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Old 22nd Jul 2017, 5:55 pm   #21
sexton_mallard
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Default Re: Kolster Brandes RV 10 (Crown)

i fitted the mains electrolytics with some jiggery-pokery and exending the wiring to allow me to solder all the connections without removing the tube from the chassis.

I powered up with a 42w lamp limiter ,then with a 150w bulb and saw signs of life, a rustle from the speaker from tuning the volume knob and tuner. Lots of nice glowing valve heaters and no smoke apart from a slight wisp from the dropper. Left the set running with a nice line whistle. Then switched to full mains via a isolation transformer. No light from the tube and the U26 EHT rectifier valve is cold with no signs of a lit heater. I checked the U26 which seems to have heater continuity and the valve base looks OK. So it could be the LOPTX heater winding?
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Old 22nd Jul 2017, 6:16 pm   #22
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Default Re: Kolster Brandes RV 10 (Crown)

The EHT rectifier (U26) heater winding is just a couple of turns round the line output transformer core in most cases. It has a very low DC resistance. So with the set unplugged and the CRT discharged, measure the resistance between the heater pin contacts in the U26 holder (valve removed). If it's open then there's a problem with the heater winding.

I don't know about this set (vintage TV is not my 'thing') but it's not unheard-of for there to be a low-ish value resistor in series with the rectifier heater winding. It's often hidden in the the rectifer socket housing. It can go open-circuit.
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Old 22nd Jul 2017, 6:33 pm   #23
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Default Re: Kolster Brandes RV 10 (Crown)

Check for RF at the EHT rectifier with a screwdriver.

Lawrence.
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Old 22nd Jul 2017, 7:58 pm   #24
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Default Re: Kolster Brandes RV 10 (Crown)

The heater winding seems to be fine with a low impedence, and again I checked the heater continuity on the U26. I traced the heater wires into the socket to be sure. There is no HT spark at all. I'm now thinking there is a problem on the primary side? I rechecked my recapping work and can't see any mistakes made.
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Old 22nd Jul 2017, 10:53 pm   #25
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Default Re: Kolster Brandes RV 10 (Crown)

Don't know about these sets off hand but certainly the later KB sets used a resistor in the EHT rectifier base which frequently changes value.

I remember the 21" Fernseh restored. Really was a lovely set.

Brian
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Old 23rd Jul 2017, 7:06 am   #26
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Default Re: Kolster Brandes RV 10 (Crown)

After I posted my comment yesterday, I looked the set up in R&TVS. There is no EHT rectifier heater resistor shown there. However Sexton Mallard says that the winding is continuous at the rectifier base pins and that he can draw no HF arc from the U26 anode (or at least that's what I think he's saying).

Do you know that the overwind is continuous? Although if it were open, I'd still expect to get some glow from the U26 filament. I assume the line output valve isn't showing any signs of distress (red hot anode or screen grid, for example).
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Old 23rd Jul 2017, 9:11 am   #27
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Default Re: Kolster Brandes RV 10 (Crown)

One old chestnut was the LOP valve screen feed, everyone carried a tin of 2.2K resistors.
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Old 23rd Jul 2017, 4:50 pm   #28
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Default Re: Kolster Brandes RV 10 (Crown)

The LOP valve PL81 does look to be working hard. In darkness a slight blue glow can be seen in side the electrode structure. The anode plates do look to almost be red but it difficult to see from the heater glow. I have never looked at one of these close up in a working set before so I don't really know what normal operation looks like. I appreciate the circuit has a lot of energy. Certainly the line whistle suggests this. I will take a picture later.

I could not get any arc from the anode cap of the U26, or from the connector either.
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Old 23rd Jul 2017, 9:13 pm   #29
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Default Re: Kolster Brandes RV 10 (Crown)

Can you draw a spark from the PL81 top cap using an insulated screwdriver?
If EHT is being produced then you should be able to do this.

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Old 24th Jul 2017, 12:52 am   #30
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Default Re: Kolster Brandes RV 10 (Crown)

Hi Dom,
Have you managed to obtain a copy of the circuit yet? My R&TV servicing books are up in Essex so I do not have the circuit to hand but I assume it will be fairly conventional.
In order for the U26 heater to light, the whole line output stage needs to be working correctly. If it isn't, there is not sufficient energy to power the heater so it's a good indication that all is not well

The anode of the PL81 gets its volts via the primary winding of the LOPT from the cathode of the efficiency diode (PY81 or similar) The anode of this diode is connected to HT via a small inductor.

