(18-11-2020, 08:50 AM)peter scott Wrote: [ -> ]Here's 2.5 Mhz on an MW 6-2. I think you'll have to go the extra half a meg. 
Peter
That would still be projected to something like 21 inch diagonal, but will see what I can achieve on the small CRT.
I have done a bit more playing with is best for me to add a couple of traps and have added two in the EF91 anodes off the HT. I have been trying hard to get the required frequency response, but it dawned on me this evening that I need to include the tuner and mixer stage from the front end, what I do just on the IF stage could have very little effect.
Anyway the two traps located above board in this picture:-
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attachment=20164]
Not very neat but having an effect, also included is a simple IF plot from the test set leading and impeadances are all wrong, but used for indication only. probably a third trap will be added when I get the tuner and have it working.
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I have ordered an EC900 and ECF801 from Ebay, so that the tuner can be used from a 6.3 volt heater winding. I believe the PCF801 is fu-bared anyway.
Adrian.
(18-11-2020, 06:38 PM)AdrianPH Wrote: [ -> ]That would still be projected to something like 21 inch diagonal, but will see what I can achieve on the small CRT.
Well actually that was a photo of the Trevor (murphyv310) Decca 1000 with a 60" diagonal.
Peter
Hi.
Peters photo is from my Decca 1000 that is a 4x3ft picture 60 inch. In fact on band three it gives an easy 3mhz bandwidth.
Brilliant set I have to say. Now in sunny Poland.
A bit of an update.
I received a VHF tuner from Julesomega and that arrived yesterday. It has received a good clean, removing the typical years of grime the best I could, also removed a lot of old flux left on from manufacturing, a bit of a scour with a pad and respray of the valve cans makes it look a bit better. Had the unit apart to see how things worked and the fine tuning method etc.
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I have started to sort out connections to the unit including changing the heaters over from 300mA to 6.3 Volt operation. I have a couple of valves EC900 and ECF801 on order coming in from France, hope to get them around Tuesday/Wednesday of the coming week.
There is still something I have not worked out and that is what to do with the AGC connection. The there is a spec on the unit in the Philips manual that suggests the whole unit can take up to 36mA, which I find a bit high!:-
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So I am open to suggestions at present, I am thinking a 9 Volt PP3 battery as a temporary solution, with a Pot across it, positive to chassis and the wiper via a 1 meg resistor to the AGC input, that should provide some control. But open to suggestions?
I have not finalised the detector side of the IF section yet, I have a simple diode detector (valve) followed by just a resistor and cap, so have not started on anything better until I know what levels I could expect from it and what I actually require.
ADrian
Excellent progress Adrian.
What model of Philips is the tuner from?
Not sure why you're bothering to change the valve types. The original P series valves add up to just over 12V which is easy to supply. the PC900 is one of those rare 300mA valves where the heater voltage is LESS than 6.3V.
Manual gain control as you describe strikes me as a good temporary solution. 36mA seems a lot of current for a small signal valve circuit but if that's what's needed to give good large signal handling then they had to do it.
Hello Trevor; Not actually sure, if you have trader sheets It is the same variant as in sheets numbered 1715, 1755 and 1801. I was also told it is used in am STC/KB VC4, but there are some component value differences from all of them.
Hello Jeffrey; Well the PCF801 was to air, so had to get replaced and as I am building a system based on a 6.3 Volt parallel heater string then it makes sense to me.
Adrian
Missed the bit about the manual gain pot, yes it just seems easiest at present, and it should allow me to determine a the level before, the line in the spec about the supply current does strike me as extremely high for the two valves.
I'd agree.
The current is way too much. It must be an error in the diagram.
You should be able to get an idea from the published anode and screen volts and load resistors from the HT line.
If I use the trader sheet 1749 with the 1755. The TV has a HT line of 231 Volts, dropping down to 136 Volts for the tuner via a 3K9 resistor, so just over 24 mA. Which still seems a lot to me, looks like I will have to switch to mains transformer for testing rather than my 12 volt to HT buck PSU.
Adrian.
I've looked at the trader sheets and confirmed your calculations. Dropping 231V to 136V in a 3K9 means it will disspate almost 2.5W. I wonder if that includes powering the transistorised UHF tuner too which may account for the rather high current.
Do you have a bench HT PSU? Incredibly useful for valve experiments. I'm fortunate in having 2 Solartron HT PSUs.
Is your 12V to HT a home-built PSU? I though buck converters could only do step down:
https://en.wikipedia.org/wiki/Buck_converter
I don't know if you have a variac, but you can make a simple variable voltage HT supply by using one to control the primary voltage of your HT transformer/rectifier. It won't be regulated but it's a lot better than nothing.