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jdjohn
Posts: 3,001
I'm trying to determine the reason for an RTR spindle drive motor bogging down near the end of a reel. Each motor has a little PCB with a resistor and diode, so I'm testing them.
Here are the diodes:
The one driving the right-hand motor - the one in question - measures like this:
The one for the left-hand motor measures like this:
So, the diode on the problem side is measuring about a third higher (in mV), but is this really significant? Also, the parts list for this RTR - an Akai GX-77 - does not specify how the diodes should spec, so I don't even know how to shop for replacements.
Btw, the resistors measure much closer to each at ~100 ohms...as they should.
Here are the diodes:
The one driving the right-hand motor - the one in question - measures like this:
The one for the left-hand motor measures like this:
So, the diode on the problem side is measuring about a third higher (in mV), but is this really significant? Also, the parts list for this RTR - an Akai GX-77 - does not specify how the diodes should spec, so I don't even know how to shop for replacements.
Btw, the resistors measure much closer to each at ~100 ohms...as they should.
"This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
Comments
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It appears you are testing the current flow in opposite directions between the examples.
Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk -
I noticed the displays show one diode is reversed in polarity between terminals 2 and 3 from the other. I'm assuming "Uf" is breakdown voltage, correct? I believe the right motor display is showing dropping voltage and the left motor display is showing breakdown voltage (usually around 0.6 volts).
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I found this on the Tapeheads forum:
-> Reel motors/slow FF/rewind. This is probably the most common issue with this deck. The power supply will current-limit these motors in wind mode so that they don't get too hot (1.5A when stalled). If you're careful and skilled, they CAN be serviced; the back must be rotated to align the crimps with the slots before it will slide out. I used a punch and tapping with a hammer on the metal nubs on the back cover. Once aligned, the back comes straight out, exposing the brushes, commutator, and rear bearing. The armature does NOT come out (I think the front bearing might be pressed on) but we now have access for cleaning and lubricating. My rear bearings were dry, and the brushes oxidized. The cover is replaced in the reverse way and tapped back to lock it under the crimps again.
Edit: MARK THE COVERS for orientation before you disassemble. If you get it wrong, your motor will turn in the opposite direction and you'll have a pile of tape on the floor.
After service, there is a marked improvement in the wind speed (still not perfect, but also not as bad anymore.). In my case the brushes and commutator look worn - it may just be time for a new motor and I'll be looking around for candidates. (they are Nihon Micro NM01s). Fortunately 12V DC motors are dime a dozen in many shapes and sizes, so the odds of something suitable are not completely astronomical.
It was also mentioned that JIS screwdrivers should be used to prevent stripping the screw heads. -
Yeah, I didn't measure those in the same direction. Doh!
That's a good reference chart, Jesse. I think the 0.6V drop makes sense. I measured them both again (in the same direction), and got Uf=~667mV for them.
Ken, I think that information you found about the motor brushes and rear bearings is probably the issue. I found some other information about a bad thermister being the potential problem, but it looks like that affects the capstan motor...not the spindle drive motors. The problem doesn't occur with shorter tapes, like factory pre-recorded tapes. But with 1,800ft tapes, it gets bogged-down near the end, with the uptake reel seeming too heavy for the motor to turn. I'll probably have a go at cleaning/lubing those motor parts.
This deck has an interesting design with a loading roller, driven by a cam that often seizes from dried grease over time, but I got that sorted. I also had problems with the key pad buttons, but got those working as well. These decks have 6 (count 'em, 6) heads, and I think their compact design gives them a bit of cult following. I'd like to service the motors, and then move this deck on to an enthusiast."This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon -
Service manual says the Diodes are Silicon W03B 100/1.0A. Looking up W03B (datasheet attached) suggest that they might be 100V 1A Silicon Rectifier diodes and a 1N4003 is a possible replacement. Schematic looks like it's just part of a snubber to keep the motor EMF from killing the transistors driving it.
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Good stuff @stevep I couldn't tell if it was WO3B, W03B, WO38, or W038.
@SeleniumFalcon I found that threadhttps://www.tapeheads.net/showthread.php?t=55416 Unfortunately, he left unanswered questions (or results), so not sure if he was actually able to fix the problem. There are links, however, to more information about the motor."This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon -
Weak DC motors are a common problem.
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I found someone parting out a couple of GX-77 in Belgium, but those motors may have the same issue."This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon -
Is this the right one? https://www.ebay.com/itm/114561300016
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Yep, that's the one I saw, but again, who's to say those motors won't have the same issue?
I found a potential mod that assists the unwind motor, which in turn puts less burden on the take-up motor. https://www.akaigx77mod.casteloseguros.com/home/the-problem"This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
