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Old 28th Jun 2018, 8:51 pm   #1
sexton_mallard
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Default KB 381 AC/DC radio

I acquired this prewar AC/DC set at Mikey405's excellent bash recently. I was looking for something 1930's and not too large with 'big' valves. It jumped to the front of the rountoit queue as I felt sorry for it and curiosity got the better of me.

It has been got at as expected but the work done on it probably saved it from the scrap pile. Initial checks are interesting. The set has a mains cut-out which cuts the power if the back is taken off which is a bit academic as the back is missing around the mains dropper area. A quite advanced bit of safety design bearing in mind this is pre-war design. Another innovative feature I have not seen before are the capacitor box modules. i guess assembly lines were sped up by these units being assembled separately. The mains electrolytic block has gone off the top of the chassis, replaced with a 25mfd electrolytic, and 3 more large electrolytics that might be replacements lurk under the chassis, their function and circuit placement yet to be confirmed. The main rectifier seems to be bridged by a silicon diode. The rectifier valve may have had everything except the heater disconnected inside the valve base as a bit of cloth tape seems to hold the valve base to the envelope. A few more electolytics appear to have given up the ghost. One that looked like it was cathode coupling now hangs disconnected at one end. Maybe this silenced the radio and put it away to be forgotten for a while?

Basic checks are promising, the HT choke, output transformer primary, mains dropper all testing good on the multimeter. Watch this space for more news.
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Old 29th Jun 2018, 8:21 pm   #2
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Default Re: KB 381 AC/DC radio

I repaired one of these radios earlier in the year. Search this forum, sorry cant help much at the moment as on holiday and wifi is sketchy.
Mike
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Old 30th Jun 2018, 12:27 am   #3
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Default Re: KB 381 AC/DC radio

Thanks Mike, I did find your thread here https://www.vintage-radio.net/forum/...d.php?t=143911 and found it useful. Tonight I replaced 'that cap', a tone control cap and the two 25mfd electrolytics which were corroded and bulging in places. The rubber/cotton mains lead was connected with positive to chassis . A IEC C7 mains lead sacrificed its connector end and was soldered in as a replacement, negative to chassis. The mains switch/volume pot appears to be a replacement and tests ok on the meter, the switch is intermittent but might 'clear' once some power flows. quite a bit of non original looking PVC wire is found but the work looks OK and has saved me some work. The original cotton wiring is in good nick and no shorts have been found - so far!

As for originality, this set which has already been modified/repaired is non original and is in fairly poor condition and no re-stuffing of original components will be done. If this set was in excellent condition and original that would probably be a candidate for restuffing. Next I will do some more basic checks and then show it some volts via a lamp limiter and isolation transformer. The replacement mains electrolytics look in good condition and don't test shorted so I will give them the benefit of the doubt. No across-the-mains cap has been found.
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Last edited by sexton_mallard; 30th Jun 2018 at 12:33 am. Reason: Typos and improved clarity.
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Old 1st Jul 2018, 12:44 am   #4
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Default Re: KB 381 AC/DC radio

This evening I did a few final checks and switched on with the lamp limiter. on a 42w bulb, the HT from the silicon diode slowly climbed up to 120VDC as I guess the mains electrolytics were reforming. After about half an hour, the 60w bulb was substituted, the heaters could be seen to glow in the dark. Switched off and with the set isolated from the supply, a feel around all the components were promising, the electrolytics were cool and all the valves were warm. 150w next... 205VAC at the supply, 230VDC at the rectifier diode....and lots of stations on MW , switched to LW and R4 was there along with a few other faint stations in the background. This set works very well and exceeds all expectations . The sound is thin and reedy but I suspect the speaker is either seized or the cone is torn/ partially detached from the voicecoil/surround.

For now I suspect the HT will be very high and a resistor will be placed in series with the diode to bring it down to a normal level until i can get a good valve rectifier and the silicon diode removed.
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Old 22nd Jul 2018, 12:21 am   #5
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Default Re: KB 381 AC/DC radio

The speaker was found to be seized. I carefully undone the centre nut then released the pole piece and unsoldered the field coil bobbin to clean up the voicecoil and pole piece which had got stuck with rust from the pole piece. As found by Mike with his identical radio, the voicecoil had started to detach from the cone and the tabs were glued back down with PVA applied from a screwdriver tip through the gaps of the spider and pressed down until they stuck. A few holes in the cone were patched with some card salvaged from a scrap speaker dampened in some water and fixed in place with PVA from a paintbrush. The bobbin and pole piece were reassembled and the bracket clamping the polepiece and bobbin were tightened up loosely until the cone could be pushed up and down with no rubbing noises and then fully tightened up. Once all this had dried, the cone was checked again for rubbing and now the speaker played nice and loud

The smaller capacitor box was opened to reveal that it was filled with a waxy substance. The leads inside were snipped and he contents gouged out ready for restuffing. It appears a repair was done in the past as one of the 0.1 mfd capacitors have been replaced by a rectangular (mica?) 0.1 mfd capacitor soldered to the case. The middle terminal of the block was disconnected.

