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Philco 48-200 volume weakens with time - Printable Version

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Philco 48-200 volume weakens with time - winkydink - 05-02-2017

Hello,

I have a Philco 48-200 radio which has been recapped a few years ago.  When I initially turn it on the radio plays like a champ, strong volume clear voices or music.  After several minutes the volume grows weaker and weaker, to the point where I almost have it at max volume after 10 to 15 minutes in order to have sufficient sound.  When it is in this state the sound quality becomes a little muddled.

Is this a tube problem ?  (i.e. one of the tubes growing weaker as it warms up ?) or is this something else.  I haven't checked the tubes yet, but was hoping to get some guidance from the Phorum members.

Thanks in advance.

RE: Philco 48-200 volume weakens with time - morzh - 05-02-2017

Check output tube, rectifier tube and resistors R101 and R102.
If could, measure the plate voltages, especially the output one.

RE: Philco 48-200 volume weakens with time - winkydink - 05-02-2017

Well I am starting at the real basics here.  I have done one re-cap myself with some guidance and the other radios I farmed out to have them done properly....so.... I'm not sure which tubes would be the output and which one is the rectifier.

I will consult the schematic for this radio and also do some research into SAFELY checking the resistance of a resistor.

I'm not totally inexperienced, but have a very healthy respect for electricity, so I will go slow here and ask a bunch of questions along the way.

RE: Philco 48-200 volume weakens with time - winkydink - 05-03-2017

OK, so I have found a schematic for the radio (Photofax) and have located the rectifier tube (either a 35Z5GT or a 35Y4), and the output tube ("Power Output" in the documentation - 50A5 or 50L6GT).  I'm assuming that since my problem is occurring after running the radio for a few minutes that I need to test the tube when it is "warm".

What would the best way to do this ?  Run the radio outside the case for 5-10 minutes.  Unplug, remove tube and test, or run the test on the cold tube and essentially have it run in the tube tester for 5-10 minutes.  The later will require me to hold the test button down on the tester for that length of time.  Neither is a big deal, but just asking in case one make more sense than the other.

Also if I need to go so far as checking the resistors R101 and R102 what exactly do I need to do ?  Am I just checking the resistance ?  Can I do this while they are still soldered to the chassis or do I have to remove them ?  Also in my document there is nothing that shows R101, R102 so I am not sure which resistors you are referring to.

Sorry for the rather basic questions, I am inexperienced at doing this type of work, but would like to learn (safely of course)

RE: Philco 48-200 volume weakens with time - morzh - 05-03-2017

They renumbered resistors in your sch. I went by the Riders. They also show 50L6 i/o 50A5 output.

Anyways.

The easiest way is to simply change the tube.
If tested, warm it up till hot and then failing and immediately transfer it in the pre-warmed and pre-set for this tube tester, and try to test immediately, see how it tests. Also test for gas if possible.

As for the 220 and 1200 ohm resistors, test them with a meter in-ciruit, then turn on and measure voltages while playing. Then measure them when it fails. See if any of the three voltages (on the three electrolytic caps' plusses) droop.

RE: Philco 48-200 volume weakens with time - winkydink - 05-04-2017

So I still need to unearth my tube tester, but figured that I would try to swap out the tubes and see what the results were.

The radio has the following tubes presently in it.

Rectifier:  35Z5GT  (thick pins)
Output:   35A5  (thick pins)

The schematic I have says to use a 50A5 (Morzh says to use a 50L6) for the output tube.  Question: is a 35A5 equivalent ?  I have another 35A5 I could substitute in, would that be OK.  If I purchase a new tube, which should I get the 50L6 or 50A5 ?  Any suggestions for suppliers, you can PM me with that info if the BB does not allow suggestions in the posts.

On the 35Z5GT rectifier tube, I don't have another one of those (I do have a 35Y4 but it has skinny pins instead of thick pins and does not lock into the tube receptacle ).  I will need to purchase this.

I know I still need to test the old tubes, but from a time and space constraints point of view it's going to be easier (and faster) to replace the tubes, then to try and dig out the boxed tube tester and make sure that, it is still in working order.

RE: Philco 48-200 volume weakens with time - morzh - 05-04-2017

They are not equivalent: the first number stands for the heater voltage so use what was there before.

But given this many variations in the sch I would try first to figure out what exact version I have and what tube should be there, just to be sure.

RE: Philco 48-200 volume weakens with time - winkydink - 05-09-2017

So I am back to square one.  But learning (or re-learning) new things with each false assumption that I make.

So it seems that schematic I have for the radio is the wrong one.  My model is a Philco 48-200 code 122, the schematic I had was for a code 121.

The other thing that I have now learned is the difference between an Octal tube and a LOCTAL tube (Thicker pins for the former, thinner pins and locking mechanism for the later).

The output tube in my radio is a 35A5, which has thicker pins (Octal).  The schematic for a code 121 (not mine) shows a 50A5.  This will definetly not fit since this is a loctal tube.

Question for the Phorum:  Does any one have a schematic for the code 122 version of my 48-200 ?  Can anyone tell me what the proper tubes would be for this radio.

RE: Philco 48-200 volume weakens with time - KCMike - 05-09-2017

There might not be a stand alone schematic for the code 122. You could check with Chuck Schwark about that.
You have a photofact copy of the schematic you are using that is 121 code.
Here is some info that might help you if you don't already have it.
http://www.nostalgiaair.org/pagesbymodel/627/M0013627.pdf

RE: Philco 48-200 volume weakens with time - winkydink - 05-19-2017

I'm just posting a followup so that this thread can be properly closed.

I was confused about the tube and pin types.

I replaced the output tube with a 50A5 (loctal)
I replaced the rectifier tube with a 35Z5GT (octal)

The radio works great now.  Strong sound, no weakening of volume or distortion with time.


Thanks to everyone for their help with this.

RE: Philco 48-200 volume weakens with time - gregb - 05-19-2017

A 35L6GT is not a rectifier tube, it is an audio output tube. If you need a octal rectifier tube it should most likely be a 35Z5. Is there not a tube placement chart on the cabinet someplace showing what tubes go where?

Gregb

RE: Philco 48-200 volume weakens with time - winkydink - 05-19-2017

Gregb

I made a mistake (again). It was a 35Z5GT. I corrected the post above as well.

The tube chart on the radio is all torn up, so not of much use. I found the attachment above by KCMike to be the most useful.

RE: Philco 48-200 volume weakens with time - codefox1 - 05-20-2017

Fora series set, the filaments should add up to 115 or 120 volts for today's service in theU.S.. So long as the pinout and tyoe is the same,a little juggling is usually OK; a string that totaled 130 volts would most likely be fine, but a string much below 115 volts is not recommended. Most tube types are still widely availavle, so usually switching sockets (loctal to octal for example) is not needed. I do save good loctal sockets from junkers "just in case."

If the tube sub doesn't clear up the problem it well may be a carbon resistor that goes way up in value as the set warms up. Many now routinely replace carbon resistors whilst replacing paper capacitors; easy and inexpensive;l less troubleshooting later on.