Sparton rebuild

[mashaffer]

Is it the line from the bottom of the two 3.3M resistors (from bottom of the last IF coil) over to the grid of the rf amp (1st tube)?

mike

Ah!, I reviewed another document I have that shows the rf amp and converter stages with the different switch positions and it clearly shows the AVC is applied to the grid of rf stage as I indicated and to the bottom of the tuned circuits for the converter stage. So both 3.3M resistors are involved. So I suppose one would lift the connection of the two 3.3M resistors to the 1st audio stage and apply the 3V there. Sound right? Also why would one not do that during the alignment too? Alignment procedure doesn't even mention AVC.

BTW one of those 3.3M resistors tests correct but one is reading over 4M. I suppose I should replace both huh?

[mashaffer]

The more I study the more I learn. The more I learn the more questions I have. Here is a close-up of the relevant section.



For reference the note on the schematic and voltage reference chart page says

The stage gain measured values listed above are approximate values for an average operative stage, rather than an absolute value. It should be borne in mind that it is possible to introduce so many variables into the measuring operation, such as, type of equipment used for measuring, handling and placement of probes, the accuracy of allignment, etc., that an absolute reading is impractical. AVC is made inoperative and 3-volt battery bias substituted for measurement.

Sorry for the long quote but I thought it important that you see exactly the verbiage.

Now for my observations.

The rf amp (and also the if amp not shown) appears to be cathode biased with the grid's ground reference coming through the AVC resistors and volume control pot (see schematic in previous post). In this case it would seem that disconnecting the AVC resistor for the rf stage from the 1st audio stage and grounding it should provide proper bias without the use of a battery. It would seem pointless to connect a battery between the grid and ground as the current through the cathode resistor would increase the bias beyond that provided by the battery alone. Thoughts?

The only other AVC connection is to the "bottom" of the secondary side of the tank circuit for the oscillator stage which then feeds what I will call the second control grid since I don't know the correct terminology (pin 6) through the selector switch. The first control grid (pin 4) is grounded through a 20K resistor while the cathode is essentially grounded through the chokes at the bottom of the schematic (voltage chart confirms 0Vdc at the cathode) thus it appears that this tube is biased via. grid leak through the 20K resistor and that the AVC controls gain via the second control grid. So are they asking that the AVC connection to this stage be replaced by a 3V battery (to pin 6)? Minus 3V or plus 3V?

Sorry that I am so muddled up here but this is my first venture into rf. Any guidance would be greatly appreciated.

On a slightly different topic I would assume that replacement or at least verification of cathode resistors and grid leak resistors would be a good idea yes?

mike

P.S. I feel slightly cheated by the lack of a magic eye on my unit so I will probably try to add that after I get her up to snuff.

[mashaffer]

Well some more head scratching and some potentially bad news. First the bad news...

The original speaker in this radio was a 10" field coil speaker. Some PO replaced it with a 12" field coil speaker. I was excited to get a chance to play with a FC speaker but alas I made a couple of measurements today and it don't look good. The field coil measures about 5M ohms. Too high. and the voice coil with one leg of the OPT disconnected measures 0.3 ohms. Rats, open field and shorted VC. Maybe what ever took the unit out of service took out the speaker as well.

This is a design with the basket used as a common and an insulated connector through the basket for the + VC connection so it is possible that the problem is in the pass through. I haven't disconnected from the pass through to check yet but I got a bad feeling about this one. The connections to the FC are covered by the insulating paper on the coil so I could not connect directly to the coil to check (could be bad connection). I tried to get a closer look by removing the U plate which is held on by four screws but when I tried to lift it the coil was coming with the plate and I wasn't ready to deal with disconnecting the VC and lifting the field coil off of the VC.

We shall see what happens there. I do have some PM speakers that I can substitute and a PS choke from an electronic organ so maybe I can convert without much expense. Still... would have been nice. The choke I have did not feed the plates on the organ but it did feed the screens of the 6L6s and 30+ each of 12AX7 and 12AU7 so I hope it has enough current capability. If the speaker is damaged of course the OPT may be toast as well. Again I may have a suitable (actually overkill) replacement available or can pick up something on the bay.

