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Old 28th Jul 2017, 9:13 am   #41
Hunts smoothing bomb
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Default Re: Kolster Brandes RV 10 (Crown)

What a great test using the heater Tx John, I like that a lot and will now perform this exactly as you have shown when I am in doubt about a LOPT!




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Old 29th Jul 2017, 11:39 am   #42
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Default Re: Kolster Brandes RV 10 (Crown)

I lashed together John's tester with a spare RS audio output TX and I get a lit dial bulb

The PL81 looks happy and does not seem to be overheating if I leave this rig running for a few minutes. Turning the horizontal hold which changes the line frequency also dims the bulb slightly with higher the frequency.

I'm going to triple check all my work around this end of the set - as you may have seen from other threads I have slipped on a few banana skins before!
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Old 29th Jul 2017, 11:58 am   #43
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Default Re: Kolster Brandes RV 10 (Crown)

I think I've misunderstood the purpose of the test. Is the lashup is in effect simulating the heater overwind and as the PL81 seems OK this indicates the fault is indeed in the LOPT but the of the stage is running OK?
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Old 29th Jul 2017, 1:39 pm   #44
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Default Re: Kolster Brandes RV 10 (Crown)

The test with the substitute transformer will prove that the line output stage is driving the output valve as it would with the correct transformer.
Obviously if the scan coils are S/C or there is anything loading the PRIMARY of the RV10 transformer this test cannot detect that as these components are out of circuit!

Knowing the KB chassis I would say by your remarks that the LOPT is faulty.

It's worth disconnecting all leads from the LOPT primary tags and run the test. This will definitely confirm a faulty LOPT if it is heavily damped with the PL81 anode glowing a dull red.

Hope this helps. J.

PS Did you get those crocodile clips from RS components? I like them!
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Old 29th Jul 2017, 5:55 pm   #45
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Default Re: Kolster Brandes RV 10 (Crown)

Glad you like my croc clips. Almost fully biodegradable and heat resistant.

I reconnected the top cap leads with the 'tester' still connected and witnessed the following. The bulb comes up to full brightness for about 1/3 of a second then suddenly dims when the line whistle comes up, the filament then goes down to a dull glow. on switch off the bulb then blinks briefly for about 1/3 of a second before going out. If there is a S/C I would not expect the bulb to light to beyond a dull glow or not glow at all.
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Old 29th Jul 2017, 7:23 pm   #46
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Default Re: Kolster Brandes RV 10 (Crown)

I disconnected the white and orange leads to the LOPT primary. The EHT rectifier heater came up which seem to confirms the primary is faulty. I don't want to give up on this set. I'm now thinking of the feasibilty of a rewind or a transplant.
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Old 29th Jul 2017, 8:58 pm   #47
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Default Re: Kolster Brandes RV 10 (Crown)

OK. This is odd. The test lamp cannot light unless the transformer is being driven at line rate via the PL81.

If you have disconnected the primary and you now have EHT it appears to me that either the scan coils are shorted, [very unlikely but possible] or you have a fault in the wiring.

Check C60 .5uf scan coupling capacitor, the two 40pf 5kv capacitors, C62/C64 for S/C. The later two may be of a slightly differing value. These are 3rd harmonic tuning caps the values of which were adjusted to stabilize transformer 'spread'.

Other than that it may well be the transformer but if the primary has a shorted turn, even when disconnected the circuit would be heavily damped and would not produce EHT. John.
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Old 29th Jul 2017, 10:44 pm   #48
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Default Re: Kolster Brandes RV 10 (Crown)

Hi,
this thread is very useful, as it gives me a lot of background info I wouldn't be able to get anywhere else. As I have a couple of Older sets to look at when I get " a round tuit "
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Old 29th Jul 2017, 11:08 pm   #49
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Default Re: Kolster Brandes RV 10 (Crown)

Thanks John. I ran the set in darkness and could see a blue discharge from the end of the white disconnected wire to the primary and could hear a faint hight voltage 'tearing' noise from the speaker. The PL81 looks rather impressive with a slight blue glow. I switched off and got a bright but not intensely bright spot from the screen which is encouraging as the tube could be OK .

I did look for any arcs or sparks from the scan coils which are very loud! What sort of resistances should I get for each?

The disc caps don't test S/C and nor does the .5 cap which I lashed together with 5 0.1 X caps. I shall carry on checks tomorrow.
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Old 30th Jul 2017, 8:31 am   #50
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Default Re: Kolster Brandes RV 10 (Crown)

Mmmm! OK run the chassis with just the scan coil connections disconnected at the coils themselves leaving all connections to the transformer in place. You should be able to get some EHT and a vertical line on the screen.

Remember that the frame timebase will probably be in poor condition due to leaky capacitors but once again you should be able to see some frame deflection if you adjust the height and frame hold controls.

The focused blue glow in the PL81 is quite normal as it is with almost all line output valves. If the timebase runs OK with the coils disconnected it points to shorting turns but I would not take a bet on it.

