Akai GX280D - whine/oscillation when capstan reaches full speed

[ChopperCharles]

I've almost got this unit completely worked out. There's only one small problem left. When the capstan reaches full speed, I hear a whine through the speakers. If I slow the capstan down with my hand it stops immediately - it doesn't slow down with the capstan. It sounds like an oscillation to me. It's very high pitched. It only happens at 7.5ips, 4.75ips does not exhibit the whine.

What could this be?

Thanks.

Charles

 

[Gepetto]

I've almost got this unit completely worked out. There's only one small problem left. When the capstan reaches full speed, I hear a whine through the speakers. If I slow the capstan down with my hand it stops immediately - it doesn't slow down with the capstan. It sounds like an oscillation to me. It's very high pitched. It only happens at 7.5ips, 4.75ips does not exhibit the whine.

What could this be?

Thanks.

Charles

Possibly the FG controller chip on the capstan motor drive board

 

[ChopperCharles]

Possibly the FG controller chip on the capstan motor drive board

There are no chips on this board. There are a bunch of transistors, but no chip. So, a simple pull-n-swap is not going to save the day, unfortunately.

Also, this machine is an oddball, it has the control board from a GX-280D-SS in there, even though it's not an SS. That's board KD-1096 in my machine, where the manual says it should have board KD-1087.

Charles.

 

[orange]

You've lost proper grounding somewhere, my Delco UX-1 was never hooked in just right and would have the electrical noises from the engine bay (ignition etc.) because of that.

 

[ChopperCharles]

Any ideas where? The servo control board is properly grounded - big heat sinks attach to the ground traces and then everything is securely bolted to the chassis. Adding a jumper wire from the servo control board to the chassis had no effect. I tried a few other places, but no dice.

Thanks!

Charles.

 

[orange]

Possibly via the odd board?

Something is bad somewhere and it's inducting one or both of the motors' electromagnetic fields...

are the motors in good shape or supposed to be shielded? Can you run them independently and see if it goes away if one is removed? If so perhaps you can go through that path and find the fault more easily.

 

[ChopperCharles]

Only the capstan motor causes this problem, and only at 7.5ips. (Can hear it when the deck is idling, and the only thing spinning is the capstan). No extra noises are present during playback, other than the capstan noise. (and again only at 7.5 ips). Motor is covered with a metal shield, but is open at the back (I can see the windings). There's another shield around the motor as well, about half an inch away on two sides (bottom and right, which is where the audio amplifiers and control boards are located).

Also, it's not just an odd board; the motor, pickup, and adjustment board are all correctly matched with the KD1096 board, these parts would be different on a machine with the KD1087 board.

Charles

 

[MACMAN007]

Only the capstan motor causes this problem, and only at 7.5ips. (Can hear it when the deck is idling, and the only thing spinning is the capstan). No extra noises are present during playback, other than the capstan noise. (and again only at 7.5 ips). Motor is covered with a metal shield, but is open at the back (I can see the windings). There's another shield around the motor as well, about half an inch away on two sides (bottom and right, which is where the audio amplifiers and control boards are located).

Also, it's not just an odd board; the motor, pickup, and adjustment board are all correctly matched with the KD1096 board, these parts would be different on a machine with the KD1087 board.

Charles

Charles, I have the EXACT SAME ISSUE! HOW ODD..

In Source position it's louder than in tape, you can hear the ringing of the flywheel or it's teeth as it rotates. I can hear it in 7 1/2 or 3 3/4 ips, but not as high pitched, perhaps 1/2 the pitch. Like you I see no ground or other issues, all the electrolytic caps have been replaced on all boards. The couple of 2SC458 transistors on the record boards were also replaced with KSC1845FTA's. I have spare LD3141 op amps I can install, but a I don't think they are the issue.

I've tried adjusting the servo board, the low speed and high speed are perfect now. There is a second pot on the servo card, VR251, I find no adjustment specification or procedure , not even voltage specs in the service manual or schematics.

At first I thought it was something wrong I did to the machine or something I broke during service.
Now I see where someone else has the same issue,.. I feel a little better, but what is this issue, did you ever find a cause and the correction?

 

[orange]

Welcome to Phoenix, yet another good member of Tapeheads!

Is everything lubricated correctly? Might you have bad bearings? I had three Advent 201s and every one of the had a transformer with bad windings that 'sang'.

 

[Gepetto]

Charles, I have the EXACT SAME ISSUE! HOW ODD..

In Source position it's louder than in tape, you can hear the ringing of the flywheel or it's teeth as it rotates. I can hear it in 7 1/2 or 3 3/4 ips, but not as high pitched, perhaps 1/2 the pitch. Like you I see no ground or other issues, all the electrolytic caps have been replaced on all boards. The couple of 2SC458 transistors on the record boards were also replaced with KSC1845FTA's. I have spare LD3141 op amps I can install, but a I don't think they are the issue.

I've tried adjusting the servo board, the low speed and high speed are perfect now. There is a second pot on the servo card, VR251, I find no adjustment specification or procedure , not even voltage specs in the service manual or schematics.

At first I thought it was something wrong I did to the machine or something I broke during service.
Now I see where someone else has the same issue,.. I feel a little better, but what is this issue, did you ever find a cause and the correction?

It is sometimes the Rohm BA6303 FG controller chip that governs the motor speed (not sure if the Akai uses this IC). Used frequently in Teac/Tascam decks.

 

[orange]

Charles is a Tapeheads member as well, perhaps you can contact him there, or send him a PM here.