[BrendaAnnD]

Anyone ever run into this one:

I've been doing a restore on a 48-482. Not completely finished with the recap, but extensive testing has revealed no more leaky caps (they WILL all be replaced, though.)

The AM and SW bands are pretty good and sound nice, even with a speaker with a 1 1/2" hole in it. The FM, however, sounds quite good at lower volumes, but breaks up and distorts as I turn it up.. sort of like a CRT blooming at higher brightness when there's not enough HV. This is usually caused by a bad coupling cap between the 1st audio and output tubes, but this has been replaced, and, as I said, the AM side does not do this. ss

It took me a lot of fidgeting to align the FM section, but finally got it to behave and it does have good gain and decent audio bandwidth. Any suggestions as to what I should look for? 

Thanks, and Happy New Year.

[RodB]

My limited experience is that these early FM sets are sensitive to changes in B+. So, now that you have restored the power supply and are feeding a little more voltage to it you might have to take the B+ down a notch . Try using a variac (if you have access to one) to lower the AC a bit.

[BrendaAnnD]

The B+ is actually a bit low according to the specs. Should be 250 off the cathode end of the 5Y3, but is only 230 or so. Part of this, though, I suspect, is that my line voltage is running only 110.

[RodB]

Ok, let's say B+ is fine. Looking at the symptoms suggests the volume control loads or modifies the FM detected signal but not the AM detected one. So maybe the detector tube or circuits need more scrutiny.

[morzh]

Brenda


Have you tried the new tubes yet?

[BrendaAnnD]

Continued replacing caps. I've pretty much cleared up the FM issue. Check this out: it was actually the coupling cap between the volume control and the 7B6, which has virtually no voltage on it, plus the bass and treble caps, which really have no voltage on them at all. Almost finished with cap replacement, then I'll check high value resistors that have a tendency to drift to twice or more of their rated values.

The FM on this set, now that it's stabilized (doesn't even have too bad of a drift problem!) turns out to be really quite sensitive, better than many of my more modern sets.

[Paul Philco322]

Not surprising, on crowded FM dial in Boston are my Zenith AM/FM sets from the 50's 60's are more selective and pick up weaker signals better than many recent sets.

Paul

[morzh]

Brenda,

We call it "stepping on the same rake twice" in Russian.

That will reinforce the rule that is not to be broke: ALL OLD PAPER CAPS ARE TO BE TOSSED.
When I asked of the tubes I assumed you replaced all of them.

[BrendaAnnD]

Mike,

They are all being replaced, but I'm old school and like to troubleshoot to find the actual problem BEFORE shotgunning the paper caps. It helps to keep my mind working.. sharpens the skills. It's been fun working on this radio. I have a couple others in the queue that I can hardly wait to get started on.

[mikethedruid]

I always TEST all the tubes in any set I'm working on, but I don't replace the ones which test good. That would be a waste of money, not to mention a waste of good tubes, many of which are no longer being made. I do replace all the paper condensers, and electrolytics, and usually replace all the resistors also with ones of higher wattage since modern resistors are smaller than the old carbon ones. I also make sure to have a full set of good replacement tubes for the set.

[morzh]

I only replace tubes if they test really weak. Even the borderline ones I keep.
I test them all, never relying on "maybe it's good".
In the current radio one tube has shorts, and two are very weak, so I bothered Bob Dobush. Speaking of which, I still have not received the tubes, so he gotta be looking for some.
I am yet to see a good paper cap. They are 100% leaky and 100% go up in value.
American ones are typically 50-100% up in value; the German ones I replace in Grundig are 300-400% up in value. Superbad.

Resistors....if they are not more than 10-15% up, I keep them, after all if they survived well, why change. More that that - I replace them.