Mighty 620 with a problem or 2 ? - Printable Version
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+--- Thread: Mighty 620 with a problem or 2 ? (/showthread.php?tid=349)
Mighty 620 with a problem or 2 ? - Guest - 05-23-2006
I have a 620 that has played well for several years. the original owner re-built all the cap blocks and i re did the electrolytics. Over the past year it has been having some problems. 1) the oscillator shuts down below about 600 khz. (kc for the purest out there) 2) when you turn the volume up about past half way it starts to sqeal like a little piggie.
I went in and checked and replaced all out of tolerance resistors, all but two actuallly. Checked all voltages, all with in close reason. I noticed that as I turn up the volume control the gain comes up smooth, then stops and as you advance the control it comes up fast. Then beyond that the squeal starts. Replaced the 75 and 42 with others from a working set and no change.
I think my problem with the oscillator shutting down is 150pf 250pf & 50pf micas so I am going to replace them. the biggest problem facing me is the sqeual. I have moved wires, checked resistance, voltages, etc. The radio works great at all frequencies above 600 Khz good volume and tone. but on the softer stations if you turn up the volume, Sqeal. This is the oldest set I have worked on, most are 39 and up. I must be missing something but I think I am too close to it. Any ideas would be welcome! Thanks in Advance.
- Chuck Schwark - 05-23-2006
Regarding the audio feedback; be sure the tubes shields are making good ground contact with the tube socket shield bases. Shields are on the 75, both 78s, 6A7, for a total of four in this model. Also be sure the grid cap lead of the 75 runs inside the shield.
Rust attacks cars and Radios too! - Guest - 05-24-2006
Chuck Schwark Wrote:Regarding the audio feedback; be sure the tubes shields are making good ground contact with the tube socket shield bases. Shields are on the 75, both 78s, 6A7, for a total of four in this model. Also be sure the grid cap lead of the 75 runs inside the shield.
All the cans were on and the grid cap lead for the 75 was INSIDE. I stared at it for a while, looked at the schematic to see if I missed something. Then I took my ohmmeter and checked the can to the chassis, guess what, very high resistance. I took off the can and looked inside the bottom and it was corroded a bit so I took some emery cloth and cleaned it up. Then looking at the tube socket connection with the can, it too was corroded. I cleaned that and did the same on all the cans and sockets. NO MORE Squeal. I guess I did overlook something, RUST! This chassis has been in use pretty much daily at least since 1975. Electrically it was upgraded but not the chassis. Over the years I guess corrosion built up where the two dissimilar metals contacted.
Thanks for the help in getting me in the correct order.
I did an alignment and the top two bands came in beautifully. 10 and 2.5 and 5.0 for WWV were dead on. But on the Am band while its right on the money, the oscillator still dumps out at about 640Khz. I replaced the three micas in the circuit but it still stops. I have 3 6A7's they all test good but they all work the same so I assume its something in the circuit. Any other Ideas on that?
Again thanks a lot.
David
