[Patrick]

I'll go at it again tomorrow, following your advice.

[Chas]

The injected IF frequency has to be modulated by an AF tone, can be 440hz or 1khz usually. The level of modulation should be 30%, just enough to identify the RF as a unique tone.

That "box" on the far left of the control panel, push "on" push choices of "AM" choose 1K or 400hz, press levels of arrow up or down look on screen for confirmation. Fancy generator, all my stuff is 50's & 60's tubes...

Once you get it going keep RF output very low so AVC does not actuate, then do the alignment...

Chas

[morzh]

Chas

Are you implying Patric did not modulate the IF? I thought of it but I gave him the benefit of the doubt, I think it is clear it has to be modulated, else you will hear squat.

But the fact that he said he is able to put the 500kHz+ frequency through to me means he does modulate it.

[Chas]

Without modulation, the RF signal would give "dead air", a quieting of the receiver. That "seems" to be what Patrick is implying.. Nothing wrong with dead air but without a tone an output meter or even an AVC response would be nil... I would have thought that Patrick would have mentioned the modulation and the choice of tones.

I do not own an HP and had to find an image of it to see if it could modulate. One can also select external modulation or none. If the button is depressed for EXTernal and there is no modulation into the appropriate port, like the output from an iPhone, same dead air.

The RF alone is still useful at tuning frequencies to zero beat against a received signal to verify calibration...

Nice, stable instrument, unlike my old tube stuff I have to chase up/down the band when the line voltage changes.. chas

[morzh]

Chas

This is the very first Patric's post:

>>......I can inject an RF signal at various frequencies down to 500Hz (with var. cap fully meshed), and tune to that frequency, at which point I can adjust both the 1st and 2nd IF cans. But I get nothing when I try to inject the 460 Hz IF.

To me that implies the signal (he uses the same generator, just changing the frequency) is modulated. Else he would not hear either 460kHz or 500kHz and up.

Of course I could've misunderstood that too.

[Steve Davis]

Just to make sure, when connecting the signal generator, you removed the grid cap from the 6A7 and connected the generator to the cap on top of the tube, not to clip you removed from the tube?

Steve

[morzh]

Steve


See my post #12 and Pat's answer.

[John Bartley]

Just going to toss this out there in case it helps. One of the ways I have found to inject an IF signal is via induction (no direct coupling), thru the antenna, with the oscillator disabled with a small value cap across the oscillator section of the tuning cap and the tuning cap set as close to the IF as possible ... for a 460kc IF that would probably be AM at lowest setting (tuning cap fully meshed). By limiting the amount of induction, the signal generator can be set to produce a very low level output and the IF adjustments really can be seen to make a difference. I have double checked this method by using a conventional hookup afterward and I have yet to re-adjust after doing it this way.

Anyway ... just an alternative.

[Chas]

One can inject RF unmodulated at a given frequency get a tone as it heterodynes with some signal locally. Change the frequency of the generator (again, no modulation) and have what appears to be no signal..
I have done it then discovered the generator was not set for modulation.
-----------------------------------------------------------------------------------------------
As for inductive coupling, I use that most of the time to avoid lashing up an antenna matching network for the generator. I leave what ever antenna is in my lab connected, usually 10-15 feet of clip leads and sometimes ground connected. Yep, I get heterodynes with the local stations but they are very weak, the "locals" are 100 miles away at 550 and 1500. My advantage to link coupling is the antenna is still properly matched and I have isolation for the AC-DC set, one more, I am not likely to drop a hot lead from the generator onto the B+ and taking out the generator attenuator.

The link also provides a convenient source of local oscillator if I suspect the oscillator is not working or cutting out...

When I serviced for a local shop (I was MA licensed Radio/TV technician) we did stuff that would seem unusual but it was fast and conclusive. Get the set analyzed, fixed and be writing the bill in 20 minutes. Back into the truck and across town for the next job...  chas

[morzh]

If injected in this particular radio via antenna the IF will be strongly attenuated by the Wavetrap.

