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Old 11th Jul 2018, 11:14 pm   #1
PJL
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Default Columbia 356

Latest acquisition is a Columbia 356 which is a clone of the Marconi 256. It was a blind bid of £12 at a local auction that won it and I could already see a knob was missing and the moths had taken a few bites out of the speaker cloth. As expected, when I go to pick up the PX4 has gone and annoyingly the wooden framed back is missing but judging by the dust both had gone walkies quite a few years ago.

You can often pick up ex-PX4 sets cheaply and, as I have accumulated a few PX4's over time, it's nice to be able to put one back to original working condition.

This is a 7 valve superhet all housed in a superb quality solid mahogany deco cabinet and it weighs a ton. The 'is it just scrap' tests revealed the mains primary, output primary and field coil are all intact. The chassis is a little tarnished on top but no major surface rust and it has what look like an original set of valves bar the PX4. The cabinet has a couple of dings and a few scratches and the top capacitor can is gone but this looks like an interesting restoration of a quite rare and very classy early superhet.
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Old 23rd Sep 2018, 11:37 pm   #2
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Default Re: Columbia 356

Does anyone have one of these or maybe one of the sets that uses the same chassis, Marconi 256 or 258, HMV 470, 253, 254?

I would like to make a replica of the HT reservoir/smoothing capacitor block which has been replaced by a can on this otherwise original set. The wiring has been changed to accommodate the new capacitors and I can't make out what they have done as the earth connection has been re-routed.
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Old 24th Sep 2018, 9:51 pm   #3
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Default Re: Columbia 356

Fallen at the first hurdle. The resistor layout is completely different to the trader. I will have to check every wire as there are 10 capacitors in one large can that I need to replace and I need to know how to wire them up to the 13 tags!
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Old 24th Sep 2018, 10:12 pm   #4
Phil G4SPZ
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Default Re: Columbia 356

My thoughts about making 'replica' capacitor boxes are as follows.

A box identical to the original will fool no-one! I take a pragmatic approach, and construct a good-quality box big enough to house the required modern components and fit in the original position, then create a printed label using a vintage-looking font in Word. In your case, it could say something like "PJL Electronics 'Dekatron' Replacement Capacitor Block" and give information that will assist a future owner, and perhaps make them smile!

Attached is one I made earlier.

Phil
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Optimist [n]: One who is not in possession of the full facts
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Old 25th Sep 2018, 9:16 pm   #5
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Default Re: Columbia 356

Turns out I was being an idiot. Surprisingly the resistors on this 86 year old are all within 20% tolerance apart from one which has had a hard life due to a leaking capacitor and 2 wire wounds which are o/c also probably failed from leaking capacitors.
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Old 26th Sep 2018, 10:32 am   #6
Herald1360
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Default Re: Columbia 356

Can't see any obvious evidence of idiocy in your posts so far. Care to share?
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Old 30th Sep 2018, 1:07 am   #7
mickash
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Default Re: Columbia 356

Quote:
Originally Posted by PJL View Post
Does anyone have one of these or maybe one of the sets that uses the same chassis, Marconi 256 or 258, HMV 470, 253, 254?

I would like to make a replica of the HT reservoir/smoothing capacitor block which has been replaced by a can on this otherwise original set. The wiring has been changed to accommodate the new capacitors and I can't make out what they have done as the earth connection has been re-routed.
Hi PJL,

I've got an HMV 470. If you look in my profile there are some pics of the capacitor block which I repacked. I can take some more pics of the chassis if that helps.

Michael.
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Old 7th Oct 2018, 11:14 pm   #8
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Default Re: Columbia 356

After some help from Michael, I have this set recapped and have put power on it.

It's alive and crackling but I am getting too much HT at the moment. I can think of 2 causes, a Mazda UU5 came with it instead of the U12 and it is far too efficient, and I am using an AC044 which is not an exact replacement for a PX4 so the anode current is a little low (42mA vs 50mA). Oh, and the mains is running at 248V.
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Old 8th Oct 2018, 10:32 am   #9
Herald1360
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Default Re: Columbia 356

Stick a resistor between the rectifier cathode and the reservoir capacitor and/or feed the set from a variac or multitapped autotransformer?

Altrnatively, if none of the components is being overvolted as such just ignore it!
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Old 8th Oct 2018, 11:48 am   #10
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Default Re: Columbia 356

I just checked the LT and it reads exactly 4.0V AC. This set takes around 100mA HT current as the field coil is powered from the HT. From 1932, it would have had a balloon U12 that probably didn't last long! The HT is 350V and should be 300V so I need to do something.

Last edited by PJL; 8th Oct 2018 at 11:59 am.
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Old 8th Oct 2018, 9:14 pm   #11
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Default Re: Columbia 356

Tried a U14 which gave the same result (only quicker as it's directly heated). I have added a resistor but it needs to be a higher value. It seems MOV were quite OK running their valves well over maximum ratings but I guess that is their prerogative.

After a particularly whistly start, a clean up of the wave-change switch has got the set working but not as well as it should on MW. It probably needs alignment but I will look at that tomorrow. I checked the cathode currents and the valves seem OK although the MS4 mixer is odd as it is operated at a very low anode current.

The volume pot, which simply adjusts the cathode resistance for the RF and IF stages, is next to useless as all the action is in the first 5% of the rotation (to be expected using vari-u valves. It is a replacement and it's possible the original was non-linear although it has to be wire-wound so not convinced.

There is no noticeable hum and nothing seems to be catching fire so it's looking like a good result.
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Old 9th Oct 2018, 7:41 pm   #12
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Default Re: Columbia 356

A re-alignment has transformed the performance. It now picks up stations quite happily without an aerial connected. I'll post a few picture when I have it tidied up a bit.

The gramophone volume control wires were left pushed into various crevices which left the detector grid o/c when selected. I have added a 100K resistor to set it to permanent full volume.

The speaker voice coil was rubbing. No spider on this one so you have to loosen the back plate and do the best you can but it uses a field coil so at least you are not fighting with magnets.
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Old 9th Oct 2018, 10:41 pm   #13
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Default Re: Columbia 356

A picture of the completed chassis. The capacitor block to the left of the transformer is a rough and ready brass sheet box to match the original that replaces a 1950's conventional can which was the wrong value and badly swollen.

The other picture is the capacitor block that contains 10 capacitors conveniently placed under the chassis covered by resistor tag boards. This is a vast improvement on later EMI sets as the block can be extracted whereas some sets had the resistor boards riveted on and wires taken straight from the pitch to the resistor tag board. There were a few 'danglers' tacked in mostly wired in parallel with capacitors in blocks that were badly leaking.
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