Interesting issue with 48-482
Posts: 1,107
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
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.
Posts: 897
Threads: 37
Joined: Feb 2015
City: Roseville, MN
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.
Posts: 1,107
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
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.
Posts: 897
Threads: 37
Joined: Feb 2015
City: Roseville, MN
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.
Posts: 15,444
Threads: 539
Joined: Oct 2011
City: Jackson, NJ
Brenda
Have you tried the new tubes yet?
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.
Posts: 1,107
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
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.
Posts: 4,369
Threads: 410
Joined: Jun 2011
City: Boston
State, Province, Country: Massachusetts
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
Tubetalk1
Posts: 15,444
Threads: 539
Joined: Oct 2011
City: Jackson, NJ
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.
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.
Posts: 1,107
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
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.
Posts: 1,824
Threads: 114
Joined: Jul 2014
City: Sneedville, TN
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.
Posts: 15,444
Threads: 539
Joined: Oct 2011
City: Jackson, NJ
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.
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.
Users browsing this thread: 1 Guest(s)
|
Recent Posts
|
GE Model P-975F
|
There are 2 of these on ePay and several pics on Radiomuseum. Although the front looks like a very popular Hong Kong ra...MrFixr55 — 09:26 AM |
Rare zenith 6s137 restore
|
Hi Fred,
I get things to work, but what you, Russ, RadioSvit and many others on the Phorum do with cabinet and chassis...MrFixr55 — 09:02 AM |
PT-6 antenna connections
|
Hi Bridcarl
+1 on the others. Pin 6 of the 7C6 is the AVC Diode so it is appropriate that that pin 4 of the coil con...MrFixr55 — 07:30 AM |
462ron
|
Joseph, this is beyond awesome. A lot of thought and engineering went into this and you hit a home run! Now I must save ...462ron — 07:27 AM |
Cannot Adjust an IF Transformer’s cores
|
Outstanding work indeed. I wish I could come up with solutions like this.slford310 — 11:16 PM |
Cannot Adjust an IF Transformer’s cores
|
The abilities of our 'Phorum Pholks" never ceases to amaze me! Fantastic fabrication, Joseph! I'm waiting now for...GarySP — 08:27 PM |
PT-6 antenna connections
|
Thanks. I never realized that was a notch noted in the schematic as described. I’ll recheck my wiring.bridkarl — 08:13 PM |
Cannot Adjust an IF Transformer’s cores
|
Thank you! I was quite new to the Phorum when I started this post, so I feel like it might be advantageous to publish a ...jrblasde — 08:07 PM |
PT-6 antenna connections
|
Notice in the schematic the notch at the bottom of the antenna coil is between pins 3 & 4.
Yes, pin 6 of the 7C6 is...RodB — 07:34 PM |
bridkarl
|
I have two PT-6 chassis. On both there is a blue wire that runs from the above chassis transformer to pin 6 of 7C6. So t...bridkarl — 07:23 PM |
Who's Online
|
There are currently 3120 online users. [Complete List] » 1 Member(s) | 3119 Guest(s)
|
|
|
![>](https://philcoradio.com/phorum/images/bootbb/asb/right_arrow.png)
|