Hi All,

I've had an issue with my Kriesler 79/15-B television where the image, while still watchable, was lacking in brightness and had very limited contrast. After a short period of use, it would gradually lose brightness until the screen was black. However, changing to another channel (static only) produced a nice bright raster.

I had a look at the tuner as it seemed to be the most logical place to start, and found my problem to be the 6HG8. I replaced it and got a nice bright raster. Interestingly, out of my four spare 6HG8s, this was the only one which would work. The rest all showed the same issue as the original.

I continued on replacing some ageing carbon resistors on the tuner that I could feel were overheating. I also accidentally dropping the tuner out of the cabinet once, though it was onto carpet, so I doubt it caused any damage.

Interestingly when I tested it again after replacing the resistors, the new 6HG8 no longer worked. The picture would lose all sync and signs that it ever originated from a stable and clear image. However, replacing the 6HG8 with the original or another replacement yielded the same stable image with limited brightness and contrast that I started off with.

So, does anyone have any ideas? I can't test the 6HG8 as I don't have the parameters for my tube tester. I'll see if I can get a circuit diagram so I can check the voltages around the tuner.

Chris

The 6HG8/ECF86 was one of last valve types introduced and therefore didn't make it to the ET4A settings list.

These valves have fairly high attrition rate in regards to low emission & element shorts from my own experience when bulk testing loose second-hand ones.

Testing a 6HG8 on a ET4A is easy.

Pentode section:-

1: N, 2: E, 3: F1, 4: F1*, 5: F2, 6: F1, 7: F1, 8: F1, 9: F1, 10: N.
Merit: 3 & Range: 20-25.

Triode section:-

1: N, 2: F1, 3: F1, 4: F1*, 5: F2, 6: E, 7: F1, 8: F1, 9: F1, 10: N.
Merit: 3 & Range: 20-25.

The best way to get the Range setting right is to test a new valve and set the Range to about 80% of full scale on the meter.

Thank you for the ET4A parameters! I tested the three tubes I could find on my tester with some interesting results:

6HG8 one

The needle flew off the chart when I tested the pentode, though I didn't fine any shorts. The triode section tested good with no shorts.

6HG8 two

The needle flew off the chart when I tested the pentode, though I didn't fine any shorts. The triode section tested good with no shorts.

6HG8 three (the one which was originally good).

The pentode section tested good with no shorts. The triode section has VERY low emission (2 - 5%), with no shorts. I'll let it bake for a while on a raised filament voltage to see if I can get some decent emission out of it.

Chris

The needle flew off the chart?!!?

I'd back off the merit control a bit then. I based the readings on my own tester of which the merit pot was changed years ago.

The most common type of shorts are heater/cathode.

I find the best way to check for shorts is to have all the selector levers marked F1 (except F1*) in the E position.
Then move each lever one by one to F1.

In the case of a 6HG8, the cathode, shared by pin 1 & 3, is also common for both the pentode and triode section. This is why you leave selector lever 1 at N position otherwise it would show as a short if you also had it in the F1 position. If you have selector lever 1 at F1, then you must have lever 3 at N.

Try both levers 1 & 3 together in F1 and see what happens!

Anyway, with all the levers (except 4 & 5) in the E position, move lever 3 (cathode) gently back & forth between E & F1. If the neon shorts indicator flashes on and off, or just stays on, then you have a short . If the neon just flashes once it should be ok.

Do you know what could be causing the brightness to gradually drop out? Even with the decent 6HG8 in (the one that used to work good), the brightness gradually faded out. The audio is still present however. The set has very little brightness now, and is the picture is almost dark. I'll let it cool down for a while and try again to see if the symptoms persist.

Chris

Apart from checking all the valves, I'm not sure.

From the symptoms you describe it could be anything.

I guess you are unable to test the 6U9 & 6X9 - well, you could if you had an adaptor socket.

I see the TV is a 1964 vintage. Does it still have paper caps & it's original electrolytics?

The tuner problems are probably a coincidence, while it's a fault it's not the fault you're looking for.

The thing to remember is that even with no signal the screen will be white. So that rules out the tuner as you can unplug it and the screen should still stay solid white. Noise from the tuner (static) actually makes black snow!

The picture fading out sounds like you're loosing one of the voltages for the CRT. Given the age of the TV my bet is those lovely waxed paper capacitors that are still used in a few places in your TV. While Kriesler used very high quality Philips polyester capacitors in most areas they only made them with a maximum 630v rating so paper capacitors had to be used where they needed a higher voltage rating.

The first place I'd check is C134, a 0.056μF 1600v capacitor connected to the anode of the 6AL3 boost (damper) diode. When it heats up it will become very leaky and drag down the BHT supply which feeds the 530v G2 supply for the CRT amongst other things.

How do I know? It happens in nearly every old valve TV I've worked on.

Just wanted to say that this is all very helpful information as I'll be recapping a 79-15 soon. Thanks collector. Is there any way to tell just from looking which caps are likely to be the good kind and probably ok. From cleaning up the chassis I can see there are a few different types. There are a couple of the bright yellow and black Ducons behind the EHT cage (I am most suspicious that these will be bad), a large amount of the mustard coloured caps, a fair amount of bumblebees and the big cans out the back. Plus whatever is lurking inside the tuner.

Thank you Andrew, I didn't consider the EHT booster cap to be a potential issue! My 1969 Philips Modular-17 has a similar issue with the booster cap. I've put the restoration of that set on hold until I can find the replacement parts for it. I'm going to focus on my AWA P2 in the meantime, as it seems to be developing a few assorted problems (the latest fiasco being the power switch breaking).

Chris

The Philips polyester capacitors are the "mustard" coloured ones. Normally very reliable and as good as any modern polyester capacitor so leave them alone. Same with the Simplex "Styroseal" cap's, they are normally fine still.
The ones that are a problem are the Ducon branded capacitors with the wax coating. They are yellow/brown in colour with black markings.
The American "bumblebee" capacitors are very rarely seen in Australian products. What you're seeing may be high wattage resistors. If in doubt, post pictures and someone will point out what are the problem components.

Don't play around in the tuner unless you need too. Moving parts around changes the interaction of the components with each other and therefore the tuner alignment. Most of the parts in tuners are either resistors or ceramic capacitors. Both are generally fairly reliable so it's best to leave well alone.

My own Philips Modular 17 had exactly the same problem. It ran fine for a few minutes then the picture faded and I quickly shut it down. A look at the boost cap explained all, it was merrily bubbling away and very hot! I replaced it and on rechecking the old capacitor when cold it was hard to measure any leakage in it. They fail under load with the problem getting worse the hotter they get.