Tektronix 465

[rqsall]

You need to remove the vertical board, not the main board.

Are you sure you can access the transformer and the diode by removing only the vertical board ? Have you already done it ?

Yes, I've done it. I had to replace the HV multiplier on mine. The diode is soldered directly on the input of it. The manual describes the procedure for replacing the HV multiplier, follow that, and you should be fine. If it's just the diode, you will not have to remove the tube.

Good luck.

[Grapsus]

You're right, page 54 "High voltage multiplier" looks like I need to remove the vertical board and the CRT, it's way better than removing the main board

[Grapsus]

Even worse news today 

I finally disconnected the vertical board and got into the HV can. In fact the high voltage diode seems OK. The low impedance I observed from the outside of the can seems to come from the HV multiplier block. The resistance I measure between GND and IN terminals is around 90 Ohms, no wonder the secondary stays at ground. Tomorrow, I'll try to run the scope without the multiplier in order to make sure it gets the transformer going and confirm the fault.

rqsall looks like your initial guess was right, my HV multiplier is toasted. How would you approach repairing it now ? Buying a replacement multiplier will probably cost 3 times the price I paid for the scope. You mentioned making one yourself ? Is it a classic Villard cascade ? Is so, I never designed one myself, what is the exact topology used by tektronix ? the component ratings ? Is it possible to remove the potting from the existing multiplier and replace just the faulty component ?

[tautech]

SM identifies it as 152-0442-00, Sphere have a lising that they describe as for late production models: 152-0635-00 but sold out. Described as Murata MSL8521

http://www.sphere.bc.ca/test/tek-parts/tekparts5.html

Might be another option. 

[rqsall]

Even worse news today 

I finally disconnected the vertical board and got into the HV can. In fact the high voltage diode seems OK. The low impedance I observed from the outside of the can seems to come from the HV multiplier block. The resistance I measure between GND and IN terminals is around 90 Ohms, no wonder the secondary stays at ground. Tomorrow, I'll try to run the scope without the multiplier in order to make sure it gets the transformer going and confirm the fault.

I should have checked my notes when I replied earlier instead of going from memory. The base of Q1404 sits at a higher voltage when the HV multiplier is shorted. It was 3.2 on mine apparently, which makes sense because it being a negative circuit, "collector pulled to ground" means "pulled up". Sorry for that.

rqsall looks like your initial guess was right, my HV multiplier is toasted. How would you approach repairing it now ? Buying a replacement multiplier will probably cost 3 times the price I paid for the scope. You mentioned making one yourself ? Is it a classic Villard cascade ? Is so, I never designed one myself, what is the exact topology used by tektronix ? the component ratings ? Is it possible to remove the potting from the existing multiplier and replace just the faulty component ?

You can forget about removing the potting. And yes, it is a Villard, or Cockroft-Walton, I guess that depends on whether you're French or not ;-)

First thing I would do now is to verify the CRT is still working. Desolder that diode from the input of the HV multiplier, optionally also desolder the multiplier's ground lead (you're going to have to anyway if you're going to replace it). And turn the scope on. No need putting the vertical board back in. You should see something like I posted in post #8 of this thread. I don't think fiddling with the horizontal control or beam finder should be necessary, but if you don't see anything showing up, it can't hurt to try.

If you see a trace I suggest you sign up to the yahoo tekscopes discussion group. There's a photo section on it where the module is X-rayed and also some photos of DIY multipliers. I almost went that way, bought the caps and diodes to make the cascade, but then I came across a used unit for a decent price. If I had payed no more than a few dozen euro for the scope (I got it free out of an inheritance), it would be worth it to me to shell out roughly the same (max 40 euro including shipping) for a multiplier that is known to work.... if the CRT can be confirmed to be OK.

Good luck.

[Grapsus]

No more doubt about fried multiplier. Here is what is looks like with multiplier's GND disconnected. I'm still waiting for access to this TekScopes list.

[Guni]

They have it at stock for... 48Eur
http://www.qservice.eu/shopdisplayproducts.asp?page=4&id=61
and here
http://www.ebay.com/itm/TEKTRONIX-152-0703-00-CMX426-High-Voltage-Multiplier-/380543774994?pt=LH_DefaultDomain_0&hash=item589a2d3112

[Grapsus]

CMX-426 doesn't look like the multiplier in the tek 465. Maybe it can be adapted in some way ? The exact part I need is 152-0552-00 and the link you provided says it's out of stock.

[PaulAm]

While you're waiting for the Tek  group OK, take a look at this thread:

http://www.vintage-radio.net/forum/showthread.php?t=85145

It starts out about the 7603, but includes some info on the 465.  Apparently that's a X6 multiplier.  The going price for one seems to be around $50, but you could roll your own if you're careful and follow good HV practice.  12KV is not something to be careless with.

[tautech]

You might like to try a few chemicals on the PDA module potting, MEK, IPA, Toluene and the like. 
Have a measure up first, you'll probably need to rebuild with a new enclosure.

