465M


adesilva_1999@...
 

Gents,

Does anyone have any parts for a 465M by any chance? I had my scope start blowing the F558 fuse recently. It blows the fuse if the anode is connected. I had a spare CRT and that does the same thing. I also ruled out the EHT. Now it seems like something in the HV regulator area (per the service manual where the -2kV is controlled) has failed. I am yet to check the components there but there are some diodes that seem to be rare.

Any pointers/help is much appreciated.

Thanks in advance.


Mlynch001
 

I would say that the High voltage multiplier is bad.

--
Michael Lynch
Dardanelle, AR


adesilva_1999@...
 

The EHT section of the HV unit was ruled out with a separate tripler. It generates the 10kV required. The fuse blows only if it it connected to the CRT HV plug. I believe the current limiting (per the service manual) does not work for some reason. Hope to check out the transistors in that section along with the associated components tonight.


 

Check the socket carefully. I have seen shorts develop between pins of the socket because of arcing and carbonizing.

On 4/24/2021 7:12 PM, adesilva_1999 via groups.io wrote:
The EHT section of the HV unit was ruled out with a separate tripler. It generates the 10kV required. The fuse blows only if it it connected to the CRT HV plug. I believe the current limiting (per the service manual) does not work for some reason. Hope to check out the transistors in that section along with the associated components tonight.




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Mlynch001
 

The circuitry for the DC restorer circuit is likely shorted. The 465M DC restorer is build in along with the HV multiplier as a unit of U550.

--
Michael Lynch
Dardanelle, AR


adesilva_1999@...
 

Thanks, Tom and Michael. I will give those a try.


adesilva_1999@...
 

Here is an update:
I checked the tube socket and it is clean as a whistle! The U550 in this scope failed a while back in that, the tripler section started blowing the fuse. Since this is not available anymore, I created my own U550. I did it in a way that the tripler is removable from the rest of it. That contraption of mine worked for almost a year or more till it blew the fuse last week. It only blows if the anode is connected. Assuming that my tripler has gone bad, for whatever it is worth, I rigged another 3 diode/3 cap tripler and made sure the 10kV is there and then connected the anode. The fuse blew again. It tells me that my original tripler is still good. The next thing was to make sure the rest of the U550 is not gone bad. So, I hooked up the original TEK U550 (without the EHT connection) and hooked up the external tripler. Again the 10kV is good but no sooner the anode is plugged in, the fuse blows. This made me think the tube might have gone bad and is drawing too much current. Luckily for me, I have another spare tube. So, I hooked that up. Well, it is the same result! So, now I am lost. It is clearly too much current in the anode circuit but what is causing it? I read through the HV section in the service manual and can't say it is pointing me to anything in particular. I have checked the transistors in the HV regulator and HV oscillator etc. But did not find any issue. There still could be a diode that breaks down on load while showing good on the meter. The only major item I cannot check by replacing is the HV transformer as I don't have one. But if that was bad, wouldn't it blow the fuse even if the anode is not connected? Also, it does generate the 10kV. Unless, it is shorting under load, what else can go bad?


DaveH52
 

I had a problem with my 465M that turned out to be C582, so I replaced it and C584 as well. I have another Tek scope that has a very similar HV circuit that kept blowing the equivalent fuse. The transistors equivalent to Q556 and Q553 got rather hot. I wound up replacing all the semiconductors in that entire circuit. and it seems to be happy now.


adesilva_1999@...
 

Thanks Dave for the reply. In my case, all the supplies are perfect. Even the Anode supply of 10kV is present. That is until the anode is connected when it blows the fuse. The way I see it, it is almost like the tube anode draws too much current. This is why I suspected the tube itself but even with the replacement tube, it does the same thing. It may be that something is not limiting the anode current. OR, may be the HV transformer is unable to take the load which is hard to test.

Like you did, I want to pull a bunch of parts in the vicinity and replace them just for the heck of it. If that test fails, it points the finger to the HV transformer in my opinion.


DaveH52
 

You can try to find a HV probe like the ones used back when TV sets had CRTs (like I did). The CRT beam current shouldn't be that high so that it presents much of a load. I now recall that I did have an issue with the insulation failing on the HV flying lead. I painted it with super corona dope and mummified it with tape because replacing it seemed to be out of the question.


Bill Riches
 

I have one of the Pomona HV  metered probe that I will lend if someone needs to make a measurement.
73,
Bill , WA2DVUCape May

On Saturday, May 1, 2021, 02:56:14 PM EDT, DaveH52 <ac2gl.dave@gmail.com> wrote:

You can try to find a HV probe like the ones used back when TV sets had CRTs (like I did). The CRT beam current shouldn't be that high so that it presents much of a load. I now recall that I did have an issue with the insulation failing on the HV flying lead. I painted it with super corona dope and mummified it with tape because replacing it seemed to be out of the question.


adesilva_1999@...
 

Bill and Dave,
Thanks for the replies. I have an HV probe (Kaise SK-9000: http://www.electronicrepairguide.com/high-voltage-meter.html) and that is how I checked the 10kV at the end of the tripler. It is dead on 10kV without the anode connected. I also tried 2 different tubes as mentioned. So, the issue of corona is not there in my opinion. I know some people have had the HV transformer in old scopes failing and if that is the case here, this is going to be a dead cause. But I have no way of checking the transformer-in case it fails only on load.

Since the original U550 failed and I made my own U550, I wonder if I damaged anything else while it was in use for almost 1 year.