Help/favour wanted

[pwdrive]

Would anyone have acsess to a valved broadcast receiver and a 'scope? what I am after is a typical P to P value of the local osc waveform as it is fed to the grid of the mixer, from memory I think it is several volts, receiver tuned to the low end of the MW band if poss.
I am 'scope and diode probeless at the moment.

Many thanks

Lawrence.

[tiv48]

hi lawence
i dont know if this will help but if you suspect the osc is not working ,mesure the voltage at the ocs anode the short the controle grid to eath ,if the osc was working the voltage will rise.
you could also try using another set tuned to mw in close proximity to set under test and tune set under test along the mw band you should be able to pick up the signal from the ocs (will cause a null in the noise from other set) workes better with a few turns of wire wrapped around mixer /osc with one end drapped over the oter set.
the voltage supplyed to the nixer grid should be negative
rob t

[pwdrive]

Hi Rob T, there is no fault as such, what I am after is the typical P to P value of the osc waveform as presented to the grid of the mixer in a standard valve receiver tuned to the low end of the MW band, it's to do with a 1Mhz osc project I am working on to drive a mixer so if someone was to measure the typical P to P value at the low end of the MW band then that would be close enough for me to get an idea, I have no 'scope and checking the mixer -ve grid volts on this project with my cheapo DMM is leading me a bit of a dance, I am getting a reading of -ve 11 volts but I am also getting some other effects while the DMM is conected to the mixer grid which might or might not be masking the true -ve voltage that that meter is trying to measure, it should be a large waveform ie: several volts P to P ie: if it is typically say 25 volts then I can assume that my DMM is more or less measuring correctly, allowing for capacitive loading etc.

Lawrence.

[AlanBeckett]

Lawence,
Unfortunately I'm not in a postion to measure it at the moment. Come in Al.
However, the ECH35 data, if I've translated te graphs correctly, says 36 volts RMS. It seems an awful lot.
Alan

[tiv48]

sorry for the misunder standing.
i must admit i find dificulty getting a meaninfull reading from the osc input to the mixer even using a scope .no maker seems to give the information .
rob t

[pwdrive]

No worries. I guess that if the receiver is working ok then whatever reading that is obtained with a high imp. 'scope probe would represent the typical norm.

Lawrence.

[pwdrive]

Lawence,
Unfortunately I'm not in a postion to measure it at the moment. Come in Al.
However, the ECH35 data, if I've translated te graphs correctly, says 36 volts RMS. It seems an awful lot.
Alan

Thanks Alan, yes it is a lot but from memory it is higher than one may think, on this test regenerodyne project I am working on the difference in gain from a low value osc input to a high value osc input is substantial.
Thanks for checking out the data.

Lawrence.

[AlanBeckett]

Lawence,
I've also had a look at ECH42 and ECH81 data. They both say around 18V RMS, again if I've translated the graphs properly. I suppose you might expect later valves to be less 'thirsty' due to the smaller geometry.
I guess the real method is to inject variable signals into the grids and measure the wanted output, which is presumably what they did.
The graphs are fairly flat around the maximum Conversion Conductance so it presumably isn't too critical, which is just as well as the level is bound to vary with frequency, ageing etc.
Alan

[pwdrive]

Thanks Alan, that seems to tie up with what I have just read in the Radiotron Designers Handbook, I am thinking now min. 20/25v p to p with older series mixers.

Lawrence.

[AlanBeckett]

Lawrence,

Clearly that's not the sort of level that you're going to get from the average Signal Generator, or, I suspect, from your Harmonic Generator.
A bit of amplification is called for I suspect. If the gain stage was VM could you stabilise the Injection Voltage with some rectification and feedback?

Alan