CurtPalme.com Home Theater Forum

jmetal88 · Joined: 06 Apr 2017 Posts: 108

The NTE535 is dirt cheap direct from NTE right now so I went ahead and ordered one. Fingers crossed I don't blow anything up!

I actually think this might have the correct focus tap on it as well. There's no connection in the schematic for a focus control off the tripler (focus control is done through separate coils in this design) so I don't think a resistor divider with a CTL pin is quite correct for this application.

jmetal88 · Joined: 06 Apr 2017 Posts: 108

I guess I got the *last* NTE535 from NTE direct. It says "out of stock" following my order. But it has been shipped, apparently. Hope it works!

**AnalogRocks** · Posted: Wed May 23, 2018 10:57 pm Post subject:

Don't blow it up!

jmetal88 · Joined: 06 Apr 2017 Posts: 108

I'm fairly convinced that I'm right about the intended operation of the circuit, but of course that 10% uncertainty does make me nervous, haha.

I did just do a quick sanity check to see if the tripler I currently have installed would be able to power up the tubes if I swapped the GND and CTL connections. It does, although it takes slightly longer for the tubes to warm up, so that indicates to me that I am on the right track with this. The only thing left to worry about is that focus tap, and... honestly, I don't think the Videobeam even uses that output to focus the tubes, at least based on the schematics I have. It looks like it just uses it as an additional high-voltage sense line, at the horizontal oscillator, and it goes through such a high resistance before getting there that I really don't think there's any way it could damage anything. It looks like the actual focusing circuitry in the Videobeam is low-voltage DC stuff.

jmetal88 · Joined: 06 Apr 2017 Posts: 108

Alright, the NTE535 came in. Good news is that it did NOT blow anything up. Bad news is, it is also not the correct tripler. For one thing, it is a five-stage tripler, and I was not sure whether that would matter, but it turns out it does as it does NOT put out enought high voltage to light up the tubes. I did have a spare 20kV capacitor to solder onto the input and convert it into a 6-stage, at which point the tubes did light up, but something about the tripler triggered the high-voltage protection circuitry after a few seconds. I am thinking it has to be whatever circuit is looking at the focus voltage. If I turn it off for a minute or two and then turn it back on, the tubes will light again, and then turn off again in a few seconds so whatever the problem is, it's not actually damaging anything.

I'm wondering if I could add another series resistor to the focus output to solve the problem. I'll have to analyze the schematic more thoroughly to see if that's a possible solution.

jmetal88 · Joined: 06 Apr 2017 Posts: 108

Yep, that's exactly what it was. Voltage was too high on the focus sense circuit. I soldered in a 100M-ohm resistor in series with the existing circuit and it's now running without shutting down after a few seconds.

So, the NTE535 isn't the correct tripler either, but it can be modified to work with an extra high voltage capacitor and two extra resistors (and these modifications are easier to do without inducing arcing than the required modifications to the NTE530).

So, to use the NTE535:

Connect the flyback lead to one terminal of a high voltage capacitor (I used a 10nF capacitor rated at 20kV).
Connect the other terminal of the HV capacitor to the Input pin.
Connect the Ground pin to the lead that formerly connected to the pin marked DC (this is white with a red/orange stripe on my Videobeam).
Connect a 100k-Ohm resistor to the CTL pin.
Connect the other end of the 100k-Ohm resistor to chassis ground (white with yellow stripe wire on my Videobeam)
Connect shield of former Sense lead to ground end of 100k-Ohm resistor.
Connect center of former Sense lead to junction of CTL pin and 100k-Ohm resistor.
Connect a 100M-Ohm (high-voltage, about 5kV) resistor to the Focus pin.
Connect the other end of the 100M-Ohm resistor to the resistor that was formerly connected to the Focus pin.
Splice the HV lead of the NTE535 to the HV lead from the old tripler (since the new tripler doesn't come with the proper connector attached already).

I think that's about it... I don't imagine there are too many other people who want to perform this surgery, so I won't elaborate unless asked. I will post if I find something about this that ends up not working in the future, though.