I suspect that the "4 locks" trick is at play here.
Or as one guy put it, when I leave the house, I leave 2 of my locks unlocked and two locked, and leave for the burglars to try to pick them.
Same here: if all or most tanks are strongly out of being tuned to 460kHz, the signal is not getting through and even if the particular tank goes via that frequency, you will never know that.
This is why tuning one by one in this case might help.

Of course it is a theory.

[mikethedruid]

Since others are explaining their methods for doing a rough tune in of the IF transformers on a radio where they have been tweaked badly out of alignment, here is a method I have found to work on radios which use a 6K7 IF amp and a 6A8 mixer oscillator. It can be modified to use on most any radio.

Start by turning the tuning condenser until the plates are meshed. Next remove the 6A8 tube from the radio. Turn on the signal generator. Set it for INTERNAL modulation, so the AM frequency has a tone modulating it. Let it sit for at least 20 minutes to stabilize well. Using a good, accurate frequency counter make sure of the accuracy of your signal generator's setting to the appropriate IF frequency. You can alternately use a good digital readout radio to check this using the first harmonic, 910KC for 455KC IF, 920KC for 460KC IF, 940KC for 470KC, etc. Tune the signal generator until you hear the loudest signal coming through the radio. Now, with the signal generator set for the highest output, connect the output THROUGH A 0.01 MFD CAP to the grid of the 6K7 tube, which will inject the modulated IF frequency into the IF amplifier. Now you can tune the 2nd IF transformer until you hear the loudest signal pass through.

Next put back the 6A8 and connect the same output from the signal generator through the .01 MFD cap to the grid of the 6A8 tube, and repeat the same procedure on the 1st IF transformer.

This will get you into the close ball park, and from there you can follow whatever the instructions are given in the radio's documentation for final IF alignment.

[Patrick]

Let me clarify so there won't be any confusion:

The HP 8657A has the capability of internal and external modulation, both AM and FM, and either 400Hz or 1 Khz. I always use internal AM at 400 Hz, and have aligned probably 50 radios with this sig gen. The 8657A has an inline 50Ω attenuator.

I also use a seperate gridcap for the det/osc. so the signal is fed directly into the grid of the tube.

Using an internal modulation of 400 Hz, I can inject that signal into the 6A7 grid, at ANY frequency from between 500 KHz and 1550 KHz. By tuning the var.cap to the frequency injected, I can adjust all 4 compenating condensers for maximum volume.

I can also adjust the LH and HF padders. AND, I can receive stations (however not too clear) all across the dial with an antenna.

What I CAN"T do is hear any audio when injecting the IF of 460 KHz. I can adjust the primary and secondary of both IF coils through their entire range, and still not hear any audio from the 460 KHz.

However, even though no schematics show it, the wire attached to pin 4 (grid 2) of the 6A7 from #42, a 32K resistor, first goes in and out of the osc. transformer, with a resistance of around 16 Ω (tickler maybe?). There are several variations of the 610, and several changes, but nothing makes any reference to this. You can tell by the untouched solder joints that this was factory installed, and I also found reference to it on another thread (someone else mentioned it).

All clear?

[morzh]

Patric

Yes, that is clear, and that was what I thought.
I think you might try that separate tuning of the IF cans, by whatever method you like best.

[BrendaAnnD]

It sounds to me like the IF chain is just seriously off frequency. In that case, you could still pick up a signal off the antenna or the 1st detector tube, but not be able to push through a signal at the IF frequency. The dial readout would be inaccurate in that case. The best thing to do in this case is to inject a modulated signal at the IF frequency into the grid of the IF amp tube, and align the second IF can for maximum signal, then back off the output of the signal generator and reconnect the signal to the grid cap of the 1st detector tube and adjust the 1st IF can for maximum, backing off the signal generator to the point that what you hear is a tone with some noise in it. Then go back to the 2nd IF and realign that for max, then the 1st again.

After that is finished, you will likely have to align the tuner local oscillator and antenna trimmers to make it track properly.

[Patrick]

Thank you, all. I shall do as suggested once this morning's coffee takes effect, and report back with results.