It appears I have a self oscillation in the output, however it is inaudible. If I turn the volume all the way down, it shows on the scope as a steady pattern. I measured it at ~18kcs with my counter. No change in amplitude with the audio pot. It remains superimposed on incomming audio at the speaker.
If I unplug the first audio tube, it changes slightly, but still there and occurs on all three bands.
Otherwise audio from stations is coming through, but I suspect it is mixing with the normal audio and might introduce some sort of distortion. All caps in the output are (new and tested) polyester or polypropilene orange drops.
Any suggestions from the experts where to look for this?
The sch in Nost. Air is crappy but what I can see that they have some sort of feedback from the output (from the speaker and the output transformer's secondary) back to the amplifynig circuitry.
have you swapped it by any chance?
(and could you put the whole sch in?)
(This post was last modified: 04-17-2013, 06:21 PM by morzh.)
If you have not done so already, change out capacitor 57 (.002uf). This cap is there to suppress parasitic oscillations. These R/C phase inverters make me crazy(er).
Bad news... Looks like one side of the audio xfmr going to the 41's is open, so I only have voltage on one 41 right now. I suspect his could be causing my problem, among other things. I knew the audio seemed a lot more distorted than I expected. Dang!
So, let me ask another question. What is a good replacement for this audio xfmr, and where would I find one? Hammond?
That looks like a good match, and the price is right! If I order today, maybe I can get it by next week. Thanks! Now I need to go drill out some rivets. I'm going to open this one up and see if the open wire is near the surface. If I'm lucky, maybe I can repair it.
Since I can't do a lot with it until the xfmr comes in, I wonder if anyone can give me some ideas of why I have so many birdies on SW2, 9 to 12 mcs band. I hear stations pretty well, but lots of birds between them. That and my audio transformer are the only two things I've got left to attack. I don't even know where to start with the birdies.
Are you sure they are birdies? I've never actually heard those on an analog single reduction receiver. They're generally caused by spurious signals originating in a phase locked loop system. Much more common in analog receivers are images (and there generally are only two per frequency, 910 or whatever KHz apart. Can you describe what you are hearing? The bands are so cluttered with garbage signals from things like light dimmers, cable modems, etc., that it can be difficult to sort out the real stations from the garbage.
I wonder if that oscillation in the audio section might be doing that. Put your scope probe at the volume control input and pull both output tubes. See if the heterdynes go away. Alternately, and perhaps even better, connect an external amp to the volume control and listen. Sometimes your ears can tell you more than your test gear.
Well, I got the audio transformer sooner than I thought, and have it in. It sounds very good, and I appreciate the suggestion Brenda. I had the same thought about the sqealing between stations on the upper bands being due to the audio problem, however it is still there. Of course the oscillation I was seeing in the audio section is now gone.
These birdies, or whatever you want to call them are not heard anywhere on the BC band. Now it's SW1 and SW2. I can tune in most SW stations and there is no heterodyne. I think have have something mixing with the osc, and the heterodynes are much stronger when I attach the antenna. It almost sounds like I'm listening on SSB as I tune up and down the band hitting carrier after carrier, until I find a strong station. I have moved wires around in the osc section thinking I've got one to close to something and causing the oscillation to start, but no luck. I'll keep looking.
Do you perhaps use one of those room to room internet expanders that plug into the wall? There are a LOT of things that can cause that on HF bands, including what is called BPL (Broadband over Power Line), which is in sporadic use these days, and is a known destroyer of HF reception. Hams have been fighting it for years. Another possibility is a switch mode wall wart or DSL modem (cable modems are offenders too, but less so.) Last but certainly not least, plasma displays radiate more hash than a fry cook.
Just for info, the connection from the output transformer secondary to pin 1 on the antenna terminal strip is to use for alignment. You connect your VOM or VTVM to pin 1 to peak the audio voltage during alignment without turning the chassis over to connect your meter underneath. Philco also made an adapter for one of the tube sockets (don't remember which one) so their service techs could connect signal generators to that rather than to tube sockets underneath.
It was a minor convenience that they did that, but it is still next to impossible to reach the compensators with the chassis inside the cabinet. To align the dial you need to remove the chassis, put it back-to-back with the cabinet, reconnect the loop, align the compensators, and then reinstall the chassis. The dial alignment changes a little when you reinstall, but you can get it pretty close.
John Honeycutt
(This post was last modified: 04-22-2013, 04:29 PM by Raleigh.)
You may find that if you can move the set to a different location the problem may just go away. As BAD mentioned there is a lot of thing that generate all manor noise on the HF bands. I can remember back in the 60's and 70's there was hardly any squeals beeps buzzes on the BC band but nowadays where I am there's a whole lot of it and up in the HF band too.
GL
Terry N3GTE
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