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Old 27th May 2007, 8:29 pm   #1
Colourstar
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Default Ferguson 506T- height and linearity problem

Timebase trouble to be exact. When first switched on, the height on this late 50s slimline set is on the low side, but otherwise the display looks great. After a minute or two the linearity rapidly deteriorates, and the picture starts undulating due to the onset of fairly severe hum. By this point the PCL82 (V13) frame output valve is glowing red. As all the caps in the locality have been changed, the fault should be with the valve, but substituting a couple of other PCL82s (admitted both used) produces the same on-screen effects and overheating. Any words of wisdom welcome!

And I was hoping to settle down with a Margaret Rutherford film and a beer...

Cheers
Steve
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Old 27th May 2007, 8:40 pm   #2
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Default Re: Ferguson 506T- T for trouble

If the capacitors have been replaced how are the resistors? Some could've changed value.

I don't know these sets and don't have any service data for them. It's not a fault I've experienced on any set! How are the rest of the valves when the hum bar develops, is there hum on sound too?

Hope someone out there knows the answer!
Cheers,
Brian
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Old 27th May 2007, 8:51 pm   #3
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Default Re: Ferguson 506T- T for trouble

Let the set cool down, and switch on. What are the voltages on the Anode, G2 and Cathode of the PCL 82 frame output valve. Before and after the fault.

Also, there should be 68 Volts on the Anode of the Triode. Look at C116 and R161 and see if this changes.

Check the Transformer T3 as well as Z2 and X2. I hope the transformer's OK!

The voltage on the Height Control and pin 9 of the PCL82 comes from the line output on these sets. Check C73 and C74.

Also, check ECC82 and associated parts too.

Cheers,

Steve P
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Old 28th May 2007, 7:13 pm   #4
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Default Re: Ferguson 506T- T for trouble

Thanks Brian and Steve P. I'll check those voltages. X2 has been replaced by a piece of wire by a previous owner... I believe it regulates the height as the set warms up. I actually have two identical 506T models, so if it is the transformer I at least have a second chance!
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Old 29th May 2007, 8:29 pm   #5
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Default Re: Ferguson 506T- T for trouble

Here are the results of some recent probing with the meter.
V13 (PCL82- the overheating valve): The cathode reads low at 4v (against 15v suggested on the circuit diagram), Pin 6 (anode) is at 172v (178 on the diagram).

I am slightly confused by the reading on pin 9 (anode) as this naturally varies a great deal as the height control is adjusted. With the screen fully scanned it reads only about 45v against the 170v shown on the ERT sheet. There's bags of height available - possibly too much. This may be the result of the previous owner substituting the missing thermistor in the height circuit with a bit of wire.

The ECC82 (V7b) which performs part of the frame oscillating duties has 62v on it's anode (75v on the circuit).

Still at novice level I'm not sure how to interpret these varied results...
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Old 29th May 2007, 8:38 pm   #6
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Default Re: Ferguson 506T- T for trouble

Check the PCL82 cathode resistor (pin 2), old resistors like this fall in value as they deterioate - the low cathode voltage with valve overheating point the finger squarely at the cathode circuit, overloading elsewhere would tend to raise the voltage at this point.
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Old 31st May 2007, 7:23 pm   #7
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Default Re: Ferguson 506T- T for trouble

Thanks very much to Studio 263. The resistor in question was hiding in the very depths of the set, but when removed it resembled a badly barbecued sausage. It had also frazzled down from 330 ohms to 44, so I think we can consider the case closed. I'll also replace the associated cap and try a fresh PCL82. After that we should be in business!

Thanks again
Steve
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Old 1st Jun 2007, 8:40 am   #8
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Default Re: Ferguson 506T- T for trouble

I' glad it's sorted!

This sort of repair was championed by George Wilding (sp?) in the Television magazines of the late 60's and early '70's. He called it "rapid diagnosis" and the idea was to make a few well chosen measurements, replace one component and complete the repair in one hit. In it's later (and most extreme) forms "rapid diagnosis" didn't even allow for trying the valve first ("new valves made no difference" - how many times have you read that?), it really was repairing in it's purest form.

The never managed it of course! It was the usual "blind panic - try any component you have to hand - thumb through the TV fault index - change all the electrolytics - blame the microprocessor / lopt / tube and give it back" routine on our bench a lot of the time.
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