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Old 6th Mar 2017, 4:02 pm   #1
MrVa1ve
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Default Variable Capacitor Blade Broke Off

Hi,

I have a Philips 310 with a broken variable capacitor blade. The vcap is in two sections and each section is 500pF (measured as 520pF on the LCR meter).

The broken blade is the end rotor blade of the LO section (I think - I'll confirm later). In fairness, this blade is the most delicate since the end blades in this model (they have sun-burst style gaps in the metal) are not secured by a tie-bar across the top and so are only secured to the axle. When I went to straighten the blade (as delicately as I could) - snap! ******!

I've tried soldering the broken blade back to the axle and whilst it's holding sort-of, I'm not entirely happy. It seems my venerable soldering iron isn't hot enough to out-do the thermal mass of the axle plus blade. Also, I'm not entirely sure the axle will take solder properly anyway.

Here are some of the options I've considered; feedback and comments greatly appreciated!

1) Dismantle it completely and try a better soldering effort. Does the axle readily accept solder?

2) Carry on without the blade. It measures as 500pF minus blade, so maybe I could try re-tuning the relevant section?

3) Add additional capacitance using trimmer

Regards.
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Old 6th Mar 2017, 4:15 pm   #2
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Default Re: Variable Capacitor Blade Broke Off

Remove the same blade from the other section. If you don't do this, the two sections won't track.
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Old 6th Mar 2017, 4:20 pm   #3
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Default Re: Variable Capacitor Blade Broke Off

I should add that an additional reason for not being entirely happy with my (admittedly sloppy) soldering job is that I'll need to potentially rough handle the unit to replace the worn out grommets.

Philips in their wisdom decided to pop rivet the holding bracket for the grommets making them very awkward to replace.
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Old 6th Mar 2017, 4:30 pm   #4
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Default Re: Variable Capacitor Blade Broke Off

Hi. I have scrapped a chassis some time ago for the 310/341 Philips radio. If you like I will have a look for it (assuming that you do not want to snap another blade and that the t/c is still on chassis?)
Cheers
Mike
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Old 6th Mar 2017, 4:36 pm   #5
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Default Re: Variable Capacitor Blade Broke Off

Quote:
Originally Posted by MrVa1ve View Post
I have a Philips 310 with a broken variable capacitor blade. The vcap is in two sections and each section is 500pF (measured as 520pF on the LCR meter).
This also happened to my Philips B3X90U (Dutch radio) which has a much smaller t/c, making it really difficult to see rotor plates that catch the fixed ones.
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Old 6th Mar 2017, 4:54 pm   #6
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Default Re: Variable Capacitor Blade Broke Off

Hi Mike, I'd be interested if you had one to spare. This is a first time renovation for me and I'm not confident the t/c will survive the grommet replacement op. It'd be handy to have one as backup! Cheers.
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Old 6th Mar 2017, 5:29 pm   #7
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Default Re: Variable Capacitor Blade Broke Off

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Originally Posted by paulsherwin View Post
Remove the same blade from the other section. If you don't do this, the two sections won't track.

And if you do, the actual tuning will drift HF away from the scale markings as the gang reaches its new (reduced) maximum capacitance.


If the blades (normally referred to as vanes) are typical, they'll be aluminium which is notoriously difficult to solder. You might get better results gluing the blade back to the axle with metal loaded epoxy or painting the axle/blade interface with some silver loaded paint as used for heated rear window element repairs.
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Old 6th Mar 2017, 5:47 pm   #8
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Default Re: Variable Capacitor Blade Broke Off

As it is the oscillator (LO) a simple re trim should do as the LO has a smaller frequency span.
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Old 6th Mar 2017, 6:28 pm   #9
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Default Re: Variable Capacitor Blade Broke Off

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Originally Posted by Herald1360 View Post
If the blades (normally referred to as vanes) are typical, they'll be aluminium which is notoriously difficult to solder. You might get better results gluing the blade back to the axle with metal loaded epoxy or painting the axle/blade interface with some silver loaded paint as used for heated rear window element repairs.
Good suggestion re epoxy. Any particular product you've had success with?

The vanes aren't aluminium (unusually): I'm guessing brass, but I may be wrong. They did take the solder quite easily (cleaned and fluxed). The axle is a different matter, so I think your epoxy idea is a runner.

