No o-scope display from my MI3

efhjr

efhjr

Can you be more specific?
I decided to dust off my MI3 and start using it. I took the top off of the chassis, reseated all of the tubes, and fired it up.

All of the tubes are glowing, and I saw some orange glow from the base of the 3RP1, but there's no light coming out of the other end of the 3RP1.

This is with no other component plugged into the MI3, if that matters.

What's my next step? A new 3RP1? The other tubes seem healthy (only one of the 6EA8s is a McIntosh/RCA original, which is pretty cool).
 
The next step is to verify the high voltage to the CRT, you will need a schematic and voltmeter capable of reading 1200 volts. This is a dangerous voltage so I do not recommend checking it if you are not experienced in working around these voltages.
The most common failure in the MI3 is the high voltage winding in the power transformer opening up or shorting. This is a case of getting the transformer rewound, I use
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/id48.html , he has done several for me.
 
dewickt said:
The next step is to verify the high voltage to the CRT, you will need a schematic and voltmeter capable of reading 1200 volts.
Click to expand...

Thank you, Terry. I'm not qualified or equipped for that kind of voltage.

Funny thing is, there's a piece of masking tape on the transformer with a price written on it ($75 or so, I think); so I suspect it's already been replaced.
 
Yes, don't mess with any of the MI's if you don't know what you are doing. The only pieces I stayed away from in my ownership: MI2, MI3, and MI-200 amps. Way over my skill level.
 
I dunno how these scopes are specifically done, but there are many modes of failure and malfunction possible that either skew the trace off-screen or deplete high voltage that have nothing to do with the crt jug failing, and the red glow near the base almost assures that this is not the source of the fault. As mentioned, this is not a good DIY for inexperienced or ignorant hands for technical and safety reasons.

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I have repaired many scope receivers, tuners and lab scopes- as Terry says, first thing is to check to see if the high voltage side of life is working.
 
Judging by the sticker on the transformer (045-693, $77.80 LIST), and the sloppy solder on the power switch, this thing has been on the bench before.

Might be time to send it off to someone else's bench for some work.
 

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efhjr said:
Judging by the sticker on the transformer (045-693, $77.80 LIST), and the sloppy solder on the power switch, this thing has been on the bench before.

Might be time to send it off to someone else's bench for some work.
Click to expand...

Most if not all of them have been on a bench before. I had a completely unmolested MI2 that had never, ever been touched and I sold it for $720 or so. It was really nice inside, though it needed some updates but I chose to keep it all original just because so many have been messed with.
 
CountD said:
Most if not all of them have been on a bench before.
Click to expand...

I find this surprising for a McIntosh component (but hey, I know they all can't be perfect). Were they really that unreliable?

Is there some way to upgrade the MI3 to make it more reliable?
 
Arc'ed over HV windings aren't unusual with scope tube gear powered directly from the power transformer, such as kit scopes from the CRT days. Seems that there's an upper limit for reliability of an unpotted HV transformer ca 600VAC to ground and up.
Subbing a standard power transformer and powering the scope with something like a well-shielded HV module from a checkout counter gas laser scanner might be a way out tho' the heater is typically at HV- minus with the cathode to get the dynode deflection voltage down to b+ levels in DC coupled scope ckts.
Again, depends on the specific application, scopes with AC cap coupled dynodes could run heater from the 6.3VAC heater string for the rest of the tuner.

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Pio1980 said:
Arc'ed over HV windings aren't unusual with scope tube gear powered directly from the power transformer, such as kit scopes from the CRT days. Seems that there's an upper limit for reliability of an unpotted HV transformer ca 600VAC to ground and up.
Subbing a standard power transformer and powering the scope with something like a well-shielded HV module from a checkout counter gas laser scanner might be a way out tho' the heater is typically at HV- minus with the cathode to get the dynode deflection voltage down to b+ levels in DC coupled scope ckts.
Again, depends on the specific application, scopes with AC cap coupled dynodes could run heater from the 6.3VAC heater string for the rest of the tuner.

Sent from my SM-N900V using Tapatalk
Click to expand...

There you go. I was wondering why so many MI2s and MI3s got smoked early in their careers. Out of 5 MI3s I had, 4 were messed with. Would be interested to see what kind of transformer you could run in there for better reliability. Once again, the transformer was not made by Mc, it was outsourced by some UL compliant company.
 
I'd like to see a full skiz posted here, I found a partial that indicates the jug dynodes are DC coupled to the driver anodes. This means it's almost certain that the cathode/heater is at full HV-minus and this would mean the full HV is on the heater winding for the jug, an arc-over failure in the power transformer is likely. Having an elevated jug cathode heater like this is a serious plot complication for a work-around, a replacement from an old working tube era tube scope with DC coupled dynodes might be the only fix if it can't be rewound.

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Mine with replacement 6K3VG transformer and seleniums replaced with 1N4007.

This is quite a bit of work and I couldn't have done it without a lot of guidance from Terry.

GgHUDDt.jpg


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Found another partial skiz with the crt jug and ps. Cathode/heater is at 1300V- with the forward end at ca 180/300V+. Accelerating voltage total ca 1600V with 1300V on the jug heater winding of the power transformer.
Terry no doubt know his way around these things and what subs for the pt are suitable with what adaptations.

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Should light up, check the 6U8/6GH8 tubes, they should be fairly close matched pairs otherwise you will not be able to get the trace on screen.
 
What Terry said.
One of these tubes not lighting will skew the trace way off the screen out of range of the controls as will a defective tube.
Try reseating them and check the centering as it's been in storage.
FWIW, a bad power transformer will usually have a strong burned smell unless potted, otherwise the primary fuse usually blows eventually if not immediately.

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