I'm not sure what you mean when you say' the scan coils are loud' together with the arcing from the white wire. Was the arc to free air or to a connecting tag? J.
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Old 30th Jul 2017, 9:26 am   #51
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Default Re: Kolster Brandes RV 10 (Crown)

Quote:
Originally Posted by sexton_mallard View Post
I ran the set in darkness and could see a blue discharge from the end of the white disconnected wire to the primary and could hear a faint hight voltage 'tearing' noise from the speaker.

Sounds like EHT leaking back from the overwind to the disconnected wire
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Old 30th Jul 2017, 2:10 pm   #52
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Default Re: Kolster Brandes RV 10 (Crown)

There may well be a leak between the overwind and the primary hence the sparks and arcing sound. This would become more apparent with the underwind disconnected.
Getting 'hands on' I feel would confirm exactly what is happening. J.
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Old 2nd Aug 2017, 5:30 pm   #53
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Default Re: Kolster Brandes RV 10 (Crown)

Quick update. I reconnected the primary and disconnected the scan coils and can now get a vertical line on the screen. The brightness had to be turned down quite a bit as to not risking a screen burn and there is a response in the line on one channel when a meter probe is inserted in the aerial socket which bodes well for the rest of the set. The resistance across the coils is 28 ohms and is infinite to chassis from both connections. I did disconnect from the the set end not the yoke end for now as I did not want to disturb the yoke. Boost voltage is now about 650v.
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Old 2nd Aug 2017, 10:11 pm   #54
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Default Re: Kolster Brandes RV 10 (Crown)

Quote:
Quick update. I reconnected the primary and disconnected the scan coils and can now get a vertical line on the screen. The brightness had to be turned down quite a bit as to not risking a screen burn. Boost voltage is now about 650v.
Hi Dom,
That's an encouraging result! Presumably, you now have a normal looking glow from the U26 heater? I imagine you will have as the boost volts have come up.
Have you changed the capacitor in series with the scan coils yet? If you have, I can only conclude that the scan coils are faulty. Not an item that usually fails but not impossible.
Having said that, do you know what the boost volts should be?
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Old 2nd Aug 2017, 10:46 pm   #55
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Default Re: Kolster Brandes RV 10 (Crown)

I have a funny feeling of deja-vu here. On post #12 I bodged in a substitute of 5 0.1mfd X2 caps in parallel as i don't have a 0.47mfd cap to hand... I did notice they were quite warm after leaving the set running for a while. All other wax and black hunts caps have already been replaced in the set. I will find another substitute or even pop the original wax cap in for the craic. The U26 is glowing away nicely and the PL81 looks happy and like any other valve without a slight blue glow and no slightly glowing anode plates as observed earlier. The comment earlier about loud scan coils was a mistake - the whistle is emanating from the LOPT and can be heard from the other end of the garden. Did not see any cats in the neighbourhood either.
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Old 3rd Aug 2017, 10:04 am   #56
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Default Re: Kolster Brandes RV 10 (Crown)

I don't know if this is relevant to this set or not but I remember a report about a much later VC200 chassis which gave a "dead set" symptom until the scan coils were disconnected with sound and EHT present.

Replacing the scan coils didn't cure the problem so the LOPT was suspected.

Just a thought....

Brian
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Old 3rd Aug 2017, 12:53 pm   #57
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Default Re: Kolster Brandes RV 10 (Crown)

I reconnected the scan coils but with the 0.5 mfd 'lash-up' bypassed but just using a .047 mfd cap as a test. I know this is over 10x smaller in value but I think it should show if the lash-up was damping the EHT. The result is no difference, no EHT. The U26 does not light. This sort of eliminates the capacitor(s)?
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Old 3rd Aug 2017, 6:01 pm   #58
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Default Re: Kolster Brandes RV 10 (Crown)

You use the word bypassed. I presume you completely disconnected the multiple pack replacing it with the .047uf. Nothing like enough coupling but it should have given some line scan and not damped the LOPT.

Before writing the scan coils off I would have a very close look at them. Make sure the connections are not shorting to the core or chassis due to rotten insulation or trapping.

I still feel the scan coils are OK but you never know with this old equipment. I never changed a set of KB scan coils other than the type fitted in the RV30 but this was for the poor interlace problem, never a shorted line coil. Worth having a close look. John
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Old 8th Aug 2017, 6:49 pm   #59
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Default Re: Kolster Brandes RV 10 (Crown)

I carefully marked the position of the linearity sleeve, magnet and released the scan coils from the tube neck after removing the mounting hardware. No obvious faults could be seen with the wiring or within the coils such as 'green spot' corrosion and so on. The measured resistance is 28 ohms. From the service notes I believe L46 and L47 are the coils and they are 37 ohms each and wired in parallel which works out to be 18.5 ohms.

As a last resort I was thinking of substituting the coils with a dummy load from about 20 ohms to 40 ohms with a resistor and see if the EHT comes up?
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Old 8th Aug 2017, 6:59 pm   #60
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Default Re: Kolster Brandes RV 10 (Crown)

I don't think a resistor will work. You need an inductance, a coil. The actual resistance of the winding is not too critical for test purposes. Try connecting the frame coils in place of the line leaving all connections to the frame output disconnected.
Resistance readings taken against the service manual can be misleading. They were often calculated from old style meters such as the AVO 7. J.
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