Has anyone else encountered this fault?

My set has a very long-term intermittent fault that appears as 2 or 3 bands of noise and reduced contrast that last for 10 to 30 seconds but not happen again for hours or days. So far, I have tracked it down to around the video detector but now it refuses to play up when the scope is connected!

The 209C was AWA's cost-reduction 90 degree chassis. It was the only model to use the triode part of the 6U8 3rd vision IF as a double-diode, i.e. vision detector and AGC.

I wonder is this an inherent fault in this circuit? This chassis is the only one to use this circuit, it was abandoned in the next model.

BTW, this set has its original 17" 90 degree RCA CRT. Still nice and bright, if a little slow warming up. It has, and needs, an ion trap magnet, but has an aluminised screen and what seems to be a straight gun. Strange....

Anyone need replica plastic knobs? I taught myself how to mould them in clear epoxy for this resto.

It was a long haul, this one. I can't recall ever working on a piece of equipment that had so many faults! But then, it did suffer a flooded garage at some stage.

Sounds like a tough case. I doubt AWA would have been able to get away with releasing a chassis with such an obvious design fault. I do know someone with a 209C that has been restored and he has not mentioned any issues like this.

I'm guessing you have looked at the obvious, such as replacing the 6U8 3rd IF / Detector valve and 12BY7 video amp valve in case of subtle internal faults? Have you also checked all the components around that area of the circuit?

What led you to isolate the fault to the video detector?

Hi TV Collector

Yes I tried the 6U8 and the 12BY7. Original 12BY7 was low emission and replaced.

How did I track it to around the video detector?

1. Direct video injection to G1 12BY7 - no fault.

2. Scoping video and AGC - the only capture when it happened with the PC scope connected showed increase in 30MHz level coincident with +ve excursion of video level.

I should reiterate that the fault is very long term and a bit subtle, it could be mistaken for an antenna fault. It can go for weeks without happening. Clue is 100Hz, i.e. two bands of noise. Caused me to suspect heater-cathode leakage.....

I wondered why this particular circuit was only ever used in this model. It has cost benefits... AWA are not the only manufacturer to release a chassis with design faults. Examples? where do I start?

BTW, I made one mod to the set - I added horizontal blanking. 20 turns of wire around the width coil former, connected in series with CRT G1. Because the design uses AC coupling to the CRT, burst and sync edge shows up on the black annoyingly on "dark" scenes. The mod is very effective.

This chassis is not the design orphan you think it is. It was used in a large number of sets in the late fifties as a cost reduced suburban reception chassis. The more complex (and expensive to build) chassis would have had better fringe reception performance. AWA was one of the first companies to have a stripped down chassis like this, though I'm not sure they advertised it as such.

The 209C shares the same chassis with models 207C, 211T and 212C. The same chassis with minimal changes (different 3rd IF valve and germanium detector diode) appeared in models 225, 228, 229 and 230. The same basic circuit continues to appear in later models with a few more minor changes. AWA basically had two different chassis variations for all the models made from 1956 to 1960.

AWA had close ties to RCA (closer than many might think). The original AWA 201C circuit was basically a straight out clone of a RCA design. I suspect the cost cutting variant used in the 209C (and other models) was another RCA design adapted for local production.

I agree, design faults did get released on the unsuspecting public, but most resulted in decreased reliability or minor performance issues. Your fault produces symptoms that would have most customers complaining on a regular basis. That's why I don't think it's a design fault, though I may be wrong!

When you saw the fault on the PC scope where did you have the probe connected too, the grid or cathode of the detector? I agree, the fault does sound more like an intermittent heater-cathode breakdown somewhere in the tuner or IF strip. Another possibility is instability in the tuner or IF somewhere due to an intermittent failure of a decoupling capacitor.
I'm still not sure that the detector is to blame at this stage. Due to the fault causing the AGC system to react, it complicates the diagnoses as the set is reacting to a fault and altering the IF operating conditions to try and compensate. As a troubleshooting measure, it may be worth disabling the AGC and apply fixed bias and see what happens next time the fault occurs.