Now for the scratcher... As I was replacing caps in the output circuit I was using the Sams schematic to check my work. I was careful to remove only one item at a time and keep close track of where the leads went but I got to a point where things really didn't look right and I thought that I had mixed up the connections. Specifically in the tone control circuit. As the schematic shows there is to be a variable resistor in series with a cap across the output plates and a second cap bypassing that series string. The first confusion was that on my unit the VR/cap string is inverted with the resistor to V6 and the cap to V5. No big deal as it works the same either way but it was a bit hard to see that at first given the way the P-P wiring was done. The real kicker is that my unit has two .002uf caps. Instead one .002uf cap from plate to plate there is one cap from each plate to ground. What would be the reason for doing it that way? Is it equivalent?

Kind of a detective job now. If the speaker is fried we will want to know what happened. My first guess had been that a tone control cap had blown shorting the plates together but that should not have taken out the speaker.

mike

[mashaffer]

Well I am proceeding slowly (probably best that way). Due to the difference in size of the components I am removing the old ones and then just putting the new ones in with full lead length and not soldering until I get everything in that area in place. That way I can make final layout decisions about routing components before cutting leads to length and soldering in place.

With all the resistors I have on hand you think that I would have what I need but the 470K grid resistors on the output tubes (6V6) are drifting upward and I only have 560K and 330K. 560K exceeds the maximum allowable value. Do you see any problem with using 330K or should I drop into Rat Shack and pick up some 470K carbon films? I am replacing the .05u coupling caps with .047u but that is still only 113k at 30Hz so it seems like it shouldn't be too limiting as far as frequency response unless it interacts negatively with the screen current self inverting scheme.

The other thing I have found is that several tube sockets have lost one or both of their mounting rivets so I will have to bolt those in. This is especially important as they have used the sockets as ground points. I tend to like Nylock nuts. Any reason not to use those to secure the sockets?

mike

[mashaffer]

Just to prove I really am still working on this I thought I would post and update. I have all but two of the caps that I am planning to replace done. I also replaced the output tube grid leaks and a coupe of cathode resistors along with the PS filter resistor. One of the wires to the dial lights was disintegrating so I replaced that as well.

I disconnected the can cap and cut off the soldering lugs. I used an existing hole in the chassis to mount a new ground lug for the PS caps and a plastic mounting strap that I put underneath. Then I drilled out one of the rivets on the old can cap and replaced with a screw and lock nut to mount a terminal strip for the PS cap connections.



The lock nuts that I got (integral star washers) for mounting the loose sockets would not fit underneath as there was not enough room between the screws and the socket body so I put the screw heads underneath and the nuts on top. I figured that shouldn't be a problem since nobody sees the chassis. I did notice that the plastic base of the tube hits them before it is fully seated. It only holds the tube up about a mm or so. I am hoping that this is not a problem. If it is maybe I can grind the nuts down thinner.



There is a grid resistor (R51 below) on the first audio stage tube that is supposed to be 10M but it measured 11M. I had C14 removed at the time that I took the measurement. At first I thought that the series combination of R41 and R42 should have shown up in parallel with it but as I look clooser at the schematic the top of the "22" just makes it look like there is a connection there. I left that resistor in place as it was a little hard to get to. Do you think its being about 10% high like that will be a problem? My 7B6 data sheet does not give a max grid leak value.



Thanks for your interest.

mike

[mashaffer]

Well I got all the caps done and hooked up a speaker, PS choke, OPT and the antenna. I can not get higher than 93V in before going over plate voltages on the schematic but I have traced it down to my choke having much lower DCR than the original speaker field coil.

Never the less I ran it for a bit with the variac set to keep things in line and ran an iPod into the phono input. It sounded really quite good for the test lash up I was using (guitar speaker with no enclosure). I could not get any radio reception but was getting some static in places. Will see how it goes once I have the voltages all nailed down (heaters are running at 5V right now). It may end up needing an alignment of course given the extent of parts replacement involved. Will have to get the Heath rf generator recapped before I try an alignment.

mike

[mashaffer]

I added a 600 ohm resistor in front of the choke as I realized that the DCR of the original field coil was much higher than that of my choke. This brought things much closer to the desired voltages. I was able to get AM working last night and did some DXing. From here in north central Indiana I was getting NYC and Nashville.

This afternoon I was able to get dozens of stations including several from Chicago and one from Cincinnati. Best AM performance in the house already without any attention to alignment. At 118V in I am getting 308 on the plates of the 6V6s v.s. the desired 270 but the heaters are right at 6.3V

Noting that the original OPT also had several hundred ohms more DCR I plan on adding another 250 ohms to the input to the choke which should get me down around 285V which should be fine.

mike