[MrFixr55]

Hi Rich,
The tap has to do with "loudness contour" and should not affect AVC. Loudness contour has to do with the effect of the tone control circuits at lower volumes. The concept is that people, even with normal hearing have a peak sensitivity at the middle range of the hearing spectrum, therefore low and high frequencies may be attenuated. How important this is in a radio with a simple tone control and receiving the current AM programming is certainly up for discussion. If one was listening to classical music on FM, records, tape or a digital source, through a true high fidelity amplifier with a flat response and very low distortion, through a very clean set of speakers with a flat frequency response throughout the spectrum, then loudness contour becomes more important.

The AVC and detected AF signal come from 2 different sources in this radio. The AVC is rectified IF, like the detected signal but a different path. Even when AVC and audio come from the same diode, there are different paths, both isolated by RC networks. Looking at the volume control, one notices a capacitor coupling the detected signal to the volume control, passing only audio frequencies. A resistor separates the AVC from the detected signals when they come from the same diode. Both AVC and detected signals before the coupling capacitor can be considered to be pulsating DC, negative in reference to the chassis. A much larger capacitor between the AVC line and ground "filters" the AVC signal to flat DC at a negative potential to the chassis. The stronger the received signal, the more negative AVC, the more negative the bias on the IF and antenna stage, the less gain in these stages. This is one reason that the AVC cap has to be good, otherwise, there will be motorboating.

[RichG]

Hi MrFixr55,
Thank you for clearing up my confusion about the AVC line and the volume control tap. I knew there was an RC network that filtered the AVC signal. I did replace both the filter capacitor and resistor in the AVC line so I think that part is okay. But something is not working properly as the AVC doesn't seem to be working as it should. I did go over the band switch to make sure it was clean. But now that I did that, I noted that this switch has been worked on as some of the solder joints looked new. Since I did find one wire from the oscillator coil just hanging loose, I'm wondering if everything else was reconnected properly. So I will be going over that switch carefully, especially the A wafer which controls the oscillator coil.
Thanks again for your help and insights!!
Rich

[RichG]

Hi all,
Well, I've got the compensator issues fixed. There were a couple more wiring issues on the A wafer. Turns out the BC band 1500KHz oscillator coil had no compensator connected across it. That compensator was connected in parallel to the BC band 580 KHz oscillator compensator. So that explains why neither of those did not work at all. In addition, I found that the BC band 1500 KHz compensator was not connected to anything. It had a wire connected to it, but the wire was cut off and not connected. Though that compensator did have a gimmick cap attached, not sure why. I connected the compensator to the C wafer and removed the gimmick cap. Found what might have been a cold solder joint on the SW band oscillator 12 Mhz compensator. Reflowed the joint, problem solved. So, now all the compensators work and I was able to calibrate the tuning so that the frequency generator tone was heard on the proper frequency. The SW and police bands were waaaay off, almost 2 Mhz on the SW band. So now the tuning tracking seems to work on all the bands. At least the AM band is correct. One of my local stations on AM 880 is now 880 on the dial, used to be over 900. I'll check this evening and see if WWV comes in on 10 MHz. Prior to this it came in on close to 12 MHz.
Still not quite sure about the AVC line, but haven't found anything amiss. I think a couple of local AM stations (really close and 50,000 watts) just overwhelm the AVC. The AVC seems to work better with stations that are not so close. Tuning across the AM band, the volume is controlled reasonably well with those stations that are not 5 miles away and pumping out 50,000 watts.
I think I'm calling the radio part done. Now I should work on the cabinet. Or maybe possibly move on to my Sky Rover. But I did just acquire a really nice Zenith chairside. But that's not for posting in the Philco Phorum!
Rich

[MrFixr55]

Congrats Rich!   BTW, when you get to your Zenith, we can still help.  This may be the Philco Phorum, but when you look at the main screen of the Phorum, you will see the titles of all of our discussion forums (oops, Phorums).  Scroll down and under "Various and Sundry Categories" you will find "Other Radios" (Non-Philco).  Use that forum to post questions on your Zenith and don't worry, this may be the Philco Phorum but many of us own other radios and Zenith is very popular.  Of my more than 100 radios, TVs, stereo and audio units, only 6 are Philco.  Our founder, the late Ron Ramirez had quite a few other non-Philco radios and was a big fan of vintage Fisher HiFi.  (Who wouldn't be!)  In addition, check out "Old Ron's Bar and Grill".  C'mon in, belly up to the bar, have a virtual brew and shoot the breeze with the gang!

