Dead deck coming back to life

[caution]

I have one very corroded M90 deck that I'm attempting to bring back to fully working condition, but it has logic board issues that first need attention before I dig into the mechanical side of it.

After carefully documenting and bagging parts over nearly 20 steps I've got the board separated from the mecha. One IC was removed to clean up some severe corrosion and dirt, but thankfully the PCB is largely intact, which is good because it's got plated holes, printed traces and about 60 printed resistors. The conformal coating mostly held up with only R710 visually compromised.

Only rewind, fast forward, stop and play do anything at all, and rewind only works when the heads are up (i.e. reed switch is closed). Stop will engage the solenoid (and stay engaged) even if you haven't pressed play yet. Actually the solenoid turns on and stays on with all four of my working buttons, but I've yet to figure out if this is normal, because FR704 gets too hot to touch if it's engaged for more than even a couple of seconds. The solenoid does not engage when you turn on the unit, but does click on shutoff.

So far I've done a bit of testing on the control chip to try and understand if there is an issue in this portion of the circuit. I'm currently focusing on the reel motor, since I haven't seen it spin yet, even though FF and REW buttons are "working."

The deck has extreme corrosion in some areas, the pinch roller is totally shot. However many steel parts are nearly intact. You can see how interfaces are corroded, like moisture got stuck in the crevices. Nothing so far gone as to be unusable. A previous owner or tech was in here and replaced the belts, stripped a screw and lost a couple, but nothing damaged.

 

[MyOhMy]

Are you planning to strip out all the corroded metal components for complete renovation or will you try to treat the rust with only a partial strip down? Either way, it looks a daunting task that's way overdue so I wish you well with the work.

 

[caution]

Hey thanks MoM, no I don't think I'll need to strip it down too much more to get at everything, I'm hoping I'll be able to soak whole parts of it in vinegar.

My next step is to investigate the solenoid latching. I will be checking the transistor that drives it. Also, since it uses that reflective pattern with the opto-sensor to determine flywheel position, I may have to fool with a flashlight or something. Or, fix my ruined flywheel pattern and partially reassemble things.

As for the reel motor, it has not spun up yet. It spins freely with my fingers so it's not seized. I've been discussing with Superduper the driver chip connections to the main control chip, and will need to dig a little deeper with a scope, which I'm in the process of acquiring.

 

[caution]

I tacked on some test hooks and watched the O.FF/FIN node for any activity during power-on, activation of any deck buttons, and power-off. Nothing over about a half-volt at any point.

 

[blu_fuz]

Um, yeah.

 

[Superduper]

What about during the other modes?

 

[caution]

I will check the other modes and the solenoid circuit tonight. Ran outta time last night, I got carried away fiddling around with my new toy

 

[caution]

The rewind pin never went high, either side of the diode, and the solenoid output pin reads 1.8V when active. Diode is fine, as are all of them. They're all rated at about 0.56V. Start yanking logic chips out? You sure this runs right with a dangling opto sensor and reed switch?

 

[Superduper]

Ok, I didn't know that you would be testing this with NO DECK attached. No, the deck will need to be attached or you will need to somehow emulate the effects of a working deck, harder to do than to simply just make sure that the deck is connected. If the hall element does not sense a spinning deck (magnet), it will trigger the auto-stop signal. You may be able to prevent the auto-stop signal by yanking Q703 (or emulating the proper state, see mech block diagram, page 14 of SM)... you'll need to study the diagram further to see exactly how that circuit works. And then there is the opto-coupler.... all in all, much simpler to just ensure that the deck is tethered after confirming that all reed switches are working proper condition, and all motors are working properly.

What do you mean when solenoid output pin is "active"? At what point exactly is it "active?" From my understanding, at no point should the solenoid latch "on". Instead it is always a momentary "on" or "H" since the moment that it is triggered, the deck should already shut down. If you are getting a steady 1.8v, and because that pin is tethered to the base of driver amp, 1.8v is certainly enough to trigger the transistor into conduction, although I'm not sure that it's enough bring it into saturation. When solenoid is triggered, does the plunger pull in full and the deck shut off? Something to consider.

 

[caution]

Forget about the solenoid stuff, we're past that I think. It took a while but the mechanicals are back in order. Quite a few things were fixed:

Apple cider vinegar flushed away the rust after a long soak. Some parts I left alone even though there was a small amount of rust, which would've meant a lot more disassembly/reassembly. The drive gear assembly plate alone has eight springs, ten funky nylon arms, and a handful of collars, E-rings and washers.

