Otari MX5050 AC voltages completly off?

J

Jonk

Member
Hey folks,

I'm getting way different AC voltage readings than the schematic calls for. Sorry I don't have the ability to post one page of a pdf here, but the manual (with schematics at the end) can be found here: Free Otari Schematics and Manuals - from AnalogRules.com and AudioSchematics.com under "MX 5050 B II Schematics"

Anyway, the machine is completely black (not dead because I can still read these voltages on the power supply, but black with no function whatsoever) It was working fine until I accidentally left a socket wrench on the end of the solenoid that (I think physically) prevented a fast forward function (or else shorted something) and burned the 100 ohm resistor R712. I replaced the resistor, but I got problems. At first the machine would fast forward and rewind but not play. Then it eventually went black.

In investigating, I found the following AC voltages at the connector labeled "U" which appear to be power transformer secondaries, but they're way off. I'm wondering if I damaged the power transformer?


Schematic AC Voltages:
U1 0V
U2 19.5V
U3 19.5V
U4 19V
U5 0V
U6 0V
U7 21V
U8 0V
U9 8.5V
U10 27V
U11 0V
U12 26V

My AC readings:
U1 gnd
U2 21V
U3 21V
U4 10.4V
U5 11.9V
U6 11.8V
U7 10.8V
U8 .2V
U9 9V
U10 14.7V
U11 14.3V
U12 18.3V

P.S.

I should mention as well, I measured all the fuses and found a blown one at FS703.

I also measured ohms across the diodes (in circuit) and I'm generally getting from 105K-120K EXCEPT in the case of D705, which is only around 3K.

???
 
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Maybe whatever you did pooched the power supply and now you're getting all these odd readings...?
 
What do you mean by pooched?

I guess a more simple version of my question would be, is this a case of
a. A damaged power transformer because my AC secondaries are completely wacky including voltages at pins that should read 0V? or
b. A blown fuse and a funky diode, or some such other problem in the power supply causing it to not pull the right amount of current and presenting a different load to the power transformer? (I'm reaching here, past my understanding)
 
Fuses then diodes. I doubt you did damage to the xfmr, unless yu left it on for a long period fooling around with it.

What type of meter are you using to measure the diodes? They should have a large difference in resistance measured forward vrs reverse.
 
Hey,

Thanks. It has been powered up while taking measurements which in total, has definitely been a few minutes.

As for the meter, it's a DMM from circuit specialists. I can't find the model. Maybe it's discontinued. I doubt it's true RMS or anything because they gave it to me as a package deal when buying something else if I recall. It can read ACV/DCV/ohm/ACA/DCA/transistor hFE. Similar probably to one of these: Digital Multimeter - Autoranging & True-RMS DMMs | Circuit Specialists

Anyway, yeah, there is definitely an inconsistency with that diode (D705) Measuring one way on all the other diodes gives around 100K reading and above. Measuring the other way gives 2K-5K range. On D705 I'm getting around 3K in the same direction the others measure above 100K. Measuring in the other direction on D705 yields an even lower resistance.

Would a bad diode there be causing such symptoms?
 
Is the rest of the machine disconnected from the power supply as you take these readings? Or are you leaving the machine innards connected to the power supply and reading it? (I don't think it will be harmful to turn on the PSU without its associated electronics connected as a load, but you should consult the experts (which I am not) along with the service manual.)

Some amount of higher voltage is to be expected with no load on the PSU, but not MUCH higher. I believe this PSU is regulated pretty well.

Diodes and transistors have probably failed. Knowing which ones have failed may be tricky to determine, especially while they are still in circuit.

Your time and energy might be better spent on hiring a qualified technician, or to rebuild the entire PSU yourself if you are competent enough to purchase new diodes and transistors for the PSU yourself.

As for the power transformer, it may be hard to determine if it's failed or not without finding another power transformer from another machine and measuring its secondary voltages on power-on.
 
One more note on the power transformer - I once spent weeks troubleshooting a Peavey VMP-2 tube preamp. It had a PCB-based IC regulated power supply, that was putting out entirely too much voltage, and had probably cooked the filaments of the tubes.

Anyhow, my point was that I suspected the power transformer was putting out too much voltage. It took a long time to know for-sure, as I had to find other people who owned the same preamp and ask them to measure their PT secondaries on power-on for comparison. The owner of the preamp did not with to pay the price for a new power transformer for "experimentation" purposes, since I didn't yet know at the time whether or not the voltages coming off the PT were normal, and these voltages weren't noted on the schematic.

