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Anybody good with tape counter/transport logic?
Old 4 weeks ago
  #1
Lives for gear
 
🎧 10 years
Anybody good with tape counter/transport logic?

It's a Tascam 122mkII. After fixing about five other problems, the deck is dropping into stop after about ten seconds of play, or maybe 3 or 4 seconds of FF/RW. If it has been on for a while and heated up, it will usually get to where it can play continuously, but will still drop out of FF/RW in about three seconds. I can also heat the control PCB with a hair dryer and eventually get it to work properly, but I'm not sure that method will be able to pinpoint an area or component.

- I've examined the solder side a few times.
- All electrolytic caps are new.
- The two hall effect sensors on the sensor board are new.
- The two transistors on the sensor board have been swapped with a couple others.
- The counter seems to increment smoothly with the take-up reel table, and there doesn't appear to be any stall or lag in it when stop gets triggered.

Can anybody make enough sense of that area of the schematic to guess at which (malfunctioning) components might trigger a stop condition?

Thanks either way!

PS- Not sure how the resized attachment will look, so I'll keep a nice big one here for a while: https://tinyurl.com/47x3t438
Attached Thumbnails
Anybody good with tape counter/transport logic?-control.jpg  
Old 3 weeks ago
  #2
Lives for gear
 
🎧 10 years
bump (...some of those guys have to still be around)
Old 3 weeks ago
  #3
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Radardoug's Avatar
 
🎧 10 years
Because this is an intermittent fault, its going to be hard to find. You need a scope.
Study the parts of the schematic that cause a stop condition.
The stop button.
Any end of tape sensors.
Possibly sensors relating to the position of tension arms if the machine has those.
Cassete machines often have a sensor which senses if the tape is turning, often driven by the dreaded rubber band, which wears out.
Put the scope on these points and watch for spikes.
I would check the condition of the stop button, maybe it has a mechanical problem.
Old 3 weeks ago
  #4
Lives for gear
 
🎧 10 years
Doug -Thanks!

It's not exactly intermittent. The only thing that changes is that it will allow itself to stay in play mode sometimes if it has been on a while. Heat is obviously involved, as I can get it to stop acting up if I get it hot enough, but I already melted one thirty dollar gear doing that, so I'm "twice shy".

It also seems a bit too consistent to be bad connections or flaky buttons. If I watch the counter, it will go almost the exact same number of counts each time in FF/RW before stopping (30 or so counts).

The whole thing is driven by one of those magnet wheels on a gear which is directly driven by the take-up table (no belt).

I don't mind taking a shotgun replacement approach, and just getting everything which 'might' be involved, but I myself am not experienced enough to make out what is doing what with this particular schematic. That "control" PCB serves about ten different functions too.

Thanks Again
Old 3 weeks ago | Show parent
  #5
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by Jidis ➑️
Doug -Thanks!

It's not exactly intermittent. The only thing that changes is that it will allow itself to stay in play mode sometimes if it has been on a while. Heat is obviously involved, as I can get it to stop acting up if I get it hot enough, but I already melted one thirty dollar gear doing that, so I'm "twice shy".

It also seems a bit too consistent to be bad connections or flaky buttons. If I watch the counter, it will go almost the exact same number of counts each time in FF/RW before stopping (30 or so counts).

The whole thing is driven by one of those magnet wheels on a gear which is directly driven by the take-up table (no belt).

I don't mind taking a shotgun replacement approach, and just getting everything which 'might' be involved, but I myself am not experienced enough to make out what is doing what with this particular schematic. That "control" PCB serves about ten different functions too.

Thanks Again
These kinds of problems are usually due to the tension arm on the tape. At certain speeds, the angle of the tape shifts and they lose their tension and cut off.

There is usually a tension adjustment inside the tape deck that can alleviate those problems.
Old 3 weeks ago | Show parent
  #6
Lives for gear
 
🎧 10 years
cruisemates- Thanks! This is just cassette though (no movable arms).
Old 2 weeks ago
  #7
Lives for gear
 
🎧 10 years
Man, I thought I had this stupid thing.

It turns out, if I heat the transistor at U507 (around B-3 in the schematic), that stops it from dropping into stop mode. Unfortunately, swapping with a similar transistor didn't fix it. I guess maybe the heat is actually causing it to malfunction and stopping it from doing its job. I'll have to start checking the stuff around it.
Old 2 weeks ago
  #8
Lives for gear
 
🎧 10 years
I'm still stuck on this thing if anybody has any good suggestions. I've since replaced a couple transistors, a couple caps, and a gate IC, but I think most of the next stuff will need to be ordered and I'm sort of shooting in the dark here.

Watching that main transport IC's "stop" line when play is initiated, it dips 3 or 4 times, getting lower on each one, until it passes the threshold for a low and throws out the stop. On FF/RWD, it begins to drop smoothly after about a second, until it reaches low. I guess everything chained to that stop pin is going to show the same thing and I don't know how to track it further.

Thanks

PS- Sadly, had this piece been one generation older, Tascam probably would have explained what every single part in that circuit was doing.
Old 2 weeks ago
  #9
Lives for gear
 
🎧 15 years
Does the take-up or supply reels stop moving before STOP is initiated? If so then the circuit is probably working properly and it could be that the rubber engaging the take-up or supply reel might be too old and not making good enough contact.
Old 2 weeks ago
  #10
Lives for gear
 
🎧 10 years
mbrebes-

Thanks, but no, it doesn't stop moving or anything. The reels and count are moving along nice and smooth when it goes to stop. All the rubber is actually new too (idler tires, pinch roller, and mode belt).

