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KB 888 "Standard Console Radio"

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Anonymous
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Been busy recently scanning service manuals.
But also done a little more to the 888. I have repaired the tear in the speaker diaphragm using kitchen tissue, you know the strong as bull, juan sheet stuff. Dilute PVA was used as it soaks into the existing fibres. I separated the kitchen paper into its individual ply's and used one ply at a time. most of the reinforcement was put on the reverse of the cone, taking care to not get any glue on the cloth speaker surround.

I have been thinking of experimenting on a piece of scrap cloth with diluted contact glue, painted onto the cloth to see how it dries and if it stays flexible and adds any strength. If I feel it is successful I will paint it onto the cloth to help hold it together as has gone very dry.

The tatty cloth and rubber insulated wires to the speaker were improved by removing the tatty and broken insulation and feeding narrow bore heat shrink sleeving over each of the bare conductors, then sleeving the whole damaged area in a larger heat shrink sleeve. If I had some new cloth covered wire I would have replaced the whole lot, but at least the original wiring is kept, with a little addition of heat shrink.

The copper strip that Turretslug offered me arrived today, thanks mate. it was going to be used to make a repair to one of the broken spring contacts of the wave change switch.
I cut it wide enough to fold double, and shaped it to fit the old spring.

When soldered into place it seemed some of the springiness had gone and it did not hold its shape very well (I think the heat from the soldering annealed the copper making it soft again) so in the end I used a tiny bit of the old spring to add pressure onto the new spring making it into more of a leaf spring. It seems to work well and opens and closes well with out loosing its gap when open, or contact when closed, it should be reliable.

The on off switch should be part of the volume control, but it is non functional and previous modifications had been done to add an external on/off switch somewhere.
The volume pot, which seems to test OK, also has a loudness tap in the circuit so I reckon it is going to be difficult to replace the whole switch and pot combined. But if I can find a pot with switch of the same style I could perhaps repair the switch.

Mike

 
Posted : 02/12/2015 10:01 pm
Anonymous
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Luckily the internal wiring in this radio, being cotton covered rubber, is in pretty good serviceable condition, so long as it is not disturbed too much.
The wave change switch was fitted back in the chassis and all connected up. I had one wire left over at the end, I had made an alteration to my hastily pencilled sketch but forgot to redo the line representing the wire after rubbing it out. Luckily there was only one choice where the wire could go, with out stretching it straight, or kinking it, so I am sure it has gone back on the correct terminal.

When doing anything major to the wiring in a radio chassis I now find it easier to do a sketch of wiring connections as photos are not always very clear when you come to look, 2 weeks later, to see where a wire went .

I also renewed the grommets and rubber washers that the tuning capacitor assembly fits onto. It is nice and firm on its rubber cushions now instead of clanking around on its old crumbling rubber supports.

I have been slowly going through the chassis changing all the wax capacitors and checking the resistors. So far all the resistors I have checked have been with in tolerance.
I am fitting the new capacitors into the old cases where possible, to try and keep the appearance of the components looking vintage. There may be one or 2 where this is not possible, I will see when and if I come to them.

At RWB I collected the freshly re-wound phase splitter transformer from Mike Barker. He rewound it in record time, it had only arrived at his place in the post on Wednesday.

I also bought probably the only SD4 valve available any where in the universe for a whopping £50.
This is what one looks like, you may never see another. :qq1

The output valves which came with the radio were not a matched pair, they were actually different valve types, a 7A2 and a APP4A. At RWB I managed to find another APP4A so I now have a pair of those for the push pull output pair. (which reminds me I must test them both)
The rectifier valve which came with the radio was a U10, this is only rated at 60mA, the correct ones are rated at 120mA so for now I will be using a spare IW4/350, which is a later type of rectifier but should work well for now.
The non functioning on off switch mounted on the back of volume control will have to wait.

So I am practically there now, ready to go as soon as I finish the recap.

Mike

 
Posted : 07/12/2015 2:48 pm
Herald1360
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Depending on how old that 20/- is that £50 may only be slightly more in real money terms than the new price was. :ccf

.........._______
.....___/|__|__|____
.=.( _---__|___|_---_)
.........O...Chris....O

 
Posted : 07/12/2015 8:37 pm
Anonymous
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The resistors forming the potential divider in the power supply were not so good, 2 were open circuit, and another was a long way off. I have been trying to work out the wattage required for the replacement resistors. I am a little confused as the voltages on the circuit in post 2 above do not match the voltages listed in the voltage chart, and I cant see any explanation to account for this.

I am planning to use 2 watt resistors for R 12, 13, 14, & 32 I think this should be OK.

