"Can't tell much about a schematic outtake"

I understand. I'd like to post the full schematic but it's way too big. If you PM me with an email, I can send you the 3160 x 1498 TIF file of the entire servo board. And I can use all the help I can get on this one.

Kind of addicted to paper. Panning a monitor isn't the same. I'll give it a shot though, assuming I have something that reads TIFs. This computer is 13yo.

One step forward, one step back

Today I did the servo board swap, putting the known good board from my VT-100S into my VT-100 with working video heads. I said a small prayer to the Gods of the Magic Smoke, flipped the switch... and it worked! Sort of. The heads are no longer running at double speed, but I'm having real bad sync problems. The picture is wavy all over the place. But I did see video and I wasn't getting two half-height frames on the screen anymore.

So I decided to try a different tape to see if that was the problem before going any further. I hit Play... and then - the %*^$ playback solenoid wouldn't pull in. So with the pinch roller not pressing against the capstan, the tape now no longer moves. I don't know if it's the solenoid itself, the switch, or one of the half dozen parts on the PC board that controls its operation.

Anyway, I walked away in disgust and will revisit this new problem tomorrow.

Could be a bad micro switch that tells the servo that the tape is loaded properly.

OK, did read the whole thread. OMG, you got 40yo parts fabbed? Looking at FF10 there is nothing inherently failure prone in the design. Component or process failure. (Did I mention my last 'real' job was failure analysis engineer?) In a system operating solely on 9V it should not be possible for one component to burn out another.

I take back the 'velocity loop' stuff. This ain't that elegant. It's signature cutcorner-consumer. "All in one."

I hope the 'sync distortion' you saw briefly before the pinch solenoid quit was just its death rattle. But seriously, stuff is falling off this machine faster than you can put it back on.

I see in the waveform sheet you sent that TP6 was annotated 'hi'. The new FF10 fixed that, right? The remaining problem is TP7 'lo'?

I'm extremely distrustful of PUTs. Howzbout removing that and seeing if you get the gate signal? I still can't say what every stage of this thing does. Maybe I will eventually, maybe I won't. It's the coarsest possible representation of servo principles I've been exposed to. I'm a retro kinda guy but not sure I'm THAT retro. Fun excercise trying.

"Could be a bad micro switch that tells the servo that the tape is loaded properly."

Well that's one thing I can rule out, 'cause there ain't one in the VT-100.

I figure I have three options:

1. Troubleshoot the solenoid board (PX-508).
2. Swap the board from the headless VT-100 I got last summer.
3. Move the working servo board back to the VT-100S and move the VT-100's working video heads over there too.

I'll probably start with option 1. Here's what we're dealing with now. It's only 10 parts, none of which is a custom IC. How hard can it be?
The entire text that accompanies this circuit is as follows:

"The purpose of this circuit is to reduce the battery discharge current as per the following function:

At the moment the play key or the record key is depressed, the maximum current (about 1.2 amperes) passes through part of the pinch roller solenoid. After the solenoid is energized (within about 5 seconds) , the holding current (about 0.2 amperes) passes through the pinch roller solenoid, thhus reducing the battery discharge current.

There is no adjustment part in theis circuit. However, if the current flowing through the pinch roller solenoid does not change after 5 seconds, condenser C-9001 (22/6.3) is shorted".

Update on 5/14/12

Further investigation revealed that SW-3002 is the Record button on the deck and SW-3004 is the Play button. The only difference between the two is that there is a momentary pushbutton in the Record line. This button is on the camera itself. Actual recording starts when you press that button and stops when you release it. I tried pressing Record and the solenoid engaged OK. However pressing Play causes no action. Therefore the problem is with the Play switch itself. It is no longer making the required connection to ground. I'm working around this with a jumper cable from the solenoid to ground whenever I want to play a tape.

"you got 40yo parts fabbed?"

I know it - Matthew is a genius! He's like that guy in France who makes his own tubes. And to just offer to help like that, that's what makes ARF such a great place.


' Looking at FF10 there is nothing inherently failure prone in the design."

You'd think. And yet there were two bad FF-10's on this board. One of them had already been replaced many years ago. I got another VT-100 over the summer and the FF-10's on it were missing (probably removed to use on a different machine). Don't know what the issue was, but these parts are very prone to failure.


'I take back the 'velocity loop' stuff. This ain't that elegant. It's signature cutcorner-consumer. "All in one."

Which is probably why it's so confusing.

"I hope the 'sync distortion' you saw briefly before the pinch solenoid quit was just its death rattle."

No, the picture was wobbly so I stoped the machine and loaded a new tape. The next time I hit play, it wouldn't engage. It didn't drop out while it was running.

"I see in the waveform sheet you sent that TP6 was annotated 'hi'. The new FF10 fixed that, right? The remaining problem is TP7 'lo'?"

