I have just taken out the RF deck and replaced those two capacitors. Both were around three times their specified value, but more importantly, the resistance of each was in the order of around 600k and 700k.
This is certainly enough to hamper the AGC action, and upon testing afterwards, it is noticeable how quick the set's gain alters to catch up with widely differing RF signal level strengths.
Although not an essential replacement, the performance of the set has been restored to normal.
To understand the black art of electronics is to understand witchcraft. Andrew.
I had a similar problem on a dual-standard set (Pye 11U) where I had missed a 0.1uF AGC decoupler hidden under the system switch. UHF was mediocre and I suspected a tuner fault. It was only when I started to consider the possibility of pulling the tuner to bits when I happened to notice the soggy waxy under the system switch. Looking at the circuit showed it was only switched in on UHF so I removed the cap and cleaned up the waxy mess and fitted a nice Mullard mustard in its place. UHF was then transformed with normal operation. The old waxy measured very lossy on the bridge and a DC test showed a resistance of 800K
Although I had previously corrected the fault of sound on vision, the tuning has remained rather critical and drift is quite bad, especially on the band III frequencies during warm up.
So, over the weekend, I decided to perform a complete IF alignment as per the well detailed description in the R&TV servicing book. Now, because I have a good accurate generator, I was confident I could achieve better results with the set than it was currently showing even though it was pretty good to begin with.
I spent some time following the procedure to the letter and I am very pleased that I have not only ended up with the tuning being less critical and less fussy about drift during warm up, but the video frequency response is better than before. Furthermore, the set is less prone to sound on vision and this is probably the reason that better video definition is now available when correctly tuned in.
So, with some careful setting up with equipment likely better than was used 65 years ago, I can safely say this model is capable of top performance.
Here below are some snaps of part of the procedure, the generator used and the before and after alignment results on screen. The first screen shot was with the tuning set just before sound on vision was apparent. The second shot shows that better definition is now available without the effects of sound on vision.
To understand the black art of electronics is to understand witchcraft. Andrew.
That's just brilliant! - Proper tools, proper result. I reckon one of my 53s could benefit from a tweak, but honestly, mine's not really that far off that I'd want to risk making things worse.
Give me a colour delta gun to converge - no problem.
Give me an IF to align - no chance!
Has anyone tried to align a TV VIF system using the tracking generator output of a spectrum analyser?
Only alignment I did was either the spot frequency method or using a sweep generator.
Surprising how stable in the TV’s service life IF alignment was if it was not messed with. Now after 60 years it’s reasonable to check and adjust it if required.
Frank
In my experience 60 year old TV IF alignment mostly stays as good as it was done when in the factory - which could vary of course. SAW filters fixed that issue!
I have posted this pic before but here is a PM5544 test card via the tuner of a 1956 HMV - alignment not touched. Note the response virtually flat to nearly 5MHz. When this pic was taken the TV did have other issues though.
In the 70s I had a small fleet of valve two-way radios in the business. The alignment of these was super critical if the receivers were to remain usable and required re-alignment every 6 months due I guess to vehicle vibration. Not generally an issue with TVs, although I do wonder about chassis that required frequent workshop pulls and rattled around in the back of a van for a day or so each time.
