Another possibility causing the fault of hum on sound after a set period of warm up time could be heater to cathode leakage in a valve other than the rectifiers. The PL83 audio output needs to be eliminated by substitution. Although not adversely affecting the HT level, the reduction caused by the demands of the audio output if the hum is very loud, might be enough to slightly reduce the boost HT thus reducing the height.
If there were recifier troubles, eg a heater to cathode short here, then I believe the results would be more dramatic. I think it would blow the heater fuse for one thing.
In any case, it is a process of elimination to some degree and as mentioned earlier, not an easy or obvious one. Electrolytic capacitors can be eliminated by the temporary connection of a replacement. HT decoupling capacitors at the top of the list to begin with perhaps.
To understand the black art of electronics is to understand witchcraft. Andrew.
It could of course be nothing more than a poor mechanical earth connection somewhere, but I guess this would be provoked by moving the chassis slightly rather than a set time of use.... But you never know. Stranger things and all that.
To understand the black art of electronics is to understand witchcraft. Andrew.
In Australia the 70 degree and 90 degree Philips and Kriesler chassis used the 6N3 / EY82, as did the HMV E1 and some of the F series, early Astor and Pye models, again 70 and 90 degrees. Anything that didn't use 6N3s used the huge and inefficient directly heated 5AS4 (similar to the 5U4), sometimes with 2 in parallel.
Oh yes, Bush and Ekco made TVs here. The locally developed 110 degree Ekco chassis used 6N3s, as did Bush. Ekco was "AEI Ekco" (they also made Gala washing machines) and Bush was "Bush Simpson". Simpson also made washing machines! Very good ones too.
All this was in the '50s. By 1960 silicon diodes made valve rectifiers obsolete. But there was obviously a steady replacement market. Many owners kept their 1st generation TVs going until colour in 1974. I have a 1959 Kriesler 79/2 21" console in my collection that had a new CRT installed in 1974 - obviously low hours and very sharp and bright. You can be lucky sometimes.
6N3s or EY82s are now quite hard to get. (there is a Russian or Chinese 6N3 that's actually an output pentode!)
Next time I need 6N3s I plan to try a pair of JJ 6CA4s.
Thanks for all the replies. I noticed that the anode of the PL83 sound output valve is glowing red, so have been investigating in this area. 'That' capacitor is one of the Mullard mustard ones which I hadn't replaced. It tests fine, but I've replaced it to be on the safe side. The primary winding of the sound output transformer was measuring 79k, rather higher than the 700 ohms quoted by the manufacturer, so the transformer from my scrap chassis (from a T36 console version) which measures correct resistance has been substituted. This has made no difference to the fault and the anode is still glowing red.
I will investigate the ECL80 frame output valve base for arcing and look for any poor connections to the chassis next. I will also pick up a new PL83 in a few weeks' time.
Laurence
I noticed that the anode of the PL83 sound output valve is glowing red, so have been investigating in this area.
I think you have found the fault.... Or at least very near to it. It will of course be well worth checking the voltages on the valve and it could be that the valve is faulty. It could be that the cathode bypass capacitor is going short, in which case a new valve may suffer the same problem. A minimum amount of time with the set powered is important whilst you check out the voltages. 👍
To understand the black art of electronics is to understand witchcraft. Andrew.
The PL83 is the series heater version of the 6.3v heater EL83, otherwise known as the 6CK6.
The 6CK6 shares with the 6M5 the unfortunate pin-out of Pin 1 - G2, Pin 2 - G1. With 200+ volts on Pin 1 and a high impedance G1 circuit on Pin 2, it's wide open to leakage in the valve socket.
My guess is that's the reason for your PL83 red-plating.
Also check the cathode bias resistor on Pin 3 to ground. Some types of carbon resistors go low ohms when overheated, making everything even worse. So check this as well.
Early Mullard 6M5s used silver plated pins. These 6M5s would suffer from silver migration across the glass between pins 1 and 2. That made loud bangs and crackles!
By the way, the only difference between a 6M5 and a 6CK6 (and their series heater versions) is the 6M5 has its G3 tied internally to the cathode on Pin 3, whereas the 6CK6 has it brought out separately to Pin 6.
The PL82 and the PL84 (6.3v version = EL84 = 6BQ5) can both substitute for the PL83 if you take the wire off Pin 1 and put it on Pin 9.
That's a much more sensible pinout and probably worth doing anyway!
Mullard / Philips Mustard caps NEVER fail!
Never quite sure what the PL83 was developed for, was it for projection TV sets needing a high amplitude of video drive for the CRT or for 819 line television systems? Manufactures found other uses for the valve, as a sound output valve and as a frame output valve. Brimar developed their own video amplifier valve, the 6CH6 which was also used as a audio output valve in certain Kolster-Brandes radios.
Till Eulenspiegel.
I guess many valves can be used for purposes other than their intended application as long as they are run within the limits. Such a case is the use of an EF80 RF pentode as audio output, HMV 1826. Odd choice, but there we are.
But back to the overheating PL83, and a good point made by irob above about a faulty cathode resistor is that it can of course go lower and lower in ohms as it gets hotter. Certainly worth checking out.
To understand the black art of electronics is to understand witchcraft. Andrew.
