I was told when I got this radio that a shop could not solve the problem. I did a complete chassis overhaul. All new caps, resistors, wiring etc. I've rung it out three times now and all connections are correct. I could list all voltages but they are all within spec. I've checked resistance on all IF's and they all read within tolerances. So let me just tell you what I have now.
Power up radio and there's nothing except dial lights and a soft hum from speaker. With my DMM neg. lead attached to chassis ground I can touch the positive lead to the plate of the 1st detector XXL tube and music comes out of the speaker. I am tuned to 1260 on the dial. Strongest local station. Moving the tuning capacitor changes nothing. I still hear the same station. Only difference is that the speaker seems to sound like a sweep oscillator but the station still comes in. I did an oscillator check with a known good radio tuned to 1255Khz and the 41-285 tuned to 800Khz and the oscillator is working. The volume control works when listening to this station as well as the tone pot. I disconnected one end of the .01ufd cap (#38 on schematic). I connected a test lead to the plate of one of the 41 output tubes and the other end of that lead to a .001ufd cap. A lead to the other side of the .001ufd cap I connected to the high side of the volume pot where the #38 was disconnected. I powered up the set and slowly cranked up the volume. This produced a noise from the speaker that also sounded like a sweep oscillator so I'm confident my audio circuit is good.
So this is where I'm at. I do not know how to induce a signal into the individual stages to check the IF circuits. Would this be the next step? I have a frequency generator that does give me a solid 455Khz signal if necessary. I sure could use some help with this one folks.
Thanks much,
Johnny
Sorry people but I goofed in part of my previous post. When I touch the positive lead of my DMM to the plate of the 1st IF 7B7 tube the volume is enhanced and I hear the station clearly. "NOT" when I touch the lead to the plate of the 1st detector XXL tube as I stated. Please forgive me. I know it messes things up when you're trying to help and you are given false info.
Thanks,
Johnny
Johnny, if you are picking up a station at 1st IF plate with lead attached it seems your problem is in the antenna circuitry lkeading to that stage. Possibly open antenna coils/transformer. Send in signal at antenna post and see if you get anything at grid first IF, next see if you are getting any gain on signal. If no signal or gain, that's the area to concentrate on. PL
Hi PL! This is where I am not experienced enough. What kind of signal do I send in at the antenna post? 455Khz from my frequency generator? Then exactly what am I looking for at the grid of the 1st IF? Then how do I check for gain on the signal? Sorry but I've only been doing this for a year or so. I've brought several radios back to life but have never run into a problem like this before.
Thanks,
Johnny
Johnny,
Before you play around with injecting signals, I'd try to clean your band switch contacts. Dirty contacts could easily block the signal. Are you sure the oscillator works? In the steps below I am assuming you have confirmed that.
You can probably assume for now that the radio works from the first IF coil forward since you are getting that radio station when you connect your meter to the XXL plate. If you have a signal generator, start by injecting a 455 KHz modulated signal onto pin 6 of the "first detector" XXL tube through a .1 uf cap. If you hear the tone, then you know the first detector and everything forward works. If you don't hear anything, you might try scanning the signal generator back and forth a little in case your IF coils are misaligned. (You could take the time to align the IF coils now if you do hear a tone.) If you hear nothing, replace the XXL tube. If still nothing, suspect the bypass cap or bias resistor on the XXL grid.
If the first detector works, then move the signal generator to the antenna section of the tuning condenser, as described in the alignment instructions for this radio. If you get nothing, then I'd suspect the band switch or the coupling capacitor part 4.
If you hear the tone at the tuning coil, then move the signal generator to the antenna terminal for the broadcast band. (Red antenna wire, I think, but please confirm for yourself.) Usually you can hear a 455 KHz modulated tone even without shorting the oscillator. If you hear nothing, it could still be the band switch, or maybe the antenna transformer part 3 or the antenna coil part 8. Check the continuity of those parts and see what you get. That should isolate the problem to either the band switch, the transformer, or the coil, unless by some chance something is mis-wired. If you get a tone straight through from the antenna terminal to the speaker, but still no music, then I'd suspect something in the oscillator section is open, like maybe coil 13 or caps 31 or 24.
Good Luck.