[Grapsus]

While you're waiting for the Tek  group OK, take a look at this thread:

http://www.vintage-radio.net/forum/showthread.php?t=85145

It starts out about the 7603, but includes some info on the 465.  Apparently that's a X6 multiplier.  The going price for one seems to be around $50, but you could roll your own if you're careful and follow good HV practice.  12KV is not something to be careless with.

Hey, thanks for the link. I previously found another relevant topic on the same forum:

http://www.vintage-radio.net/forum/showthread.php?t=96958

The guy replaced the multiplier with an x5 part and manually added one stage. They misleadingly talk about a "tripler", but you're right it must be a x6 multiplier. It's the only way to go from -2.45 to 14.5.

I don't know much about HV, but I thought that in a Cockroft-Walton circuit each part had only to withstand the input voltage. Why did they choose 12kV diodes ? Is it extra security ? 4 kV diodes seem to be easy to find, but 12 kV ?! those aren't even listed on Farnell, Mouser and others. I'd be ok with a DIY replacement, but I don't have an HV probe so there will be no possibility for experimentation, I'd have to built a circuit that's been proven to work. And frankly it kind of scares me. When I was getting the multiplier out of the scope, I must have touched a cap that still had some charge from the -2.45 kV rail, it made one of my fingers numb for a minute, 14 kV must be even worse and it can make sparks at > 1cm 

[Grapsus]

You might like to try a few chemicals on the PDA module potting, MEK, IPA, Toluene and the like. 
Have a measure up first, you'll probably need to rebuild with a new enclosure.

Yep, I still have hope for dissolving the potting too. But it feels like very thick and dense epoxy. rqsall says it's impossible. Maybe with an x-ray image at hand I could strategically drill around the first stage and only replace one faulty diode ?

[tautech]

You might like to try a few chemicals on the PDA module potting, MEK, IPA, Toluene and the like. 
Have a measure up first, you'll probably need to rebuild with a new enclosure.

Yep, I still have hope for dissolving the potting too. But it feels like very thick and dense epoxy. rqsall says it's impossible. Maybe with an x-ray image at hand I could strategically drill around the first stage and only replace one faulty diode ?

Possibly.
Potted stuff I have re-worked was of a barely transparent nature that allowed one to inspect as you go in good light. X-ray image should be a great help, you'd imagine the units to be all very similar, but they might differ in immersion depth. 

Gentle work with a Dremel and by hand would be my choice if chemicals won't work.
Take photos and measurements first, it might end up unrecognizeable. 

[Grapsus]

Apparently that's a X6 multiplier.

The guy replaced the multiplier with an x5 part and manually added one stage. They misleadingly talk about a "tripler", but you're right it must be a x6 multiplier. It's the only way to go from -2.45 to 14.5.

My bad, it's called a tripler because it multiplies 4900 Volts peak-to-peak by three. The additional section must have been added because the replacement multiplier was too weak. It also explains the 12 kV rating of the diodes, it's two times the peak-to-peak voltage.

I have the x-ray images now, I will try to somehow disassemble the broken multiplier before looking for a replacement or a DIY build.

[SoundTech-LG]

I have a spare blown tripler off a 465 as well. Are you going into business? 

[Grapsus]

I finally got access to the TekScopes group. The x-ray images are publicly accessible here: http://pp5ms.com/tek_hv/1.jpg (it goes up to 9.jpg)

I also found a long discussion about building a replacement multiplier from discrete components. And there is like a 30 post discussion about whether it's a "tripler" or a x6 multiplier. It was really confusing. So I watched carefully EEvblog #469 "Voltage Multipliers" and I finally understood the circuit. On the x-ray photos we clearly see 6 capacitors and 6 diodes. Therefore the circuit is a three-stage Cockroft-Walton multiplier. It multiplies the input peak voltage by 6. The input voltage is 2.45 kV peak. Hence the HV output is 14.7 kV which agrees with the service manual. Also we see in Dave's video that each stage gets the previous stage DC voltage and the input AC voltage riding on that DC. So each stage has to withstand the input peak-to-peak voltage around 4.5 kV which is 10 kV with a 2x security margin. That explains why 12 kV diodes and 10 kV caps were used.

I tried to disassemble my dead multiplier to make sure it was the same. I confirm that it's hopeless trying to remove the potting. Acetone and IPA didn't work, I don't have toluene. But it really looks like a polymerized kind of stuff that is almost inert once it settled. I tried drilling it and even at low speed it got so hot that it started burning. Desperate, I smashed the thing with a hammer and it took a lot of hammering to see something.

My HV multiplier was a little different from the one on the x-ray photos. The arrangement is the same, but the capacitors are bigger so that the circuit occupies the whole box. I managed to measure one of the capacitors, it just had a small corned chipped so I could access both plates. They are 1800 pF. Also I found a 470k carbon powder series resistor before the HV output. We can see the same resistor on the x-ray photos, it appears as a gap in the output line.

