EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Cadmandu on October 09, 2019, 01:11:12 pm
-
Hi All,
I have a Heathkit IG-42 RF generator, Fluke 1900A counter, and a Tektronix 453.
I want to monitor my generator output while aligning the three IF stages of a HQ-140X receiver.
Questions about this procedure.
1) Do i use a T connector on the bnc output of the IG-42 one for the counter and one for the injection into the IF section of the HQ-140X
2) Do i use a Tek 2 watt 50 ohm terminator 011-0049-01 on either or both t-connector outputs?
3) Is it better to install a second bnc to the generator coming off the output tube before the attenuator controls?
4) when do you use a 50 ohm attenuator ? not the feed through type.
Thanks
-
Hi All,
I have a Heathkit IG-42 RF generator, Fluke 1900A counter, and a Tektronix 453.
I want to monitor my generator output while aligning the three IF stages of a HQ-140X receiver.
Questions about this procedure.
1) Do i use a T connector on the bnc output of the IG-42 one for the counter and one for the injection into the IF section of the HQ-140X
2) Do i use a Tek 2 watt 50 ohm terminator 011-0049-01 on either or both t-connector outputs?
3) Is it better to install a second bnc to the generator coming off the output tube before the attenuator controls?
4) when do you use a 50 ohm attenuator ? not the feed through type.
Thanks
Are you using the 453 to monitor the input signal to the HQ140X IF?
If so, you can just use a bnc tee with no termination at the input of the 'scope, with a second cable to the IF stages.
You can feed the counter from the "channel 1 output" bnc on the back, totally isolating it from the rest of the equipment.
There will be mismatch between the impedance of that output if your counter doesn't have a high impedance input, but it should still work OK.
If you are using the 453 to monitor the 455kHz signal into the detector, you won't need the tee.
As the 453 is still looking at the same signal, having the counter connected as above will still work.
If you are looking at the level of demodulated audio after the detector, or at DC levels on the AGC line, it will obviously, not work
In the latter two cases, you will need to place the tee on the counter input, instead of the 'scope .
If the counter does not have a high impedance input, you are pretty much stuck with a double termination (once at the counter, then another at the far end of the cable to the radio.)
At 455KHz, such things are pretty academic, as, say, a quarter wavelength at that frequency is around 165 metres, so reflections are fairly unlikely to be a problem.
Here, you run into another problem, though, the IF stages are not nice modular devices with a standard 50 Ohm input Z.
You have to either feed the 455kHz into the mixer tube grid where the RF input normally is, or at the output of that device.
They are both high impedance points, & the latter one has the additional delight of the tube HT.
You need a 50 Ohm to high Z pad, with a capacitor in series with the coax centre conductor.
You would probably get away with a normal 50 Ohm termination, followed by a 1MOhm resistor, in series with a 100nF cap.
Back in the day, such "special" terminations were common, & often called "dummy loads.
Mr Carlson may have a video showing both this device & its use for aligning vintage radios.
-
Thanks alot VK that is a real mouhful. I will be awhile understanding all this.
-
But wait!, There's more!
After I wrote the above, I found a thread started by Radio Tech referring to his Youtube video about repairing & aligning a Hallicrafters S53A, which uses very similar circuitry to your receiver.
https://www.eevblog.com/forum/other-blog-specific/my-video-had-a-unexpected-high-view-count-what-the/msg2724466/#msg2724466 (https://www.eevblog.com/forum/other-blog-specific/my-video-had-a-unexpected-high-view-count-what-the/msg2724466/#msg2724466)
This contains a link to the video, which is pretty much a tutorial on what you want to do.
Watching it, it appears that Hallicrafters' normal procedure is to connect the 455KHz to one of the sections of the tuning gang.
There is evidently enough coupling to inject the IF at this point.(Thinking back, I may have used this method,but it's years since I aligned a tube radio!)
This means you don't get caught up with the tube HT, or messing around underneath the chassis.
He still uses a series 100nF cap, but doesn't seem to have done anything much about matching, so it really looks like you can just stick a capacitor on the end of your coax, & "Bob's your uncle"!