The metallised valve screen connection on the valve base on V2 also appears to have been connected to chassis. Is this expected?

This has been a most interesting radio to rummage around inside. The safety features, capacitor blocks, rectangular high value mica capacitors and the dial disc which appears to be made from a plastic together with the dial glass which is also appears to be plastic are all unexpected and new to me. Could the dial 'glass' be made from mica or nitrocellulose?

As for the age of the set, were these sets manufactured after or even during the war explaining the apparent modernity of the materials used?
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Last edited by sexton_mallard; 22nd Jul 2018 at 12:35 am. Reason: corrections and typos fixed.
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Old 22nd Jul 2018, 1:32 am   #6
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Default Re: KB 381 AC/DC radio

An interesting read, your ministrations appear to be making good progress. It must be said, there appear to be copious quantities of 'iron oxide' to deal with at some point.
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Old 22nd Jul 2018, 2:01 am   #7
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Default Re: KB 381 AC/DC radio

Sounds like you're making good progress on the restoration. Cant wait to see the finished set in all its splendour.

Thanks,
Peter
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Old 22nd Jul 2018, 3:40 pm   #8
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Default Re: KB 381 AC/DC radio

Quote:
Originally Posted by sexton_mallard View Post
The metallised valve screen connection on the valve base on V2 also appears to have been connected to chassis. Is this expected?
What valve do you have fitted as V2, is it a Cossor 13VPA? I cant find any information showing if the metallising is connected to a pin. It maybe, pin 1 seems to be unused so the metallising could be connected to pin 1 and thence to the chassis on the socket.
Both my KB 381 sets have 9D2 valves as V2, and there is an aluminium screen fitted to the raised metal shroud around the valve socket.

So it was originally intended to screen the valve by external valve screen.

Mike
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Old 22nd Jul 2018, 4:13 pm   #9
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Default Re: KB 381 AC/DC radio

Thanks for the encouragement guys . The valve is a Mullard VP13C at V2.
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Old 13th Aug 2018, 11:11 pm   #10
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Default Re: KB 381 AC/DC radio

The two capacitor boxes have been restuffed, the contents of were a solid mass of wax with a thin layer of bitumen at the top. I have saved this 'vintage audiophile' wax for now .

The howling stopped as soon as the valve metalising pin on V2 was connected to chassis, the wire for it reinstated.

The set performs well for about 4 minutes then the audio begins to fade away to a faint volume. I never had the set running long enough before repairs to reveal this fault. A bit of investigation with my AF and RF signal generators working backwards from the audio output to the aerial narrows the problem down around V1. voltage checks for the anode and screen grid are only about 20v higher than the 120V and 55v quoted in Trader 533. A check of all resistors around V1 only uncovered R5 to be 2x higher than spec, this was replaced with a 2k7 replacement. This made little difference. I'm now suspecting valve V1 but I don't have a substitute to try (a Mullard FC13C).

I'm trying to see if a voltage drop occurs or a current increase occurs as the fading happens. Is there any more voltage and current figures for V1 (the frequency changer) in any other data for this set?

EDIT: valve data found at http://www.r-type.org/pdfs/fc13c.pdf

Can you suggest some more tests/checks to try? Thanks in advance.
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Last edited by sexton_mallard; 13th Aug 2018 at 11:26 pm. Reason: typos, readability and then finding more valve data
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Old 14th Aug 2018, 7:24 am   #11
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Default Re: KB 381 AC/DC radio

The KB service manual gives a bit more detail regarding the valve voltages.
Have you checked the AGC voltage as the volume decreases, check the grid voltage on V1.

Mike
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Old 15th Aug 2018, 11:15 pm   #12
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Default Re: KB 381 AC/DC radio

Quote:
Originally Posted by crackle View Post
'....So why am I getting -12v on g1 of V1, and does this really matter/make any difference.
I'm getting -10v to -16v on G1 as well! None of the voltages changed as the fade sets in. In fact after a few more switch on/off cycles, any audio has gone altogether. my gut is screaming 'Duff V1 valve!!'. the voltage on the other side of C5, a 0.0001mfd mica cap is +5v. The signal generators coming though nice and loud from V2, V3 & V4 as before. IF is at 130kc/s.

