Adjustment of Cassette Engine Speed in National RX-7000

[Magnum]

Sooooo, I finally had the time and the guts to open my boombox.

I made a few experiences, which I'm happy to share with you.

1.
The screws on the back (for opening the front panel) are really difficult to screw back in.

2.
As a reference, I played a cassette recorded on a different tape deck and tried to sync it with the same song played on YouTube at the same time.

3.
The song was "No tengo dinero" by Righeira (extended version). I heard it for the next few hours and I thing it's fair to say that I heard it enough for the next couple of years.

4.
I noticed that moving the screw in the motor block even the tiniest bit substancially altered the speed.

5.
I only managed it to get the two songs in synch for about max. 3 minutes. After that, it developed into a hall effect and eventually the two versions drifted apart.

6.
The previous owner told me that he had the belt renewed and that it was completely cleaned and serviced presumably up to the lube level.

7.
I finally quit the operation and closed the box.

So the problem still exists. I lost half a day and "No tengo dinero" got wiped off of my list of favorite songs.

Any suggestions what the problem might be (apart from new belts). New motor????

Here are a few images:

First step: Opening of the front cover

I left a cassette in it, in order to test speed with my computer.

Interesting new look at it:

A closer look at the cassette mechanism:

Removal of the switch circuit board:

The key step:

 

[baddboybill]

Most likely bad pinch roller and since your at it check if there are and rubber tires that are dry and cracked

 

[Superduper]

Several possibilities:

(1.) Mechanical friction: If the previous owner lubed the deck, and depending upon how it was lubed, it is sometimes possible for the motor to work loosely at first, then tighten up as the motor spins off the lube, expecially if it was a thin lube like light machine oil. Sometimes, it is necessary for the deck to operate for awhile before speed can be stabilized enough before final adjustment. Unfortunately, if there was excess lube that got flung off, that could also affect the belts.

(2) Mechanical friction: Loose or slipping pinch roller. In normal operation, the forward reel pulls tape at a slight tension against the pinch roller. This tension is necessary to keep the tape reel tight and prevent the tape from bunching up and despool or get eaten in the process. The speed of the tape as it navigates the tape path therefore, should be dictated by the speed of the capstan needle. Under certain conditions which may include glazed pinch roller, or insufficient roller tension, the tape could slip as it is being pulled by the forward reel which would cause it to run too fast. However, if you are also seeing the tape run too slow as well, then this probably isn't the cause.

(3.) Internal motor speed controller. Inside the motor, there is speed control circuitry. The mini pot that you rotate with a jewelers screwdriver is part of that circuit. It is sometimes possible that oxidation builds up in the potentiometer contacts which changes the value of the pot and therefore the speed. If oxidation is severe, it may not be possible to get a stable value without some help. You can try a tiny spritz of oxidation solvent inside the hole and work the pot to see if you can get the motor speed to stabilize. However, the mere presence of the solution itself might have a destabilizing effect on the control since the solvent drenches everything so if you try this procedure, it may be necessary to run the motor until the solvent dries up some. Still, the adjustment pot is only 1 part of the control circuitry and if that is not the culprit causing instability, then repair of that tiny pcb might be necessary. For most people, the easiest solution is to simply replace the motor itself if the speed control board is bad.

(4.) In most boomboxes, the cassette motor is powered on by a mechanical leaf switch. However, in this case, the motor is controlled by a solid state design which sinks the motor ground through a transistor driver circuit. The motor driver and pre-drivers Q623, Q624, might not be operating stable. You will need to monitor those during operation, especially if they are heating up noticeably during use. To confirm, you can monitor using an appropriate test instrument, or sometimes see if heat is having a destablizing effect by artificially heating and cooling those 2 transistors to see if there is a pronounced variation in the motor speed with the application of heat or freeze spray.

 

[Magnum]

Thanks for the feedback!

So basically we are talking about either a mechanical problem (bad pinch roller) or an electronic-related one (motor, motor driver and pre-drivers).

What strikes me is that the speed seems to change only very slightly. Like I said, once synched with the real speed it got out of sync very gradually up to the point that I wouldn't notice it hadn't I monitored it with said method and while keeeping a very keen eye (or ear that is) on it at the same time. There are no harsh speed fluctuations.

Doesn't that indicate towards an electronic problem?

Replacing the motor seems a bit overkill considering that it works fine besides the speed problem. I think I can live with the fact that the box will have "Play only" status in my home with no recording.

And if I would like to take action:

Any special considerations when replacing a bad pinch roller?

And let's suppose I decide to do something about the motor. Is it even possible to obtain a completely new motor for it?

 

[baddboybill]

You can purchase new motors but there are different kinds and you need to see what voltage, speed and direction your is. But i would 1st replace pinch roller as its easiest and most likely needs new one anyways.