[TV MAN]

Stinkkin intermittents! If anyone has ever dealt with one of these, you know what I mean! Anyways, I still have a volume coming and going issue. Thought I had it traced to the tuner HF trimmer - not. I've had to (more or less) table to project for now, and will be real hit or miss over the next few months as to how much I can get done as I am now 'owned' by my job for the season. About the only chances I will have will be a rainy Sunday afternoon.

I have spent quite a bit of time tracing things out, thinking the problem may be around the 1st detector area. I disabled the osc by removing the 27 tube, and have been connecting the gen at IF freq' to the grids of the different tubes up to the 1st af/2nd det. I get the most noted effect from the 1st det grid, and RF tube. Seems those two tubes work back and forth (one affects the other and vice a versa. ) It is difficult to tell since I think I hear some change no matter where I inject the signal, but it is most noted in this area. Could be that if a tube current is changing (due to a bad connection) it will effect others on down the line. I havent pulled the RF tube yet to run checks without it in the circuit.

All coils check, tubes swapped around, resistors, voltages, etc. One issue in trying to check things is checking plate voltages - kills the signal, then I cant tell if it is acting up when I am watching voltages.

I think I am down to resoldering connections and taking bakelites back out in the area to check my work. Any thoughts?

[morzh]

Tim

Yeah, every doc hates them unspecific intermittent complaints. My mom was good at those, this is why he was so loved by patients. She taught me a lot about troubleshooting, believe it or not.
Meantime my heart goes out to ya , be brave, you will conquer it, even if it takes a few years

[TV MAN]

I "took" an hour and did some more troubleshooting. I have thought several times I had it narrowed down, but it keeps eluding me. Now - m a y b e ... I have really narrowed it down. I removed the RF tube - eliminated that thing. Still have the osc tube out, so it is disabled. I inject the signal at the grid of the 1st detector tube at the 260kc IF freq. I connected the signal tracer to the grid of the 2nd detector/1st af tube. I also have Fluke meters connected to the voice coil of the radio speaker and signal tracer speaker - it can see what my ears cant hear. At this point, the tracer follows the radio. I moved it to the grid of the IF tube. At this point, the tracer does NOT follow. It stays steady, the radio fluctuates. I just finished tracking voltages on the IF tube, plate-screen-cathode. Dont notice any changes following the fluctuations in volume (and mild static).

I overcame killing the signal on the plates when meter is connected by cranking up the sig gen. I have to wait for things to cool again, then I am going to connect the tracer to the plate of the IF tube and see it things track or not, next is the cathode of the 1st det tube. Still need to track voltages on this tube.

http://www.philcoradio.com/tech/images/70a.jpg

Dumb question - when I did the IF alignment, I didnt have a freq counter, now I do. I think I aligned the IF at something close to 263kc instead of 260. I havent corrected this yet. May not be an issue - but, after that stinking tube shield fiasco - ... ??? I dont want to mess with that until I have this intermittent resolved. Unless ... ??? this IS the problem???

Yea, Mike - years.. I hope not.

[TV MAN]

Ok, I just moved my signal tracer tracer to the plate of the IF tube and connected through a small cap - now it tracks the radio! I can also hear the static through the signal tracer (it is more sensitive than the radio), so I'd say I have it narrowed to the IF stage, maybe more specifically, the IF transformer. I will do some careful checks in this area, but the only thing I see on the plate circuit is the primary of the transformer (#20). Sounds like it's time to pull the IF transformer. To double ck, I went back to the IF grid with the tracer - same, does NOT track the radio and I dont hear the static. I double checked the plate and cathode voltage (no changes), but this may be such a small change that it affects the signal, without affecting a measurable voltage. Just to post, plate sits at 305v, cathode is at 10.6v, G2 is 119. I still have the volume control bypassed with a series of fixed resistors. Thoughts?

[Steve Davis]

Tim, before you pull the IF, check trimmer number 19 on the second IF to be sure it has a good ground. A few years ago, I had an intermittent problem with the IF on a 70. It has been a long time but, it seems that there was a steel tab on the trimmer that had some rust that was causing the ground connection to be intermittent. You might try connecting the trimmer to ground with a clip lead to see if the problem goes away.

Steve

[TV MAN]

I pulled the IF coil, and much to my surprise, I found a "modern" looking coil.
   

