Mcintosh mpi-4 cathode tube replacement

[grange45]

Hi all,
This evening my mpi-4’s display suddenly stopped working. When I turn it on and play music I only get the green glow with no oscilloscope action. I believe this is the cathode tube needing to be replaced. It probably has over 1000 hours on it since I bought it 6 years ago along with the hours that the past owner had on it.

Can someone steer me in the right direction for a replacement tube?

 

[chazix]

I only get the green glow

If you see anything on the CRT face, I believe it is extraordinarily unlikely that the CRT is at fault. I think someone will need to troubleshoot the deflection circuits, or perhaps the power supply for those circuits.

That is - and pardon me for suggesting it, but it's fundamental - if you're sure that the problem is internal to the MPI-4. Have you tried anything to confirm that signal is getting to the MPI-4 inputs?

There is a McIntosh-specific forum in AK, BTW. You could expect to find the highest density of MPI-4 experts there. (I'm not one, more of a generalist.)

 

[w3jn]

Indeed, the CRT is undoubtedly good if you're getting a green glow. Are you getting a horizontal line when in L, Dual trace, or R? Does changing the sweep speed make a difference?

Note that if you're in Multipath mode you'll just get a dot on the face unless you have the appropriate connections to your tuner.

If you'd like this moved to the McIntosh forum just hit "report' and we'll move the thread for you.

 

[gadget73]

+1 on glow = good CRT and high voltage supplies. If there is no dot on the screen at all, fair guess one of the deflection circuits is goofy pulling it off track. If the V and H circuits are doing nothing you should just get a dot in the middle.

CRTs rarely "go bad", they usually just get weaker and weaker over time until the display is dim or fuzzy. It is a vacuum tube at it's heart and its got a heater that can die but when that happens it does absolutely nothing on the screen.

 

[grange45]

+1 on glow = good CRT and high voltage supplies. If there is no dot on the screen at all, fair guess one of the deflection circuits is goofy pulling it off track. If the V and H circuits are doing nothing you should just get a dot in the middle.

CRTs rarely "go bad", they usually just get weaker and weaker over time until the display is dim or fuzzy. It is a vacuum tube at it's heart and its got a heater that can die but when that happens it does absolutely nothing on the screen.

Thanks for all your replies!

After messing around with the buttons, the only time I get something is when it’s in the stereo mode selector. It’s very faint but the green signal is there when the screen is red.

I didn’t have this problem until after keeping it on the same mode for years I selected this one for a change…so I’m guessing one of the deflection circuits? If so is there any troubleshooting that I can do before sending it to someone with an oscilloscope?

 

[w3jn]

Dirty switches is the first thing I’d suspect.

 

[gadget73]

^^ inclined to agree here, if they haven't been used in a long time contacts can get pretty crusty.

 

[grange45]

That makes sense and appreciate all the responses.

To give an update: I found that it has a very faint display for about 15 minutes and then something just clicks and it starts working again until I turn it off. I’m planning on taking it out it’s wooden case to clean the switches. I’ll let you know once I do it.

 

[grange45]

Now there is nothing but a faint display. Planning on opening it up during the holiday weekend.

 

[grange45]

Update: I cleaned all the switches and pots and still no change. The display has progressed to nothing when the display is positioned to the center but when I move it vertically or horizontal there is a little brighter display. Does this make it more than likely it’s the tube? None of the filter caps or resistors look to be in bad shape.

 

[cademan]

None of the filter caps or resistors look to be in bad shape.

You cannot tell by looking at capacitors if they are bad unless the metal can itself is grossly bulging and/or leaking electrolyte fluid.

Find a schematic and use a multimeter to troubleshoot. If you can find the voltage listings for the tube, then you can measure it and find out if you need to have it tested.

Simple things first. When you said it would come on after 15 minutes, that kinda reminded me that you might have a loose connection or cold solder joint somewhere that when it gets warmed up, it kicks in. If you can solder, check it out!

 

[grange45]

So over a year I finally sourced an nos crt tube. The old tube was getting dimmer and dimmer and finally showed nothing. I installed it and am getting a very bright green dot but no activity even when switching different functions. Is there a setup that needs to be done or would it doing this show a more internal problem? Thanks!