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41-258 B+ Problem - Printable Version

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41-258 B+ Problem - Guest - 03-06-2009

Hi folks. I have just finished recapping, replacing all resistors, and completely rewiring a Philco 41-258. The set plays well, except for some problems with "motorboating" in the upper part of the AM band. I ran a check on the B+ voltage, coming off of the filter caps, and it's way too hot. I am getting about 106 VDC. I tried turning down the line voltage to about 90 VAC with my variac, and the problem went away. So, I installed a 10K dropping resistor on the line right after the last filter cap. I got the voltage drop I was looking for. But, the set now has all kinds of whistles and motorboating on the entire AM band.

I have noticed that this set's oscillator seems to be very sensitive to even minor changes in the power side of the radio. Is there another location, or strategy, that I can use to drop the B+ voltage? Here is the schematic, over at Nostalgia Air:
http://www.nostalgiaair.org/PagesByModel/411/M0013411.pdf

Re: 41-258 B+ Problem - codefox1 - 03-07-2009

The elevated B+ is caused by the average line voltage increase we see now (I sometimes see 125 volts) vs the 110 or 115 volts that was common 50 years ago. In and of itself it shouldn't be a problem, but you can use a bucking transformer or dropping resistor (say 25 ohms, 20 watts, and yes it'll get warm)to lower the voltage coming into the set. If there is also hum, one of your replacement filter caps may be open, but the chances of this are small. This would also benefit the life of the tubes, keeping their voltages closer to their design specs.

I assume you aligned the set after the rebuild and have the antenna loop attached properly.

The motorboating can be just a matter of lead dressing or placement of the newly installed. Double check to make sure that you didn't mix up any B- and chassis ground returns on the caps. You could try gently adjusting the leads near the oscillator an antenna coils with a non metallic stick, a new plastic chopstick is fine for this, and btw you can also file the ends on one of these to make a screwdriver on one end, and alignment tool on the other.

Let us know how you fare!

Re: 41-258 B+ Problem - rghines1 - 03-07-2009

Hi,

By adding the 10K resistor you may have effectively isolated the spurious RF signals that were being filtered out by the last power supply cap. Could move the 10K resistor between the choke and rectifier tube or add a 0.1 uF poly cap after the 10K resistor, connect negative end to the B- bus not chassis.

Saw you got the hum problem solved too. Nice work!

Richard

Re: 41-258 B+ Problem - Guest - 03-10-2009

Richard, I can't thank you enough. You were dead right about that resistor isolating the RF signals from being filtered. I put a resistor directly ahead of the filter caps, and the choke, and it looks like I am good to go.

When I got this radio, it had suffered a good deal of corrosion. The only stage that worked was the RF amp! All the other wiring and most of the components were shot. It has taken about six months. But, now it plays just like it did back in '41. I am strictly a newbie at radio repair. This is the first time I have attempted a complete rewire of one of these old "rubber wire" sets. This one was definitely a challenge.

I tried aligning the set, when I first powered it up after the rebuild. But, there were so many birdies, images and motorboating, it made alignment just about impossible. Now that I have the B+ issue worked out, it looks like I am finally going to get this bad boy out the door Icon_biggrin Thanks again for all your help.