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Old 7th Oct 2006, 7:23 pm   #1
adibrook
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Default Marconi 264 progress

Today i decided to finally do somethign with thet 264 that i bought a while ago.

I thought it will be a total nightmare because of the electronics. The caps being embedded in a block of tar and stuff.

My origonal plan was to get it working how people usually suggest, but disconnecting everythgin, taking off the terminal block, tog et acess to the block of tar.

BUT, i decided that i dont really care about authenticity, and because modern caps are so much smaller, i can get away without even moving anythgin.

So i just disconnected all the leads going to the block of tar, and voilently ripped them out, taking some of the tar with it. Then i proceeded to jusy wire up caps between the resistors.

So far sucess. I'v got the HT up to 300V ish and the op pentode works.

Everythgin around the af preamp looks ok, but for some reason it refuses to amplify anything as yet. I still havnt foudn the ''tone correction cap'' between the grid resistor and earth.

But i am making progress. I think with healthy HT and a working o/p stage it cant be that far off working.

The PSU took a while for me to work out how it actually works. I got very confused when what i thought was the HT rail was at -600V, but that wasnt the HT rail, that was the unconventional back to front smoothing system where it connects to the center tap of the mains transformer.

I'm really not used to working with this kind of circuit. Its kind of like point-to-point, but with no caps. But after a couple of hours it just clicked, and i think i'm making progress.

I havnt replaced anythign smaller than 0.1uf yet. Are the square mica caps usually reliable?

I wander if its worth it replacing all the resistors?

And what the hell will i do with the horrible horrible cab, with its peeling vaneer and rusted chrome trim .
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Old 7th Oct 2006, 9:39 pm   #2
Ed_Dinning
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Default Re: Marconi 264 progress

Hi Adi, resistors over about 47K are suspect, change them if they are more than 10% out. Low value ones under 100R sometimes suffer this fate as well but they usually look cooked.
Paper capacitors down to 0.01 should be changed.
Mica caps - flat square things are usually OK.

Cover the cab with Rexine?

Ed
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Old 8th Oct 2006, 5:23 pm   #3
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Default Re: Marconi 264 progress

Hi Adi

I had to replace a couple of resistors on mine. ISTR 20K ohms(22k will be fine) and 33 k ohms,which was the anode resistor for the audio triode and had risen in value to about 500K!!
I found the set worked with all the wax capacitors replaced, including those in the box. Electrolytics were fifties types but re-formed fine. The Bakelite encased mica caps seem to be OK, unless cracked.
I would also recommend you disconnect the wire to the mains aerial socket, fed through a capacitor mounted under the connection strip on the mains transformer.

All I have left to do with my 264 to do is apply the transfer and tart up the knobs. Hopefully to appear soon on Pauls Stenning other website................
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Old 8th Oct 2006, 7:35 pm   #4
adibrook
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Default Re: Marconi 264 progress

Well...i'm slowly replaceing all capacitors. I replaced them with a mix of electrolytic and poly caps mainle from scrap tv pcbs.

Maybe the anode load of the af triode has risen too much in value. I will check.

Also, i will need to fit a vary-mu pentode. I tink getting an original may be hard, so for now i will use a EF80 with a seperate heater supply.

By the way, does anyone know of any vari-mu rf pentodes which have 4V heaters? Fitting another valve socket above the original will not be a problem, and is easily revirsable.
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Old 8th Oct 2006, 7:44 pm   #5
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Default Re: Marconi 264 progress

Hi Adi,

Sorry, forgot to send your stuff - there is a vms 4 here for the set.....

I will get a parcel in the post tomorrow.....

Cheers
Sean
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Old 8th Oct 2006, 11:33 pm   #6
adibrook
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Default Re: Marconi 264 progress

Oh...yeah...i forgot too. Thanks m1ecy!!!

More progress. I think i managed to change most of the caps in the set appart from the mica ones. I even got the preamp working for a short time! I found the anode load resistor to be open. So i bypassed it by a modern resistor (for now). It worked for a few mins...and then guess what? The cathode resistor went open . So i bypassed that too...and guess what now? The other anode resistor went open. I dont think those resistors like me.

Anyway, bad news is the rectifier just comitted seppuku.

It was working and suddenly fireworks. One of the cathodes got plasma cut in half, which shorthed the heater and is making it put exessive strain on the mains tranny.

However, for now thats not a problem. I can use another ht source or something. Or a couple of diodes and resistors.

I think i'll just order a load of resistors from maplin and replace them all.
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Old 9th Oct 2006, 3:12 pm   #7
Tim
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Default Re: Marconi 264 progress

Quote:
got the preamp working for a short time! I found the anode load resistor to be open.
glad to be of help........

Perhaps you had better build a lamp limiter................
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Old 9th Oct 2006, 5:22 pm   #8
adibrook
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Default Re: Marconi 264 progress

I have a lamp limiter, which gets used alot for all sorts of dodgy circuitry.

Today i will use an Ez80 with a seperate 6.3v supply, and probably an EF80 too, to try and localte any other sources of problems.
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