Eico model 460 oscilloscope question

[J Luth]

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]

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.

Here's the manual http://bama.edebris.com/download/eico/460/460.pdf
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.
nosaj

[J Luth]

nosaj

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]

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]

Enzo

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]

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]

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.

[pholmes]

I have 2 EICO model 460 scopes, which I used decades ago. I'm now not allowing myself to turn them on until I have "re-capped" them and carefully tested them when turning on. If your goal is to "have an oscilloscope for use", don't restore the model 460. For less money you can buy a more modern used one from a seller that guarantees it is working. If you are short on indoor space, what you want is a "usb port oscilloscope" that plugs into your computer. It's tiny, about the size and shape of a laptop power block, with almost no switches on it, and uses your computer for controls and display. ... Back to the model 460 - I have mixed feelings. I liked the large 5" diameter screen, bigger than many low-cost scopes from those decades. But know that your transformer problems are not a rare coincidence. From my own experience, and from reading on-line, EICO did not use good conservative engineering design practice on these scopes. There's inadequate cooling through the case, and the transformer in particular gets HOT, and has a short life if run for long periods of time, over years. Get an idea what the current buying & selling price is for these, it's not that high. One could buy another junk-model-460 that has a working transformer and swap, but other bad ones might also have a bad transformer. Watch out about there being more than one "sub-model" with slightly different voltages for the high voltage transformer. The transformers are not labelled themselves, you have to get the schematics that came with it, and see if it has the added sheet saying modifications have been made for higher voltage. You could swap transformers between 2 of the lower voltage ones, or swap transformers between two of the higher voltage ones, but don't mix them. Many other parts have to have slightly different values in the circuit. The same applies to the main picture tube. Confirm it's really the same tube, if swapping. My 2 scopes are from the different time periods of EICO model 460, so I cannot swap transformers.

[pholmes]

J Luth: From your posts on this thread, it sounded like your first priority would be to get another multimeter, at least for resistance measurements. Have you already decided what to do with that EICO Model 460 Oscilloscope, back in February? (like maybe toss it in a dumpster?) On the off-chance you might still mess with it, or other people might be searching the internet, like I did when finding your thread, I'll say a few more things: Besides the nice 5-inch screen, I really like the Model 460 because I think it has a pleasant large layout of many controls, for a crude simple cheap scope from that time period. It can go up to 2 MHz, silly by modern standards, but great for audio. Note that it doesn't have a real triggered sweep, it has "synced sweep", where it tries to stabilize (synchronize) to the major hump or wave of a repeating waveform, once you dial the sweep frequency close enough. It allows a lot of scope picture tube factors to be "raw input from a jack" if you want. Now, I had a question on your diagnosis of buzzing: Did you unplug both V9 (6AX5) and V8 (1V2) tubes, and then with that centertap still attached to circuits, get buzzing. Or even better, when you had all transformer wires unsoldered, then with only input 120VAC power and that infamous center-tap wire attached, did you get buzzing? If not, then what additional wires attached cause the buzz to start? When you talk about 840 v center-tapped to two 420 v's, you should know that that's only what I would call "the medium high voltage". The "really high voltage" is at the two wires going to V8 (1V2). Sure, between those 2 1V2 wires is very little, just a heater, but that pair of wires has "really high voltage" against the center-tap of the 840. That 840 is never used, it's really a pair of 420's into a dual-rectifier tube so as to get full-wave rectification out of 2 opposite polarity 420V's. That becomes the voltage used by most of the tubes in the design, less voltage at some tubes. By contrast, accelerating electrons down the picture tube to the screen requires the really high voltage, (not much current) produced by half-wave rectification in V8 (1V2). That voltage varies based on which sub-model of Model 460 you have. In older early-1960's Model 460, it's 750 VAC from the 2 wires between transformer to V8 (not between those wires, they only have a tiny heater voltage between them, but between them and the center-tap between the pair of 420V's. Then coming out the other side of V8 is -950 V DC (pulsing). In newer late-1960's Model 460 they left the pair of 420V's the same, but jacked-up the picture tube voltage so it's now 1140 VAC from transformer to V8 (1V2), with pulsing -1500 V DC coming out of it! Let me say the following to anyone reading this: Don't measure these voltages unless you have a meter and probes suitable for such high voltages, and you follow good advice from the internet and others with experience with this stuff, and you are wide-awake, and not in a hurry! J Luth: I know you've been messing around with this transformer with the power-ON a lot already, but be careful from now on, knowing that some of the voltages-to ground are much higher than the 420V you had been talking about. If the transformer can run without buzzing or overheating with only one of the voltages removed, for a long time, there might be the possibility of providing the other missing voltage with an added transformer. Alternatively, if there was another Model 460 that was "dead" for a different reason, like burned-out main picture tube, one might be able to get it's transformer, if it has the same V8 voltage.