Now, what do I do with this thing??? It doesnt look bad. I connected my meter to ohm it. It checks around 75 ohm on the primary, 55 on the secondary. I probed the wires coming from the coil to see if the resistance changed when I moved wires. Nothing. Now, interesting - as this coil warms (from my hand) it increases in resistance. I warmed it with the light bulb over my bench, and I could watch the resistance increase - to about 80+. Not sure if this means a thing or not. The secondary also did the same, just not as much or as fast. I cleaned the connections and resoldered, then reinstalled temporarily. Same problem. Seemed weaker even. I changed the IF tube again. Same. I am waiting for things to cool again and will try the ground on the trimmers. I did try tightening the bolts. I also resoldered the socket connection on the plate pin.

I did inject the signal at audio freq at the grid of the 2nd detector just to double check the AF section. Aside from a hum, it sounded ok.

I dont think I can rewind this coil myself. Any more ideas?

[morzh]

Thermal coeff. of copper is about 0.4% per degree C. That is if you have 75 ohm at say 25C then at 36C (which is your body temp) it will become roughly 4% more that of 75 would be 3 ohms. So yes it is close and your hand will bring the resistance of 75 ohms close to 80.

[TV MAN]

Going to make some current checks and maybe bypass this coil and try to figure out if this is guilty or innocent. I am going to clean the ground screws on the trimmers and see what I get. Going to take some time. Hasnt acted up with the amp meter in series with the B+ to plate circuit. Wanted to monitor so see of the current follows the fluctuations in volume and noise.

[morzh]

I forgot...have I asked you if the coil cans make good ground contact?

[codefox1]

Sounds like you are doing fine. Once you are setled, bring the set up very slowly andcarefully with a variac with an old fashioned analog milliampmeter ahead of B+divider to measure the load jut to be sure. Well, that's what I do. Best!

[TV MAN]

Now things are starting to get interesting! I measured the current draw of the IF tube by opening the cathode. I saw flucuations that followed the volume and noise. I thought for sure the problem was in that stage, more speficially the plate/coil circuit. The cathode of the IF tube is connected directly to the volume control (bypassed with fixed resistors, control out of circuit), then down to the 70 ohm bias resistor to chassis ground. It also connects to the K of the RF tube. Soooo, I pulled the IF tube, replaced the RF tube (with a different tube), fed the gen into the G1 of RF, tracer at the plate, milliamp meter still in the circuit (opened on the volume control slider wire). Huh... My ears, the output meter and milliamp meter agree - it is in this tube as well. Cathode circuit? Now, where???

Voltage checks reveal nothing. Everything ohms out. I have the ground of the bias resistor jumpered to another ground. There are 3 sections of this resistor. I didnt note any voltage changes on the G2.

Just to note: K voltage is about 15v with only the IF or RF tube in socket. Current on the IF tube is around 25-30 microamps, depending on how strong I set the generator. RF current is around 8 milliamps (because I have the grid cap off). If I am reading things correctly, I have cleared the IF stage, and pinned to the the common cathode circuit.

I dont think further current checks will help me, aside from maybe the G2 current. I think I am down to susbitution of the sections of this resistor. Any thoughts or insight?

Ps: I did do the twist the cans trick. Didnt seem to make any difference at this point.

[morzh]

Tube socket cleaning?
Have you tapped on the tube and see if this affects it?

[TV MAN]

Ok, I dont want to get my hopes up just yet, but, I bypassed (subbed) the 70 ohm bias resistor to ground. I am not getting the flutcuations I had before. I do get fluctuations in the current draw, however, now they are smooth and slight. I reconnected the original resistor, and almost immediatly I noted the erratic fluctuations in current draw and volume. I have the sub back in. I am going to test this off and on for the next week before I shout victory or 'onward'...

I have tapped and swapped tubes. I dont think it is the socket. I dont get any change when I move the tube around. They seem good and tight in the sockets. I am getting the same thing on 2 different sockets, with 2 different tubes. The screen and cathode voltages are the only thing these 2 have in common.

Something else I noted is a wire 'whisker' between the open section of the bias resistor between points F & G. I read aprox 500k on this, and it is not on the diagram. Anyway, I will fire back if this has licked this irritating problem or not. Thanks for the advice.

[morzh]

F-G should read 250 Ohm.

[TV MAN]

I have tested this on and off as much as I could this week. I reconnected the original bias resistor last night, but I didnt notice any major difference, although I thought I heard 'the sound'. This evening, I powered up and immediatly heard the noises & saw erratic current fluctuations. I powered down and temp installed a 68ohm resistor. So far so good. With it being intermittent, it is really hard to tell quickly of the problem is really solved or if it is just 'deciding' to work for the time.

When I checked the F-G, I had one of the wires disconnected. I think the problem is in that bias resistor. I dont know if I should replace only the bad section or if I should replace it all. Tell ya, if this is the problem, this one should go in the record books!