[Guest]

This is my first of what I hope will be many posts on the board.

I 've been working on a Philco 90 mid-model (single 47 tube type) for a few days now. I'm still green when it comes to restoration (I had the good fortune of having two agreeable projects in the past, a Crosley tombstone and an Airliner floor model that only required recapping, a bit of tuning, a resistor and a couple of replacement tubes).

The 90 didn't work when I got it, but after replacing the cord, rebuilding all the Bakelite blocks (that really cleaned up the squealing and the signal!), replacing the bad volume control, replacing a few rotten wires and tweaking the tuning, I've got it working. Signal is strong and I'm picking up all my stations. My problem is, looks like someone tried their hand at replacing the electrolytics at one point. There were two replacement 10mfd caps in the place of the old cans. I look at the parts list for the unit and it shows two cans, each with a 10mfd and a 6 mfd electrolytic in them. Am I safe to leave these two 10 mfd caps in? How were those cans wired originally? Where does the 6mfd caps in the parts list come in?

And a second question: there is a slight 60 cycle hum to the radio too. All caps have been replaced with the exception of the tone control. Should these be replaced? Also, do the power transformers have any caps in them?

These are probably rookie questions to you guys, and I'm sorry for asking so many but any help would be appreciated.

[rghines1]

The 6 uF cap was used for 60 Hz line frequency, 10 uF for 25-40 Hz power. Leave the original cans in the the radio for looks. Electrically they must be disconnected as they are likely leaking and would damage the power transformer and rectifier in short order.

A slight hum is normal. Rebuild the tone control too. Check the Tech Section here or philcorepairbench.com for the cap values.

No caps inside the power transformer. Caps you are seeing on the schematic should be inside a bakelite block. Rebuild this block too. Congrats on getting it working.

Richard

[Ron Ramirez]

Hi and welcome!

While the original electrolytics were 6 uF, this is no longer a standard value. You may use 10 uF, 450V electrolytics to replace the 6 uF units with no problems. Disconnect the original cans as Richard stated.

Be sure you watch polarity on these! If you look at the schematic on my website:
http://www.philcoradio.com/tech/images/90b.jpg
you will see that I have marked the positive ends for you.

In addition, note that while the negative lead of the output electrolytic (57) goes to chassis ground, the negative lead of the input electrolytic (56) goes to B-, not chassis ground. If you ground this lead, you will definitely have hum issues.

I suggest that a barely perceptible hum is acceptable; anything more indicates a problem.

[Guest]

Thank you for your quick responses. I appreciate it. I guess I should have been more clear. The original cans are gone (no longer on the radio) and in their place are two electrolytics, each at 10 mfd. These are the only electrolytics on the radio, put there by a previous owner. The negatives of each are soldered to the BC resistor: one at the far left of the resistor and one with three wires on it, soldered to the terminal to the right of that. Were these the proper termination points for the original electrolytics? If not, where were they?

[Texasrocker]

"humm"?... if I read your original post correctly francocleef, seems that you replaced all the other by-pass type condensers (caps-bakelight blocks) but didnt replace the "previously replaced" eclectrolytics? If so, those (electrolytics) were most likely subbed in place of the orig "missing orig electrolytics" several yrs ago also? Most modern type electrolytics are very small in comparison to the originals. Since you didnt replace those, why not try replacing them also?, ( following Rons correct polarity instructions)!! New 10 ufs @ 450 volts may be all you need to finish a full re-capping job? If the electrolytics you left in place are also older types, most likely they are bad also. Doesnt cost much to go ahead and replace those few electrolytics caps also to see if it rids the hum? Just a idea. Perhaps I read all this wrong? If so,please excuse me, as Im getting to be a old-f*rt these days!

[Guest]

Thanks to all of you for the feedback. Those two 10mfd electrrolytics are now replaced with brand new ones. Still getting the slight hum so i'll have to dig deeper. I've been studying the Radio-tician Radio Repair Course on Nostalgia Air and it looks to be just what a rookie like me needs. Time to hit the books!

[radiomikee]

The negatives of the two main electrolytics go to the same point, the point where the high voltage center tap meets the multitap resistor. If you don't have them wired this way ( that is one negative on the chassis), you will have a hum.