There is also a 'boost capacitor', one side of which is connected to HT, the other side goes to another connection on the LOPT. The boosted supply is often used to feed the frame oscillator, focus volts etc.
The boost is a useful place to measure what is going on in the output stage.
If I had the set in front of me I would proceed as follows:

1) Identify the boost cap from the circuit and ascertain which side of it goes to the HT supply and which side is the 'boosted' side.
2) Connect a meter to the 'boosted' side (1000V range) and switch the set on.
3) As the diode (PY81 etc) warms up you should get a reading of 200V or whatever the HT supply is.
4) As the PL81 starts to conduct and you hear line whistle, the reading on the meter should climb to about 650 to 750V. At this point, the U26 heater should light.

If you get no reading or a very low one on the meter, check that there is HT on the anode of the diode (PY81 etc). If there is, then the PY81 is probably faulty.
With the set running, check for volts on the screen of the PL81. As has been mentioned, the screen resistor (2k2 ono) can fail.
The PL81 itself could be faulty and as has also been mentioned, change the boost cap and also the coupling cap to the grid of the PL81.

If all this fails, then my suspicions would fall on the LOPT itself, but there are still a few more checks that can be done.

If someone would like to post the relevant part of the circuit, I could be much more specific as to where to measure and exactly what voltages would be expected!

Hope this helps and remember to be careful when measuring these high voltages.
All the best
Nick
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Old 24th Jul 2017, 7:52 am   #31
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Default Re: Kolster Brandes RV 10 (Crown)

Radio and TV Servicing. 1960/61.

Not a lot to wrong here. Check for man made faults when you carried out the capacitor change. Check screen grid, pin 8 of PL81 for voltage and scope the line drive waveform to pin 2. Run with the U26 [R20] disconnected to eliminate an internal short in the valve damping the LOPT. Does the LOPT get warm after 15 mins? I had a number of underwinds [primary] Fail on these models. John.
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Old 24th Jul 2017, 11:02 am   #32
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Default Re: Kolster Brandes RV 10 (Crown)

Thanks John, At least I can see what's going on!
The boost cap is the one next to the inductor in V8's anode (PY83). The boost supply from this is used to feed the frame oscillator via the height control and also the focus supply.
Other likely culprits are the coupling cap to the line coils and the two tuning caps in parallel across the primary.
Change the boost cap and the coupling cap in the grid of the PL81
Is the voltage on the triode section (line osc) of the PCF80 correct?
As John said, what is the screen voltage on the PL81?
Try changing the PCF80, PL81 and PY83.
Check that you get a low resistance between the top cap of the PY83 & PL81 through the primary of the LOPT (with the set switched off!!)
By the way, don't try and measure the voltage on either top cap- there are very high peak voltages and your meter will not like it
Let us know what happens after doing all that.
Good luck
Cheers Nick
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Old 26th Jul 2017, 9:10 pm   #33
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Default Re: Kolster Brandes RV 10 (Crown)

Thanks for all the guidance gents! Here are some observations so far.

1. No arc can be drawn from the top cap of the PL81.

2. Resistance checks from the top cap of the PL81 and PY83 to chassis is about 2M Ohms, cap to cap about 10 Ohms.

3. Voltage from boosted side of boost cap is 235v. Removing the U26 makes no difference.

4. Pin 8 PL81 is at about 180v and the screen resistor R68 (4.7k in the book) mine measures 7K.

I left the set on for 10 mins to see if the LOPT was getting warm but the PL81 was definitely 'red plating' so I switched off! The LOPT remained cool despite being grilled from the PL81..

I shall carry on prodding when time permits. Watch this space.
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Last edited by sexton_mallard; 26th Jul 2017 at 9:22 pm. Reason: correct typo PL81 not PY83
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Old 26th Jul 2017, 11:17 pm   #34
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Default Re: Kolster Brandes RV 10 (Crown)

Hi Dom,
If the boosted side of the cap is 235V that's just the HT being passed through the PY83 diode. So the PY83 is likely to be OK. If the PL81 is glowing, it's conducting heavily and as the screen voltage is reasonable, that current is flowing through the anode via the primary of the transformer. As you can't draw a spark from the top cap of the PL81, it would seem like the LOPT is being heavily damped. Sadly, this is most likely to be due to shorted turns either in the primary or eht overwind. Sorry about that
It's still possible that the boost or other caps are faulty- I would change all the ones I mentioned in the above post if you haven't already done so. Also try replacing the 4k7 screen resistor, although with 180V, I doubt if it will make much difference.

Is it possible to disconnect the EHT overwind where it joins the tag for the top cap of the PL81? If the overwind is faulty, this can remove much of the damping and allow a spark to be drawn from the top cap of the PL81.

