I'm having a problem calibrating the dial on my 41-295. When I adjust compensator 20A and 10A with the dial set at 1500 KHz, I find that the adjustment to 10A (adjustment on the tuning capacitor) has little or no effect. With the generator and dial set to 580 KHz, 20 seems to adjust normally. The end result is the dial tracks normally, at the low end of the broadcast band, but begins to lose it's tracking at 760 KHz , and the station that should be received at 760 KHz, now comes in at 780 KHz. The adjustable 10A appears to be intact, but not functioning properly, even though the mica appears to be in place, and the adjustment screw appears to be the original slotted hex head screw. If you fully tighten the adjustment screw the signal drops very slightly, but as you loosen it, the volume returns slightly but never gets any stronger through the remaining range of adjustment.
What else can I check? All the tubes in the set test good to excellent.
What are you using for an output indicator? Your ears are not good enough to trust as an accurate output indicator during an alignment procedure, and I don't care how many "restorers" claim all you need are your ears. They do not know what they are talking about, period, end of story. You should be using an analog output meter (good) or oscilloscope (better). Ideally this output indicator should be connected across the AVC line; if this is not possible, alternate (but less accurate) methods include connecting between plates of the output tubes, between plate and screen of one of the output tubes, or across the speaker voice coil.
Is your signal generator known to be accurate? Many aren't, which is why I now run mine through a frequency counter to obtain the best possible alignment on every radio that crosses my bench.
Now to address your questions...
Adjustment (20A) is for setting the oscillator to 1500 kc on the AM band; (10A) tunes the antenna circuit for best resonance at 1500 kc. You may not hear a lot of difference when adjusting (10A), but again, an output indicator is the only accurate way to tell what effect this adjustment has on the circuit.
Adjust (20A), then (10A), then switch to 580 kc to adjust (20).
Now when you adjust the low frequency padder (20), a different procedure is utilized. This is known as "rocking" the tuning condenser. What you do is tune in your generator signal of 580 kc, which may - or may not - come in at 580 on the dial. Now turn the padder (20) slightly, and move the tuning thumbwheel to follow the signal. If the signal is getting weaker, turn padder (20) in the opposite direction slightly, then follow the signal with the tuning thumbwheel. The goal here is to get the signal as high as possible; when making this adjustment using the "rocking" method as outlined here, you will reach a peak where the signal reaches its strongest point, then will begin to drop off. You want the adjustment to be at the point where the signal is loudest, even if the 580 kc signal is not coming in at 580 on the dial. This adjustment is not meant to make 580 come in exactly at 580 on the dial, but to make the oscillator track with the set's IF circuit.
Once you finish this adjustment, go back to 1500 and readjust (20A), then (10A).
After that, proceed to adjust the SW bands as given in the set's alignment procedure.
This all assumes you performed IF alignment before beginning to do the RF alignment.
Thanks, Ron
I'll dig out my scope and give it one more try. Haven't calibrated my signal generator. I'm really pleased with the performance of the radio, but it bugs me to have the dial be off frequency. The pushbuttons, of course, are "dead on", and that's what I normally use.
Henry
Not all consumer radios had dials that tracked perfectly. My 18B cathedral has a lot of markings on its dial, done by the original owner no doubt as Philco encouraged that in the early 1930s. WSM is marked at 64 on its dial. In 1933, as now, WSM was/is on 650 kc. When I restored and aligned this 18B, and adjusting the low frequency padder by "rocking" as outlined above, the low end of the dial is 10 kc off on the low side. Judging by these markings, it was that way when it left the factory. Since it has these markings on the dial, I left it that way.
Again, for best performance, it is best to adjust the low end so that the oscillator tracks accurately with the set's IF frequency. This is why it is more important to reach a peak on the low end, whether or not it throws off the low end tracking.
Just like Ron said, always test your RF generator feeding into a Freq Counter to make sure your test gear is accurate or your chasing yourself round in circles on alignments. For serious radio restorers using vintage RF generators only, a Freq meter is a must have item to keep accuracy. Most just get close with a tone generated from older RF gens on one end of the dial sometimes. If you dont have a freq counter, a portable radio with digital crystal controlled tuning can be used to track your RF generator for spot on first, seconds, & third harmonics. I have a old Radio Shack model DX-375 that covers most bands I keep on my bench so I dont have to bring out the Freq meter. Just takes a little practice. In all my many restorations over many years, I only had one radio that needed the mica replaced from rot on the tuning cap. It was a Zenith 7G-605 that had been used during WW2 and stored in a damp environment for many yrs. I only use my ears to "peak up" the I.F. transformers , and never trust my vintage RF generators, even though they produce a fairly accurate signal across the bands. I have a old dual trace scope here I need to restore, but a freq meter works great for servicing old radios and making sure your RF gen is spot on. I never cared for tube testers much either, even the best Hickoks high-dollar models can lie. The best test for tubes is under proper operating conditions in the set(s) the tubes were designed for. Just my .02
I fitted my lowly EICO 324 sig gen with a capacitive/resistive coupling output to a BNC on the back, taken from the top of the output attenuator to have a fixed level output to my freq counter. Never have relied on the dial calib.
Chuck
Thanks Guys,
I've got some surplus equipment, but nothing has been calibrated in years. I have a Lodestar Signal Generator/Counter, an HP 5315B Universal counter, and a Tektronics 465B Dual Trace scope. All I can say is that the Lodestar and the Hewlett Packard agree within decimals, so I have to assume that the generated frequency is pretty close. With the last re-alignment, I ended up with the lower end of the band indicating "dead on" but the upper end off about 40 Khz. I end up using the pushbuttons, exclusively, so in reality, the whole thing is just an annoyance.
I'm thinking that, at this juncture, with my limited understanding of how the circuits work, and precisely how to affect a solution, I'm probably going to have to live with what I've got.
Henry