Discussion in 'Amateur Radio Amplifiers' started by KC5DFP, Sep 26, 2019.
It appears someone nicked it with a tool, cracked the edge, and epoxied it.
I have designed many input "pi" networks for amps. Use the formulas quoted in the several handbooks for amps. You need to know the input Z of your tubes, which is often quoted- depending on the tube type and configuration (GG, etc).
I decided to pull and test the 10M toroid using my scope and function generator. I found that the toroid was a short and it lost its
permeability as it showed as a short, even after rewinding the core with new wire. I’ve read this can happen if a core is exposed to being overdriven or a lightning strike. Just to confirm what I was measuring I had a wound toroid for another project which I was able to measure with the scope/generator.
So, at this point I’ll order some toroid cores and see if this is the issue, on if I one step closer. Not sure which core to get but I think this one should be good, but let me know.
Why not just wind an air core coil? You can add or subtract coils and massage it until you get a perfect match. Do you have a way to measure Inductance? Start with an air core coil of about .15uh see what it gives you with the caps you installed go up and down the band and see which way the swr is going this will tell you to add or subtract windings.
More likely "overdriven," which caused overheating in the core. Once a core exceeds its Curie point, that's it and it will never work again as anything other than a "coil form."
I suspect "somebody" was using your amp for CB operation.
The Dentron CM-1 or CM-2 (or whatever it was -- it's been many years since I dealt with one) used an air-wound coil for input tuning on 10m. They used to sell that as a "kit" for several different amplifiers, back in the day.
Thanks for the response and suggestion. I’m trying to keep the form factor the same and air core generally has less bandwidth, but not sure if it would make a difference seeing it a single band.
Thanks for the response too. Either a CB’er or an overzealous Ham operator wanting to get a kilowatt out of the amp. Drive power for these should be 65-70 watts but perhaps a older rig was used that put out twice as much.
Anyway I’m glad I’m finding this issues and getting g the amp back to working order.
I still have an issue with the current draw being 100mA on standby. I did put a known good set of tubes in and got the same reading. D1 checked out fine, so hoping to get some suggestions on this one.
Pull the tubes and check for 47K ohms to ground on any pin 3 of the tube sockets. I'm suspecting that K1A may be stuck on the diode (providing operating bias) and not switching over to R1 for cutoff.
That could easily be it.
Also, if R1 was shorted or greatly reduced in value, that would cause the same problem even if the relay is fine.
Or someone stuck the wrong R in there...
Let me count the ways...
In any case, very simple circuit and easily de-bugged.