The NTE530 is definitely simpler to hook up, but the big problem with it is that you need to make a 3-way splice on the HV lead instead of a 2-way and you need a separate potted resistor box to get that 300M-Ohm connection that's necessary for the Sense divider. I think I prefer the NTE535 with the above mods for the time being.

jmetal88 · Joined: 06 Apr 2017 Posts: 108

Additional info: The HV socket that the 3-way splitter for the anode leads plugs into appears to be an Amphenol Alden F310RX"L". I don't know why the L is in quotation marks, but I'm told by a representative of the company that it can be ordered with various cable lengths, so maybe that's to indicate you should specify a length when ordering? Anyway, I was able to get my hands on a sample with a 24" lead and can confirm it's nearly identical to the original connector (with the exception of the plastic being opaque instead of translucent), it fits on the mount, and it mates with the splitter in exactly the same way as the original connector.

I figure this might be useful information for anyone who needs to hook up a new tripler but doesn't want to cut the lead off their old tripler to make that happen.

EDIT: Actually, I've just had a look at the bag the connector came in, and yes, the "L" is where you're supposed to specify the length (they sent me a 24" cable and the bag is labeled "F310RX24").

jmetal88 · Joined: 06 Apr 2017 Posts: 108

I now have a full service manual for the Videobeam, and based on some further notes contained therein about the over-voltage protection circuitry, I believe an 80-Megaohm resistor in series with the focus tap on the NTE-535 may be more appropriate than a 100-Megaohm resistor, although I would have to experiment to confirm.

EDIT: Based on some voltage measurements I just took, I'm now fairly confident that 100-Megaohms is probably the correct value for this tripler. I was basically looking to see what voltage I was getting across a certain resistor that was said to be 'directly proportional to the anode voltage' in the service manual (it didn't say by what ratio, but since it implied you should measure across it if you lacked a high-voltage probe, I assumed 10000:1). I know it's not good to make assumptions, but under the assumption I'd made, a reading of 3V is what you should get across that resistor with the high voltage properly adjusted. Mathematically, I thought 80-Megaohms in series with the focus tap would get that, but I guess the tripler's focus output is at a higher voltage than I was expecting, as I wound up getting 3V across that resistor with my 100-Megaohm resistor in place.

jmetal88 · Joined: 06 Apr 2017 Posts: 108

Well, I found another note in the service manual that doesn't agree with my previous finding. It mentions a test point in the over-voltage protection circuitry that should read "20 to 25 volts" and with my series resistor mod, it only reads 12 volts. However, it is right in range if I take out the series resistor I added and just hook the stock resistor already in the machine straight to the focus tap. Unfortunately, that means I might have something else wrong with the over-voltage protection circuitry as it still shuts down a few seconds after power-up that way. Or it's possible that the epoxied-down set point pot just needs adjustment after installing a new tripler, but I'd have to replace it in order to adjust it. Or I guess it's also possible that the high voltage coming out of my new tripler actually is slightly too high and needs to be adjusted down (which may also be why my focus controls seem out-of-whack, I suppose).

**AnalogRocks** · Posted: Fri Jun 15, 2018 3:28 pm Post subject:

jmetal88 · Joined: 06 Apr 2017 Posts: 108

I thought I posted a second update last night, but I guess I didn't get to it... Anyway, as I was checking out other things in the circuit I happened to stumble across a transistor with a broken leg on the horizontal output board. I replaced it and could immediately tell that it had dropped my high voltage output. I readjusted the picture to be watchable on the lower voltage and was able to attain much better focus as well.

Then I checked it with my high voltage probe (which I had been assuming was way out of calibration) and it reported 29kV back to me. So I guess I was actually getting the high voltage too high, even though I thought the measurements on the voltage dividers were okay! From here, it should only need minor readjustment to get the levels to where they're supposed to be.

**AnalogRocks** · Posted: Fri Jun 15, 2018 9:13 pm Post subject:

This is starting to sound like the journey I had with my Ampro 4000. That turned out to be blown whoosit-whatsits on the thingamigigs Very Happy

jmetal88 · Joined: 06 Apr 2017 Posts: 108

It's alive!

barclay66 · Posted: Fri Jun 22, 2018 8:34 am Post subject:

Hi,

That looks awesome! I wouldn't have believed that the picture quality would be that good. My experience with this kind of technology didn't produce such a sharp and well-defined picture.
I had one of these: http://obsoletetellyemuseum.blogspot.com/2011/09/grundig-super-color-cinema-9000-gsc900.html

Kind Regards,
barclay66

Edit: An internal view of the Cinema 9000 http://obsoletetellyemuseum.blogspot.com/2011/09/grundig-super-color-cinema-9000-gsc900_24.html
I admired its modular design. Still have three TV sets and about 150 modules for this type of chassis...