'Dog-bone' Ceramics: Swap or Not? - UK Vintage Radio Repair and Restoration Discussion Forum

russell_w_b · 18th Apr 2015, 4:55 pm

Is there any advantage (or disadvantage) in swapping out the 'dogbone' decoupling ceramics which don't leak but are out of tolerance (600 - 700pF instead of 1nF) with new, in-spec single-layer ceramics?

These being around the 37.5MHz amp / 2nd mixer of my RA-17L.

evingar · 18th Apr 2015, 5:44 pm

Personally I'd leave alone - both in terms or originality and also because they aren’t likely to degrade anywhere near to the extent of paper types. As decouplers, the value won't be that critical.

Sideband · 18th Apr 2015, 7:23 pm

Agreed...leave them! A nominal 1000pF ceramic reading 600 - 700pF is OK for RF/IF decoupling and in practice, probably hasn't changed much during the capacitor's life.

Skywave · 18th Apr 2015, 7:33 pm

I agree with the remarks given by the above members. However, are you trying to locate a fault or are you doing 'routine' preventative maintenance?
If either, a new thread might be advised to avoid this one going off topic.

Al.

russell_w_b · 18th Apr 2015, 7:51 pm

I'm just seeking overall improvement, so 'routine preventative maintenance'. I've replaced the components (Rs and Cs) in the 1MHz oscillator and seen an improvement 'on paper' insofar as I now measure 2V RMS at the 1MHz O/P (1.3V RMS on acquisition of the set) and 1.8V RMS at TP3 when cycling through the Megs with V9 removed, as opposed to the half-volt or so I had before.

I've also replaced the Cs and the out-of-spec Rs on the little board in the 2nd VFO (other components in there were in spec) just so I don't have to go in there again unless I have to. Wasn't it yourself who suggested the board might've got a bit hot with being enclosed?

I digress, however...

I'm inclined to leave them in that case, but really was curious to know if modern single-layer ceramics are a suitable replacement for the 'dog-bone' ceramic ones.

Radio Wrangler · 18th Apr 2015, 11:37 pm

A -40% change in a decoupler might just have a small effect on tuned circuits, but I think the question is whether the reduction in capacitance is a sign of impending accelerating change.

If you've done enough work to isolate and measure it, you've also disturbed it enough to have changed it.

David

19th Apr 2015, 7:58 am

Hi K ceramics do loose capacitance over time, you can anneal them and regain the original capacity by heating to a few hundred (check!) degrees.

Skywave · 19th Apr 2015, 9:01 am

Quote:

Originally Posted by russell_w_b

I've also replaced the Cs and the out-of-spec Rs on the little board in the 2nd VFO Wasn't it yourself who suggested the board might've got a bit hot with being enclosed?

Yes, indeed it was. I have met more than one example where that board was very badly burnt.

Al.

18th Apr 2015, 4:55 pm	#1
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	'Dog-bone' Ceramics: Swap or Not? Is there any advantage (or disadvantage) in swapping out the 'dogbone' decoupling ceramics which don't leak but are out of tolerance (600 - 700pF instead of 1nF) with new, in-spec single-layer ceramics? These being around the 37.5MHz amp / 2nd mixer of my RA-17L. __________________ Regds, Russell W. B. G4YLI.

18th Apr 2015, 5:44 pm	#2
evingar Octode Join Date: Oct 2003 Location: Newbury, Berkshire, UK. Posts: 1,770	Re: 'Dog-bone' Ceramics: Swap or Not? Personally I'd leave alone - both in terms or originality and also because they aren’t likely to degrade anywhere near to the extent of paper types. As decouplers, the value won't be that critical. __________________ Chris Last edited by evingar; 18th Apr 2015 at 5:50 pm.

18th Apr 2015, 7:23 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: 'Dog-bone' Ceramics: Swap or Not? Agreed...leave them! A nominal 1000pF ceramic reading 600 - 700pF is OK for RF/IF decoupling and in practice, probably hasn't changed much during the capacitor's life. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

18th Apr 2015, 7:33 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: 'Dog-bone' Ceramics: Swap or Not? I agree with the remarks given by the above members. However, are you trying to locate a fault or are you doing 'routine' preventative maintenance? If either, a new thread might be advised to avoid this one going off topic. Al.

18th Apr 2015, 7:51 pm	#5
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: 'Dog-bone' Ceramics: Swap or Not? I'm just seeking overall improvement, so 'routine preventative maintenance'. I've replaced the components (Rs and Cs) in the 1MHz oscillator and seen an improvement 'on paper' insofar as I now measure 2V RMS at the 1MHz O/P (1.3V RMS on acquisition of the set) and 1.8V RMS at TP3 when cycling through the Megs with V9 removed, as opposed to the half-volt or so I had before. I've also replaced the Cs and the out-of-spec Rs on the little board in the 2nd VFO (other components in there were in spec) just so I don't have to go in there again unless I have to. Wasn't it yourself who suggested the board might've got a bit hot with being enclosed? I digress, however... I'm inclined to leave them in that case, but really was curious to know if modern single-layer ceramics are a suitable replacement for the 'dog-bone' ceramic ones. __________________ Regds, Russell W. B. G4YLI.

18th Apr 2015, 11:37 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 'Dog-bone' Ceramics: Swap or Not? A -40% change in a decoupler might just have a small effect on tuned circuits, but I think the question is whether the reduction in capacitance is a sign of impending accelerating change. If you've done enough work to isolate and measure it, you've also disturbed it enough to have changed it. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

19th Apr 2015, 7:58 am	#7
Guest Guest Posts: n/a	Re: 'Dog-bone' Ceramics: Swap or Not? Hi K ceramics do loose capacitance over time, you can anneal them and regain the original capacity by heating to a few hundred (check!) degrees.