Mystery AVC issue on 38-9
Posts: 18
Threads: 6
Joined: Nov 2018
City: Severance
State, Province, Country: Co
Hello all. I can use some help on another mystery problem, this time on a Philco 38-9
http://www.nostalgiaair.org/Resources/262/M0013262.htm
This is a customer radio that will drop the volume after about 10 min playing. Low to moderately strong stations drop out completely, while stronger stations are still at a listenable level, although equally diminished.
Measuring the AVC voltage tuned to a moderately strong station, at the junction of resistor 15 and cap 3, I see that the voltage starts at about -5 VDC then slowly rises to about -.5 VDC, where the radio abruptly loses volume. Turn is off and let if cool, for a few minutes and and cycle repeats.
I have replace all Electrolytics (twice), all other caps (including the two micas ) and most resistors including 43
I have swapped tubes multiple times
The power transformer voltages are stable
The voltages on all tubes are within tolerances
I have spent hours and hours on this one. I would appreciate any suggestions where I might look as to the source of the disappearing AVC voltage.
Thanks Steve
Posts: 1,824
Threads: 114
Joined: Jul 2014
City: Sneedville, TN
You haven't said whether you have replaced the paper condensers, I would start there. I suspect the problem is in #23, or #19a, 19b, or 19c. You also may have a resistor which is changing in value as it heats up, especially #20 or #21. It could also be #28, #31, #34, or #36. First step is to replace ALL the paper condensers if you haven't already. If one is sliding in value as it heats up, it means that ALL of them are not reliable any more. I always replace ALL the paper condensers as well as the electrolytics before I do anything else, even before I turn on a set of this age.
Posts: 1,401
Threads: 70
Joined: Oct 2007
City: Linn Creek, MO
The AVC voltage is not the problem, it is a response to the problem. Something in the front end or the IF is causing the signal to drop, the AVC is trying to compensate. You might try a can of circuit cooler to locate the problem.
Steve
M R Radios C M Tubes
Posts: 18
Threads: 6
Joined: Nov 2018
City: Severance
State, Province, Country: Co
Yes, all the paper capacitors have been replaced. Steve
Posts: 18
Threads: 6
Joined: Nov 2018
City: Severance
State, Province, Country: Co
Thanks Steve, I totally agree. I need to find root cause. I'm not familiar with circuit cooler or how it's used. Can you fill me in.
Posts: 93
Threads: 20
Joined: Aug 2014
City: Greenville, TX
Here is a source:
https://www.govets.com/maintenance-repai...VNEALw_wcB
Do not keep it on one component very long. Quick spray is enough.
Posts: 18
Threads: 6
Joined: Nov 2018
City: Severance
State, Province, Country: Co
Good stuff. Have you ever tried cooling the major components: transformers, variable caps, tubes, etc ?
Posts: 15,527
Threads: 545
Joined: Oct 2011
City: Jackson, NJ
Canned air (duster) when the can is inverted act as a cooler.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Posts: 18
Threads: 6
Joined: Nov 2018
City: Severance
State, Province, Country: Co
It was the 6K7G. As you all know, Philco radios, with the heat shield socket in tact it will only allow 6K7Gs. 6K7s or 6k7GTs wont fit, at least not without some encouragement from Mr. vice grips. I had 3, count 'em, 3 6K7G's test ok, but all would result the exact same symptoms in the radio. Out of desperation i ground off the rivets holding the socket and shield and put in a metal 6K7, ..... Bingo.
Thanks to all for the help. There are alot of great ideas here I can use next time. And one thing is for sure, there will always be a next time.
Steve
Users browsing this thread: 1 Guest(s)
|
Recent Posts
|
Correct model 84 Ac plug an cord type
|
I remember a bakelite "lampshade" (a fancy lampshade) style plug being available in the hardware store when I ...MrFixr55 — 10:39 AM |
Correct model 84 Ac plug an cord type
|
Hello Arran
Yes, I also like the lampshade style plug !
Sincerely Richardradiorich — 08:46 AM |
Restoring Philco 38-14
|
Hello morzh,
Nice explanation on the x and y capacitors and glad you are narrowing down the issue with this set !
Si...radiorich — 08:43 AM |
Correct model 84 Ac plug an cord type
|
From what I read Philco went to using brown rubber power cords in 1936, so cloth was used in the 1935 and earlier models...Arran — 11:25 PM |
Correct model 84 Ac plug an cord type
|
Hi Bruce,
I have not worked on an 84, but in general, power cords for almost any radio prior to 1938 was cloth covered...MrFixr55 — 08:34 PM |
Restoring Philco 38-14
|
OK, well, so after staring at the sch again, I realized the problem had to be the #12/12A padder.
I put a scope on the o...morzh — 08:15 PM |
Correct model 84 Ac plug an cord type
|
Hello Bruce,
I mostly use brown far as my cloth powercords go !
Here is the plug that I have used bakelite Acorn style...radiorich — 08:05 PM |
Correct model 84 Ac plug an cord type
|
Looking for as correct as can find the right Ac plug an the correct colored cloth line. Ive seen many old photos but mos...Bruce — 04:13 PM |
Restoring Philco 38-14
|
Rod
Two Y-caps make sense if you use them from L/N to the chassis; this is only makes sense in the transformer radios w...morzh — 10:11 AM |
Restoring Philco 38-14
|
Mike, I'm curious about a specific use of he Y cap. Lately I've seen a single cap across the power transformer primary. ...RodB — 09:01 AM |
Who's Online
|
There are currently 3428 online users. [Complete List] » 1 Member(s) | 3427 Guest(s)
|
|
|
|