The control plate on the drive gear is now restored thanks to my cutting machine. As is usually the case the dimensions were simple - the corner radii are 1mm, the inner slot is 3mm, and the diameters are 8, 19 and 42mm. The angles are weird though, but that may have to do with how much time it takes the mechanism to complete each function. The first attempt used chrome mylar only barely 2 mils thick, and was not stiff enough to stop surface aberrations from showing through. Switched to 8 mil chrome and now it's flat as a mirror.

Replaced some damaged screws and had to replace one of the screws that holds the front plate on, but as luck would have it I found a screw with the same thread, head style, length, and it's black, which I wanted so it would match the silkscreen around the hole on the front plate.

Fixed some improper wire routing, from a previous repair session. Nothing major, but the way some wires were bent made it obvious they needed to go a different way. Thanks to someone inside the deck before me, the idler tire and belts are brand new. It was done competently as everything I've noticed was reassembled correctly (some Sharpie marks were drawn as part mating guides). There's a sticker on the frame that says "NEW AM OSC. 12/2015" so it may have been a technician.

Replaced the rubber on the pinch roller bushing. This took considerable trial and error, but now I have a way to do them if I need to later. The brass bushing has a 2.47mm dia. pin, and was larger than any of my scavenged pins and rollers. On top of that, there is a clear resin barrier between the bushing and the tire, which I left on there. Acetone and acrylic solvent wouldn't touch it. It's for the best anyway because I wanted an even but slightly rough surface to work with for the new tire attachment. All my scavenged tires have holes too small so I chose the best quality one and bored it out to slightly smaller than the bushing. Drill bits and diamond coated cylinder bits were a failure, they just spun inside it. What finally worked was one of those arrowhead-type bits for tile and glass. A little bit of cement and a final evening up of the outer surface and she's done! In the end I lost about 10-15 mils on the tire radius, but shouldn't be enough to affect the spring tension. Tapes play fine.

Numerous spots inside were all but devoid of grease, so I restored them with Super Lube.

I replaced the clear capstan washer that holds it in place, it had enlarged and the flywheel was falling out under its own weight. Found an exact replacement from a previously scavenged deck.

The reel motor had a fully rusted outside, so I just wiped off what I could and wrapped it with some aluminum foil tape.

So here's where I sit...

Works:
Play
Stop
Pause
Fast-forward
Rewind (only if it's already playing or fast-forwarding)
Eject
Auto-stop at beginning or end of tape, during play or FF
On powerup, solenoid/drive gear resets without tape; briefly runs capstan motor with tape

Not working:
Rewind from idle state
Pause LED
Record
Record LED (assumed)
Record mute (assumed)

 

[caution]

I think the control module membrane is shot. The gold contacts are shiny and clean so I don't think that's where the issue is. With it disconnected the resistances are a few hundred ohms if pressed very firmly, but usually in the range of a few K (or more) if pressed with normal pressure. I have no idea what this resistance should normally be, as it's represented as a switch on the schematic. There are already series resistors on the inputs, so this may be adding too much in some cases. The differences in resistance may be a factor here, as rew and FF use 100 ohms, play and stop use 220, record is 470 and pause and record mute are 1.2k.

It seems like the inputs demand a lower resistance than they're getting right now and it's at the point where the resistance is tolerable in some situations but not all of them anymore, and needs a lower resistance in an idle state to work properly.

At idle, the play, stop and FF buttons work immediately and with a light touch, while rewind needs to be pressed harder and for at least a second or two.

The record and pause LEDs are fine, as a quick test with a resistor and battery confirmed. The record button works from idle as well, I just didn't notice before because apparently you have to hold it down and press pause to put it in monitoring mode, from which you can press play to begin recording.

The record button stopped the deck when I held it in firmly for a couple of seconds while it was playing, but that may have been a confused input.

I will find a recent TV or stereo remote with a similar contact arrangement and transplant it.

 

[Superduper]

Well, the good news is that the main IC appears good. Remember what I told you about the Fwd and Rwd functions. For the Reel motor IC driver to function, both of the F & R inputs need to be in the correct states. F-H/R-L = Fwd, F-L/R-H = Rev, both F-H/R-H and R-L/R-L will have no response. So at idle, you will need to ensure that the logic chips are behaving properly. Unfortunately, there is no logic flow chart to show what is supposed to happen to the logic circuits further down from the main IC.

