portastudio 424 strange speed issue

[brujo]

Hello, I 'm here trying to get some help with a 424 portastudio. I replaced the old capstan motor because not able to engage the play command, I replaced it with a Mabuchi EG-510KD 2B 12V CCw motor that should have the same specs of the original ( as my understanding), but it is a little bit bigger. the behaviour that I am experiencing is that the tape is spinning extremely slow, when the slow speed is choosen it is slightly faster than normal or fast, also the pitch control pot is working backwards: turning slow speed it up and viceversa. Looking at U502 with a scope, the clock is running , the reset pulse when turned on is engaged and also I can see the pulse at pin1. strange Voltages on some of the other pins some have like 9v when play is engaged, others are at like 1.5V. I know it is a very long shot here but maybe someone will try to give me some help so I tried. I do some diy repair on amp or audio equipment but I don't have any experience with tape related equipments.
For me the goal here would be to learn more about these machines and to let this portastudio works somewhere near to where it should.

ps. the belt and pinch roller looks ok to me.

thanks for your time and have a good one.

 

[Jack Saturn]

I've never seen / worked with the EG-510KD-2B, but in my experience, the Tascam 424 doesn't seem to like to play nice with motors that aren't the original MMI-6H2LWK model. I've made attempts to swap in an EG-530KD-2B, which meets the same specs, but experienced the same issues you did, where the speeds do not react as they should and the pitch control is either erratic or doesn't respond at all.

I can only assume that there's something specific about the internals of the original motor which makes it specifically work with the 424, and I suspect it is because the 424 is the only model that has three speed settings, while the vast majority of 4-track machines have two speeds at most.

If you have any breakthroughs, let us know!