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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 31st Jan 2009, 11:37 pm   #1
valvekits
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Default Quad II Main Smoothing Capacitor

I repaired one of my Quad II’s main smoothing caps today by cutting open the top very carefully with a dremel cutting disc. I was able to use a hot air gun to gently melt the wax potting compound and remove the leaky caps inside. I replaced them using caps in series with a couple of balancing resistors. Finally, I used some JB Weld to neatly glue the top back on and it all turned out fine.

Eddie
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Old 1st Feb 2009, 12:43 am   #2
GrimJosef
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Default Re: Quad II Main Smoothing Capacitor

Or alternatively you can cut a 1.5" diameter hole in the dead centre of the base and, once you're through the paper inside, you'll see the bottom of the old capacitor. Apparently this can be extracted with a corkscrew (it takes a bit of wiggling) provided you've remembered to desolder the wires from the top connectors of course. F&T make a 16+16uF 500V cap which fits back into the same hole, or you can increase to 32+32uF if you want a bit more ooomph and don't mind compromising Peter Walker's original design. (Actually I think the F&T caps are shorter, so they might need a bit of packing underneath.) Resolder the wires, check it works, glue the disc back into the base, make good and Robert's your mother's brother

Cheers,

GJ
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Old 3rd Feb 2009, 5:37 pm   #3
BassoonBloke
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Smile Re: Quad II Main Smoothing Capacitor

Hi All,

Just to let you know. If you do decide to increase the caps to 32+32uf, you will probably need to change the rectifiers to GZ34's as GZ32's do not like the extra capacity and will quickly wear and die (from personal experience !).

Cheers,
Alan.
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Old 3rd Feb 2009, 11:48 pm   #4
GrimJosef
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Default Re: Quad II Main Smoothing Capacitor

Quote:
Originally Posted by BassoonBloke View Post
If you do decide to increase the caps to 32+32uf, you will probably need to change the rectifiers to GZ34's as GZ32's do not like the extra capacity.
I guess the problem is that there isn't enough resistance in the anode circuit to allow the capacitor upgrade. I've just made some measurements on a Quad II mains transformer and the half-secondary resistance is about 60 ohms. The primary adds slightly less than 11 ohms multiplied by the turns ratio squared (310V/240V=1.3, which squares up to 1.69) i.e. about 18 ohms. That makes 78 ohms in total, which will increase a little as the transformer heats up.

The Mullard data sheet for the GZ32 says it needs a minimum of 100 ohms if it's going to be loaded with 32uF. It only needs 50 ohms if the first cap is 16uF (the one after the choke is much less of a problem). One solution would be to insert a couple of 22 ohm resistors, one in each anode. However this would drop the HT voltage a little. Changing to a GZ34 is a much better solution. Not only can it handle more capacitance (even 60uF only needs a series resistance of about 55 ohms) but it also draws less heater current than the GZ32 so will be slightly more gentle on the mains transformer.

Cheers,

GJ
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