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KB 888 "Standard Console Radio"

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Anonymous
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The bench didn't quite look right after clearing the last job off it, so I have removed the chassis and speaker from the KB 888 I recently acquired.
I think this radio is probably the best looking in my collection. It has already had some restoration work done to the cabinet and is looking very smart.

It has a massive 12 inch energised speaker, which has 2 transformers mounted on the basket. One is the output transformer the other is the phase splitter transformer.The phase splitter is probably mounted on the speaker because the chassis is quite crowded on the top, with 8 valves and 5 large coil cans, a massive tuning condenser and mains transformer. The speaker has a faux suede fabric surround and the diaphragm does move but it could possibly benefit from being more supple, if there was something that could be used to treat the fabric. Any recommendation?

At the moment the previously replaced "Radiospares" electrolytics are reforming, I think they may be OK given a bit more time.

And I have been going though it visually and checking a few of the transformers.
There are lots of capacitors that will need replacing.

The mains transformer windings seem at first test to be OK but I am going to go through all the transformers again now that I have printed out the coil resistance data.

The on/off switch on the volume control has been rewired to what was probably an external switch but this is not visible any where now.

One leaf of the wave change switch has broken off and was in the bottom of the chassis shelf, it can probably be re fixed with a bit of phosphor bronze soldered on to join it back.
One valve was missing in its can, a SD4 which is a diode tetrode for the inter channel noise suppression circuit. I have placed a wanted add in the forum wanted section.
More later
Mike

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Topic starter Posted : 27/11/2015 1:11 pm
Anonymous
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Bad news, the intervalve (phase splitter) transformer has an open circuit secondary, it should be 4k ohms.
The other coils all measure up OK. The mains transformer has had a little soak on full mains and there is no sign of even getting warm.
The smoothing electrolytics are now up to 350 volts and the leakage is only 0.3mA each.

I have got the wave change switch out and been thinking about how to do the repair on it, what I need is a long thin piece of springy brass (phosphor bronze) about 4mm wide and 50mm long. I cant think of anything around here that can be used.

. The idea is to solder a new spring onto the terminal I have already tinned.

In the meantime here is the circuit, if anyone wants to attempt to explain the operation of V4, the SD4, which I am looking for.

Mike

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Topic starter Posted : 27/11/2015 9:16 pm
Cathovisor
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Looks relatively simple to me. The diode section of the SD4 demodulates the IF and produces a variable DC potential on the control grid which is used to pull the grid of the TDD4 negative and into cutoff - note the anode of the SD4 is directly coupled to the control grid of the TDD4. The threshold of when the cutoff action occurs is set by varying the screen potential of the SD4.

Under no signal conditions, the cathode and grid potentials of the SD4 are the same (-110V) so the valve conducts heavily. The supply for the anode comes from R2 - which I am going to stick my neck out and say is in circuit when the noise suppression is switched on. When a signal arrives at the diode of the SD4 it produces a negative bias on its control grid which reduces the anode current of the SD4 and allows the anode voltage to rise, and in turn this affects the voltage on the grid of the TDD4 allowing it to conduct. Note that the anode voltage of the SD4 will only ever rise to a nominal negative value to suit the grid bias requirements of the TDD4, but in the scheme of things the anode is still positive w.r.t. the cathode.

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Posted : 27/11/2015 10:16 pm
turretslug
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I have got the wave change switch out and been thinking about how to do the repair on it, what I need is a long thin piece of springy brass (phosphor bronze) about 4mm wide and 50mm long. I cant think of anything around here that can be used.

Is it still possible to get "Atomic Strip" (the springy copper alloy strip that used to be used for draft-proofing around wooden door-frames)? It stays very springy over thousands of cycles and solders wonderfully. I've got the remnants of a roll here, I'd be happy to drop an envelope-length piece in the post.

That's what I think of as a "proper" radio! That speaker assembly must weigh a ton. <Dons anorak> I do find these 'thirties sets that illustrate superhet evolution interesting, though I find 'thirties rubber-insulated wiring less welcome.

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Posted : 27/11/2015 10:48 pm
Cathovisor
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That's what I think of as a "proper" radio! That speaker assembly must weigh a ton.

You want a heavy loudspeaker? Try the cast-iron lump fitted to the Murphy A40C, or the Rola G12-a-like fitted to some late 30s RGDs...!

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Posted : 27/11/2015 10:56 pm
EDDINNING
(@eddinning)
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Hi Crackle, these I/V transformers are usually 11, I re-wound one for Til some time ago and it works well.
Lots of turns of fine wire though. The better quality ones are wound on Radiometal cores and are quite small. Be careful not to bend the lams if they are of this type as it damages the magnetic properties.

Ed

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Posted : 27/11/2015 11:17 pm
Anonymous
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Hi Ed
This one is a bit more than the normal small inter valve transformer, its as big as the OP transformer (well the core is). It is constructed if e's and i's and has 3 separate bobbins on the former. I have looked to see if it was a matter of corrosion by the start of the winding, but although there was a break in the thin wire I could still not get a reading, so there must be another within the windings.

I will also take a photos and post here.

With 10v on the primary there is 14v on the good secondary. But I suspect the primary may be faulty as the manual says it should be 1.9k ohms but it only measures 1.6k ohms, each secondary is supposed to be 4k ohms the one which does have continuity measures 3.8k ohms.