As for the control module membrane -- can you show a picture of what it is you are talking about? Is that a rubber membrane backed with carbon?

 

[BoomboxLover48]

My Good Lord! Only Eric the great Magician can do all these tricks!

 

[caution]

Thanks Royce! Your pinch roller restoration thread helped with ideas.

Yeah, it's nice that things cleared up the way they did after everything is back in place. I'm certain the Hall effect sensor made all the difference, pulling it out of a constant state of auto-stop. From what I can see it looks like if it doesn't transduce a signal, then auto-stop regulator Q703 does a logic version of holding down your finger on the stop button. The emitter is tied to the Hall element and the collector passes through inverter IC705-D, whose output controls inverter IC704-E, whose output ties directly to /STOP at IC701. Once it isn't being engaged constantly, the control IC's output pins run the reel motor properly like you point out.

Here's the membrane out of the module. I tried to zoom in to show how hard and knobby they feel, they must have been exposed to water or chemicals in water.

It's got two rows of contacts 15mm apart. The columns are 17mm apart except in the gap between the contact pairs for play and stop, which is 15mm. With such an odd setup I may have to convert the thing to use metal click-domes held on with tape or glue together membrane chunks.

Oh and you'll notice that spots remain on the eject plate. I realized a few hours into it that two of the three brass studs on it have copper collars. I noticed that they started to look a lot like that pink color copper gets when it's eroding quickly (I used to run a PCB etching machine) and since they were already pretty beaten up from prior corrosion I decided to take it out and be done. Good enough.

A 100% flush generally needs about two weeks to address any deep pitting. The staining pattern on the metal was from when I took it out for a couple minutes to take a stiff plastic brush to it to try and get loose rust off to help it along. In that time the air reacted with the vinegar and flash rusted the part and turned it a tinge of yellow, but was removed once put back into the bath.

 

[caution]

It looks like it won't be much work to repair the membrane. Caig makes a silver paint, and ButtonWorx makes stick-on replacement carbon pads as well. I think the silver paint option may be more lasting (and conductive).

 

[caution]

As it turns out that the belts were pretty new but too big. I noticed the black pulley for the capstan slipping once in a while, so I took them off and they all measured way over what they are supposed to be. I had all the right sizes so now it's working fine.

So far everything that water hates - steel, paint, and rubber (except the capstan motor grommets) has had to be repaired.

Oh, and a thought about the solenoid's fusible resistor FR704. I think it overheats when the belts go, because without them, the control IC can't get a signal from the Hall effect sensor and just latches on the solenoid like it did when I had mine detached from the deck, unable to turn off/reset. Although mine still had a good resistance it was pretty charred so I replaced it. Mouser carries Yageo brand 1/4W fusible resistors.

 

[Superduper]

It looks like it won't be much work to repair the membrane. Caig makes a silver paint, and ButtonWorx makes stick-on replacement carbon pads as well. I think the silver paint option may be more lasting (and conductive).

Eric, I have restored many remote controls that had exactly that type of membrane and PCB with carbon based materials (brush on, not stick on) designed specifically for that purpose with good effect. They are designed for membranes which are flexible. I'm pretty sure the silver stuff is best suited for used on PCB's and other stable substrates that aren't flexible. Also, the carbon component is not as conductive as the silver stuff but the system doesn't expect it to be 100% 0-ohms. The built in resistor suggests that it isn't critical. However, one characteristic of carbon is that it is pressure sensitive meaning the resistance goes down as more pressure is exerted.

BTW, if you recall back somewhere in the PM stream, I asked you to check the input by switching directly at the IC input pins instead of going through the control circuitry to isolate that portion and eliminate it as a potential source of issue. And I believe you didn't understand why that would be necessary....... So THIS is why, lol.

 

[caution]

If I had only listened! But hey I got there eventually though!
You are a life saver, thank you so much for all the help

 

[jimmyjimmy19702010]

'Listening' to Eric and Norm talk is like listening to French, I've no idea what is being said, but gee it sounds strangely satisfying!!

 

[baddboybill]

Now that is awesome job so far Eric. Very cool brother!!!