Posts: 825
Threads: 113
Joined: Aug 2012
City: The Villages
State, Province, Country: Florida
Hi Guys,
Here's my latest project Zenith Model G725 . When I got it all it did was have a loud hum at any volume. I replaced all the electrolytics and wax caps. Replaced any resistor out of specs. I tested all the tubes and they check ok.
So, now, with the volume turned all the way down I get a loud hum. Once I move the control any amount, the hum goes away and the radio sounds great-AM and FM.
Any thoughts on what might be causing this?
Thanks,
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
Posts: 4,109
Threads: 311
Joined: Nov 2013
City: Kings Park NY
I am far from a chassis guy so I wont give any advice but I like to respond to posts I read.
Kirk
Times I have been electrocuted in 2021
As of 1/01/2021
AC: 4 DC: 1
Last year: 6
Posts: 13,776
Threads: 580
Joined: Sep 2005
City: Ferdinand
State, Province, Country: Indiana
Heater-cathode short in a tube? That's the first thing that comes to my mind...
Also make sure all leads are properly soldered to the volume control; if the low end of the control comes unsoldered from ground or B-, it could also produce this effect.
--
Ron Ramirez
Ferdinand IN
Posts: 825
Threads: 113
Joined: Aug 2012
City: The Villages
State, Province, Country: Florida
Thanks Ron,
I swapped each tube with known good ones, one at a time. I resoldered each terminal of the vol control. Could it be the vol control itself?
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
Posts: 13,776
Threads: 580
Joined: Sep 2005
City: Ferdinand
State, Province, Country: Indiana
Could be...measure across the two outer terminals and see if you get anything close to the rated resistance. If not, then you have a problem.
Preferably using an analog VOM, measure between the center terminal and one of the outer terminals. Holding the test leads in place (alligator clips come in handy here), slowly turn the control from one end to the other. If the resistance does not change smoothly but jumps open here and there...then you have a problem.
--
Ron Ramirez
Ferdinand IN
Posts: 2,023
Threads: 365
Joined: Jun 2010
City: Dover, OH
This might sound silly, but did you clean the controls? Sometimes a dirty control pot or even tube socket can cause hum as well.
No matter where you go, there you are.
Posts: 825
Threads: 113
Joined: Aug 2012
City: The Villages
State, Province, Country: Florida
No problem with any resistance readings, Ron. Jayce, I wish it were that simple. That was done while replacing all of the components.
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
Posts: 15,444
Threads: 539
Joined: Oct 2011
City: Jackson, NJ
The box of the pot is groundedvi a a cap.....have you cheked it?
Also, the quality of the speaker voice coil grounding, look at it.
(This post was last modified: 12-26-2014, 12:02 AM by morzh.)
Posts: 825
Threads: 113
Joined: Aug 2012
City: The Villages
State, Province, Country: Florida
The cap checks good, Mike. Voice coil ground is ok.
Now, for the good news. There's a 1k resistor that goes from the low terminal of the volume control to ground. The 1k resistor was right on. I jumped it with another resistor and the hum went away. After some experimenting I settled on a 470 ohm resistor to replace the 1k. I'm sure someone can tell me why it works, but it sounds great now!
Thanks everyone for your input. I hope you all had a good Christmas and looking forward to a healthy and happy new year.
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
Posts: 15,444
Threads: 539
Joined: Oct 2011
City: Jackson, NJ
Could be an inductive wirewound resistor you had before that picked up noise.....which then could mean that if you re-oriented it the noise could've been attenuated. A wild guess.....
Posts: 825
Threads: 113
Joined: Aug 2012
City: The Villages
State, Province, Country: Florida
Nope. Carbon resistor.
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
Users browsing this thread: 1 Guest(s)
|
|
Recent Posts
|
|
Cannot Adjust an IF Transformer’s cores
|
| Joseph, now you are an engineer!!RodB — 06:37 PM |
|
PT-6 antenna connections
|
| Hi, reading your explanation I can see the confusion. If you take it one connection at a time it should result in a desi...RodB — 06:34 PM |
|
Cannot Adjust an IF Transformer’s cores
|
| It works! I received the PCBs this morning in the mail, and put everything together. I reused the base from the original...jrblasde — 04:32 PM |
|
PT-6 antenna connections
|
| I seem to have lost my way on this PT-6.
I see the antenna aerial transformer schematic - I’m reading pins 1 through ...bridkarl — 12:30 PM |
|
40-150 General questions
|
| Good thing you're a good electrician ... the pictures show a lot of rewiring in your future! Take care and BE HEALTHY!...GarySP — 11:52 AM |
|
40-150 General questions
|
| I think RobB has it right. It is a part number and looks like its stamped on the chassis and that wasn't something you ...klondike98 — 11:41 AM |
|
40-150 General questions
|
| It's most likely a part number. Most of Philco part numbers are in that format. I looked at the parts list for this mode...RodB — 08:10 AM |
|
40-150 General questions
|
| Hi Kevin,
No, I don't but I like your Quote about being a good electrician. The way I look at it is "The fact ...MrFixr55 — 08:06 AM |
|
Philco 38-7 Speaker
|
| 7K should be fine. While not a perfect match (This transformer is meant for 6V6 tubes), Antique Electronic Supply (AES)...MrFixr55 — 08:03 AM |
|
40-150 General questions
|
| I discovered a stamp on the underside of the breadboard on my Philco 40-150. It is"56-1417' in black ink with numbe...Kevin_D — 07:14 AM |
|
Who's Online
|
There are currently 2701 online users. [Complete List]
» 1 Member(s) | 2700 Guest(s)
|
|
|
|
 
|
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