Perhaps you could please PM me a price for a rewind.

cheers
Mike

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Topic starter Posted : 28/11/2015 12:31 am
Cathovisor
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Is it still possible to get "Atomic Strip" (the springy copper alloy strip that used to be used for draft-proofing around wooden door-frames)?

Indeed it is, but its name has changed:
http://www.diyandbuilding.com/proper-co ... 29-6m-pack

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Posted : 28/11/2015 6:55 pm
Anonymous
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Gerry from Crowthorne valves has suggested a diode pentode, would that work in this circuit.
Which one would be best?

My list of valves says I have a AC2/PENDD which is B7 4 volt.
But is there a smaller type that would do a better job?

Thanks
Mike

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Topic starter Posted : 28/11/2015 8:37 pm
Cathovisor
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Gerry from Crowthorne valves has suggested a diode pentode, would that work in this circuit.
Which one would be best?

ITYM a double-diode pentode.

Trouble is, the valve you have uses twice the heater current. As to whether it'd work? No idea. I'd be inclined to go for something like an SP4 and a germanium diode. But in all honesty Mike, put a post up on Radiomuseum for an E444 - over 250 listed sets used it.

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Posted : 28/11/2015 9:19 pm
occiput
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If all else fails, you might always look on ebay.

For example:

http://www.ebay.fr/itm/LAMPE-RADIO-MULL ... Swl9BWJTtm

('ware the line wrap)

although by the looks of it, you'd have to negotiate with the seller over delivery to the UK.

709379

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Posted : 28/11/2015 9:46 pm
Anonymous
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They have a 6 pin base how is that going to fit?
What about an AC/VP1 with a diode, released 1934, I have one with a 7 pin base.
Less messing around with the chassis fitting a 6 pin socket.

Mike

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Topic starter Posted : 28/11/2015 9:52 pm
Cathovisor
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The AC/VP1 is vari-mu. You need a 'straight' valve, like the SP4 I have suggested previously.

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Posted : 28/11/2015 10:21 pm
occiput
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They have a 6 pin base how is that going to fit?
What about an AC/VP1 with a diode, released 1934, I have one with a 7 pin base.
Less messing around with the chassis fitting a 6 pin socket.

I have looked back over my post, but can't see any suggestion of changing the socket in the set.

The two obvious approaches seem to me to be:
(i) rebase the valve
or
(ii) make an adapter.

I would do the latter, though I concede this would mean sourcing a six-pin valve socket. However, that shouldn't be impossible. The same website as is advertising the valve seems a good starting point to me, though given the surliness of your reply, I'm disinclined to help you any further with your problem.

As, and for the reasons, others have explained to you at length, if you wish to wait to source an SD4, you may be waiting a long time. You are therefore facing a choice of redesigning the set around a different valve, disabling the function, or getting the right valve from somewhere where they seem to be obtainable and dealing with the consequences of it coming with a base which does not match your set. Your call.

709379

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Posted : 28/11/2015 10:56 pm
Cathovisor
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I'm inclined to agree with 'occiput' regarding the best approach of dealing with this matter, although personally I'd go for re-basing an E444 (or RENS1254, I believe).

As you're an admin at Radiomuseum Mike, I'd also suggest that you tell the tube admin that the SD4 is NOT equivalent to the E444 as has clearly been demonstrated and to amend the corresponding pages appropriately. For instance, the base shown on the SD4 page is also the B6 base, yet we know the valve to be B7.

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Posted : 28/11/2015 11:03 pm
Anonymous
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though given the surliness of your reply, I'm disinclined to help you any further with your problem.

I think you have read the wrong meaning into my reply, but that's OK if you dont wish to help, thanks for your input.

Mike

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Topic starter Posted : 28/11/2015 11:59 pm
Anonymous
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As you're an admin at Radiomuseum Mike, I'd also suggest that you tell the tube admin that the SD4 is NOT equivalent to the E444 as has clearly been demonstrated and to amend the corresponding pages appropriately. For instance, the base shown on the SD4 page is also the B6 base, yet we know the valve to be B7.

Maybe there are there 2 types of SD4! I have a few valves with different bases but same type code.

Mike

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Topic starter Posted : 29/11/2015 12:06 am
crustytv
(@crustytv)
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I think you have read the wrong meaning into my reply, but that's OK if you dont wish to help, thanks for your input.

Mike

Mike however unintentional I unfortunately read your short reply in the similar way as Occi did. That's the trouble with forum's and the lack of face to face contact. Things typed may be interpreted in a way they were not intended. Lets hope it has not put occi off from contributing further to the forum and your thread. This post is not to single you or occi out, just an example of how we all must be aware of how our posts may come across to others.

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Posted : 29/11/2015 12:06 am
murphymad
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Crackle,

Why not buy a real Mullard SD4 valve. Yes I know they are rarer than golden chickens teeth.

I believe there is going to be one at Royal Wootton Bassett on Sunday. A REAL one in its original box!

I do have this on very good authority.

Come and see me on Sunday when you pick up your transformer which has arrived this morning...

Mike...

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Posted : 02/12/2015 3:13 pm
Anonymous
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That sounds like a plan, I'll see you on Sunday. :bba

cheers
Mike

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Topic starter Posted : 02/12/2015 5:02 pm
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