Delving Into the World of A Denon DN-T620 – Part 2

When we last looked into the problems of a Denon DN-T620 we discovered that the OPU (optical pickup unit) seemed to be faulty. The OPU is the module in the gray metal framework in the photo to the right.

In the case of this OPU, the model number is clearly marked on a small circuit board. It is an H8147AF manufactured by Sharp.

This item is no longer available from Sharp and original manufacturer’s replacements are pretty much impossible to find. The good news is that duplicate replacements are available from several suppliers. They are, as you might guess, not made in the USA, but come from across the Pacific. The prices are reasonable, so I ordered one.

The new OPU arrived after a 2 1/2 week wait. In the photo on the right you can see the old pickup on the left still in the frame. On the right is the new OPU. Except for the Sharp text labeling it looks identical.

Replacement if a bit tricky as it is necessary to remove the anti-static protection piece. In this case it is a very tiny bridge soldered between two electrical contacts. This step is crucial and takes a steady hand.

After reassembly it was time to test the CD player. Much to my surprise, the CDs that I used all skipped much worse than originally. What could possibly be wrong?

With the cover off of the machine I examined the movement of the drive gears very closely. I noticed that instead of stepping evenly, the drive gears were stalling and jumping. On two occasions they actually moved backwards! (The stepping movement is a function of the tracking methodology by which the OPU follows the CD data track. I am considering a post about this function.)

I dismantled the machine again and removed the OPU for a close inspection. It turned out that there was a problem with the teeth on the side of the sled. It looked like they had been badly molded. Unless these teeth are perfect, the sled will not move correctly. They function much like the rack in the rack-and-pinion steering assembly in a car.

While I was waiting for the OPU to arrive from across the ocean, I had read that occasionally skipping is caused by dirty motors, gears, and pivot points. Sometimes the factory applied grease, a lubricant, hardens and prevents the parts from moving smoothly. I had recently run into that exact problem when I serviced an old Zenith record player and, later, a Sony reel-to-reel tape machine.

I dismantled the OPU and exposed the gears and other drive components. I cleaned everything with alcohol and then lubricated the various points with a special oil I often use. I replace the “new” OPU with the original one and then put the whole shebang back together.

The cleaning did the trick! The Denon T620 worked as good as new. In the video on the right you can hear how nice the machine sounds. I have since played it for many hours and am pleased to report that it is super.

In case you can’t see the video on the right you can click here to see it on Youtube.