Thanks

Mike

 
Posted : 10/12/2015 9:18 pm
Anonymous
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I have had a good result with the on/off switch on the volume control.
You may remember the on/off switch was not functioning and someone had wired in an external switch which was nowhere to be found but there was a short stub of the added wiring.
I took the volume control apart and carefully drilled off the rivets which held the switch to the body of the control. When it was apart I found a spring which causes the contacts to snap over was broken.
I have a few spare springs and found a closed one of similar size and gauge, I carefully bent out the ends out leaving 2 turns of the spring. (the original spring only had one turn, but I thought 2 turns would give less strain on the metal of the spring) I fashioned the ends to match the original spring, and with a little experimentation of the angle of the 2 ends got the switch to toggle nicely.
It was assembled using solder to hold the rivets where I had drilled them out. It seems to work reliably.

I have also completed the recap, all caps bar the smoothing electrolytics and one mica block capacitor have been changed. Even the 100pf and 500pf caps in the tuning circuit were wax paper types so they were also changed. I have tried to hide the new caps in the original capacitor cases.
I have some 22uf caps on order from Farnell (free postage now) to replace the 2 cathode bypass capacitors.
In the end only 2 resistors were changed, one resistor, a 3k wire wound, I managed to repair as the damage was at one end where it connects to the end collar, it now reads 2.9k, but I think that should be OK.
I have given some of the rusty metal work, valve shields, and mains transformer shield a good brush with a wire wheel. I will upload some photos of the underside of the chassis when I have fitted the 2 new 22uf caps.
I have fitted the new mains cable and have been debating whether to fit the earth or not.

Mike

 
Posted : 12/12/2015 11:06 pm
Anonymous
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Some success and yet not success. I have had the radio working on and off for a day, all be it with a rather quiet and distorted output. It actually sounded better if I disconnected one feed to the output pair. However the HT was down to 190v, it should be 270v on the anodes of the output valves. This may have been caused by a weak rectifier. But when I went back to it yesterday morning with a new plan, I had hatched over night, to use an oscilloscope to check the phasing of the newly wound inter valve transformer, the radio had developed a fault. The HT would not rise above 60v, something had changed on the chassis and was holding the HT down.
Not wanting to stress the speaker energising coil and HT winding, I have been testing it using my electrolytic reformer connected directly onto the HT rail with all the valves removed. This can supply up to 5mA at a max of about 350v, and I have been going round un-soldering likely components to see which may have failed. I am going to alter the reformer circuit to allow more current to flow, say 15mA.
I did find that one of the original (replaced in the past) 8uF smoothing caps had failed and I now propose to change all of them. But there still seems to be something causing a high current to flow in the circuitry. I am beginning to wonder if it could be a cap in one or more of the IF cans. I had already found that for 100pf and 500pf capacitors, wax paper types had been used. So today I plan to continue isolating parts of the circuit and testing components and to look at dismantling the IF cans.

Mike

 
Posted : 16/12/2015 8:05 am
Anonymous
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There are no capacitors in the RF & IF cans, just coils, all adjustments are made by external trimming caps.
The total current taken by all the circuit, less valves is 9.5mA which seems OK and agrees more or less with what is expected by taking into account all the resistors that are connected from HT to chassis.
After a lot of experiments and fiddling, I found that the unexpected high current taken by the circuit only happened when I plugged in the loudspeaker and energising coil to the socket on the chassis and this was further isolated to the +ve HT wire which goes to the OP transformer CT.
In theory there should be no connection between the OP transformer primary and chassis but I was getting a reading of between about 100r to 2k.
When I checked the resistance of the windings they measure OK almost the same as what is quoted in the service manual.
So the transformer had to be unsoldered and removed from the loudspeaker chassis.
Undoing the nuts holding it on was not made easy by the red paint which had been dabbed on to help secure them, but I found that with a little WD40 on the red paint and thread it made the job easier.
Once it was removed from the speaker chassis and I had removed the shielding covers from the transformer I found the problem, one wire from one end of the primary had been trapped between the bobbin cheek and the metal shield and it had rubbed through the insulation and was gently touching the shield case.
I can repair that so it has saved having to have another transformer rewound.

I am going to change the smoothing caps and the plan is to use two 450v 22uF caps in series for each of the 8uF capacitors as the HT could under some circumstances rise to about 480v and 500v caps are not so common.
I will also try and make a cardboard box to mount them all in so it looks similar to what I expect it should have been originally.

I will have a look around at some of my other early to mid 30’s sets and see if I can find a similar capacitor block.
Ah, here we are, I have found one I did earlier from a KB 850.

But has any body else please got any more good photos to hand of a multi capacitor block from this era?