Point: you may very well have a failed power transformer but it's not too likely that happened. Failed ICs, transistors and diodes on the PSU are a more likely cause.
 
I double-checked the schematic, and there's not a whole lot going on upstream from that power transformer. You're measuring all of the secondary windings of the power transformer, looks like. So the key is - is the Control PCB assembly connected, or disconnected to this?

Earlier I mis-spoke.. the power supply does not do any AC/DC conversion. All of that is done on the Control PCB assembly.

Having the schematic is not the same as having the service manual. Hifi Manuals - all manuals for free has it, if you register for a free account.
 
Hey Muckelroy,

Thanks so much for the response. Sorry it took awhile to get back to it. I actually ended up getting another control pcb board off ebay that the seller said was in excellent working condition.

And yes, those previous readings were with the control pcb connected.

Anyway, with the new contol pcb, here are the new AC readings at the "U" connector

U1 0V
U2 21.3V
U3 21.3V
U4 11V
U5 12.5V
U6 11.7V
U7 11.7V
U8 5.3V
U9 5.3 V
U10 14.5V
U11 14.8V
U12 19.5V

So, pretty similar to the previous control board.

Bummer.
 
Hey Folks,

I'm bumping this one more time. I guess I'm just looking for ideas concerning how to test the power transformer. I'm wondering, since it's not the power supply pcb (part number PB-46Q) is there really anything else it could be?

Is there something up stream from the ps pcb that could be pulling current to the extent that it would cause those weird differences in AC voltages on the power transformer secondaries? Or do you guys think it is definitely the power transformer?
 
Hopefully a member who has a MX5050 will chime in. I know a couple at least use them. They will be the most help.
 
Can you post the schematic?

Im not so sure the transformer is pooched or what ever it was called earlier.

Looking at the voltages it looks like a ground problem possibly.

Does the schematic show all the 0V "U's" connected together on the power supply board close to where the transformer wires attach to the P/S PCB?

EDIT: Aha! i found a scan thats so bad I cant read all the numbers. Did you move the ground lead of your meter to the appropriate "0V" for each voltage measurement? The are not all hooked together. Use the correct U number for "0V" (meter black lead) for each voltge you are measureing.
 
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Well, ask yourself how much you're willing to spend on this machine, then weigh that with finding an Otari authorized technician, or someone who regularly services these.

As for going about it alone, if you were willing to go so far as to replace the Control P.C.B. PB-46Q, why not replace the power transformer as well? It is the next most suspect item at this point.

You asked where to get a service manual. Hifi Manuals - all manuals for free has it.

I'm assuming all fuses are replaced? Do you see any other fried or questionable components on the amplifier, or Timer PCBs?
 
I know first-hand how frustrating it can be when power transformers are failing or doing wacky things. I've seen power transformers put out too much voltage in a Leslie 122 amplifier, and in a Peavey VMP-2. It was weird, because other people say PTs fail in such a way that there is less voltage, not more voltage coming out. I suppose that if there is a partial short in the primary side, this can cause more voltage to come out of the secondary side of the transformer.

The only way I learned the PT was the culprit in the above two repairs was to compare from a few known-good PTs. I feel your pain, and it would be nice if a fellow owner of this machine could test their voltages and chime in. Until then, a new PT may be your only option aside from consulting other technicians.
 
Id already posted to find out if the transformer was tested correctly, but no response yet. They arent that hard to trouble shoot.
 
You were asking whether I had the power supply pcb connected when I tested it right? I did. Same thing when I put in the new power supply board.

I could test the AC voltages without it connected to anything but I'm pretty sure that won't get the right readings either, unless (as has been mentioned) I can compare to what someone else is getting unloaded. That's a lot of hassle for you folks though.
 
no, what I asked is if you measured between the right Us. You CANNOT just use one of the 0V Us for checking the voltages of all the transformer taps. If you do that you will get reading that are wrong. Your original post doesnt show any missing voltages, just some that were divided over other terminals.
Im at work, so I cant go looking for a scan of the chassis to tell you which combinations you should be using to measure these voltages.

Edit: Shame I just threw one of these machines away a couple weeks ago. :-(
 
Okay thanks

I'll research the proper ground references for each of the secondaries. I didn't realize that could be what I'm doing wrong.

I'd post the schematic but I can't figure out how to upload one page from a pdf. . . . .
 
Measure:
U1 to U2 (17V)
U1 to U3 (17V)
U4 to U5 (19V)
U10 to U11 (27V)
U6 to U7 (18V)
U6 to U12 (24V)
U8 to U9 (8.5V)

report back
 
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