Pageant P50
 

Sorry for a dumb question but now that I have removed the chassis how do I get to the underside? Is it safe to turn it over and rest it on the tube:o?

TIA
David

Re: Pageant P50
 

I always rested them on their side. I looks like the tuner side would be the best.
Many years of doing it this way. :thumbsup:
Dave, USradcoll1

Re: Pageant P50
 

Same here, I usually use a suitable length of wood to prop it in a secure position while working beneath the chassis.
:beer:
Mark

Re: Pageant P50
 

I managed to get a dim raster on it today and I had time to measure the eht at 13.5kV but then something went phut and eht dropped to zero.

The eht rectifier (a NOS U25 had been fitted) now has a faint blue glow and I can't see the heater glowing. The line whistle is still there but it sounds different. The sound of the line whistle and the brightness of blue glow does change if I alter the line hold.

Any thoughts greatly appreciated:)

David

Re: Pageant P50
 

If the chassis used a non coated tube I would go for the Visconol EHT smoothing cap. It looks as if it does not require one as the tube is coated.
First suspect is the U25. Disconnect the anode and see if you get a strong spark from the connection to the overwind when you brush it with a well insulated screwdriver. J.

Re: Pageant P50
 

The Pageant P50 is electrically identical to the Defiant models 410 and 710.
Information can be found in the 1957/58 Radio and Television book on pages 317, 318 and 319.
Looks like you've got another faulty U25 and as John suggests disconnect the anode wire and check for a flame like spark.
The boost capacitor might have gone short circuit. Circuit reference is C65 in the R & T book. Disconnect the top cap connection of the U191, if the EHT comes up then C65 is S/C. C65 is 0.25mfd.

DFWB.

Re: Pageant P50
 

I removed the anode connection of the U25 and got lots of sparks from the overwind.

Remembering my troublesome Murphy V310 and all its faults I thought I'd check the heater winding on the LOPT and it's hardly outputting anything. I removed and connected the U25's heater to a bench supply and they glowed nicely.

The heater winding looks very accessible. Can it be replaced or should I use a EHT stick rectifier, like I did with the Murphy?

Regards
David

Re: Pageant P50
 

Hi David,
You can use an EHT rectifier stick. As for the U25 heater winding I can't ever remember having to replace one but if you have any doubts about it's condition go ahead and replace it.
But before you do that reconnect the anode of the U25 to the transformer and disconnect the CRT anode connector and check the spark at both ends of the U25. The spark at the cathode of the EHT valve is quite different from that on the anode. If the spark at the cathode is the same as the anode then the U25 is faulty. Of course if there is no spark at the cathode and you are sure you have a good valve then it's likely the heater winding insulation has broken down.

DFWB.

Re: Pageant P50
 

Very early production sets were supplied with a Mazda CRM171 which doesn't have an external graphite coating so a separate EHT capacitor was required.
Your set has a CRM172 so I assume your set doesn't have an EHT capacitor.

DFWB.

Re: Pageant P50
 

Ha! ha! here we go again! The heater winding may have broken down to the transformer core. That might have been the noise you heard when the picture went off. You may also have a faulty U25. Certainly try a stick rectifier to prove everything is working OK. Just wire it from the transformer direct to the tube for test. Use crocodile test leads as a bench test.
Only two faults can cause an EHT rectifier to glow blue.
1 A faulty valve itself.
2 A short or very heavy leak on the EHT supply itself such as a breakdown of the heater winding. [Easily rewound as it is generally only a turn or two.]
It is also possible that the tube has an internal short. This is extremely rare and another story we can set aside. Let me know via PM if you would like a NOS U25 foc, John.

Re: Pageant P50
 

I was expecting to see something melted but I've removed and cleaned the heater winding and it looks fine. There was something between the heater connections but I doubt it was tracking? I don't understand why the winding only measures about 0.1Vac on my DMM?

Yes there's no EHT capacitor. The tube is coated but it's not in great condition. It's a bit crinkly for some reason?