Unfortunately, my 41-255 is not on display right now so I can't play it.  I do have one local station which is strong in my area and even overwhelms my car radio at times.  Your best bet is comparing the performance to your other radios.  

There are not 1 but 2 AVC circuits in this set.  Refer to the schematic in the Library.  These 2 lines are labeled "C" and "D". One diode in the detector tube provides AVC for the 1st IF Amp on Line "C".  The 2 diodes of the 1st AF Amp tube provide the AVC Line "D" that controls the Mixer (1st detector) tube.  It may be possible to measure the AVC when tuning between stations.  The stronger the station, the more negative the signal should be.  Whichever AVC line does not get more negative on stronger stations is causing this issue.

Check resistors 5, 33, 46, 47 and 61, caps 6, 29 and their associated wiring. 

If the problem is with AVC Line C, consider replacing the 100 pF cap #37D, which applies RF to the AVC Diode portion of the 7A6 detector tube.  The problem is that this cap is part of the "3rd IF" transformer (the transformer between the 2nd IF and Detector.  Also look at the tube.  It appears that both diodes share the same cathode.  If this is so, then the tube itself may not be an issue but the connection between the pin and socket can be.

[RichG]

Hi MrFixr55!
Yep, I figured out that there were two AVC lines...finally. Every single resistor (except for 61) and capacitor (except for the mica caps) has been replaced. So, the AVC line filters should be good to go. The time constant on those filters is pretty small (~.1 second) so the AVC response should be fairly quick. Now, I did note that the schematic shows that on AVC line C the filter capacitor is connected after the resistor and this is the way it was wired initially. The schematic shows the filter capacitor on AVC line D is connected before the resistor. I thought this was weird, but I replaced the resistor and capacitor as they were initially wired. It's possible that this may be the issue. I probably should revisit that wiring to see how that AVC filter is physically wired. I think the capacitor should be after the resistor. Or is there something about the AVC D line filtering that is different?
I did check resistor 61 and it is well within tolerance. I did a general check on the negative bias on the AVC lines while tuned to a dead area on the band and it looks okay. But I did not check it while tuning the very strong stations (probably should have). Maybe for another day! I did clean up all of the tube pins and socket contacts so I think we're good there. I did note that there is one 47K resistor that is part of the 3rd IF xformer. Unfortunately, I did not check the value to make sure it was still okay.
I did a quick check with my ZTO, recently restored, and run into the same issue with the strong stations though it's not nearly as bad. So, I may do well checking the AVC bias on both lines. However, checking voltages in the chassis is a bit of a pain. The radio has to be connected to the internal antenna. When I extend the antenna connections, I notice a marked change just moving my hands around and keeping them short, there's not a lot of room to flip the chassis on its side.
I did some searching around the Phorum and noted that there is a section non Philco radios. I will try to post some info when I start the Zenith 12h650. I have my Sky Rover (made by Belmont) out of the case. So, I may post some info on that as well. I have not yet checked on Old Ron's Bar and Grill!
Thanks so much for your help and guidance!
Rich

[MrFixr55]

A 12H 650 and a TO? Which one? I'm envious. You have a great eye, lucky as heck or you have some money and an understanding spouse, all of which I lack. I do have an 8Goo5, first of the post war TOs. Seems that the newer ones are more desirable, but it seems that 1L6 convertor tubes used in the newer models are getting pricy. Mine does use relatively common 1LC6 or 1LA6 convertor and IMHO has a closer relationship to the Clipper / Bomber pre-WWII version.

The only reason that they have put a cap before the resistor on Line D is because the AVC Diode does not serve as the detector. Yes, the resistor before the cap does cause a time constant, but since Physics II in college was 50 years ago, don't ask me to calculate. It is also interesting that there seems to be a rather fixed bias there also, from R61. Note that the center tap of the HV supply is not B- in these sets, chassis ground is. Since the HV PS center tap is more negative (by about 65V, thanks to R61, this is almost akin to C-, and since this is tied to AVC line D through 470KOhm and to Line C through 1.47Megohm. there is a pretty good negative bias on these stages already. I have not seen AVC like this before.

