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-   -   UHF tuner use. (https://www.vintage-radio.net/forum/showthread.php?t=157805)

radiograham 6th Jul 2019 1:14 am

UHF tuner use.
 
Hi I am hoping someone can advise me about this,i have a uhf tuner u943c made by Philips i think.This is the tuner sold by Maplins and used in their Nicam tv tuner in feb/mar 1990 magazine.This is meant to be fed to a saw filter,i was hoping to use it with a valve if strip but not having much luck.Can anyone tell if me if it's possible to use it in this way or am i wasting my time? Graham.

Restoration73 6th Jul 2019 9:45 pm

Re: UHF tuner use.
 
In general, any varicap uhf tuner has a lower gain compared to a mechanical type, and even that often used the vhf tuner (on a 405-625 set) switched to "U" to give additional
IF gain. Therefore you may need an outboard IF preamp based on a transistor or MMIC
to overcome the losses. This site gives excellent background development of tuners;
https://www.maximus-randd.com/tv-tuner-history-pt4.html

Nuvistor 7th Jul 2019 3:25 pm

Re: UHF tuner use.
 
RBM used transistor or valve UHF tuners straight into the IF strip in their Bush TV125U and TV135RU televisions. The 125 used EF85/80’s the 135RU used EF183/184’s.

Others makes used the VHF Mixer for extra IF gain, perhaps if you look at Bush circuits it may help.

radiograham 7th Jul 2019 11:49 pm

Re: UHF tuner use.
 
Hi,i was wondering if this tuner which is fed into a saw filter rather than a conventional if strip whether the if output would be suitable for use with a valve if strip. Graham.

Nuvistor 8th Jul 2019 12:51 am

Re: UHF tuner use.
 
Direct drive SAW filter Tuner units such as the u943 have a low impedance output, this will need matching to the IF strip, some valve iF inputs probably have a much higher impedance although those Bush tuners were fed by 75 Ohm coax.
You could possibly cure two problems with a transistor matching stage between tuner and IF, it will give some extra gain and match the units.
The matching is the problem you have to solve.

G6Tanuki 8th Jul 2019 2:33 pm

Re: UHF tuner use.
 
Yes, the impedance-matching is likely to be an issue!

Either use a tuned-circuit with a tapping for the input, or maybe consider using an emitter-follower. Do you know the impedance of the SAW filter the tuner is designed to drive??

See fig.4 here:

https://www.robkalmeijer.nl/techniek...e71/index.html

for an idea though when I've done this for HF-band crystal filters (either homebrew ones using ex-TV crystals or commercial KVG ones) my trick has been to remove what's shown as "Rterm" and then replace what's shown as the two 22K base-bias resistors with a pair of resistors each double the value of Rterm.

radiograham 10th Jul 2019 1:58 am

Re: UHF tuner use.
 
Hi thanks for all the help,not had a lot of success using this tuner for anything other than its original purpose ie with a saw filter,in this it works fine.Think i need to find an earlier type. Graham.

Nuvistor 10th Jul 2019 7:32 am

Re: UHF tuner use.
 
Is the IF strip home made or from an old TV set and by not a lot of success do you mean poor pictures or it just doesn’t work?


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