Hello all,
I'm working on a Coronado (Gamble Skogmo) model 527A. The schematic can be found here
http://www.nostalgiaair.org/Resources/060/M0007060.htm
I've whittled the trouble list down to one item and I am in need of some guidance. I cannot get the oscillator adjusted. When the tuning gang is completely open at 1720 kHz and the signal generator set at 1720 kHz, I can only hear the modulated signal by tuning the radio to about 1200 kHz. This is with the oscillator trimmer backed all the way out. Since the radio would not tune any stations initially,I don't know if this problem is a preexisting condition or not.
I can tune in the usual stations for my location but they show up about 500kHz low. The radio seems to play well other than the tuning being so far off.
I have substituted an NOS 6A8 and that resulted in no change. I also checked the wire dress against the pictures I took before starting work and they are very close to the pictures. Capacitor C2 was out of tolerance and replaced, but that had no effect.
I'm stumped and can only think of one or two unlikely things that could cause this problem. Any help would be appreciated.
Did you perform an IF alignment first? Is the output of your signal generator correct? Other then that maybe there is some fault in the oscillator tank circuit, but I would check the more obvious things first. Did you check R20, the 50,000 ohm resistor?
+1 on Arron's comments, something is wrong with the oscillator tanlk circuit. The oscillator seems to be running at a much higher frequency than it should be. This is kind of a stumper. A few things to look for (IMHO, and I do mean humble)
Does either the oscillator coil or the main tuning cap look like it was replaced? Is the tuning cap Osc section the same size as the RF section? (The older ones were, the newer ones have smaller plate on the oscillator section
If you have a digital all band radio like a Sangian or a Grundig, use it to validate the signal generator.
If memory serves, the local oscillator should be at 465 KHz above the desired frequency. Calibrate the tuning dial (With cap fully meshed, set the pointer to 535KHz, with cap fully meshed, scale should read 1650 KHz (or whatever the highest broadcast frequency is). Tune the radio to a known station on the lower end of the scale. Use your digital radio to test for the frequency of the local oscillator. tuning the digital radio, it should squeal, then get quiet at 465 KHz above the frequency tuned on the Coronado. My guess is that you will find something to be way off.
More to the puzzle:
Nost adjustments to RF, IF and Local Osc allow only minor adjustment. For example, if the IF Frequency is 465 KHz, you shouldn't be able to adjust both primary and secondary of both IF Xformers to resonate at 200 Khz. If the oscillator is running at 1000KHz higher than it should, you should not be able to adjust RF resonance to correct for this much difference.
Thanks to the both of you for the replies.
I did perform an IF alignment first and it didn’t take much to peak the transformers.
I probably checked R20 and will look at my records to be sure.
The dial was calibrated using the alignment instructions which called for fully opening the tuning cap. When the cap is closed, the dial is at it's lowest reading.
The tuning cap for sure looks original. I’ll look at the oscillator coil to be sure but I don’t recall it looking like a replacement during my initial inspection.
Oscillator does run but my really good multi band radio is not working right now so I’m stuck with one that has a crummy dial.
I’ll post the further results this evening or tomorrow morning.
Even the "crummy" one should work, as it seems that something is Waaaay off here. Glad it is not the IF, but do verify the sig gen against another radio.
I have checked my signal generator with two different frequency counters and it is dead on.
R20 was changed when I was doing electronic repairs.
The oscillator looks original based on wire dress and soldering.
The oscillator section of the tuning cap has the same number of plates as the other section but the rotor plates are smaller.
If I tune this radio to 800 kHz I can hear the oscillator at 1520 kHz on another radio. Keep in mind that the Coronado's dial is not made for precise measuring.
There are no nearby stations below 920 kHz so not being able to tune the bottom end in is no loss. However, I’d like to find the cause of this problem.
Would it be worth measuring the capacitance of the tuning cap and the inductance of the oscillator coil and calculating the resonant frequency?
There's only 5 parts that are in play here; the tube, C2, R2, tuning capacitor and oscillator coil. The first three have been replaced and I assume that R2 was replaced with a 47k resistor and C2 a 250pf capacitor. I also assume you have scrubbed clean the tube socket pins and reflowed the solder in all the connections. No band switch to clean. So now we're left with the L and the C. It sounds like you have the correct section of the tuning condenser, the one next to the speaker, connected to the oscillator circuit. And maybe the trimmer on the tuning cap needs to be taken apart and cleaned. Most of us on the Phorum have seen or heard how delicate 70 and 80 year old rf coils are. If the resistance readings are off then so is the inductance. If the oscillator is running at the wrong frequency then these are the things to look at.
Possibly a shorted oscillator coil, but most of the time, that would kill oscillation totally. You are correct, the Oscillator section of the tuning cap should (but not always) have smaller or fewer plates than the RF section. The oscillator sounds like it is running at approx 250KHZ higher than it should. If the coil is not slug tuned, I would try to replace it.
On most sets I have worked on the tuning condensers with the smaller sized plates in the oscillator section offer minimal means of adjustment, just a trimmer to set the high end of the dial. The ones with equal sized sections have either a slug tuned oscillator coil, and the small trimmer on the tuning condenser , or a larger adjustable cap called a "padder", most schematics call it a "600 padder" rather then an iron slug, as well as the small trimmer on the tuning condenser. I much prefer sets with the equal sized sections in the tuning condenser, it's much easier to get the tracking spot on, with the other type it's sometimes hit and miss. Getting back to the IF transformers was there a single peak or a double one when you were adjusting them?
Regards
Arran
I’ll investigate the suggestions in the next day or two.
I was thinking about the IF transformers last night as I have a radio that did have a double peaks and it wouldn’t tune a station at all on one of the peaks. Since the IF transformers were so close to being peaked on this radio, I hadn't considered them as contributors to this problem.
There was no double peak on the IF transformers but I did find out that three of the four trimmers were just about bottomed out and the fourth was about half of a turn from the bottom.
Trimmers were cleaned when I had the tuner assembly apart for repairs to the push buttons.
I also measured the oscillator coil, out of circuit and found the the coil resistances were within 20% of what the schematic shows. DC voltages were checked and were well within tolerance.
I think I’m going to set this aside until the weekend. Then I will open the tuning caps all the way, hook up a signal generator and oscilloscope to the oscillator coil and see exactly where it peaks out. I’ll post the results of my experiment.
I can finally declare victory.
I hooked up a scope and the oscillator coil resonated at 2.7 mHz, 2.2mHz and 2.5mHz. Each time I took a measurement I would turn the signal generator to 1.4 mHz and would tune the radio to see how far off the dial was. Then when I checked the resonant frequency again, it resonated at a different frequency.
I removed the coil and found that the mounting bracket screw that attached the coil to the chassis and provided the ground connection was slightly loose as was the rivet that held it to the coil. I sanded the bracket and the chassis at the points of contact and reflowed the solder at the rivet. Wired it back in and Shazam, it oscillated at 2.158. The trimmer brought it up to 2.185 and the dial now agrees with what I set on the signal generator.
Thanks to Arran and MrFixer55 for your help. This was an interesting problem and a good experience.