Quote:
Originally Posted by Sealtest
I do have an update on this set. I spent hours trying to adjust it with no luck, but something interesting did happen. After shutting the set down and walking away for a while, I powered it back up. The flyback sounded like a modern tv and the screen lit up, then as the set warmed up it went back to the normal darkness. It really has been a frustrating project.
I have not tested the CRT yet actually since it did used to kinda work and my CRT tester never has an attachment that works. Yesterday I did get 4 different CRT testers in various conditions so they need to be tested.
Also I got a few oscilliscopes. I see test points in the schematic but am not sure how to actually do the tests. Do i remove the chassis and poke around with the scope? I don't see any way to hook up a scope and adjust the horizontal at the same time any other way. Sorry for the questions. This is the first tube set I've had.
|
Weak CRTs typically get better the longer they are on in my experience.
It sounds like a component is drifting at warmup and messing up the H osc operation, or shifting the bias to the H output...Shifting CRT base bias could also cause that if the HV holds steady through the blackout.
First, verify that one of your scopes work. Most ones made in the last ~40 years have an internal square wave generator with a terminal on the front panel...Hook a probe to it and an input and check that you can make that square wave appear on the screen with a decent wave shape and amplitude that makes sense relative to the scale on screen, the voltage setting and multiplier of the probe.
If your scope is good there are a few ways to do the procedure. Some RCA sets have 2 cores in the H osc coil one accessible via the back or top of the chassis the other only available underneath the chassis...If there are two slugs you will probably need to adjust both. Assuming a transformer powered set you can pull the H output tube and adjust the osc with the chassis on the bench (on a series string reconnect only the heater leads of the tube using clip leads).
If you ever need to scope something with the chassis in the cabinet there are
options for that too...You can get tube socket savers/extenders with test points on them to probe test points on tube pins. You can also get alligator clip leads to bring test points out from under the chassis (and avoid putting your probes at risk of mechanical damage). If you can't get the scope close enough to the set for the probes to reach you can get BNC extender cables for the probe(s)...Make sure to get the right impedance cable (most scopes/instrumentation use 50-ohm impedance cables).