|J Luth||2/13/2018 4:35 PM|
|Eico model 460 oscilloscope question|
Anyone know if there is a replacement transformer available for Eico model 460?
Have one here that is showing an apparent bad short, on the bulb tester, bulb glows bright and the transformer has a sizzling sound. Tried removing all the tubes still does it. Disconnected all leads except main power and high voltage output, no tubes and still does it. Disconnected the high voltage and tester shows no short and there is no noise from the transformer. Took a reading on the high voltage and got approximately 840 cross the two leads and 420 between each and the center tap, readings are approximate, the meter is not very good. Reconnected high voltage leads same problem. Lifted the center tap, no short and no noise. Disconnected all the high voltage caps so that the high voltage leads are only going to the rectifier socket and the caps no longer connected to the same place as the center tap, no tube, still does it. My meter is acting up so I am not sure any ohm measurements I take would be accurate but will check. I know it would need to be recapped but not going that far if it is indeed the trans. Not sure if the way I am doing the testing is correct.
|nosaj||2/13/2018 5:40 PM|
Looks like there are several taps on the transformer. Do you have big power resistor? You should be able to connect the HV lines to it to see if the PT is giving issues under load or if the issue is farther down. But the sizzling noise would have me pretty much considering a new PT or a another Oscilloscope preferable a solid state one.
|J Luth||2/13/2018 9:18 PM|
Thanks, I have the schematic and the manual for the scope. What value resistor would be needed?
I actually disconnected every transformer wire from the scope then reconnected one section at a time. Started with the main power, nothing else connected then tested it was OK. Next was to connect the filament section again no problem. Then the high voltage and had the problem. At this point there are still at least two other taps in the trans that are not connected. None of the scope circuitry is connected to the transformer, I have it all disconnected. That's why I am thinking the trans is gone.
This scope along with a bunch of storage bins full of electronics parts came by this weekend for free.
So I figured I'd mess around with it and see if it worked.
As far as another scope, a solid state is on my list.
A lot of cool ooolld stuff in the bins definitely from an old tv/radio repair shop. There are things in there that I have never seen. A lot is still new but very old. Maybe I'll start a small museum.
Who knows maybe some of it is still good.
|Enzo||2/13/2018 9:27 PM|
|Disconnect high voltage from what, and how? Your sizzle description sounds to me like a corona arc. Turn all the room lights off and power up to sizzle, do you see any tiny blue spark/light anywhere around the CRT or circuits?|
Roll up a magazine or something and hold to your ear and use like a stethoscope. You can then listen closely all over to find out EXACTLY where the sizzle noise comes from.
|J Luth||2/13/2018 9:59 PM|
The high voltage side is disconnected from the rectifier tube socket and the center tap from the resistor to ground by un-soldering the wires. I did that to try and get a voltage reading which I was able to get. As far as corona arc there is none, I did do as you state about turning off the lights I did that over the weekend before I started disconnecting the trans. Don't see how there would be any corona arc seeing as how everything is disconnected from the trans including the CRT. When I have the high voltage reconnected to the tube socket there is no tube installed in the socket and all the wires from the other side of the tube socket that go to the filter cap and the other HV 1000+ capacitors are disconnected even though that probably does not matter with the tube out. I have checked the rectifier socket to make sure that it is not shorted, it's OK All other leads, the CRT and everything else is disconnected from the trans. I have also tried with only the primary and the HV connected no filament or anything else connected. The only way the sizzle stops is if I lift the HV center tap. The sizzle is right at the transformer. Only been turning it on for a few seconds at a time, figure it will blow another fuse, I'm getting low on them and can't get them without traveling a bit, if I leave it on for long. Plus if the trans does go up in smoke, the wife will not be a happy camper, and we all know how that goes.
|Enzo||2/13/2018 10:11 PM|
|MY thought was that if it only sizzles when connected to a circuit, then the circuit may be at fault.|
I used to have a bunch of little circuit breakers - the silver rectangular kind with the red post sticking out - the kind the post pops when tripped. Used to see them in old TV sets and other cheap consumer goods. I wired clip leads to them and hung them nearby. They came in everything from half an amp up to several amps. I clipped them in place of fuses for just this sort of situation.
|J Luth||2/13/2018 10:30 PM|
|When I heard the sizzle, I thought the same, that's why tried it with the lights off and then tried disconnected everything. I even pulled the transformer and removed the end cover on the HV side to see if one of the wires was bad but didn't see anything but I covered them with shrink tube anyway. One of the wires did break while I had it open but not the HV section, stupid mistake. I was able to reattach the wire to the coil. When I check the leads with an ohm meter I don't get any signs of a dead short but the meter is acting up so I can't check for accurate resistance in the windings. And as I posted above when the HV tap is disconnected and I check the voltage, there is no sizzle and I get roughly 840V cross the two leads and 420V between each and the center tap.|