Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Closed
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Pulling hair over Apex 7A!
#6

Yes, that would be very helpful if you could post the voltage readings of each of the taps on the voltage divider resistor and the pins of the AVC tube. Make all measurements referenced to chassis ground.

This is a difficult and confusing set to troubleshoot because of the floating voltage divider supplies. RF and IF tube cathodes are floating at about +90V, and are actually directly connected to the converter tube's screen grid whose cathode is referenced to ground.

So if the RF and IF cathodes are at +90V, the control grids will have to be at about +87 to give an effective negative bias of -3 V for full gain. On a strong signal, the control grids will be maybe at +75 V for a effective AVC bias of -15 V.

The AVC tube supplies the variable +87 v to + 75 V grid bias from its plate circuit, controlled by its grid voltage obtained from the detector cathode. The localizer control should set the plate voltage to +87 (3V less than RF and IF cathodes) with no signal so the AVC feedback can then reduce the plate voltage further as the received signal increases.

This is actually DC amplified AVC and probably keeps the audio level very stable with varying signals, but its very complicated and finicky as to adjustment. RCA used a similar circuit in its first AVC equipped radios, but you can see why it never became popular.


Messages In This Thread
Pulling hair over Apex 7A! - by BrendaAnnD - 10-27-2013, 02:53 PM
RE: Pulling hair over Apex 7A! - by Mondial - 10-27-2013, 06:26 PM
RE: Pulling hair over Apex 7A! - by BrendaAnnD - 10-27-2013, 08:33 PM
RE: Pulling hair over Apex 7A! - by Mondial - 10-27-2013, 09:10 PM
RE: Pulling hair over Apex 7A! - by BrendaAnnD - 10-27-2013, 10:03 PM
RE: Pulling hair over Apex 7A! - by Mondial - 10-27-2013, 10:56 PM
RE: Pulling hair over Apex 7A! - by Radioroslyn - 10-27-2013, 11:38 PM
RE: Pulling hair over Apex 7A! - by BrendaAnnD - 10-28-2013, 08:45 AM
RE: Pulling hair over Apex 7A! - by Mondial - 10-28-2013, 09:12 AM
RE: Pulling hair over Apex 7A! - by BrendaAnnD - 10-28-2013, 04:40 PM
RE: Pulling hair over Apex 7A! - by Mondial - 10-28-2013, 04:48 PM
RE: Pulling hair over Apex 7A! - by morzh - 10-28-2013, 09:52 PM
RE: Pulling hair over Apex 7A! - by Arran - 10-29-2013, 02:46 AM
RE: Pulling hair over Apex 7A! - by BrendaAnnD - 10-29-2013, 09:17 AM
RE: Pulling hair over Apex 7A! - by Chuck Schwark - 10-29-2013, 09:23 AM
RE: Pulling hair over Apex 7A! - by Eliot Ness - 10-29-2013, 09:41 AM
RE: Pulling hair over Apex 7A! - by codefox1 - 10-30-2013, 07:41 AM
RE: Pulling hair over Apex 7A! - by Gene Pederson - 11-01-2013, 04:08 PM
RE: Pulling hair over Apex 7A! - by Arran - 11-01-2013, 10:27 PM



Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Philco Battery-WWII vintage
Hello Bob, What Amazing find! Sincerely Richardradiorich — 10:19 PM
Testing a speaker and output transformer Trutone A2-G
Hi Murf, Do this: Monitor between ground and one plate of the rectifier while playing the radio.  Do the same with ...MrFixr55 — 09:51 PM
Philco Battery-WWII vintage
Mike; I take it that Soviet cars did not come equipped with block heaters, hence the hot oil change? Where I lived in...Arran — 08:42 PM
Philco 42-345 Restoration/Repair
The more the better. (Within reasonable limits.) 2-5 times more is no problem in this case.Vlad95 — 03:50 PM
Philco 42-345 Restoration/Repair
Thanks, RodB and Vlad95, I was mainly trying to figure out this capacitor to locate a replacement, Vlad thank you for...osanders0311 — 03:44 PM
Testing a speaker and output transformer Trutone A2-G
Would an output transformer if weak, cause the problems I am seeing with lower B+ voltage? murfmurf — 01:22 PM
Testing a speaker and output transformer Trutone A2-G
Hello murf! I merged the threads. Please do not start new threads regarding the same radio. Take care, - GaryGarySP — 10:21 PM
Philco 42-345 Restoration/Repair
Regarding picture 5 on page 1 of this thread this capacitor is #40 on scematics diagram - 4mF+4mF. Black wire is "-...Vlad95 — 09:01 PM
Philco 42-345 Restoration/Repair
That's a dual 4 MFD, red and green are both 4mfd and black is common negative. You should be able to see them in the pow...RodB — 08:58 PM
Philco 42-345 Restoration/Repair
Can anyone help me figure out a capacitor that has 3 wires coming out of it red and green on one side and a black wire o...osanders0311 — 08:14 PM

[-]
Who's Online
There are currently no members online.

>