[RichG]

The ZTO is a 6T40Z chassis model 600. I think 1955 vintage. And yes, I was lucky to get it at a good price along with 3 big boxes of tubes that the seller just wanted gone. It was a good thing because I found a couple of replacement tubes for the ZTO. The Tolex was in pretty good shape, just a good cleaning and a spruce up with Meltonian creme polish. The shoe polish has a pleasant odor that doesn't linger. Mine uses the unobtainium 1L6 tube and it was dead. I did find a guy that replaces that tube with a solid state tube of his design. Somewhat similar to the old Fetrons that Heathkit used to sell as replacements for their VTVMs. I think my 1L6 replacement used to be a 12AU7. He cuts the tube off at the base, cleans out all the guts and replaces them with small surface mount PC board and then glues the tube back together. The solid state tube works great.
I know what you mean by the very different AVC lines. I scoured some books on various AVC schemes but did not run across this particular method. Yes, I did know that the center tap is not B-. I've run across similar negative biasing techniques like this one.

Rich

[Vlad95]

MrFixr55, RichG
You may be surprised, but instead of the rare and expensive 1L6 tube, all Trancoceanic models can use a regular 1R5 tube.
This is really interesting, since the internal architecture of these tubes is very different: 1L6 is a pentagrid, 1R5 is a heptode with different grid pinouts and their functional purpose.
But in fact, instead of 1L6, 1R5 works correctly. Perhaps there is one limitation - decreased sensitivity in the upper part of the SW range, at frequencies above 9-12 MHz.
But this is not critical, since this range is now almost empty.

[MrFixr55]

Vlad, I don't know if anyone else in the Phorum knows this! GREAT INFO! Without rewiring?

The 8G005 has a similar issue. If you can't get a 1LC6, you can use a 1LA6. They are both pentagrid convertors, but I presume that the 1LC6 has better high frequency performance.

A lot of shortwave is dead because BBC, Vatican Radio, etc., no longer beam toward the USA and Canada as they can be heard on the internet. Vatican Radio concentrates on Africa and other developing areas. I assume that BBC and others do the same.

[Vlad95]

Absolutely nothing to do! Just insert 1R5 into the socket instead of 1L6 and enjoy. It's hard to believe, but it's true. Tested on all my Transoceanics.
Loctal heptodes are easier to change. All of them - 1LA6, 1LB6, 1LC6 - have the same architecture and pinout. Only some parameters differ. But I think they can all be changed without problems.

[RodB]

When you look at the difference in price, then it's easy to believe. A few years ago I restored a TO and checked out the price of replacing the tubes. I ordered all but the 1L6. Thats when I discovered the 1R5 could be subed in. Turned out the one in the set was good.

[Arran]

I think it depends on the particular front end of the radio but in most cases a 1R5 will work as a substitute, though some suggest that two of the pins need to be rewired, or a pin cut off in certain cases. Brand Z had a number of single band sets that also used used the infamous 1L6, my guess is that they did so to achieve some economy of scale since they were the ones who contracted with Sylvania to manufacture the 1L6 to start with.
The story I read is that Brand Z spent a lot of time, and money to develop the pushbutton activated front ends for the T.O sets in the mid 1940s, and they designed it to operate with a 1LA6, by 1949 seven pin miniatures (finger lamps to our former Soviet friends) were the tubes everyone was using. So rather then redesigning the T.O front end from scratch, Brand Z approached Sylvania and had them make a seven pin mini tube with similar interelectrode characteristics as the 1LA6, and so the 1L6 was born.
I have a G-500 T.O and the chassis looks like it was originally punched out to accept octal/loctal based sockets as all of the socket holes are 1-1/8'' rather then 1/2-5/8 of an inch. I honestly can't see why they bothered, even on the 600 series, they didn't make the chassis, or the set any smaller, it's still a chunky suitcase sized thing, and was until the quit making them in 1961.
Regards
Arran

[RichG]

Hi all,
Wow, wish I knew that the 1L6 could be replaced that easily. I did replace one 1U4 with a 1L4 which requires realignment due to slightly different interelectrode capacitance. Something I was going to do anyway so it was a non issue. I will keep the 1R5 in mind just in case I need a replacement for the 1L6.

Rich

[bridkarl]

I was curious how you handled the line cap? These generally seem to use something like an .04 mfd cap.
What is best for replacing with a safety cap - an .01 Y cap?? Or as close as possible to
.04??

I’ve done two 41-255 and I think that’s what I did but I see people using lower .01 values which seems wrong.

I can never remember if I’m supposrd to use an X2 or a Y2 cap.


Everything is getting packed so I can’t get at those units to check.

[RodB]

It's not real important to copy the line cap value. Most common value is 0.01mfd X1 safety capacitor. The way to remember it is, X is across the line, Y is the symbol they make with the transformer primary. (Each cap connects to a primary lead, then both remaining leads connect to chassis ground, Y shape.) You can buy X1Y2 caps so you don't need to buy them separately. They're nice, compact disc caps.