speed problem on Riso GR3770

**bcp** · 07-26-2008

I have a Riso GR3770 which works fine at the slowest speed, but at any speed above the slowest the unit makes a sound like the drum is hitting something at the same spot (same time each revolution) If it is run at higher speeds the quality of print remains unchanged as well as reistration, but still it makes the sound. I realize it hard to know what problem is but just wondering if anyone has experienced this. thanks

**techathens** · 07-26-2008

You probably have an issue with a drum support roller [000-01169-xxx] - the three rubber rollers on the operator side of the drum that the drum rests on, or is supported by (on the frame - Technopedia: GR Series Parts Manual; pdf 19011 / Fig. 11 part # 42). Look at them - at least one of the three probably has lost its rubber or has developed a flat spot (caused by an air bubble during the mfg process that has been exposed from wear).

Also look at the drum flange (that the support rollers ride on) to make sure you do not have a build-up or foreign matter (usually ink and paper dust) that the support rollers "hop" over.

There are other things that may cause a sound like this, but go with this first.

Good Luck!

**bcp** · 07-29-2008

I'm sorry I should have mentioned that I have 4 drums and they all make the same sound. I have had a close listen and the noise is when the paper ejects out and you can feel the bump as well as hear it.

**techathens** · 07-30-2008

Sounds as if you have an issue with the suction unit, specifically the gear that drives the exit belts. On the rear side of the suction unit there is a belt running from the front to back of the unit - the rear gear that drives the belts is attached to a shaft with an allen or hex screw, which will loosen over time causing a "popping sound" at times. This is the first place I would look, if that is not it then inspect the moving parts (and supporting parts) of the suction unit (not including the fan) to see if there is a bushing or bearing worn or out of place, belt misaligned, etc or for an obstruction (rare). Sometimes bushings or bearing will pop out of the suction ass'y frame causing a misalignment in the shaft(s) that the exit belts attach.

Another thought on something to look at: the Separator Assembly - specifically the small roller area that rides on the rear drum body support and lifts the separator ass'y up at the 12 o'clock position (so it will not come in contact with the clamp ass'y).

Good Luck!

**bcp** · 07-31-2008

thanks will let you know how things went

**bcp** · 08-09-2008

Hey thanks for the information it was the hex screw loose causing sound. I have another problem tho. I have purchased a drum from the USA and it has the same part number as the other 4 drums that I have, but when I insert it into the machine message comes up " incorrect size drum " It is a Gr drum A3 size, but doesn't work. Any ideas. Perhaps its programmed for imperial and not metric?

**techathens** · 08-09-2008

I will check on that - I believe while this is an A3 cylinder that the switches (on the Drum Control PCB2) are set here in the US to Ledger [11"x17"], not A3 [11.69"x16.54"].

What the differences between a US and European machine that may cause this are I'll have to research - it's been a while since I've delved into the operation theory of the GR.