Motor speed issue with Portastudio 424 MKI after replacement

[Trent]

Hi,

I'm having an issue with my 424. When I got it it needed a new motor, the old one was dead and was too weak to move anything or engage the play / record head. I replaced it with a model EG530YD-2BH, since I was unable to find the original model anywhere. This replacement has the exact same specs as far as I can tell; 2000/4000 rpm, CCW, and 12V. The issue is when it's now turned on, the motor will spin at full speed, as if the A and B pins of the motor are open. When the motor is disconnected, the resistance across the A/B wires on the board are around 30K ohms. Additionally, the speed switch does nothing to change the motor speed, though the pitch knob does speed it up or down slightly. The motor seems to be working fine, and if I short the A/B pins the motor will slow down to it's minimum speed. This makes me think that the motor is not the problem, but it is something in the speed control circuitry. If I can supply any additional details I would be happy to.

Thank you!

Trent

 

[SkywaveTDR]

I hate to say it but when putting in these third party motors it is not always plug and play and might require a bit of engineering to change circuits to allow them to work. I don't recall doing a 424 Mk II yet but I should keep track of the changes I have done as they are many.

 

[Trent]

So I got the motor working, I ended up taking the original motor apart and replacing one of the IC's in it (AN6605). The original was badly burnt. Now I'm having an issue with the tape head not staying engaged. It engages fully, but will promptly disengage. I think it's something to do with the electromagnetic actuator not staying on. Any ideas as to what could be causing this?

 

[keef]

Just a guess but could it be the latches in the corner that engage with the tape cassette?

 

[Trent]

Here is a short video showing the problem:

I'm wondering if there are any trim pots that control the actuator mechanism.

 

[-mjk-]

It's not a motor speed issue. Notice carefully on your video, that the takeup motor is not engaging. You'll see a loop of tape develop on the takeup reel and then it immediately shuts down. The transport is operating correctly in that if the takeup is not working, the transport shuts down. Yes, I realize that FF is working, but the logic is separate for those functions. It's probably not the motor, but rather something in the logic.

 

[Trent]

Hmm yes. That sounds like a possibility. Any idea on where to start diagnosing this problem? Or some tests to check if it is what's happening?

 

[-mjk-]

What I stated is fact. If the take-up reel does not engage, the transport will stop. That prevents the transport from spilling tape. As for diagnosing the problem, if you're a qualified tech, the next step would be to obtain a service manual. The issue could be mechanical, or could be electrical. Personally, I would take it to a service center and be done with it.