Ge f-74 Tube radio problems/restoration - Page 1
Forum section: electronics / repair

author: CoopedUp
author_rank: Regular Contributor
timestamp: July 02, 2019, 10:40:23 pm
content: So I'm repairing this radio and in its state the amplifier functions good with a little hum(I might add more capacitance to fix that) but what I'm wondering is why the radio part is very weak. Is this something that would be adjusted in the if transformers or whatever they are called or is there no internal antenna. I don't know what style antenna would be inside I might just be oblivious to the antenna in it.Problem two, I believe that the radio occasional starts to oscillate loudly. I don't know if that's another adjustment thing or if a component is bad. If I find more problems or solutions I will keep this updated.

author: CoopedUp
author_rank: Regular Contributor
timestamp: July 02, 2019, 11:08:17 pm
content: I found a mistake I made while recapping the radio, one of the mains capacitors wasn't hooked up right so I fixed that and there's less hum and I can find a few more stations but they seem to fade in and out of focus

author: vk6zgo
author_rank: Super Contributor
timestamp: July 03, 2019, 08:16:00 am
content: Quote from: CoopedUp on July 02, 2019, 10:40:23 pmDon't mess with the IF transformers or any similar devices.This is the number one "trap for young players."Lack of sensitivity in such radios may be due to "low emission" in the 6A8 converter, the 6K7 IF amplifier, or even the 6H6 detector tube.Your 80 rectifier tube could be low emission, too, so the HT line could be much lower than it should be, preventing the tubes from operating correctly.As far as antennas are concerned, the radio is almost certainly designed to use an external antenna.Such an antenna doesn't have to be a full scale outdoor one---often, a few feet of wire hanging off the antenna terminal is adequate for local MW AM stations.Be certain you are switched to "MW", as Shortwave reception is much more variable depending upon time of day, sunspot count, & a raft of other things.I assume the oscillation appears as a squealing sound at the speaker output.You can "fault find" as follows:-(a)Make it oscillate, then remove the 6H6If the oscillation continues your fault is in the audio section.If it stops, go to (b)(b)With all the tubes back in, make the radio oscillate, then remove the 6A8.If the oscillation stops, your fault is in the RF & mixer section.If it continues, go to (c).(c) With all the tubes back in make the radio oscillate, then remove the 6K7.The oscillation should now stop.If someone had "fiddled" with the IF tuning, that could be a possible reason for an oscillating IF stage.Many years ago, a friend built a nice little "superhet", but because he was poor, as were we all, used parts scavenged from all over.He couldn't stop it oscillating, so passed it on for me to try.Try as I might, I couldn't.In passing, I noticed the IF transformers were from two different manufacturers.Something about the part number of the second IF transformer rang a bell, & when I checked the next day at work, it turned out that it had been designed for use in a 292 kHz IF.It could be tuned to 455kHz, but appeared as highly inductively reactive.This turned the IF stage into a "Tuned Plate,Tuned Gridl" oscillator!