The telly only lasted a few minutes but in that time I made some other observations, which may or may not be connected to the problem (although I bet they're just additional ones :(). I couldn't get a picture from my Hedghog converter. The brightness control worked but the contrast did nothing. The height control couldn't get the frame to fill the top and bottom of the screen.


Regards
David

Re: Pageant P50
 

"I don't understand why the winding only measures about 0.1Vac on my DMM?"

Even if a mean value of 2volts is available from the heater winding you won't get a correct reading on the DVM because of the shape of the waveform from the LO transformer. The valve heater is actually supplied with pulses which are 9uS wide within a duty cycle of 98.8uS.

DFWB.

Re: Pageant P50
 

I'm over the moon! After cleaning, reassembling and trying another U25 I've got a full screen picture, of sorts. I also noticed a wire from the line linearity coil was broken. Could this have caused the problem?

Anyhow, It's supposed to be showing a grey scale but this is all I can get that's stable. I'm not sure what channel my telly uses. Channels 1 to 4 on the hedghog seems to have different effects on the picture but channel 1 produces a crackly audio tone as well and the picture shown below.

David

Re: Pageant P50
 

It just keeps getting better, once I remembered to use an attenuator:)! The contrast still does nothing though.

David

Re: Pageant P50
 

Check the decoupling capacitors on the vision AGC line. C33 0.05mfd. C12 0.5mfd. C27 0.002mfd. C50 0.1mfd. C4 0.001mfd.
Also, MR1.

DFWB.

Re: Pageant P50
 

MR1 measures nothing in either direction. I've never seen this type of diode and there's two of them in my chassis. I did start a thread here questioning what they were and they've been identified as Brimar M1 selenium diodes (I've attached some data below). I'm really not sure what I should change them with?

Thanks
David

Re: Pageant P50
 

Almost any basic diode will make a satisfactory replacement for the 'metal peas'.
1N4148, BY126 series any cheap as chips diode. Just replaced an M3, the RF AGC delay diode in Ferguson 405T this very morning. I used a 1N4148 and it's fine. John.

Re: Pageant P50
 

Partial success. The picture isn't inverted anymore but the contrast control still doesn't work. All the parts suggested by DFWB have been checked and replaced, where needed. In addition I cleaned and checked the contrast pot, and changed R45 4.7M and R14 1M (EHT Trader Chart) as they were both way out of tolerance.

The picture below doesn't look good to me. If I turn up the brightness I get bright edges. There seems to be bright and darker patches and the verticals have kinks in? I haven't got time now but next week I'll put on a test card C but I've got a feeling I'm going to be told I've got a tired tube (again:().

David

Re: Pageant P50
 

Agree. Use the familiar test card C. If the contrast control does not work and appears to be floating around mid setting you will have to measure the voltage on it's slider to determine if you have a positive offset voltage derived from the H.T. line via the pot.

The AGC system is simple mean level with a negative voltage derived from the sync separator grid to bias the common IF valve and the RF valve in the tuner. The offset voltage is applied to the AGC line reducing the bias voltage and therefore varying the gain of what is called the 'controlled stages.' A few voltage checks will prove the point and you will be able to see where it is going down.
Plessey manufactured receivers for the Co-oP always employed Mazda tubes and valves where at all possible. The CRM172 is one of the better Mazda tubes and if it is a bit tired, maybe after 60 years it is not surprising.
Regards, John.

Re: Pageant P50
 

I couldn't wait till next week! Two pictures attached, the first one was within a few minutes. I thought it looked quite good, apart from some wobbles in the vertical lines. The second one was after a 5 minutes when it seemed to loose something? I need to find the source of some crackling noises which makes the picture twitchy, hopefully it'll be a dirty valve base somewhere.

I measured the voltage on the wiper of the contrast pot and it goes from a few mV to 194Vdc. In total darkness the contrast control does have a very very slight effect, but not enough to do anything.

Regards
David