Hey thanks for jumping in John. I need all the help I can get with this thing. I've been doing this for about a year and a half and have learned a great deal but this radio is stretching my knowledge of these classics a bit far.
As for the band switch, I had it completely out and cleaned it quite thoroughly. The oscillator is working and I have verified this at least 4 or 5 times now. And as I stated in my "OOPS" post, it is the plate of the 1st IF 7B7 tube that I touch with my DMM lead. NOT the plate of the 1st Det. XXL.
I did check all the resistances on the Ant. Trans. #3 and the Osc. Trans. #13. Only anomally I see is the reading between pins 5 and 6 on the Osc. Trans. #13. This reading S/B <1 ohm and I am seeing 4 ohms. Is this a problem?
Now I will perform these other checks you have outlined for me.
Thanks much,
Johnny
As I recall, all of the tubes in this radio save for the audio output and rectifier tubes are those d**ned loctals.
You will have to not only clean the pins of each of those loctal tubes, but also clean each hole of every loctal socket. Carefully scrape each pin of every loctal tube with a knife until they are shiny. As for the sockets, a small brush intended to be used between the teeth works great in this application, along with the use of spray contact cleaner. Spray contact cleaner into each tube socket pin hole, one at a time, working the brush into the hole and moving it up and down several times.
Just remember this: Clean one tube and one socket at a time! Don't pull all of the tubes and then forget which one goes where!
Dirty loctal tube pin contacts can cause all sorts of strange problems in radios that use them.
Thanks Ron! All this has been done. Thoroughly cleaned every tube and socket. I know the problems they can cause so what the heck. I'll do it again. I can find nothing else wrong with this radio short of the 4 ohms across pins 5 and 6 of the Osc. Trans.. They should be <1 ohm.
Johnny
Hello again,
So I cleaned those pesky loctals again but to no avail. The same problem still exists. The pins and the sockets for the loctals are very, very clean. I really need to know how to induce the signal into the individual stages to test the IF circuits. I have the heath IG-102S generator but I don't see a signal on the jack that is supposed to be the "modulated" RF output. I do have 455khz from the "RF" output jack. I really need this procedure explained to me or reference a web site that does so.
Thanks much,
Johnny
Johnny, if you have the manual for your signal tracer it should explain the procedure. I'll try to give you a basic idea. When you placed your probe on the 1st IF Det and got music, what happened is the probe was acting as an antenna and sending the signal from that strong local station to the next stage in the radio and all the way to the speaker (you heard the music). Actually what you inadvertantly did is signal trace using that strong local station. You put it in at the plate and it went through to the speaker. This would indicate that the oscillator is working as well as the IF and audio section of the radio. You actually lucked out in having that strong a station nearby.It was the same as if you used your signal generator to actively inject a the signal at that point. What it did also point out is your RF section is not functioning correctly. It should be picking up that station, amplifying the signal and sending it to the grid of the 1st Det. The RF section is supposed to select a particular signal frequency, amplify it and send it onto the next stage. This is why your turning of the tuning condensor did not vary the station you received because it affects the RF section and no effect on later stages (that's not entirely true but for the present keep it simple). To check the RF section using your signal generator you would select a freguency, add the internal modulation and set the output voltage. You then apply that to your atenna input. Going by the schematic for the set you would then trace the signal to the first component and see if its there (usually antenna transformer). As an aside, if you did not have the signal generator with a high output voltage you would not get enough voltage from that local station to be able to read it (low millivolts). Then you would go to the secondary of the antenna transformer, see if you are getting the signal there (and at a slightly higher voltage) and continue on to the grid of the 1st Det tube. Some where along the line you are going to lose the signal and now you have narrowed down the problem area. Fixing the problem is another chapter but the above is the bare bones outline of what you would be doing. PL
Well here's what I've got now. Induce the 455khz signal into the grids of both XXL tubes and I get nothing from the speaker.
Induce the same signal into the grid of the 1st IF 7B7 and I get nothing.
Induce the same signal into the plate of the 1st IF 7B7 and I hear my expected noise from the speaker. Same with the grid and the plate of the 2nd IF 7B7. Yes! I do get my noise at the grid of the 2nd IF 7B7.
