.

Last edited by Skywave; 1st Jul 2009 at 12:02 am.
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Old 1st Jul 2009, 10:54 am   #5
XTC
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Default Re: Racal RA17 MKII - Very hot filter coil L46

Hi Al,

I've got three cct diagrams, each on two A3 sheets. RA17 (early), RA17 MKII and RA17L.

RA17 (early) and RA17L both show the four heater filters at the bottom right hand side below the transformer and rectifier. It's quite clear from these that L46 connects to V3, V5 and V7, RF amp , 1st VFO oscillator and 1st mixer, which are all on the same chassis IIRC.

RA17 MKII shows the four filters near the sections where they are used. The L46 filter connects to positions 'x' which are the heaters of V3, V5 and V7. V1, V2, V4, V6 and V8 (xtal osc, harmonic generator, harmonic mixer and 37.5MHz amp before the BPF) have heaters connected through L44 and are marked 'z'.

In respect of the heater filters, the diagrams are the same but drafted differently.

So, I'm as certain as I reasonably can be. I've never had to do any work on that part of the circuit so I'm going from the diagrams.

If your diagram is different, Racal did do odd things, maybe for particular customers, and they were probably always trying things to reduce the cost. Maybe my RA17 MKII diagram is out of kilter with most MKIIs?


Which valves does L44 supply on your cct diagram?


As for the mil spec transformers in RA17s, they were probably designed to deliver twice their marked current in an orchid house in Death Valley, but having them deliver into a short circuit is still something to avoid.

Pete.
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Old 1st Jul 2009, 2:37 pm   #6
Skywave
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Arrow Re: Racal RA17 MKII - Very hot filter coil L46

Hi Pete,

I get a bit confused over this "Mk. 1, Mk. 2" business of Racal RA17's.

I read the OP's post as referring to an RA17L. This was the cct. diag. that I referred to, L46 feeding eight valves. My cct. diag. for an RA17 shows a different heater supply arrangement.

Anyway, whatever the config. the OP has, there may - or may not - be a fault with the heater cct. in his version. We've both made useful suggestions; let's see what he comes back with.

Al. / Skywave.
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Old 1st Jul 2009, 5:52 pm   #7
tim_mills
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Default Re: Racal RA17 MKII - Very hot filter coil L46

Hi Al and Pete,

Apologies for not getting back sooner. Many thanks for your responses to my dilemma it was greatly appreciated.

I managed to have a look last night and sure enough, there appears to be a very thin thread of solder connecting the brown wire that comes from the 1st VFO to the chassis . I will do the obvious but if I still encounter the same problem, I'll follow your recommendations.

To answer your question Pete, L46 started smoking and burnt through the shellac insulation, to a point where it has flaked off. I don't know if I can get away with this or may it cause problems in the future?

I will let you know how I get on.

Thanks again,
Tim
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Old 1st Jul 2009, 7:33 pm   #8
XTC
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Default Re: Racal RA17 MKII - Very hot filter coil L46

I don't think there was much difference between the RA17 and the MK2, just cosmetic changes and minor circuit changes. Maybe layout changes.

The RA17L is one of the MK3s and there were more significant changes to the filtering, the AGC and the RF front-end. There were other odd variants such as the RA17N which had a 1.6MHz IF rather than 100KHz. Then there are the American sets which are sort of covered in the RA17L manuals I've seen. I don't know whether they had manuals written specifically for them.

As for the heater filtering, check out the cct diagram in the BAMA RA17L manual. It's drafted differently to mine, but looks essentially the same.

I suspect there were many odd variants and redraftings of the documentation. I'd guess that sometimes they just issued a standard manual and a type written addendum.

To get back to Tim, the O/P, I assume he's got a manual and cct diagram for the MK2, but from what you say, he'd do well to trace the heater circuitry and check that it's in line with his cct diagram. He'll pretty well have to do this anyway to check for shorts and faulty caps.

I'm dying to know what's at the bottom of his problem.

Pete.
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Old 1st Jul 2009, 9:43 pm   #9
XTC
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Default Re: Racal RA17 MKII - Very hot filter coil L46

Quote:
Originally Posted by tim_mills View Post
Hi Al and Pete,

I managed to have a look last night and sure enough, there appears to be a very thin thread of solder connecting the brown wire that comes from the 1st VFO to the chassis . I will do the obvious but if I still encounter the same problem, I'll follow your recommendations.

To answer your question Pete, L46 started smoking and burnt through the shellac insulation, to a point where it has flaked off. I don't know if I can get away with this or may it cause problems in the future?
I missed your earlier post.


I think you'll be OK with L46 as it is. If you notice odd instability, or you come across a scrap chassis for a few quid (it does happen, especially if you are not looking), get a replacement. You could even rewind the inductor you have. It looks like about twenty turns of (18 swg?). I'd be tempted to rewind it. The wanted section on this forum is worth trying.

I'm glad you sorted the problem.

Come back and chat again, or to help someone else.

Pete.
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