Also, I see from the circuit diagram that the cap in both the RF and LO sections is teamed with a variable inductor. Is it possible to re-tune the tank using this inductor?
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Old 6th Mar 2017, 9:29 pm   #10
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Default Re: Variable Capacitor Blade Broke Off

I'd try to use some small side cutters to 'nip' some ridges into the area where the blade fits into/onto the shaft and apply pressure to get it to seat. Thereafter a few dots of epoxy to keep it in place.
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Old 7th Mar 2017, 12:37 am   #11
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Default Re: Variable Capacitor Blade Broke Off

I note that this occured when you were straightening a blade for some reason.

Could I caution that many tuning caps have the outer blades bent by the makers to correct the ganging of the capacitance and should not be straightened?

Epoxy should save the day, wedge paper/card in the gaps whilst it sets to get the spacing correct.
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Old 7th Mar 2017, 12:55 am   #12
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Default Re: Variable Capacitor Blade Broke Off

I thought of that Sam but didn't have the confidence to relate it to the OP's set. Seems a bit crude but it probably served it's purpose at the time-if that is the case here that is.

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Old 7th Mar 2017, 9:47 am   #13
MrVa1ve
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Default Re: Variable Capacitor Blade Broke Off

Quote:
Originally Posted by Boater Sam View Post
I note that this occured when you were straightening a blade for some reason.
The reason was the blades were shorting out the cap. It seems to have got a slight ding at some stage and bent inwards (and not out, as you'd expect in a trimming operation). Looks like epoxy is the way to go and I see jb weld seems to fit the bill nicely.

But, it'll have to wait until I sort out these grommets.
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Old 7th Mar 2017, 10:08 am   #14
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Default Re: Variable Capacitor Blade Broke Off

If there is space that epoxy "metal" you get as a sort of sausage is good. I used some on a joint on my classic car's radiator and it lasted for years. Otherwise I would use the slow setting Araldite. I have never had any luck with the quick set stuff. You need to sort out some sort of support first of course as it has no strength at all before it starts to set.
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Old 7th Mar 2017, 11:18 am   #15
MrVa1ve
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Default Re: Variable Capacitor Blade Broke Off

Bluetack to the rescue! Solves a lot of my "third hand" requirements (that and pipe cleaners). Bit of cardboard in between, lump of tack across the blades and maybe a strut to stop sag. There's plenty of clearance to the stator blade, so I don't have to leave the epoxy layer too thin.

I'll let you know how it goes.
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Old 7th Mar 2017, 12:15 pm   #16
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Default Re: Variable Capacitor Blade Broke Off

Quote:
Originally Posted by MrVa1ve View Post
Good suggestion re epoxy. Any particular product you've had success with?
Sorry, no. My suggestion was just a thought rather than actual experience.
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Old 7th Mar 2017, 12:20 pm   #17
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Default Re: Variable Capacitor Blade Broke Off

Araldite Precision has never let me down.
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Old 13th Mar 2017, 2:10 pm   #18
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Default Re: Variable Capacitor Blade Broke Off

Quote:
Originally Posted by MrVa1ve View Post
I have a Philips 310 with a broken variable capacitor blade.
Hi. I have found a good replacement t.c from a Philips 341A radio (later modification to the 310A). Hopefully this is suitable?
If it is of interest, please p.m me?
Cheers
Mike
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Old 20th Mar 2017, 9:04 pm   #19
MrVa1ve
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Default Re: Variable Capacitor Blade Broke Off

Hi Mike,
I couldn't find a way to attach pictures to the PM, so I'll reply here instead. Anyway, here are the images as promised. You can see the cap is screwed to the chassis with the base separated from the main body by rubber grommets. The base is held in place with a bracket, the two tabs of which you can see poking out above the screws. The bracket was riveted to three aluminium standoffs beneath the body of the cap. Hope this helps.

Regards. D.
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Old 21st Mar 2017, 5:11 pm   #20
MrVa1ve
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Default Re: Variable Capacitor Blade Broke Off

For the record, I can report that the metallic epoxy fix was a total success. I opted for JB Weld (regular stuff) in the end since I read somewhere that it was slightly conductive, which is an advantage in this case. Also, it gave me plenty of time to position the blade.

The trick was not to cover the site where the blade sat completely. That way, you can align the blade to the ends of the site and get your positioning correct. Then use a couple of lumps of blue tack at either end and let it set. The capacitance now reads roughly equal to the other section, so a good result. Thanks for all the advice guys!
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