KB VC4 dual standard TV.

[ryanbrownin]

Hi,

Last night I got my first dual standard TV and I must say it is one of the nicest sets I've had so far (cosmetically) the cabinet is in grade A condition and it looks almost brand new, it also came with some brackets for legs which I discovered when I looked at the bottom of the set, this is perfect as I have ordered a set of original chrome legs that didn't have brackets so fingers crossed they fit.

Anyway onto the electronics side. In the past I have just plugged TV's directly into the walls but with this set I have turned a new leaf as this set isn't the usual riffraff I buy. This morning I built myself a lamp limiter, gave it a test with various different items and it was doing its job correctly. After a thorough inspection inside the TV to check for any obvious damage or warning signs, I plugged it into the lamp limiter and nothing happened. I checked and changed fuses but nothing happened.

If anyone can give me some advice/suggestions on what to check I'd appreciate it.

Thanks.

Ryan

[Jonster]

I seem to remember line output transformers are a common failure point on these sets, make sure you can get a raster and EHT before spending too much time or money on legs and stuff. Otherwise it's just the usual capacitors that need to be changed.

[Station X]

Originally Posted by ryanbrownin

If anyone can give me some advice/suggestions on what to check I'd appreciate it.

Thanks.

Ryan

Assuming it has a dropper resistor and valves, check for continuity from the L pin to the N pin of the mains plug.

[matspar]

If continuity is absent then start with the basics. If there's nothing going on at all then some simple meter checks will determine if mains is reaching the power supply. Make sure the heater chain is intact (I suspect it isn't if no valves are lighting up) check the mains dropper resistances, you may have a section which is o/c. Check for an HT voltage. Trace the heater chain connections around the set and check any heater chain connections between sub-assemblies. Check the mains voltage tapping arrangement, look for any wires broken off and make sure your fuses are intact. It shouldn't be too hard to diagnose. Good luck!
Matt

[Heatercathodeshort]

Check that your newly built lamp limiter is supplying power. Check for mains at the centre tag of the dropper and then each end. Check the heater thermistor and first port of call the continuity of the boost diode pins 4/5. [PY800/801 etc.] J.

[AC/HL]

Stuck mains switches are sometimes a problem in long dormant sets too. Have you got the service data?

[ryanbrownin]

Hi,

I took the back off again and it was apparent that I had not been as thorough as I had thought when conducting my inspection, I noticed that some wires travelling from the mains dropper were charred and the resistor attached to the dropper could be removed by hand I had missed this as the wires were coated in dust and my view was obstructed by a valve. One of the wires that was charred was suppose to travel to a fuse but the wire had burnt up in the middle leaving to separate pieces.

Does anybody have any suggestion of what the cause of the burn up could have been?

Hi Bill, unfortunately I don't have the service data, it would be very useful if did.

Kind regards,

Ryan

[high_vacuum_house]

Looks like the heat from that valve could have damaged the insulation on the wiring and then incorrectly fused (or wired out) mains supply caused an arc to form between the wiring and the nearby metalwork and caused damage before the fuse should have blown. The wiring would have been live and the chassis at neutral and therefore approx 240V between the conductors and chassis. If the insulation had cracked going round the metalwork a short circuit could easily occur.

Christopher Capener

[AC/HL]

The circuit doesn't appear to be in the Trader series, but it is in Radio and Television Servicing 1967-68. These books are occasionally offered on the Forum, so you may get lucky.

[TonyDuell]

I may well have that volume of R&TVS (I have most from 1952 to 1981). If you get no other help, I can probably be convinced to scan the appropriate pages, but not for a few days I'm afraid.

[1100 man]

Hi Ryan,
I've just dug out the 67/68 volume of R&TV servicing. The circuit is incredibly small and I don't have scanning facilities, but I have photographed the circuit in 4 pictures and it looks to be (just about) readable.
If you need any other parts, layouts etc, please let me know.
Hope that helps,
Cheers
Nick

[Nuvistor]

Is this any clearer for the PSU circuit?

[ryanbrownin]

Hi,

Thank you very much for the service data guys, much appreciated... I'm currently trying to get my head around reading it. Can anyone provide a layout diagram of the top of the chassis?

I've looked at a couple of images of other vc4's and I've noticed that the wire wound resistor on the mains dropper in my set isn't on other vc4's (I've only seen the chassis of two other vc4's) can anyone give a suggestion as to why it is there?

