Hi all, well trying to wrap up this radio from H**l. The last problem I am having is in the FM section. I can't get anything but my local station, which is coming in on several positions on the dial. Seems like the oscillator isn't running as tuning has no effect. I've tried listening for it with a portable but nothing. The portable picks up the oscillator from other FM receivers I have. As you can see, the oscillator circuit isn't very complicated. Not many components at all. Original measurements showed voltages way off on Q1, the FM osc. The original transistor had collector to emitter leakage. I've replaced it and now the voltages are more in line but still no difference. I have replaced R1, 2, 4. I have replaced or subbed C7, 9, 10, 11 and 17 even though they test good. I have tested C15 and the tuning gang. I have disconnected VC1 the afc diode then reconnected later. L3 is the only inductor in the circuit which is only a few turns of coiled wire. The spacing of the turns is loose so that I can see there's nothing shorted and it does ohm out. Q1 cross references to an ECG 161 as per parts list notes which I installed. I am totally stumped on this one. I keep coming back to the osc transistor and have subbed 3 other NPN silicon FM RF or oscillator capable transistors in addition to the ECG 161, but still no difference. Even subbed a UHF type! Perplexed isn't the word. Sorry for the long dissertation.
I do not have experience in FM but what you are describing is not necessarily the ocillator.
If you have a scope, see the osc output while varying the frequency.
Also try it without AFC (it uses the Varicap).
Take a look at all tuning elements.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Thanks for your input Mike. My scope is only good to 15 MHZ. I have disconnected the Varicap diode with no difference. I have checked, double checked and even triple checked all the components in the oscillator. If you have read my other posts on this radio, you saw how a previous guy hacked and butchered thus unit. That what I'm up against. Also, I can't hear the oscillator in a portable I have.
The osc relies on those L4, R8, R9 and C23 at the output being there. Check they are ok. not disconnected etc.
PS. I think it is Colpitts circuit. I simulated it, it works fine. But only if the mentioned liad circuit is connected. Without it there is no capacitive feedback divider. Of course, there might be other reasons.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 01-24-2019, 12:28 AM by morzh.)
Hi Mike, yup, as far as those parts and connections you mentioned, been there and done that multiple times. This is a fairly uncomplicated circuit made with reliable disc ceramic caps, all of which were tested or subbed anyway. Even tried scoping the B+ supply and adding .01 mfd caps on the supply line temporarily just to be sure it is bypassed well at RF frequencies! I even disconnected D16 the RF overload diode and tested it, thinking if it was shorted or leaky it could load down the osc and kill it but still nothing! I am beyond stumped.
Could you disconnect the Q3 and the C18, this would isolate the generator.
Then see if it oscillates.
And, yes you could use the scope, it will not accurately measure the amplitude but should show the signal, even if attenuated.
PS. Put the lead on the right side of C10, use 10x probe.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Ok Mike. I will try my scope anyway. I have disconnected and reconnected different paths that I can't remember anymore if I had C18 out of circuit or not. I know I tested and subbed it but don't remember if I tried the osc with it removed. But rest assured, I will be trying that later today, thanks!
Ok, I've disconnected both C18 and Q3, used my scope on both sides of C10 to ground and still nothing. I setup my portable around 100 MHz, tried listening for the osc on another FM radio I have and was able to pick up the osc from 3 feet away clearly when tuning the other set to around 90 MHz, which was what I expected. When trying to listen for the magnavox osc at only 1 foot away I got nothing anywhere up and down the dial. Tried subbing C7, 14, and 23 again and still nothing! I am out of ideas!
Q1 is an NPN transistor which I replaced with an ECG161, another NPN. Way back in the beginning, the voltages on Q1 were way off. It had collector to emitter leakage. After replacing it, the voltages are much closer to what they should be. After racking my brain, I tried 3 other transistors in its place , the voltages still good but not working. The ECG 161 is back in now.When I first got this radio, the previous guy had most of the components removed from the top of the board, why I don't know. He then remounted them on the foil side. So I unsoldered them, tested them and reinstalled them on the top side again. Makes me think he had a problem with the osc too. Practically all the solder connections have been redone during the process of remounting the parts. Haven't been able to track down any cracks in the foil. Earlier I isolated the tuning gang and tested it for shorts as well as using my cap tester on it to make sure it was changing capacitance in a smooth manner as I rotated the tuning knob. Where the pcb mounts to the metal frame is where there are some ground points that look fine. I will have a go at resoldering those points and hope for the best. I have spent a ton of time on this unit over the last month and just can't believe this is so illusive! Can't remember the last time a radio gave me this much grief!
That's transistors for you. Finicky, finicky. Pernickety even.
Also, see if there are pools of rosin flux between traces, dirt, green stuff etc.
This could be sensitive and those things could throw the balance off.
Is the board dry? I mean, not surface but the laminate itself. It is probably the brown phenolic paper laminate, it could potentially absorb moisture if kept in such place. Baking at low temps as the last resort.
Check the outputs of the transistor against the datasheet, if different from the original.
It gotta be something simple.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 01-24-2019, 04:37 PM by morzh.)
Ok, went over any solder connections that looked iffy, removed and cleaned any flux overflow between foil traces, checked continuity from end to end of the foil traces, looking for breaks. Lastly, reflowed solder on the grounding points and tabs on the pcb and reflowed solder on the tuning gang on the circuit board, all futile. No oscillations yet. This set has always been in a heated living room I am told so can't imagine there's a moisture problem in the circuit board. I would be hesitant in trying to warm it in an oven for fear of warping plastic parts, especially the dial. I keep getting a nagging feeling that Q1, the osc transistor is critical. That is what led me to try 3 different transistors with no luck. The original Q1 did have collector to emitter leakage! The Sams parts list does have the ECG161 as a replacement. Maybe, if I could get an original Magnavox transistor that would work. I don't know if it's that critical. Anyway one last tidbit of info on this radio. According to the woman's son, he said the set never really played that great. I don't know what to think!!
One thing I noticed: on the sch Vbe is 0.44V. But you have 0.7V which is typical Silicon.
Tne sch which shows either N750 or 1N7501, can't discern and cannot find the datasheet, implies Ge transistor.
I wonder if the sch was calculated for a Ge and not Si type. Not sure if it makes any difference here, when I smulated I used just NPN transistor with no specs.
Last resort : either get a Ge transistor for 800Mhz or the sch might need to be adjusted to the Si transistor.
Else it makes no sense.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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