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I've got an old Lafayette Comstat 35 that will receive for about 5 minutes, or until it gets warm. The receive "just wont stay up" . Any ideas on what might be the problem. I'm assuming that its probably a flat receive tube, but i'm clueless about tube type radios. As far as I know it will still transmit regardless of time or temp. It is a SUPER NICE and CLEAN radio, I would almost consider it mint, this thing is original...all the way down to that ugly **Censored** orange channel selector knob . Seriously, I would like to git er' goin' again and shoot some dusty ol signals over the air. Any help would be appreciated. Thanks.
It's really hard to say what is causing the receive to go away. Consider the age of the radio, the first thing I would check would be the caps. Although it doesn't always work, you could get a can of "liquid freeze" and see if you can locate a bad cap using it. When the receive goes away, spray the caps one by one and if you "freeze" one and the receive comes back you have found a bad one. If you are thinking of keeping the radio though, it may be worth the investment to have them all replaced. Dry caps are pretty common in older radios, and even if you fix it by replacing a bad cap (if found) chances are pretty good the others will follow suit in short order...
It's most likely a tube but it could also be could be a component in the recieve section. Since it still transmits after it warms up that rules anything out in the xtal synthesizer or transmit section. I would get some freeze spray and spray it around the tubes in the receive section and see if it comes back up that could help you locate your bad tube if you don't have a tube checker. Don't spray it too close to the tubes after they warm up or they will shatter. If I had a schematic for that that radio I could tell you which tubes to check. It has to be one of the tubes in RF or IF stage.
As far as the caps go if they were bad you would either have a fireworks display or a bad hum. But since that radio is nearly 40yrs old it would be a good idea to check them and eventually replace them.
When your receive dies, it would be useful to note if you're still hearing background static as you would if the antenna were disconnected and the volume turned up. Should give you a clue if its the audio stage or the RF stages.
If you're serious about the radio, you should order SAMS 72.
Unlimited power permitted on CH1 to CH27 as per FCC CFR 47 part 18: 18.305(a) and 18.301
That radio has somewhat of a schematic on the bottom cover plate. Its been so long since I looked at it, I don't remember what it illustrates. I'll try to scan it on my printer and post it. LOL! Maybe it'll turn out clear enough to make something out of it.
I guess my main question is...Can you still get replacement tubes for that radio that are compatible with the originals and if so what #'s on the tubes to you use to match them up?
I did notice a while back when I got the radio that several of the tubes had the exact same #'s on them, some were different though.
Also, about how much do new tubes cost these days? If they're expensive I may just leave the radio like it sits...as a good antique conversation piece.
I have heard about the capacitor problem in the old radios. I've gone through that entire radio looking for bad solder joints and leaky caps to no avail. I think I'll try the freeze spray technique. Good Idea!!
I'll try to post the diagram later today, after work. Thanks for the GREAT INPUT. Keep it coming.
Lost Sheep wrote: As far as I know it will still transmit regardless of time or temp.
Does it have audio,modulation on transmit? The audio stages on that radio are shared on both receive and transmit. As far as the tubes you can still get them price varys. The numbers on the tube are written on the glass envelope on the tube usually 6au6 12ax7 as an example
Good question. To be honest, I,m not sure about modulation. I'll hook er' up tonight and see and I'll also check on those tube #'s.
Can anyone direct me to a location or website where I can purchase tubes? I seriously doubt that our sponsors carry ANYTHING related to tube equipment.
Thanks!!
. Any ideas on what might be the problem. I'm assuming that its probably a flat receive tube, but i'm clueless about tube type radios. As far as I know it will still transmit regardless of time or temp. It is a SUPER NICE and CLEAN radio, I would almost consider it mint, this thing is original...all the way down to that ugly **Censored** orange channel selector knob 
. Seriously, I would like to git er' goin' again and shoot some dusty ol signals over the air. Any help would be appreciated. Thanks.