Kindest regards,

Also any advice on re-wiring and does anyone know what the correct fuse on the mains transformer is supposed to be?

Ryan

[Station X]

What mains transformer?

I must confess that I've never repaired an old TV in my entire life, but a circuit diagram is a circuit diagram whether it relates to a radio, TV or something else.

Look at the circuit in post #12. Would it be that difficult to start at the L pin of the mains plug and using a meter on Ohms range check for continuity through the mains lead, OFF/ON switch, dropper resistor, dropper resistor tap and heater chain back to the N pin of the mains plug?

Having done that you could check for continuity through the fuse, series resistors and rectifier diode up to the smoothing capacitor.

As for the extra resistor, it's probably there to replace an open circuit section of the dropper resistor. Testing will reveal all.

Rewiring of the PSU section will probably mean reinsulating the wires and keeping them a reasonable distance apart.

[ms660]

The transformer with the fuse on top looks like the sound output transformer.

Most droppers were run flat out, the appendages would quite often multiply...

Lawrence.

[1100 man]

Hi Ryan,
Here are a couple of pictures of the chassis layout top and bottom from the book. Also the relevant part of the power supply in better detail.
The 'mains transformer' with the fuse on is actually the speaker transformer (L61/L62). The fuse is mounted on it for convenience but has nothing to do with it electrically. The fuse is shown as a 1A delay. That will be a T1A 20mm type (T for delay type)
Mains droppers have a very hard life. They run very hot and the wiring usually goes brittle and falls apart. Soldering to them with normal solder ends up eventually with dry joints. A high melting point solder is usually specified.
As was said above, it's a case of chasing through the wiring with a meter to find out where the problem is. The first place to start is the mains switch itself- they often go open circuit.
Also don't forget to remove the mains filter cap (C142, 0.1mfd) from live to chassis. It will often fail with a large bang!!
Make sure that the NEURTAL of the mains supply ends up connected to the chassis rather than the live- check continuity with the meter. You don't want the chassis to be 'live'
Good luck
All the best
Nick

[Focus Diode]

Hi Ryan,

The mains fuse should be 1A "delay" type. The fuse looks to be intact in your set but should be replaced if higher than 1A of course.

The layout in all the versions of the VC series up to the VC100 is similar. I have the VC2 manual here. Unfortunately I'm having problems uploading images via the phone but will try again later.

Would be nice to see a pic of the front of the set.

Cheers
Brian

EDIT - While attempting to upload image I see Nick posted giving similar information at the same time!

[boxdoctor]

I would suggest that you cover all the wires to the mains dropper with a good high-temperature sleeving, for the first 3 - 4 inches at least from the dropper, over the original covering. Special care is needed where they go round that mains dropper supporting bracket. - It looks to have sharp edges! Tony.

[ryanbrownin]

Hi,

Thanks for the tips/instruction and thanks Nick and Brian for the layout diagrams. My apologies for my confusion with the sound output transformer, I'm still learning

Just a quick update, so far I have only got round to doing the re-wiring and testing the continuity of the mains lead, it probably wasn't wise to start this project 5 days before I go off to university. The mains lead tested OK after rewiring around the mains dropper, I also tested the fuse holder and it was OK too.

As you requested Brian, here is a photo of the set.

Kind regards,
Ryan

[Focus Diode]

Lovely looking set Ryan. Looks to be the same model my friend Steve has although his was in a far worse condition.

Please don't let some seemingly negative comments put you off. To someone without a circuit to hand and finding the fuse mounted on the sound output transformer it was hardly surprising you thought it might have been a mains transformer. We were all beginners at one time. I once scrapped a set due to the mains filter capacitor "blowing up" but unknown to me at the time. If only forums like this were around back then. Something I deeply regret to this day but not worth dwelling over as the past can't be undone.

With your set it's apparent one or more sections of the mains dropper has gone open circuit (or burnt out) these run extremely hot so don't have an everlasting life in any set. It seems a previous repairer has already substituted one burnt out section with a resistor. This itself could have gone open circuit.

Using your multimeter with the set unplugged from the mains it's easy to check the continuity of the wires to the first tag on the dropper from the on/off switch (R169/R170 which should read short circuit) and measure the resistances of each section. If a section is found open circuit a high wattage resistor of the same or close to the same value of the section can be fitted.

You've already has good advice about high temperature soldier and the care of positioning the leads from others.

Good luck and let us know how you get on. Don't hesitate to get in contact if you need further advice.

Cheers
Brian