Hello everyone,

I have a Riso HC5500 (yes, old, but still in service) and it's having a problem with the Magenta head.

Those of you who are familiar with the HC5500 know that it has 24 printheads arranged in 4 banks of 6 heads each. Each bank of 6 heads is one of the C,M,Y,K colors.

I am having a problem that is confined to the Magenta head bank. C,Y and K are not affected, only Magenta.

Four sample images are attached. The photos are crops of 8.5x11 sheets fed in landscape mode so all 6 heads print across the sheet. (1 and six are partially "not there" because the machine can actually print 12 inches wide, but the 8.5x11 was enough to show this problem in landscape.)

When you start printing, Head 4 looks OK but within 10 sheets of this test bar, it completely drops out.

The images below show the dropout sequentially (Image1, Image2, Image3, Image4) as it is just beginning and then finally, when the entire head is not firing at all.

If you run HyperCleaning on the Magenta head bank, it comes back, prints the full bar, but then does the exact same thing: within 10 sheets, that head is completely gone, like it's out of ink.

The other heads have small dropouts on some of the nozzles but that's not my concern here. I'm wondering:

1) WHY Would only head 4 have what appears to be this ink starvation problem? All 6 heads are fed through the same ink distribution unit, 1 tube per head.
2) Has anyone seen this kind of behavior before and do you have any idea how to fix it?

I know I'm probably whistling past the graveyard on this but I would be very, very appreciative if anyone has an idea to share with me.

Thank you in advance,
Alex
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