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Philco 90 - Printable Version

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Philco 90 - fields 100 - 10-28-2014

I tried to use the set after 2 months of it sitting on the shelf.There is a low hum present with  the volume down and even 1st AF tube out. The radio has been  100% restored  recently. Checked grounds, all seem to be OK.Middle tr-r tap for the 47 tube filament is grounded as well. E-caps do not show any leakage albeit checked with  a regular VOM (still working on my cap. checker). Any other places should I check? Thanks.
P.S. I just read JPH 14 post from 10/05/2014. Does not look relevant to me, as the issue, I believe, is after 1st AF (27) tube. I did try another 47 tube with no improvement. Should I bump up the second E-cap?

http://www.nostalgiaair.org/PagesByModel/881/M0013881.pdf

RE: Philco 90 - Phlogiston - 10-28-2014

If you are saying that it did work when you put it away, then, an intermittent contact is most likely. Check tube sockets, grid caps and the volume control for correct function.

RE: Philco 90 - fields 100 - 10-28-2014

The volume control is before I st AF type 27 tube. As I said, the hum is present with that tube out. 47 tube -2nd AF has no grid cap.

RE: Philco 90 - morzh - 10-28-2014

Fields,

Pull the chassis out, try to flex it a bit, see if this affects it. If yes, look for wires touching chassis, like filament wire maybe.

Try to see if the centertap of the B filament is grounded.
Check grounds.

Also try to see if another radio nearby with the same antenna type also hums. If you have a small tranistor radio, this is a good indicator.

RE: Philco 90 - TV MAN - 10-29-2014

I'd not worry about the e-caps just yet. My 70 has a low hum with the volume turned down, and that is with new caps. If a little flexing around or moving tubes doesnt bring it to life, you may have simply failed a tube. They still go bad. If you have a tube tester, test 'em. Start simple. If they all check ok and it still doesnt come to life, then make a few quick signal checks , then voltage checks to get an idea where the problem may be. 

RE: Philco 90 - morzh - 10-29-2014

Oh one more thing....your transformer may simply buzz. I have this in my 70. There is not much one can do about it short of disassembling, re-shimming, varnishing, clamping the shell (the latter I tried to no avail).

Could it be a failed tube? Possibly it could. Though I hadn't seen a tube acting like it and I would have to understand how a class A single tube amp can produce hum due to the tube's parameters' changing. But...all is worth checking.

One more thing: try to change the 80 tube (and check it). You have full wave rectifier with the center tap, and if one half starts going weak and another is strong that can produce hum due to increased ripple.

RE: Philco 90 - fields 100 - 10-29-2014

(10-29-2014, 04:06 PM)morzh Wrote:  Oh one more thing....your transformer may simply buzz. I have this in my 70. There is not much one can do about it short of disassembling, re-shimming, varnishing, clamping the shell (the latter I tried to no avail).
So, you are referring to the mechanical sound being simply heard from the wooden box or the  one being amplified throughout the AF stage?
I tried the other 80, no improvement. However bumping up the first e-cap did decrease the hum I'd say by 35- 40 %.

RE: Philco 90 - morzh - 10-29-2014

Bumping up the first cap will also increase your B+ voltage. See how much it is and if it is still within specs (the tubes will run hotter still).

Another thing to try could be using a 0.15uF or so cap in parallel with the filter choke (100V-rated will do) that is between the pluses of the 1st and 2nd electrolytic caps. You can variate the value, see if that helps. (seems counter-intuitive but it does help sometimes).

Could it be that when you restored the radio you simply did not hear the hum that was there, since it was (and is) low?
Could you possibly run it via Variac at 110V and now you are running it directly from MAINs at 120V?
There may be many things that were simply overlooked at the time of shelving it which now you pay attention to.

RE: Philco 90 - fields 100 - 10-29-2014

(10-29-2014, 09:08 PM)morzh Wrote:  Bumping up the first cap will also increase your B+ voltage. See how much it is and if it is still within specs (the tubes will run hotter still).

Another thing to try could be using a 0.15uF or so cap in parallel with the filter choke (100V-rated will do) that is between the pluses of the 1st and 2nd electrolytic caps. You can variate the value, see if that helps. (seems counter-intuitive but it does help sometimes).

Could it be that when you restored the radio you simply did not hear the hum that was there, since it was (and is) low?
Could you possibly run it via Variac at 110V and now you are running it directly from MAINs at 120V?
There may be many things that were simply overlooked at the time of shelving it which now you pay attention to.

Sorry, I bumped the one with a neg to the ground, hence it was the #2, and the voltage remained the same. I must agree with a last sentence of yours - when I do alignment, I usually keep the speaker face down on the table next to the chassis to afford connection to the latter. Adding to that, it gave me that particular time so much pain in the back getting it aligned that I  probably did not care much for that hum. All the same, now it is much lower and I think quite  tolerable.

RE: Philco 90 - morzh - 10-29-2014

Yes, #2 bump-up is benign, if it helps, then good.
Still try that small cap across the hoke just for giggles.

RE: Philco 90 - TV MAN - 11-02-2014

Sorry Fields, I hadnt read things clear, thought I had my post deleted. My bad. However, in my 70, I do have a low hum with the volume turned down, and felt this was normal considering the small value e-caps used. Just wanted to clairfy.