Looking at the valve data for the Mullard FC13C, some of the voltages are being exceeded, for example, G2 oscillator anode is 120v from service data, my set it's 150v, the mullard spec is 90v. would running the valve this way shorten its life considerably?

The valve lineup for the other valves is V2 Mullard VP13C, V3 Tungram DOT13 (?) V4 Brimar 7D5 and V5 Vario TW1 (?).

I think it's time to go valve shopping for a new V1 and V5 next.
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Last edited by sexton_mallard; 15th Aug 2018 at 11:19 pm. Reason: Typos and improved clarity.
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Old 16th Aug 2018, 9:48 am   #13
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Default Re: KB 381 AC/DC radio

V1 should be a 15D1 but I am sure the FC13C can be made to work.

The cathode resistor of 150 ohms means they are pushing V1 pretty hard. The trader says the total is 14.5mA for the 15D1 but the 5mA anode current is well above the recommended for the FC13C.

Measure the cathode voltage and R2 resistance and calculate the total current. If it is above 10mA, I would try replacing R2 with 300 ohms.
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Old 7th Sep 2018, 6:45 pm   #14
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Default Re: KB 381 AC/DC radio

Thanks PJL. Measurements of cathode voltage on V1 showed a current of about 6 milliamps. Today the postman delivered 2 15D1 valves. One went in place of the FC13C and full performance was restored with the set running nice and stable for a few hours . Attention now turns to the tuning gang. The reduction drive is slipping. I did lubricate the tuning gang lightly but the drive still slips. Any experience of this with this range of KB radios?
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Old 8th Sep 2018, 6:52 am   #15
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Default Re: KB 381 AC/DC radio

Do you mean the tuning knob spindle slips as you turn it and the vanes do not move.
I have had this problem, there is a note about it in the service manual.
See page 17 - 18;
Under the reduction gear housing there is a hole through the shaft. In the hole are 2 ball bearings and a small spring, be careful if you remove the housing.
If you do remove the housing the centre spindle can be removed for cleaning, but watch out for the ball bearings falling out.

Mike
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Old 8th Sep 2018, 8:50 am   #16
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Default Re: KB 381 AC/DC radio

This tuning drive, having only 2 ball bearings, can tend to slip. The later concentric reduction drives had 3 balls and always seemed to be reliable. The reduction drive may actually work better without any oil or grease on the ball bearings and cover.

Something else noticed on a radio with this same type of tuning drive was the moving vanes were poorly balanced and any vibration tended to cause gravity to make the vanes drop. To counter that I cut a sliver of lead about 12mm x 8 mm and 2 mm thick and folded it lengthways over a metal edge. Once it was formed into a channel I gently crimped it to the edge of the celluloid dial disk to counter the weight of the moving vanes, it did help a little, but watch for it fouling on the chassis and on the inside of the cabinet.

Mike
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Old 8th Sep 2018, 8:13 pm   #17
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Default Re: KB 381 AC/DC radio

Thanks Mike! I removed the dial disc and did a bit more lubrication around the tuning gang and spindles and now the reduction mechanism is working fine. the dial disc appears to be impregnated with dirt from the rear which I can't remove. This dirt is seen with the dial bulbs illuminating the disc from behind. I will leave it for now in case I damage the disc face. With this out of the way, the dial bulbs were cleaned and some crumbly wiring will be replaced. The case got a good dowsing in woodworm killer after removing and cleaning the dial 'glass' and bezel. There is a card stapled in the top of the case... what is this for?
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Old 8th Sep 2018, 9:25 pm   #18
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Default Re: KB 381 AC/DC radio

Yes it is like a thin chipboard, about 1/8th inch thick. I think it is to help deflect heat to protect the top of the cabinet.
I have not seen a dial as dirty as that before, be careful cleaning the dial disk they are quite brittle, how do I know that?
I have never tried this but it may work, if it is supported on a flat board with a hole in the centre for the boss to go through so it is lying flat, you may be able to use something like Vim or Cif to clean the back of the dial. But be careful to not wash the black ink out of the impressed markings on the front.
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Old 5th Apr 2019, 11:52 pm   #19
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Default Re: KB 381 AC/DC radio

I noticed that a good portion of the MW band was deaf and overall the performance a bit lacklustre. I attempted a full realignment as per Trader 533 (which is an update done in 1941). This has brought the whole MW band back and the the dial is reasonably accurate.

Performance is still a bit lacklustre. The wafer/trimmer variable caps C33 and C34 do look the worse for wear with the clamp screw backed right off for best performance on one of the trimmers - which is hardly ideal with a dog-eared stack of wafers flapping around. connecting a wire to the top cap of V1 shows a big increase of sensitivity but of course no selectivity. The coils in the cans look OK. I will do some resistance checks across L1 L2 and L3.
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