There doesn't seem to be any replacement part available right now, so I guess I will have to build my own multiplier. rqsall pointed me to the ebay store "high-voltage-hv" which has 100 nS 12 kV diodes and 10 kV caps. Do you think 100 nS recovery will be enough for this 50 kHz multiplier ?

Regarding the output resistor, why do you think they used a regular resistor instead of a HV one ? Does it have to be carbon powder or can I use a metal film as well ? I found those on Farnell :

"OHMITE  SM108035003FE  RESISTANCE 20KV 500K 1%" 6 euros piece
"OHMITE RESISTANCE COMP CARBON 470K 0.25W 5%" pretty cheap

Any suggestions about the potting compound ? I don't think paraffin is a good idea, the scope gets pretty hot. Maybe silicone or polyurethane ? something that can be removed later without destroying everything.

[oldway]

You can try to use an ERO multiplier.
Look for "ero kaskade" on ebay.de

I think an ERO HV multiplier from an Hameg HM604 or HM605 (BG1899-882-201) could do the job.
Hameg is still selling spare parts for those oscilloscopes.

http://laehnversand.de/repair/ersatzteile-audio-video-tv/ero-kaskaden.html

[tautech]

Any suggestions about the potting compound ? I don't think paraffin is a good idea, the scope gets pretty hot. Maybe silicone or polyurethane ? something that can be removed later without destroying everything.

https://www.eevblog.com/forum/testgear/high-voltage-oscilloscope-probes-make-or-buy/msg652364/#msg652364

There is a good choice of potting compounds available from all the major suppliers.
Dielectric strength rating will be most important IMO.

It would seem it won't cost much for componentry, probably the potting will be the most expensive.
Might be a good idea to buy enough components to make a couple more in case you need to.
If you get the "recipe" right firt time you can make some more from the left-overs and flog them off.
Best you get a few months use in first. 

[rqsall]

There doesn't seem to be any replacement part available right now, so I guess I will have to build my own multiplier. rqsall pointed me to the ebay store "high-voltage-hv" which has 100 nS 12 kV diodes and 10 kV caps. Do you think 100 nS recovery will be enough for this 50 kHz multiplier ?

Regarding the output resistor, why do you think they used a regular resistor instead of a HV one ? Does it have to be carbon powder or can I use a metal film as well ? I found those on Farnell :

"OHMITE  SM108035003FE  RESISTANCE 20KV 500K 1%" 6 euros piece
"OHMITE RESISTANCE COMP CARBON 470K 0.25W 5%" pretty cheap

Any suggestions about the potting compound ? I don't think paraffin is a good idea, the scope gets pretty hot. Maybe silicone or polyurethane ? something that can be removed later without destroying everything.

I've heard from people on tekscopes that used beeswax, or would use it. But the thought scares me. Not being able to find affordable potting compound that I trusted is why it took me so long to get started that I eventually found a replacement.

As far as the resistor, I don't think it needs to be high voltage rated. There's barely any current going through this circuit, and even if it was 1 mA: V = I*R => 470000*0.001 = 470Volts and I doubt the current is more than a few dozen uA. Since it's so low a current, I doubt it matters if it's carbon or metal film. But someone should correct me if I'm wrong.

100ns for the diode is what I bought and I remember reading somewhere that was enough.

[Grapsus]

I ordered 12 kV diodes and 2200 pF capacitors from this ebay store high-voltage-hv. It ships from China, so I guess I'll have to wait for a week or two 

Regarding the potting compound, how about this one ? http://fr.farnell.com/pro-power/ppc124/potting-resin-pu-250g/dp/1776512

It's super cheap. It's polyurethane, so it may be removed if maintenance is required. And it has a dielectric strength of 11.5 kV/mm. If I'm careful about leaving at least a centimeter between every terminal it will give me a 100 kV security margin. Is it enough ? I really don't have any experience with HV.

If I get the right recipe and the resulting module proves itself working in the long run, I'll be glad to make extra units to save some fellow scopes.

[rqsall]

I think that potting compound should do it. Not sure if 250 grams is enough though. I guess I could weigh my old (broken) one.

[Grapsus]

I read on TekScopes that the compound used in the original multiplier was filled with glass or ceramic dust (that's why it's so hard to cut) so it is extremely dense. I think it would be more appropriate to estimate the required mass of compound with the volume of the box. But even if I need two bags of compound it's still pretty cheap.

Also I'll start looking for a kind of vacuum tupperware that will be necessary to get the bubbles out of the compound before it settles.

[tautech]

I'd consider heating the potting and pot to reduce viscosity to let bubbles rise to the surface more easily.
Datasheets of the compound you use should give guidance to application temps and conditions.
How you construct the trebler might also help how the compound to "floods" the enclosure.

[Grapsus]

Victory !

[tautech]

That potted real nice.
What diode did you use?

To avert any risk of the GND and HV lead breaking off, you could have used an eye terminal and solder leads to it, if you have the room around the potted trebler. Thinking of the old stand-off PCB test point type terminal. Then a snug fitting sliding sleeve over the joint.

Anyway looks fine for purpose.