 
Posted : 16/12/2015 12:32 pm
Anonymous
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All working again, the HT is up to full voltage, more volume, distortion is difficult to ascertain, it is better but the speaker is upside down with one edge propped up on a block of 4x2.
But there was a problem with instability when he HT was at full. It was OK if I kept the AC mains turned down to 200v giving an HT of around 230v, and with a little tweak of the "tone" control.
This led me to swap one of the APP4A valves for a APP4C and this seemed to stop the instability, and it is now OK on the full mains with an HT of 270v.
The function of the SD4 valve of silencing the inter station noise seems a little disappointing, it seems more like a general gain control, reducing the station volume as well as the background volume. I think I will check again how this valve is wired in, as modifications had previously been made to bypass the missing valve, which I had to reverse.

I think I may need to look around for a decent matched output pair.
Mike

 
Posted : 16/12/2015 6:29 pm
Anonymous
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The chassis is now all finished, all new capacitors were fitted, the remaining old electrolytics were either re-stuffed or replaced in a home made electrolytic block. All other capacitors were replaced and the new caps were stuffed into the old cases. The types used by KB made this operation quite easy, if not a little messy. It is advisable to give the outside of the capacitor a good wipe with an oily rag (moistened with white spirit) to clean off most of the dirty wax.
Only 2 resistors needed changing, one I used was a 7 watt green enamel type which I left as it was the other was a 10k 2 watt which I fitted inside a braided tube, as this was a method that KB had used else where.
The new capacitor box was made out of the backing for a foolscap note pad. Based on the design I had for a PP1 battery cover, with the dimensions changed to suit the space available and the metal bracket. A separate cover was designed and printed out on A4 paper then folded and stuck to the card box, it even has a nice waxy feel as I melted wax onto it and blew it around with a hair dryer.
So all in all the underside and the above the chassis now looks almost as original.
I will be fitting the speaker and chassis back in the cabinet tomorrow, I expect it to sound suburb. The speaker sound good on the bench now it is face up.
Oddly the radio sounds much better during the daytime than at night when the audio sounds a little confused and distorted. I wonder if this is a result of the very low IF frequency. (130kHz)
The tone control has a good effect. The "loudness" tap on the volume is not noticeable, but it may be more so when it is back in the cabinet.
I gave the 3 trimmers on the tuning cap a little tweak to realign the HF end of MW to squeeze in BBC Essex. This also improved the sensitivity some what, and also brought R4 on LW nearer the 1500m mark. The only remaining thing is, if I get the opportunity to fit a nice matched pair of output valves, I have still ended up with different types, a APP4A and an APP4C. Total HT current now is about 80mA, and the plate voltage is slightly over the 270 given in the service manual. There is no hum at all with the doubling up of the choke and energising coil, and it all seems to run fairly cool, even the rectifyer and OP valves can be touched after half an hour of being on. I believe there should be 3 more metal cans for the valves, but as they are metallised anyway I don't think this matters too much.

Mike

 
Posted : 17/12/2015 5:10 pm
Anonymous
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Oops, can any body see the mistake I made on the underside of the chassis, I have only just noticed it myself when adding some photos to the Radio Museum, and I was comparing the before and after restoration images. It looks like I will have to remove the chassis from the cabinet again.

Mike

 
Posted : 19/12/2015 9:58 pm
Cathovisor
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Unterminated earth wire from the mains lead?

 
Posted : 19/12/2015 10:49 pm
Anonymous
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No not the mains lead, that was left because I was undecided whether to connect it or not.

It was the electrolytic on the top component board, it is C8 25uF, it needs to be connected +ve to the 0 volts rail. because the -ve is connected to a less than 0v point in the circuit.

I removed and tested it, it was drawing current when connected to my bench supply via a mA meter so I broke apart the old RS case and replaced it with another new cap.
(The one I took out did seem to be slowly reforming so I left it for a while and it seemed to have recovered from its short bout of being reverse connected, well it was a 63v electrolytic in a less than 25v circuit so I guess no permanent harm was done to the electrolytic.)

I have to admit that I have not noticed much difference in the operation of the radio, now that the capacitor is connected the correct way.
I wonder what function the electrolytic did.

I also connected the earth up to the solder tag I had fitted for it, but left it disconnected in the plug, as I believed with it connected there was a little more hash in the background.
Mike

edit
Is C8 a cathode bypass capacitor for V4?

 
Posted : 20/12/2015 4:57 pm
Cathovisor
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Is C8 a cathode bypass capacitor for V4?

No; it decouples the anode supply for V4 and inter alia the grid bias for V5.

 
Posted : 20/12/2015 5:31 pm
Anonymous
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Ah yes, I see it now, thanks.

Mike

 
Posted : 20/12/2015 10:03 pm
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