Oops - those notes are from a previous session with this board a long time ago. I just forgot to erase them. Just ignore those.

"I'm extremely distrustful of PUTs. Howzbout removing that and seeing if you get the gate signal?"

Well, that's an idea. Right now that board is out of the machine while I play with the known good one from the 100S.

The pinch roller circuit is Akai's take on a common electromagnet control. It uses
a far greater current (more ampere-turns) to pull the armature across the distance
and when sealed (pulled in) and a lower current to hold.

We used to repair those units and I have the service manual on file here

From time to time I view your posts and think of getting the file out. But I don't

"From time to time I view your posts and think of getting the file out. But I don't "

That's probably a very wise move if you value your sanity

Just a bit of lore.. We repaired the units on the second floor of a building that is now Tom Lee
Music in Vancouver BC. When heads were replaced, it was impossible to see through the
microscope because the building shook every time a trolley bus went by.

"it was impossible to see through the microscope"

You actually had one of those custom Akai microscope things, with the four eyepieces?? Wow, you wouldn't happen to know what happened to it would you?

Yes we had the alignment fixture to replace the polepieces. It was an Akai factory
service shop. I left in 1978. No clue what became of shop gear. Firm west bust in the
early 80's and I wasn't till the 90's I met one of the techs at a Sencore School.

Well I took a look at the solenoid today. The coil resistance looked OK so I applied 12 v. to it using a bench power supply. It pulled right in and held just fine with 0.15 A, so that part looks OK.

I then pressed Play (with the power disconnected) and measured the resistance from ground on the PX-508 board (the bottom of the coil) to chassis ground. I got a strange reading. It started off at zero, and then slowly rose to 0.7 ohms, just like when you measure resistance across a capacitor/resistor pair. I made the same measurement on my donor VT-100 and I got a steady 0 ohms (which is what I'd expect).

How can I not be getting zero resistance from this board to ground? My guess is that the 2SC1061 transistor (the one closer to the coil in the schematic) has an E-C short and I'm actually measuring the resistance across the 12 V power supply, and charging up some filter cap in it. Does that sound about right?

Akai was notorious for things that didn't make sense. Like, WTH is the purpose of the SW array? Here's a mechanical example. One audio model was capable of 3 speeds. But it required a sleeve over the capstan and a different diameter pinchroller, which stored under the hinged head cover when not being used. It also auto-reversed by a solenoid mechanically relocating the R/P head downward. Now here's the rub. If you stored the spare pinchroller under the headcover upside down--and the manual didn't warn you of this--the roller would impinge upon the mobile R/P stack and cause it to play all 4 tracks at the same time.

Guess how I found that out. OK I'll tell you. Customer brought it to me saying it played backwards tracks at the same time as forwards. I never heard it happen but he insisted. So I replaced all the transistors and most of the mechanism and gave it back to him working perfectly after extensive testing. But I always tested it with the head cover in the open position. Where the upside-down accessory pinchroller wasn't pushing on the mechanism. You see how scatterbrain Akai engineering was?

Now if TEAC had designed a video recorder we probably wouldn't be having this conversation. This also explains why Akai couldn't make a reliable module with 2 transistors, 2 diodes, a few resistors and caps (FF10). They're idiots. Seldom knew their ass from a golfcourse. Just so you know who you're up against. Remember their glass-ferrite audio heads? I installed those new out of the box and they were 6dB down a 3K because the notion was great but the execution was abysmal.

Today I did the one test I should have done first yesterday. I turned on the power and simply grounded the solenoid board directly with a jumper wire. The solenoid locked right in. As far as I'm concerned, that's good enough for now. Apparently, the fault is in the Play switch, and that thing is buried in the middle of the machine. You literally cannot see it from any angle. Removing it would be like reaching the heater core in your car.

So I resumed my diagnostics with a different tape. I'm still getting video but the vertical oscillator is way off. I'm wondering if this may be due to the fact that I'm using the VT-100S servo board. Since the VT-100 has no tracking control, I left that section disconnected. I can try jumping it with a 50K resistor and see if that changes anything. The 100S tracking control is a 0-93K pot.

What does your CT pulse look like?

Yeah, that sounds like a good place to start. Unfortunately, I didn't have time to work on it today. We'll give it a shot tomorrow.

You probably should not do anymore troubleshooting (except for the play switch) until you wire in a tracking control. You really need a pot so that you can adjust the tracking to your tape. It is somewhat surprising that you got anything resembling a picture with that connection left open.

I have to agree, Tom. If there's a plug for TRKG CTRL it's expecting a nominal resistance and 'infinity' ain't nominal.

Actually, you're right, ad that point had been nagging at me. Here's the only place where the two servo boards differ. The first clip is the VT-100, the second is the VT-100S. Right now, I'm running with nothing connected to the three wires that go to the tracking control.