I am having a difficult time getting this radio finished. It plays very well but it has an intermittent problem where the reception drops off leaving a weaker signal.
What I have done to this radio is replaced all caps, ecaps, micas and resistors. The canned ohm is measuring good. I have run wires from grounding eyes to new ground points. I have cleaned ground straps on tuning condenser. I even took out the antenna coil looking for problems finding none but resoldered the contacts while I was there. When the signal drops out I can still detect the oscillator on another radio. The signal on a weaker station is gone but on a strong station it is there but much weaker. I have replaced the 6A8 tube with two others and the 6H6 and 6F5 with one other. When the signal is weak touching the 6A8 cap increases the signal.
Two problems I see is plate voltage on tubes is lower than listed by about 25 volts. Second is the AVC voltage does not vary much between no station and strong station while playing normally reading about -2.2 to -2.7 so very little change. I'm measuring on pin 8 of the 6H6 tube.
Using my signal tracer is no help as soon as I touch the RF probe to anything the signal returns to normal.
This may seem like an obvious question but did you resolder, or replace the grid cap leads at all, they often go flaky where the wire breaks internally, or the solder joint goes bad on the clip end. This is a common fault in radios with double ended tubes, I've repaired many a battery radio with this ailment. This may not be the issue but it really doesn't hurt to do it anyway, same with resoldering the grid caps themselves, especially if they are loose.
Regards
Arran
Hi Arran, Failed to mention that. Yes I cleaned and resoldered the grid cap wires. The wire from the tuning cap has been replaced as well as the gimmick. In this radio the gimmick makes a difference. The 6A8 and 6K7 tubes are both metal tubes as the tube shields are missing. One of the 6A8 tubes I used was glass and I cleaned out the solder and scrapped the little wire then resoldered. I have fixed many 75 tubes doing that.
I wonder if maybe someone forgot to solder a terminal at the factory? Finding intermittent faults is tricky sometimes, is there any pattern to the reception dropping off, like it starts acting up after the radio is playing for a time? Is it a mechanical thing, such as if you thump the bench, or tap things the reception goes away, or comes back? Since you have already swapped the 6A8 and 6F5 tubes, what about the 6K7? Are any of the tube sockets loose, I think these used wafer sockets which are not great at the best of times?
Regards
Arran
I forgot, have we been through a very similar problem very recently (like less than a month ago) with you or someone else?
Grid caps are one thing, the actual grid caps that are on the tubes themselves may have the same problem, the short wire that is on top developing a cold solder spot in that cap.
Try to wiggle the grid cap wires, swap tubes etc.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
I have swapped the 6K7 tube as well as the 6H6 and 6F5. Wiggling wires or tapping on chassis and parts does not upset the radio. The tubes are quite tight in the sockets. I have even wiggled them. There is no pattern to the drop out. Sometimes it will play for hours and sometimes act up within 10 minutes or be very weak on first power up for the day. I can't make it fail no matter what I do but touching a wire with a probe will bring it back immediately. Mike I did resolder tube caps scrapping wire going to the cap. The 6A8 and 6K7 in the radio now are metal.
I guess my next step will be to resolder all connections although I have done many of them already. Why does touching the grid of the 6A8 being in reception when it is in the weak state and then goes away when I remove my finger? I don't even have to actually touch it but get close.
Dan, something came to mind while reading this. It may be a mica cap in the 6A8 circuitry. They rarely cause any problems but usually when they do it's a weird thing. Trouble is, when disconnecting them it's risky. Maybe just cut the wire lead, substitute another part, if no change then repair the cut lead.
Maybe a flaky winding in one of the IF or oscillator transformers. The Philco rf windings get corroded and change resistance from the use of cellulose winding insulation. Maybe there's a similar case here. Just thinking outside the cabinet.
If you try to make voltage measurements is performance restored just from touching?
Sorry for being a broken record but if you install a Radio / Aux switch you can try an external input to at least determine if the issue is Audio, power or the "radio" RF, Mixer, IF, Detector, AVC. I have a Zenith chassis from a 6S27 / 6S52. That schematic shows the mod needed to add a "Radio / Phono Switch". This mod was common back in the day and if done with period parts, does not detract from the value of the set, IMHO.
You also said that if you touch the grid cap of the 6A8, the signal gets louder. Does it stay loud when you let go?
Maybe you have some funkiness in the AVC circuit.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
MrFixr, I can make voltage measurements without reception returning. There is no change. When I touch the 6A8 the signal gets louder and does not stay louder when I let go. I have wired in a aux connector without the switch. I will check to see if that play when the set does it's thing again.
I did not turn the set off but what bought it back was disconnect and reconnect the antenna ground. I wonder if this radio does not like my antenna. The antenna is about 100 feet of RG59 out to a tree. From there it splits off where the shield is attached to a long piece of rerod driven into the ground. The center connects to a 200 foot wire strung from tree to tree. I used RG59 to keep house noise off the antenna. I have used this same antenna on all my radios when they are on the bench. Without the ground the set is noisy.
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