Try changing the caps first though and also measure the anode volts of the PCF80 line osc. Do you have a 'scope? If so (as John has already said) it would be useful to measure the drive waveform on the PL81 grid.
I really hope it's not the transformer but I have to say, its not looking too good!
All the best
Nick
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Old 26th Jul 2017, 11:28 pm   #35
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Default Re: Kolster Brandes RV 10 (Crown)

Quote:
Originally Posted by sexton_mallard View Post

I left the set on for 10 mins to see if the LOPT was getting warm but the PL81 was definitely 'red plating' so I switched off! The LOPT remained cool despite being grilled from the PL81..
All the current flowing through the PL81 will be flowing through the primary (between the two top caps). As this is only 10 ohms, the power will be quite small and probably won't generate much heat in 10 mins.
If the coupling cap from the anode of the line oscillator to the grid of the PL81 is leaky, the PL81 will certainly get hot and bothered- certainly worth changing.
Cheers
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Old 26th Jul 2017, 11:35 pm   #36
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Default Re: Kolster Brandes RV 10 (Crown)

Clutching at straws, you earlier said that you have a line whistle. Do you also have -50 volts or so on the grid of the PL81?
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Old 27th Jul 2017, 8:23 am   #37
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Default Re: Kolster Brandes RV 10 (Crown)

Removing the actual connections from the overwind will have no effect if it has a s/c turn. To confirm this put a single loop of wire through the core of a working transformer and short circuit it while the receiver is working.
You will have to actually remove the overwind [not possible with the RV10 without destroying it] to make this test.
As Bill has mentioned, check the line drive waveform with a scope if you have one or check for a negative voltage [not quite so conclusive] at pin 2 of the PL81.

If you have any old LOPT from a scrap set, disconnect the line output and boost diode top caps and substitute the same connections on the spare transformer. Leave everything else in place. If EHT is now available at the spare transformers anode connection to the EHT rectifier, it is a good bet that the original has S/C turns.

This is probably why the RV10 was set aside a long while back. It's my guess that a Jellypot or other Thorn transformer would work OK but of course you have to experiment and understand what is happening in the circuits of the RV10 and the donor transformer. Recheck the boost capacitor connections just to be sure. John.
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Old 27th Jul 2017, 12:40 pm   #38
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Default Re: Kolster Brandes RV 10 (Crown)

I get about -45 volts on the grid/pin 2 of the PL81. I do have a 'scope, i have never used one on a TV before but I am keen to learn I will dig it out to try later on. This might also seem a daft question but as I don't have a standards converter yet, I have been running the set with no actual signal. I seem to recall that TV timebases need a proper signal present even to trigger the 25 or so fps (frames per sec) screen refresh. Is this relevant to this model of set?
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Old 27th Jul 2017, 6:52 pm   #39
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Default Re: Kolster Brandes RV 10 (Crown)

No. You should be able to get a complete raster without any signal applied to the input socket. Changing channels should produce clicks, pops and flashes on the screen. If you have a signal generator you can get something more interesting by feeding 45mc/s into the aerial socket, modulated that is and obtain bands on the screen and a sound signal at 41.5mc/s, channel 1 of course.

Don't worry too much about that at the moment. The line drive sounds good so I suggest you try a substitute hook up with another LOPT and let us know the results. John.
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Old 27th Jul 2017, 7:54 pm   #40
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Default Re: Kolster Brandes RV 10 (Crown)

This evening I thought I might clarify a simple substitute LOPT test utilizing practically any line output transformer of any make.

The example I have here is from a Ferguson 506T I scrapped years ago. Before you start mourning it was filthy, disgusting, knobless and the cabinet was bashed to bits!

As you can see all I have done is remove the top cap connections from the PL81 and PY81 in this case a PYE V14C and connecting them via some croc clip extension leads to the corresponding connections on the Thorn 506T transformer. No other connections or disconnections were made.

As you can see good EHT is developed in the test transformer to the extent that the EY86 EHT rectifier lights up producing healthy EHT. This can be seen as I draw an arc from the anode connection. [Try holding a smartphone and drawing an arc at the same time]

I played around a bit more. If you don't have a scrap LOPT, delve down your junk box and find an old audio output or mains heater transformer. Connect the primary to the PL81 and PY81 [or any other timebase valves that may be incorporated in the test receiver] and the secondary to a 6.3v .3amp scale light lamp. As you can see with the line output stage working correctly the lamp will light.

This test will work with any receiver that has an independent line oscillator not dependent on any feedback pulse from the primary of the lopt.

It's a crude test but practical and will determine if the damped line output stage is due to shorted turns in the line output transformer itself.

With a little practice you can get to 'read' the results and arrive at a satisfactory conclusion. John.
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