Risograph GR 3770 Offset problem

[Guest]

We've been experiencing a new issue with huge > 1" offsetting of image that varies from page to page. The paper feed might be at fault, because speed of paper feed seems to affect the offset. We've checked the feed tires and stripper pad, which are fairly new, and adjusted the tension in numerous ways. The timing wheel/registration is in position, which, if I'm not mistaken, will only cause a regular/fixed amount of offset anyway. So if it's really a paper feed issue, I'm not sure what else to look at.

At the same time, there seems to be something occasionally failing where the drum clamp can be manually opened and closed (not manually, but in test mode), but sometimes will fail to lock in during normal operation. The T14 error occurs multiple times and then will go away. The clamp safety switch on the back seems to operate fine.

Can this be related? I wonder if anyone has any ideas?

[cobiray]

Sounds like separate issues IMO.

The registration could be a few things. Clean the timing and guide rollers, check the one way gear and spring on the timing roller in the back for wear. If the inner race of the spring is shiny, replace it. Clean the one way gear and look for a wear mark where it rides on the shaft. On especially high mileage units this roller will need replaced.

For the T-14, check the magnet on the clamp. This is on the back of the drum It should be 180 degrees opposite the clamp itself.

[DNORMAN76]

Will check these items on the feed. Thanks
Re the t-14 errors, the behavior occurs when any drum is loaded.

[cobiray]

Could be the solenoid to engage or the switches may be dirty/worn. You can clean the switch contacts, plunger and solenoid body.

[joelsa12]

Here is my 2 cents if it is worth anything. I agree with all the cleaning that cobiray has mentioned. Do all that. Is this bouncing of the registration happening on regular 20 lb bond? What size is the paper? Depending if you have the old style paper feed clutch( it will have a brown wire), you can take this apart and clean the clutch. Dirt builds in there and will cause the clutch to stay engaged too long causing your registration problems. Also check the the hubs that the feed tires are on. They are one way locking hubs. They can get dirty and cause only one of the feed tires to opperate improperly. You can take them off the saft and clean the inside of them. If this is happening on only 11 by 17 paper there may be another issue. Keep us posted

[DNORMAN76]

Thanks for all the tips.
Have not identified the paper issue -- but we probably have resolved the clamp/t14 errors!
Upon disassembling the entire clamp motor/assy, we found a broken tooth on the drive gear. Hard to find, easy to fix if we can get the part.
Now we will go on to the other paper feed suggestions - i.e. the clutch and the feed tires maybe binding up or something like. We'll first run some tests on other paper sizes -- we mostly run envelopes and 8.5x11 on the machine, but we haven't analyzed the movement across different papers.

[ianmck]

3750 offset problem solved like this....

I don't know how siilar the 3750 and 3770 are but I had a problem with unstable print position on a 3750. I found the P Disc needed adjustment as it was misaligned. When this was corrected the printed image appeared consistently in the same place.

[DNORMAN76]

Hey, thanks. I fixed the clamp issue, bought a new piece of rubber for the stripper and the machine is working great, but continues to have the "unstable" print position (good description!). What

is the P disc? The timing/registration wheel is aligned. We've had to adjust it before so I know what to do with it (align the small holes...) Is this what you mean?

[ianmck]

Location of P disc

I am honestly not sure if the GR3750 and 3770 are alike on this issue but on the 3750 the P disc is on the outside of the axel the guide roller cam and cam pulley.

The disc passes between through the paper feed clutch centre mounted at 3 o'clock to the axel. The is a notch on it and you aling the notch to the upper corner of the sensor for B4/A4 paper or the lower corner for A3 paper.

I have a diagram of the 3750 setup with instructions on how to align I can send you if you email me c/o ianmck@bethere.o.uk

[cobiray]

If the registration varies (unstable as you say) you likely have an issue with the timing and guide rollers. If you pull the drum out they are on the left. Start by cleaning them. If that doesn't affect the output you likely have a worn one way gear or tension spring. It is also possible the shaft is worn out and the roller assy may need replaced.