Interesting difference in a B9A heater pin out.
https://www.radiomuseum.org/tubes/tube_el183e.html
Frank
Never quite sure what the PL83 was developed for, was it for projection TV sets needing a high amplitude of video drive for the CRT
Pretty sure it was this, Till. I don't know if you've any older copies of the "Television Engineers Pocket Book" - a compendium of useful information edited by Pat Hawker in its early days (I actually have Pat's own copy of one edition, purchased from his son) but in the chapter on projection television I'm certain it mentions specifically that valve, along with all the elaborate protection circuitry to prevent tube damage in the event of scan failure.
It is difficult to measure the voltages on the valve as the screen tube does not come out with the chassis, being mounted in the cabinet. It has been suggested that I try disconnecting the HT from the sound output stage to eliminate it, as the fault is also visible on screen. I tried this tonight and the fault is still present with no HT to the sound stage (HT to output transformer and to g2 of valve disconnected), and the anode of the PL83 is still glowing red without the HT.
The cathode to chassis resistor measures okay cold (only slightly out of tolerance at 227 ohms when it should be 200), although I'm not sure how it is behaving when hot. The cathode to chassis electrolytic has been replaced. I can't see any visible signs of arcing having occurred on either the base of the PL83 or the ECL80 frame output valve.
Laurence
Does the PL83 anode glow red hot when the volume control is set at minimum which should effectively connect the control grid to chassis?
From the Mullard Maintenance Manual, information for the PL83. With a supply voltage of 200V the and with a grid bias of 3.5V the anode current is 36mA and the screen grid current 5mA, the cathode bias resistor can be as low as 82 ohms. It's 200ohms in the TV33. Have you measured the voltage across the bias resistor? That done the actual cathode current can be calculated.
The operating conditions for a video amplifier can be ignored because it is assumed these are for positive going video applied to the control grid.
Till Eulenspiegel.
If it's still red-plating then HT must still be getting to it!
You won't see any sign of arcing or charring between pins 1 and 2. It will be happening deep inside the socket insulation. It only takes several meg-ohms of leakage at 200 volts to cause havoc. A bit of moisture will do it. That's why the choice of pinout is such a bad one.
Carbon composition cathode resistors that go low as a result of a gross overload generally stay that way - they don't tend to vary with heat. We had one brand in Australia (Ducon) that was notorious for this but it was known to happen to most other brands. At least IRC resistors went high, or open circuit! Philips carbon film types just burnt their paint off, but generally stayed the course. I've even seen them glowing at orange heat but still measuring OK after the power was removed!
Pin 9 of the PL83 is given as "Internally Connected".
Is anything actually connected to pin 9? Bush were not unknown to make connections to pins they shouldn't have, the UL41 in the DAC90A being the most infamous example.
I'm surprised there's not a removable cover on the base of the cabinet to allow you to take measurements and perform maintenance tasks without removing the chassis.
I've made a stupid mistake here in that I was looking at the cathode, which glows brighter on this valve than on the others, in that the metal structure itself glows whereas you can just see the filament glow from the top of the cathodes on most of the other valves. I'm not quite sure how I've managed to misunderstand this having worked on valve radios and TVs as a hobby for the past 25 years. Apologies - at least I've learnt something anyway (and there is nothing connected to pin 9 of the PL83).
So it's looking like I have a fault in the frame output stage. The set does have a removable cover on the underside, but the tuner/IF chassis is mounted on the frame beneath the main deck making access difficult.
Laurence
Laurence
I thought the leads on the tuner/IF unit were long enough to let you move it out of the way?
So it's looking like I have a fault in the frame output stage.
I wonder how that is causing a loud hum on sound then?
To understand the black art of electronics is to understand witchcraft. Andrew.
@pye625 good point. I've taken the voltage readings from the pins of the PL83 and I've got the following:
Cathode: 3.4v
Anode: 184v
HT to g2: 205v, but seems to be varying a bit.
I only have the manufacturers values for the TV43, which are C: 4.3v, A: 184v g2: 198v. The readings were taken with a DMM.
@cathovisor The main deck is reasonably accessible with the bottom panel removed, but you are correct in that if the contrast control is removed from the cabinet the tuner/IF chassis could be swung out to the side.
Laurence
After having got a bit stuck (and irrational) with this problem last year I put this set aside for a while. Having recently completed a few other projects, I returned to this last week. The sound I get from the speaker is a ‘splutter’ similar to what you get if a mains plug is making poor contact.
I thought I’d try observing the chassis in darkness whilst the set was briefly powered up via an isolating transformer to see if any arcing was visible in any of the valves or elsewhere. None could be seen, but the heater in one of the pair of PY82 mains rectifiers suddenly lit up as if there was a heater-cathode short. A substitute PY82 was tried and this too exhibited the same behaviour. The heater in this valve is next in circuit to the thermistor and resistor in parallel, so I initially suspected the thermistor. The thermistor actually looks fine, measuring around 1k when cold and the 400 ohm wire wound resistor which is in parallel with it has continuity but does look to have had a hard life, with the green paint bubbled up and flaking off a bit.
I’m now thinking that the arcing may be taking place on the base of that PY82. I now have a Megger, so will try testing between base contacts to see if this reveals anything.
Any thoughts or comments anyone has on this would be very welcome.
Thanks
Laurence
