R-91 magazine motor buzz

[wdln]

My R-91, which has worked quite well over the last year, has developed a mechanical buzz that I've determined to be coming from the magazine motor.  It still worked fine though; the magazine rotated normally and it played the correct records with no jams.  This morning I discovered that the buzz will disappear, then I get errors 30 and 32 reported when it tries to rotate the magazine.  The buzz will come back after 10 minutes or so and it will work normally again.

Whether the buzz is there or if is timed out from heat, the magazine motor LED is not lit.  The motor LED only comes on when the magazine is rotating, or if in the timeout, when it's SUPPOSED to be rotating.  However, I measured the voltage across the motor and it's receiving 20VAC just sitting there.  When I hit the cancel button to rotate the magazine (when it's working properly), it jumps up to about 27VAC as the magazine rotates.  It kinda seems to me like the motor is getting power all the time instead of just during magazine rotation, and it's eventually timing itself out from overheating until it cools off for a bit.  Or maybe whatever feeds it power is sensing the extra load and is timing out.  The sprag is holding it back from letting the motor turn and it's overheating.  Since I hypothesized that this morning, I've now got the unit turned off to avoid damaging the motor.  But perhaps that ship has already sailed if it's timing out like that.

So the question is, what do I need to check for a failure?  Is a relay or solenoid stuck on somewhere?  It seems like a bad connection somewhere wouldn't default to leave the motor on (instead of off), but it may be possible.  The motor LED is not lit when the buzzing is occurring, so maybe it's downstream of the computer control.  I would appreciate any direction from the collective!

Dave Lennie
Peoria, Illinois USA

[wdln]

OK, using some info from the field service manual, I've determined that the Central Control Computer is sending the correct signal to the Mechanism Control to operate the magazine motor... Quiescent voltage is 8VDC, and it drops to 1VDC when the magazine motor should be operated. This is correct. Pin 6 on the P207 multi-pin connector from the Mechanism Control board is what connects to the motor. Removing the connector from the board, this pin is outputting 20VAC all the time, and 27ish VAC when the motor should be activated. More accurately, I think the pin provides a path to ground, as I am measuring between this pin and the 28VAC pin on the power supply connector. The magazine motor is wired between these two points.

My field service manual does not have schematics, but looking at the bottom of the circuit board, it's pretty easy to see that pin 6 from P207 is connected directly to the output of one of the power transistors attached to a heat sink that sticks up out of the board's cover. I suspect that has failed and is allowing a partial connection to ground all the time, keeping the motor partially energized and causing the buzzing (the sprag keeping it immobilized).

This component is labeled Q203 on the circuit board; Q202 is identical and is connected to pin 7 on P207, which drives the transfer motor. It makes sense that those would both be big power transistors. The transistors themselves are labeled Q681 across the top, MEX vertically on the right, a T logo across the lower part (it does not look like the Texas Instruments logo) and F7 printed next to the T. I cannot find a datasheet or listing for it using any of those numbers. The pins are labeled G, A and C on the board, not the E, B and C I was expecting for a transistor, so I'm not sure exactly what kind of component this is. I'm not an electrical engineer and I'm about at the end of my usable knowledge here.

I think if I could replace this transistor I'd be back in business. I did find another complete Mechanism Control on ebay but it is not quite the right part number, and it lacks the I and H pots that adjust the optical switch sensitivity. There's a refurbished one for $140 that is the right number, but that's a little steep. I suspect this one only needs a <$5 component to be fixed!

So does anyone have a shop manual for an R-91? I'm looking for more information on this transistor.

[wdln]

BTW, the Mechanism Control board looks to be in good shape; I don't see any evidence of anything overheating or burning up, including around the questionable transistor.

The cover for the Mechanism Control indicates part # 4-07221-05. The board itself says Rowe AMI 6-08708-05. The back (solder side) of the board says 6-08709-04. Perhaps that's the part number for just the board itself (without the components).

[wdln]

I'm apparently determined to solve this myself... More information I found out below. I'm hoping that someone will jump in with more or this will help someone else fix their similar problem.

