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Hey all,

I just acquired a 556, but it's having some fairly serious issues. Before powering it on, I checked all the supply rails for shorts with a meter, and checked the power plug for shorts as well. I then proceeded to slowly turn the scope on with a variac. Once at full voltage for a few moments, I heard a bang and smelled that something had gone wrong. Surprisingly, the main rails are all healthy (350,225,100,-150). The high voltage on the upper beam is at it's nominal voltage, but the high voltage on the lower beam is around 100 volts. Boooo.

Okay... so now I wonder... if the post-deflection acceleration (supplied by the lower beam supply only) is dead, will the CRT still light? What are the chances that I blew up the HV transformer itself and will need to wind a new one? What components are most likely to have exploded in that area?

Evan

Apologies... two missing details... I couldn't see any obvious damage under the HV cover, (no charring or whatever), and I don't clearly remember if it's +100 volts or -100 volts that I measured on the -1850 supply for the lower beam.

I have never played with a 556, so I don't know the specifics..but assume
you have checked/replaced the caps where necessary.

regards
Rajesh

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/Rajesh

Okay, I've found some additional evidence of what's gone wrong. Both R1300 and R1301 have exploded. I'm curious what I should try replacing and how I should turn the thing back on after the pieces have been replaced. I don't want these things to explode in my face if something else is causing them to fail.

Okay I managed to get my hands on a tube tester, and it looks like V1300 is dead. There's a very good chance that V1300 just simply gave out at high current... but I really want to rule out a short elsewhere on the HV side... potentially inside the transformer or further down the line. Does anyone have any suggestions for how to test the windings on T1301... or any insight as far as what components in the HV box might be shot?

From bitter experience I can confirm that both HV transformers in the 556 are prone to the thermal runway problem that has been discussed here ad nauseam with regard to the 547 and some other scopes. T1301 produces the PDA accelerating voltage for the CRT so if it's not working you won't see any trace. The only cure is a replacement or rewound transformer. It is possible that a failed transformer could cause the oscillator tube to fail and that could be what you are seeing. I had both transformers in my 556 rewound by a friend - not an easy task and one requiring many iterations on a test jig before acceptable results were obtained. The mechanical arrangement of those two HV boxes makes working on them quite a challenge.

Sorry to be the bearer of bad news :-(

Morris

Morris?Can you get the info on wire size and turns required to rewind transform from your friend that did them , was thinking of trying to rewind my 547 and 556 in case my 556 hv went down , thanks .
On Thu, Aug 9, 2018 at 1:57 AM, Morris Odell<vilgotch@...> wrote: From bitter experience I can confirm that both HV transformers in the 556 are prone to the thermal runway problem that has been discussed here ad nauseam with regard to the 547 and some other scopes. T1301 produces the PDA accelerating voltage for the CRT so if it's not working you won't see any trace. The only cure is a replacement or rewound transformer. It is possible that a failed transformer could cause the oscillator tube to fail and that could be what you are seeing. I? had both transformers in my 556 rewound by a friend - not an easy task and one requiring many iterations on a test jig before acceptable results were obtained. The mechanical arrangement of those two HV boxes? makes working on them quite a challenge.

Sorry to be the bearer of bad news :-(

Morris

I have the parts to build a winding machine if necessary. What did the test jig look like?

I'm mostly curious how you determined if the wound coil was good without installing it inside the scope.

I have the Tek specification drawings for both the 556 HV transformers - I'm not sure if they're in the group files but if not I'll upload them.

When I mentioned a jig, what i was referring to was a test jig for the transformer to avoid the fiddly job of having to refit it into the box until it was working properly. It's necessary to make sure the rewound transformer is capable of withstanding the high voltages without arcing. To do that I lashed up the circuit on an old chassis I had lying around with the correct tube and other parts. I ran it from a suitable vacuum tube era bench power supply. I didn't include the feedback system. I just set it up to deliver roughly the correct output voltage by adjusting the oscillator screen voltage. When I was happy with it I reinstalled it into the box and replaced the PDA rectifier diodes with suitable fast high voltage silicon diodes which were not easy to find.

Of course it helps to be experienced in working with high voltage vacuum tube circuits and also to have a suitable power supply for testing. This is NOT a job for a newbie!!!!

Morris

I have uploaded scans of the hv transformer drawings to a folder in the Photos area called (not surprisingly) "556 high voltage transformers".

Good luck!

Morrios

Thanks for the info morrios , do you have anything points that you think will help in this process ??? Things to pay attention to.?
On Sat, Aug 11, 2018 at 5:22 AM, Morris Odell<vilgotch@...> wrote: I have uploaded scans of the hv transformer drawings to a folder in the Photos area called (not surprisingly) "556 high voltage transformers".

Good luck!

Morrios

do you have anything points that you think will help in this process ?? Things to pay attention to.

Only to be careful with the high voltage and the complex reassembly process. My replacement transformers were layer wound but the originals are pie (universal) wound. Chuck Harris on this group has wound beautifully made replacements for the 547 transformer that is similar but not identical. I don't know if he might be able to do a pair of 556 trannies.

Good luck!

Morris

Hello Evan,
I have what is left of a 556 scope. I just checked and both high voltage (plastic rectangular) enclosures are still in the chassis. The front panel and low voltage transformer are also available. Don't know what condition. Free except whatever it takes to get it out of here. I already took the tunnel diodes and the tubes are missing. Would like to get rid of it. Plus lots of other Tek stuff.
Carl (W9CJH)

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