Pulling hair over Apex 7A!
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I sure hope someone here knows these radios. Schematic (or sorts) is available on N/A.
When I got the radio, it was DOA, totally shorted input filters. After a lot of fun, I finally replaced literally every paper cap and every composition resistor. The radio now works, sort of.
The problem is, it has NO AGC action whatsoever. I can't connect even the tiniest antenna to it without it overloading on most all stations (and by overloading I mean the front end is totally swamped, cut off completely!)
Add to this, when I turn up the volume, it starts to either warble or motorboat, depending on station strength.
The AGC section of this thing honestly confuses the he!! out of me. (Shoot, those 'power detectors' are sort of confusing, too.. but at least I've figured out how they work...)
So... if anyone has some suggestions, I could sure use them.
TIA
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Does the localizer control have any effect on the overloading?
This pot sets the level of bias on the RF and IF tubes with no signal by setting the resting grid voltage of the 24 AGC tube. The 24 grid is also connected to the cathode of the 27 detector. When a signal is received, the 27 cathode goes more positive, making the 24 plate less positive. The 24 plate is coupled to the grids of the RF and IF tube, so their grids also less positive thereby increasing their negative grid bias. This reduces the gain of the RF and IF which provides the AGC action.
The whole AGC system depends on the correct voltages on all the controlled tubes, so if the voltage is off anywhere in the circuit, it will throw everything off and the set will overload.
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All of those control voltages seem wonky to me. The localizer control changes the cathode voltage on the AVC tube, but very little, perhaps 2-3 volts. The control itself has a voltage swing from around -6 to 50 volts or so on the wiper. The best I've ever had for difference is changing the speed of the warble between a trill and a chirp.
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The voltage on the AVC tube cathode should not change much with the localizer control because it is set by the tapped voltage divider resistor. The slider of the localizer control should only change from around -64 V to about +12 V ( -64 from power trans center tap line and +12 from AVC tube cathode). Are all sections of the tapped resistor good and within tolerance?
The cathode of the 24 AVC tube is grounded through the 290 ohm section of the tapped resistor so there never should be 50 V on the cathode. Cathode should be at about 12 V, so if you have 50 V there something must be wrong with the tube or the tapped resistor.
(This post was last modified: 10-27-2013, 08:25 PM by Mondial.)
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I'll have to check again. The tapped resistor is all perfect. I'll try to get all the voltages and post them later. Should I be taking them from the HT center tap, or the chassis? What few voltages are listed on the schematic are K-P or K-G.
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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.
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Dang! My hair is already smoking!!! And I'm not even working on it!! It's nice to have a rocket scientist on board. They don't even draw circles around the toobs, it took me a little while to find the 24A down it the power supply.
GL guys
Terry
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This is the most I can get:
AVC (24A) K- 3-5V, G1- 0-4V, G2- 6-9V, P- 36V
Det. (27) K- 3-10V, G1- -16, P- 142
RF (24A) K- 36, G1- 35, G2- 215, P-262
If I change the value of the cathode resistor on the detector tube (120K) to a lower value (around 85K), I can start to get a degree of AGC action, but nowhere near enough.
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Something looks funny on the grid of the 24 AVC tube. It is directly connected to the cathode of the 27 detector, so the voltages on the AVC grid and the detector cathode should be identical.
Your voltage list shows AVC G1 at 0 - 4V but det K at 4 - 10 V ???
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Yeah... I know. That's one of the reasons I'm pulling my hair out. It's very difficult to trace anything in this set, it has those really nice bundled wires with the string gussets. And far too many of those are the same color or faded enough that they look the same. I don't really want to undo those gussets, but I may have to. So much that doesn't make sense in this set, but I will keep at it.
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Brenda, what are the voltages across each section of the 4300 ohm tapped resistor?
This resistor supplies B+ to the main voltage divider and the RF and IF cathodes. 35 V on the cathodes of the RF and IF seem very low
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Terry
I'll be honest, once I looked at the sch without the circles, I relused myself from the further dealin with it - I starte3d having a headache.
I hate badly drawn schematics: not only you have to figure how it works but you also have to interpret some extremely bad drawing habits; it's like hacking a piece of spaghetti code that is not commented at all as well; sorry Brenda, my head hurts enough from work 
PS: you know, there is something to be said about the engineering documentation discipline. In the old USSR we had a thing called "ГОСТ" - transliterated as GOST (sounds almost like "ghost"). Stands for "state mandated standard". It actually dictated exactly how every type of part was to be drawn. Heck, I never had problems reading those schematics. Even those that were 70 years old.
(This post was last modified: 10-28-2013, 08:53 PM by morzh.)
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That standardized things after a while here too, particularly after the Radio Manufacturer's Association was formed, it's now known as the EIA. After the RMA came into being a lot of the whacky and non standardized schematic drafting came to an end, along with oddball capacitor and resistance values. The difference is that the RMA was the private radio industry coming together to agree to some standards rather then by government mandate. The automotive industry had a very similar organization called the Society of Automotive Engineers, or SAE, they are part of the reason why we have one standard of machine screw threads. Either way works I suppose but it shows that mutual interests can often accomplish the same goal. I'm not surprised about hair pulling over an early 30s set, they used some rather whacky ideas at times.
Regards
Arran
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WELL NOW....
In the process of taking a bunch more voltages in order to get more troubleshooting help, I discovered something that I had missed: the 3200 ohm section of the OTHER Candohm was open! I had not caught this previously, since there was voltage at that point, and since there were no voltages for several points in the power supply train, I just assumed it must be the right voltage. It was ~35 volts.
I decided that I would try to figure out what was supposed to be at that point, and so took a reading on that section of the Candohm. It measured into the 100's of thousands of ohms. AHA! I went and got a 3300 ohm 3 watt resistor and subbed it across. I fired up the radio and VOILA! It's a radio! Great AGC action, even better gain than before. In fact, it's pretty amazing in the stations it can receive now that it's working and hooked up to my longwire.
I want to thank you all for the help you've given me on this set. I'm very happy that I've been able to make it work with all it's original values back in place. Now I just need to find a nice matching set of knobs and finish repairing the cabinet.
(This post was last modified: 10-29-2013, 08:18 AM by BrendaAnnD.)
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Congrats! Now you won't have to disturb all that lacing cord. 
Chuck
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