The AM Forum

Ok guys on to the next project... I acquired a NC300 super cosmetics with crystal calibrator. Issue HUM of course its the caps tubes tested good except the 5y3 which I am going to replace with a solid state and I could not test the ballast so I am going to assume I will replace that also. Now question is on the caps what have you guys in general changed out??

John W9JCM

All paper, all electrolytic. I won't turn a radio of that vintage on until I've done that. Transformers and coils are HARD to find.

dont turn it on again until you change the filter caps. never turn on a old radio out of the blue after who knows how many years being off. No quicker way to kill a transformer.

and the radio does not have to hum or buzz to need all new electrolytics. One of the great myths of radio work. Any radio with more than roughly 15~20 years of service time most likely needs all new replacements.

There's a guy on ebay selling new cap kits for classic radios containing every electrolytic cap inside the radio in the correct quantity for an excellent price. I just got 2 of these and will be replacing every single one inside both my Yeasu's:

http://cgi.ebay.com/Yaesu-FT-101E-HF-Transceiver-Capacitor-Replacement-Kit_W0QQitemZ130219827623QQihZ003QQcategoryZ40069QQssPageNameZWDVWQQrdZ1QQcmdZViewItem (http://cgi.ebay.com/Yaesu-FT-101E-HF-Transceiver-Capacitor-Replacement-Kit_W0QQitemZ130219827623QQihZ003QQcategoryZ40069QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)

The value is that you dont have to futz around with looking up all the values, finding vendors, etc. well worth it. Just order a kit and go.

Good luck finding a replacement ballast.  If it works, just use it.  The 4H4C can be directly subbed by a 6V6 in the same socket.

If the rectifier is bad chances are the filter caps are bad also.  Don't neglect heater-cathode shorts, etc., these will cause hum. 

There are a million causes of hum... some of the more esoteric:  hum modulation on the oscillator (replace the osc tube); open vol control; hammy hambone re-wirings (I had an SX28 where some idiot tacked the fil voltage to the grid of one of the 6V6s); bad RF bypass caps.

Once finished replacing or reforming the electrolytics, you may find a tube with excess heater to cathode leakage causing some hum.  Substituting tubes is recommended, tube testers won't always show this type of leakage.

My NC-300 is pretty durable.  I only had to replace a few capacitors (been 13 years, don't remember which and my notes aren't handy) that measured leaky.  I did reform the power supply electrolytics.  From the current school of thought the reformed caps may not stand the test of time, so I may do the can opener thing and stuff replacements in later this summer.  I like the NC-300, big and good looks (to me) even if the performance (AGC in particular took some modifications) is lacking in some respects.  I seem to recall alignment of the 2215 Khz crystal filter somewhat problematic but may re-address that now that I have a spectrum analyzer and tracking generator.

Mark K9TR

Do the filaments in all the tubes light?  If so the ballast tube is fine.  It regulates filament voltage to the oscillator tube and a couple of others.  If you want to check it with an ohm meter just pull the tube and use the ohm meter across the two pins with wires attached (ballast tube).

Chances are the cathode cap in the audio section is bad.  If it hasn't been replaced, then do so and check the resistor it is parallel with.  If the cathode cap has been changed and the ballast tube is open, then that can be where the hum is coming from.  Other places include bad grounds or component placement.

The transformer in that thing is small and gets hot, so check the electrolytics and I strongly suggest you replace them if they aren't modern design.  If the caps are old and do form, they will become leaky in a year or so of regular use.  The more current they leak, the larger the load on the transformer.  Eventually the load will become more than the transformer can handle and it will quit.

Decent receiver but there are some errata notes on it.  One is the removal of L6 and replacement of it with a crystal.  I strongly suggest you perform that mod if yours doesn't have it.  If it does the slug in L6 will not be there.  That is the second conversion oscillator tank circuit and even moisture in the air will cause it to shift frequency affecting calibration and sideband selection.

I had a hum in an NC-300 that drove me nuts. I finally chased it down to a bad tube. I know it was the one that 1/2 amplifies for the S-meter circuit and I think it is also the first audio tube...

Carl /KPD

Thanks guys will do her right. And as far as turning it on I used a variac to bring it up yea not the best either.  ;D  Let you know how its going as time goes here.