So is my signal not getting through the 1st IF 7B7? This tube has been swapped out with two other known good tubes. Sockets on "ALL" loctals have been cleaned thoroughly and I even checked continuity between pin hole and terminals of all sockets where the components and wires solder to. All is good. And of course all the tube pins have been cleaned too.
Why do I get nothing from the grids of the XXL tubes?
Once again I have to say that I have rung out all circuitry and checked all connections of components and checked that all components are good in their values etc. All IF's have been opened up and checked for proper resistance.
I don't know what to do next. Suggestions are very welcome.
Thanks much,
Johnny
Johnny, set the generator in the broadcast band not at your IF frequency, your canceling out. Try a mid range frequency or the frequency of the strong station you were getting. You use the IF frequency when you are aligning radio, another story for later. PL
Hi PL,
You know I've tried adjusting the signal generator all up and down the scale to try to do exactly what you're saying. This thing is just baffling me. It sure acts like my 1st IF transformer is not working even though the internal resistances are reading good and all associated circuitry is good.
How critical are the resistances on the 1st IF transformer? I have another IF I would like to slave in to see what happens but the resistances are 22ohms and 23ohms primary and secondary. My schematic says my existing 1st IF is .5 and 11.5 respectively.
Thanks,
Johnny
Johnny,
Typical diagnostic procedure is to identify the last point you can get a signal through to the speaker and then test the stage before it.
You've made progress by showing that you get a tone from the speaker when putting a modulated 455 KHz signal into the grid of the 2nd IF tube. You've proved the radio works from that point forward. The plate of the 1st IF is the input to the 2nd IF transformer, so if you get a tone from there to the speaker, you've proved that the 2nd IF transformer (#34) works and that the radio works from that point forward.
But If you get no tone when putting the 455 KHz modulated signal on the 1st IF grid, logic tells you that the 1st IF stage is not working for some unknown reason. You first must find the fault in the 1st IF stage. Putting a signal into the antenna terminal will not get past the fault in the 1st IF stage, so that is not worth trying until you repair the 1st IF stage and verify that it works by putting a signal on the 1st IF grid.
Here are some of the obvious things to try.
1) Connect your signal generator again to the 2nd IF tube. Verify that you get a tone as before. Leave the generator connected and swap the two IF tubes. If you get no tone, then the tube now in the 2nd IF socket is probably bad. If you get a tone, then the tube is good and the fault lies elsewhere in the 1st IF stage.
2) Check for 180 Volts DC at the plate of the 1st IF tube. I assume you have done this.
3) Check for 100 Volts DC at pin 3 of the 1st IF tube.
4) Check that the filament is heating properly. (Tube should get a little bit warm after a while.) Or you could remove the tube and measure across the socket pins 1 and 8. You should get around 6 volts AC with the tube removed. Another way is to leave the tube in place and measure across those same pins. You should get a small AC voltage, maybe less than 1 volt AC, across the filament. If it is near 6 VAC then the filament is not drawing current, either because it is burned out inside the tube or because it is not making contact with the socket.)
5) With the radio OFF check the continuity between the 1st IF tube pin 7 (cathode) and ground. Should be close to zero ohms.
6) With the radio OFF check the continuity between the first IF socket pin 6 and the blue wire of the 1st IF transformer part #32. (Blue wire connects to resistor #33 and bypass cap #29). Should be 11.5 Ohms. Then measure from pin 6 to ground. I can't tell exactly what you should get because my schematic is blurry, but I think it should be around 3.75 Mega-ohm.
Let me know what you find. Again, if you can't hear a tone with the signal generator putting a modulated 455 KHz signal on the 1st IF grid, then no tests on the antenna or earlier stages are going to tell you anything.
Johnny,
I'm not sure how critical those transformer resistances are, but they sound really high to me, especially the primary. Did you measure them with the transformer wires disconnected?
I think I'd try to sub in your spare transformer. Did you replace all the rubber wires inside the transformer can? Wires probably aren't the problem, but you might try that and see if coil resistance changes. Otherwise, I'd say you have at least one bad winding in the transformer, probably both.
The transformer could be a problem, but it is likely that something else is keeping you from getting a tone when you put a signal on the 1st IF grid. The resistance change in the secondary of the 1st IF is not enough to have much impact on the DC bias of the 1st IF grid.