Ekco T344 plus another one
The detector out polarity look correct.
Not much more progress to report right now, except that I tried the detector the other way around out of interest - the sync was virtually non-existent and the picture inverted. Also, I received the OA91s, but fitting one has made no improvment - so it looks as though, for reference, a modern silicone diode may work as a detector in these sets! Interestingly, I notice that on the circuit for the Ekco T368, the detector is labelled "MR1" as though it's a metal rectifier there rather than a germanium diode.
One thing to note for anyone else tackling a T344 is that there is an error on the service sheet's diagram - R26 is labelled as 470k whereas it is in fact 470 ohms. I was about to take mine out and replace it with the "correct" value until I checked on the very similar T368 diagram and saw it labelled there as 470R.
I did also take some voltages from the tuner stage, and the valve voltages there are all pretty normal except that 125v on the anode of V1.
Something I tried this evening, which I think rules out anything beyond the IF stage as the cause of low contrast, was to inject an amplified signal into the grid of V8, the video amp. All I had available to do this was an audio preamp. Feeding in the video signal from the Hedghog worked with the simple vertical grey bands test card, which seemed to be just within the limit of the amp's frequency response. The sync was iffy, but what this proved was that with a higher amplitude signal being passed into the video amp, high contrast blacks and whites are possible.
I haven't spent much time on the TV lately, I've been a bit stalled with it to be honest - and I have had a side project, a 16mm movie projector about which I've just posted elsewhere on the forum.
I measured voltages in the IF parts of the circuit, and they all look more or less spot-on; I've also tried swapping valves V1, V2, V3, and V5, nothing made any improvement, so I'm thinking that it's worth removing the tuner box and trying to sort out that low voltage at V1's anode (125v versus the 165v specified).
A very quick update on this - I'm trying to get back into it again, after a summer of other projects & duties. As mentioned previously, I'm trying to get to the bottom of the low voltage anomaly in the tuner section (because despite evidence to the contrary at this being to blame, I'm a bit stuck at what could be causing the poor contrast). I've just replaced R9 which was a little high, at about 126k rather than the rated 100k, but unfortunately the voltage has now dropped further to about 107v at the anode of v1 🙁 .
What happens if you apply an external AGC voltage?
That's a good idea - and something I haven't tried. I've only tried increasing the AGC to 0v with respect to chassis, and that didn't make enough of a difference to affect the contrast greatly (from what I remember, this was months ago now!). I'll give that a go, it may be a few days till I get the chance however.
0v on the AGC line and not enough contrast sounds like low gain somewhere, which would also explain the low HT on the tuner since the RF amp is running with low or no bias.
I'm in Oz so I don't know that set. What's in the IF? 3 x EF80s?
Hmm. Special Mazda valves in the 2 stage vision IF! They will be clapped out by now.
Looks like the closest to the 6F19 in the first IF is the EF85 or 6BY7 in Oz speak. 2nd IF is EF80 / 6BX6.
Since it's a 2 stage IF you could consider replacing them with the newer, higher gain, pin-for-pin frame-grid types, 6EH7 (EF183) and 6EJ7 (EF184) in that order.
Thanks for the suggestion, I'll have look to see whether I already have any EF183 / EF184s at the weekend. Although I have already been through all (I think) the valves in the vision circuits and tried substitutes (several in some cases!).
It's definitely a gain problem, I had the scope on the video amp back in summer, and the peak voltage before it started clipping was way lower than it ought to be. Logically the problem ought to lie in the IF stage, however the lack of any voltage anomalies there is what got me stuck.
Ah I see. So it clips early? Since you already tried the valves and the voltages check out, that leaves the detector diode. Drop a 1N4148 in there and see how you go!
Those newer frame-grid valves were developed to provide good performance from a 2 stage IF. Earlier generation valves were pretty marginal when there was only 2 of them.
Could you try injecting a video signal into the stages and work back from the video output valve to see where it drops.
You mean a video signal modulating a carrier at the IF frequency?
You can use an ordinary VHF sig gen with AM modulation of about 500 Hz which the generator may already have. That will give you horizontal grey - black bars, about 10 on the screen. No sync of course....
The fact it clips at a low contrast level does sound very much like the diode.
Thanks, both of you, and sorry I haven't replied before now - I've only just got to sit down at the computer this weekend, and it's a bit late to start on the telly now unfortunately. I just cannot find the time to get back into this properly at the moment, especially with making more of an effort to get out of the house every weekend due to being shut in for months...
I need to read back through my own replies on this thread to really get back into it. I have already substituted the detector diode, but I can't remember now what type I ended up with. I do remember though that the set seemed to work better with a silicon type rather than a germanium one, perhaps that's a clue as to what's going on.
I see from my inventory that I have one single EF183 in my stock so I will give that a go, just in case.