* The T logo stands for Teccor, which was acquired by Littelfuse in 2003. The Q### numbering format fits with this manufacturer's labelling. I found a whole list of linked semiconductor logos here: www.westfloridacomponents.com/mm5/graphics/manufacturer-logos.pdf
* The component in question is not a transistor but a TRIAC or thyristor, a semiconductor used to electronically switch AC loads like motors. Like a relay but with no moving parts. The A and C terminals are the supply and load connections and the G terminal is the gate, or trigger voltage.
* Nobody appears to have a datasheet specifically for the Q681 that would allow me to match the specs to something modern.

The differences between different thyristors seem to be the case type, the amount of voltage and current it can handle switching, and the trigger voltage and current. I think I'm going to try an NTE 5638, which is equivalent to a Teccor L2001L7. This thyristor is in the same TO-220 case and there's already another L2001L7 on the Mechanism Control board that drives the turntable motor, and it is in turn driven off the same type of IC that drives the magazine motor's thyristor. It handles up to 400V and 8 amps; I can't imagine that motor draws more than 8 amps of current. Wish me luck...

www.weisd.com/store2/NTE5638.pdf

[Ron Rich]

You have found the answer-- PLEASE SIGN, and State your location (country, at least) in the status, or signature section, so it shows on each post !  I STRONGLY suggest you remove that motor and OIL it, (fill the felts--stay away from the rubber grommets!), with 20 wt ND oil, in both bearings, before it freezes on you-- Ron Rich

[wdln]

Added my name and location to my signature. Thanks for the advice on the motor! Do you think the NTE 5638 will work in this application then?

[Ron Rich]

Hi Dave,
Thanks--sure, it's not a critical application. Ron Rich

[wdln]

Excellent, thank you! I'll post my results in a week or two.

[amiman66]

Hello wdln,

Usually when there is buzzing on the transfer or carousel motors the fault can be the two photocouplers Z200 & Z201, you here the buzz and if you watch the transfer motor you will see it moving very slowly in the case of carousel motor if you hold the spagwheel linkage this might also turn very slowly.

Regards
Alan

Alan Hood
ami-man
UK

[Ron Rich]

Hi Alan,
Thanks for the above--I never considered that an opto could "leak" that way--I know TRIAC's sure can !!
Ron Rich

[amiman66]

I find this to be the one of the most common faults on the mechanism control units I have repaired other than the IC that controls the opto switch and burnt out resistors.

I came across a R-84 to R-88 jukebox once that was burnt out, my guess is that the power supply fuses had been changed to very high rated ones, the burning source of ignition was the mechanism control that was obliterated, the jukebox was a right off of course.

Regards
Alan

Alan Hood
ami-man
UK

[Ron Rich]

Hi Alan,
"Hi rated" fuses ? Naw, that does not happen in the UK too? I thought you guy's sent all of the "ninkenpoops" here, to change fuses ! Ron Rich

[amiman66]

Hi Ron,

Back in the day of my earlier career, I worked for a electrical company, I used to wind transformers as part of my job from small va up to 7KVA 3 phase transformers, one of our lines was a power tool transformer complete in a case.

We had it returned as being faulty, on inspection the fuse mounted on top of the transformer had been replaced by a brass rod, needless the windings had shorted out and the customer was given short firth.

Regards
Alan

[Ron Rich]

Hi Alan,
I thought a brass rod, cut to the correct length was perfect --You know AMi actually did that on one of the 50's models. The changed the electrical location of a fuse on a component which did not lend itself well to be wired to the old fuse holder, so they placed a new holder onto the chassis at a more convenient location and jumped the old holder with that rod ( also scratched out the fuse value silk screen, that was jumped). Ron Rich

[wdln]

Thanks for the extra info (and stories), Alan... the triac (NTE 5638) arrived today and I just got it installed. Success! The buzzing is gone and the motor works when it's supposed to. I was concerned that maybe the switching speed of the 5638 triac might be different than the Teccor chip, necessitating readjusting the mechanism and sensors so that the right record always plays, but it looks like my fears were not warranted as it seems to be working correctly. I will take out the motor and make sure it's lubed up.

I'm also about halfway through re-foaming the speakers, as I found while I was in there that they were completely disintegrating. I should be back up and running later today. I discovered all these problems when I was putting in some newly-acquired 45's... I'm looking forward to being able to hear them!