Does anyone have a suggestion on where to start and how to proceed in finding a solution to strong motorboating in a model 41-295? The unit was not functioning when I found it. I have since re-tubed and re-capped it, and now I find the output section apparently working just fine, but no signal, except the motorboating gets through. When I pull the XXL tube the motorboating stops, but of course, there is no radio signal.
I need general plan on how to proceed.
I managed to get the motorboating stopped. Everything now seems to be functioning properly except the oscillator section. If anyone has any suggestions on how to properly troubleshoot that section, please let me know. Both the oscillator and mixer tubes (XXL's) have been checked, and filament and plate voltages are normal. Looks like I'm going to have to tackle the dreaded four section, ganged switch. Suggestions?
Henry
Make sure you did not move or re-postion any grid or plate wires that could cause feedback in the audio stages. The audio circuitry in this set is not your simple AA5 type design and it's crowded in there.
Even putting a replacement cap or resistor in another location, relative to the old part's position can cause problems. If you replaced bad wires, they need to be exactly where the originals were. The new B+ bypass caps and new electrolytics should have taken care of power supply bypassing.
Also be sure you checked and replaced any and all resistors that are more than 20% vout of nominal value.
Chuck
Henry,
Been working on a 41-295 also. In addition to checking the tolerances of all the resistors....there are two paper caps. hidden under the audio outut transformer...maybe you found these? They are C70 & C75, .006mfd, coupling caps to the 42's. Also check C19 the 2,000pfd. Micromold cap, part of the AM oscillator circuit. Although it looks like a mica it could be a paper cap if it is a Micromold. Also, check the 3,000pfd. cap inside the 1st. IF can. This was a Micromold 'mica' in my radio but also could be a paper unit! And, check C24/25 the 370 pfd micas on the swithing tuning assembly. This combined unit could be bad..it is an odd looking block cap.
If you have rubber insulated wires into the IF cans they should be replaced also to prevent possible shorts. Lead dressing from the volume/tone controls can be critical.
Best of luck. John
Agree with all said. Lead dress and placement of components is very important. Also make sure the screen and bias voltages are correct. If this set has 2 XXL's swap them. Microphonics in a tube take on a whole new meaning when they are in IF/RF service, but sometines you can still tap them gently with a rubber eraser and get a clue.
Blast the switch with deoxit, work it back and forth several times, and let it dry out overnight. I like to take an old dry tootbrush to clean out any crud while it's still wet with the solvent. I'll also swab a tiny bit of WD40 on the bushings and detents with a Qtip to make sure the action is free and easy. If these places are really dried out, I apply a miniscule amount of lithium grease with a toothpick and then wipe a little more WD40 on them.
Many cases of motorboating ultimately come down to a problem in the B+ chain; one stage not being properly isolated from the next. If either the resistor in the voltage divider has changed value, especially if it mearures low or has arced over, this can be it. Rarer, is a replacement filter cap that is open or too small in value, or mis-wired.
Finally if the problem is caused by a very strong local station, something needs to be done to reduce the signal coming in through the antenna.
Thanks for your prompt reply, John. I will check out all of your suggestions. I believe the set is close to working, in that I can inject a signal at the volume control and I get a good clean result. I found and replaced the two .003mfd 1000v caps under the output transformer as well as all of the exposed electrolytic and paper caps. I will look into the Micromold like caps. I didn't dig into the multi-wafer components, for obvious reasons. I did that once in a 41-300, that I now have working, except for the push button option. I'll get back to that. This 41-295 popped up, looking to be in near perfect condition, and I got distracted.
This set had an XXL and a 7A4 installed as mixer and oscillator. I have tried replacing with XXL's I have checked to be good, but inevitably I end up with 36.2v on pin 2, and .212v on the grid of the mixer and 193.5v on the plate; 1.8v on the grid and 7.4v on pin 7 of the oscillator. As you know, every other pin on both tubes is either not hooked up or is grounded. These voltages are being checked with the receiver set to "Broadcast". When I scratch the antenna terminals, I get no response from the set.
Just for grins, once I get this thing going, would it be productive to punch a hole and add that 7A6, that the 41-300 has in the enhanced AVC circuit? So far, I haven't heard a 41-295 in operation. Is there a significant difference in performance of the two sets?
Henry
Thanks, Codefox,
I've done the XXL swap and microphonics check to no avail. The multi-wafer switch looks pretty good, and I have blasted it with deoxit and scrubbed down what I could get to without removing the complex. As I mentioned, the motorboating problem was corrected when I found a portion on the B+ circuit to be not yet soldered - a function of the late night repair activities.
I live in the East county of San Diego, in the local mountain community of Julian, and AM reception in not the greatest. I get probably half a dozen listenable AM stations, and of course the ever present evening blasting and over-riding stations that come in from Mexico. See my response to John for more information.
Henry
All's well that ends well.
Disregard all of the above listed queries. I've found the enemy, and it's me! Looks like I got a couple of other wires crossed, but now I've made the corrections and the thing is working perfectly. There's no accounting for stupidity, and the resultant self-imposed dilemma!
Thanks again, guys for your help.
Henry Younce
Hey, don't beat yourself up too much. I went thru the same "learning curve" myself
with goofy mistakes and such. Comes with the hobby. You won't do that again. :-)
Chuck