Take Care
Old 2 weeks ago
  #11
rez
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rez's Avatar
 
1 Review written
🎧 5 years
Hello!

In this tapedeck there are several places that could cause a stop condition.

- if the Stop button is pressed
- if the torque sensors sense high torque or tape tension
- if a Stop Memo position on the counter is reached
and maybe more...

The IC U501 is mainly a logic array for the several different and excluding play, cue, ff and rev, rec and stop controls from the buttons.
This IC BA843 has one more condition that forces a stop condition and that is a power cycle, so maybe that chip looses power at some point for a short moment?
This IC should give out stable hi or low signals at its outputs - if the output signal decreases there might be something wrong with the IC.
If any logic IC does not give a stable output i first check if the power rails are stable and maybe replace all Electrolytics in a device that old.
If that does not fix the problem i would look at the input side of the IC if there are unplausible signals that cause the outputs to fluctuate.
If the power and input signals are good - the probability for a bad IC is very high.
Most of the logic stuff is CMOS devices which are very easy damaged with static electricity.

The Transistor Q505 which is controlled from the opamp U507 has a direct connection to the stop input of IC U501 and can cause the same action as the stop button itself as it connects to ground if the collector-emitter becomes conductive.

You say that the problem is worse if the tapedeck is cold and gets slightly better if it warms up - that may point to the torque sensors, because the old grease in some places might actually lead to higher torque values that get smaller as the grease warms up and gets more fluid.
Measuring or adjusting the torque is a bit difficult without the proper test tools to say the least.
If you have put new grease in the machine the set values for torque might no longer be right, depending on how they were adjusted in the past.

I have a 122mkII here which has the sped-up motor problem, caused by leaky capacitors on the motor control board, but the transport controls are working so i might be able to measure some points to compare values so we might be able to find differences.

peace, rez

Last edited by rez; 2 weeks ago at 05:34 PM.. Reason: more info added...
Old 2 weeks ago
  #12
Lives for gear
 
🎧 10 years
Much thanks Rez for all the info. I've been watching this thread like a hawk, as so much time and money has gone into this deck, I feel like I can't give up on it.

I'll check into the stuff you mentioned.

The torque I believe was in spec., but I'll double check that. All the electrolytic caps are new, including those on that capstan board. Removing the capstan board may actually have been the start of this. It's a long story, but the lockwashers holding that plate down scratched into the paint and I ended up with 10.8 volts riding on the metal cage the transport is housed in for a while. It could have hurt one of the logic chips as you suggested.

Quote:
The Transistor Q505 which is controlled from the opamp U507 has a direct connection to the stop input of IC U501 and can cause the same action as the stop button itself as it connects to ground if the collector-emitter becomes conductive.
That's why I was thinking it was something to the right of that Q505 transistor in the schematic, as heating up U507 (actually one of those 1N241 transistors) fixes it until it cools back down. Q505,U507, and U502 have all been replaced, so it's not them. I only did basic multimeter checks on the diodes surrounding them though.

BTW- There's something screwy about this schematic. It seems to use amp symbols in a few places which should be transistors. There is in fact another U507, which is a DIP8 opamp over on the opposite side of the board, but the diode at D510 does connect in real life to the emitter of the "U507" transistor.

I'll let you know if I can find anything interesting when I get back on it.

Thanks Again!
Old 2 weeks ago | Show parent
  #13
Lives for gear
 
🎧 10 years
Quote:
Originally Posted by rez ➑️
If any logic IC does not give a stable output i first check if the power rails are stable and maybe replace all Electrolytics in a device that old.
Rez,

It looks like in my case, that main BA843 chip is running off only 3.3V. The datasheet I dug up looks like 4.5 is their minimum. Other than that, it sits solidly at 3.3, and doesn't dip or move as the transport does its "stop thing".

<edit> Definitely <3.3 right at that P104 power header (supposed to be 5V). I'm also getting values a bit too high on the +/- 15 rails, but I'm not sure that's causing any trouble.

Last edited by Jidis; 2 weeks ago at 04:46 AM.. Reason: more findings
Old 2 weeks ago
  #14
Lives for gear
 
🎧 10 years
Hell yes!!!!

Feeding a real 5V from my bench supply to that P104 line stops it from screwing up and going into stop.

I guess I need to see what's creating the 5V line that might have gotten broken.

Much thanks again Rez!
Old 2 weeks ago
  #15
rez
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rez's Avatar
 
1 Review written
🎧 5 years
Hello!

The power supply in this device is a bit complicated as the engineers thought it would be great to need +15, -15, +12, -12, +8, +7.5, -7.5, +7, -7 and 5v at various points in this device...

From a quick look the 5v rail is generated on the mother pcb in the section around the power transistor Q111 which is a 2SD-213 NPN.

This power transistor is controlled by the transistor U133 RT1N241 which looks like a litte ic with three legs.
That transistor in turn is dependend on the same transistor type U135 which controls the NAND-Gates in U118 which is a TC4011BP CMOS type that then control U133.

Be careful working in that area of the device as you are near the dangerous high voltage stuff! Everything on the low side of the power transformer T001 should be fairly low voltage but it is still AC and can cause heart failure when touched!
After the rectifier bridges it is way less dangerous. Just be careful not to get zapped by one of the big reservoir or filter caps.

peace, rez

Last edited by rez; 2 weeks ago at 10:56 AM.. Reason: cosmetic changes...
Old 2 weeks ago | Show parent
  #16
Lives for gear
 
🎧 10 years
Quote:
Originally Posted by rez ➑️
From a quick look the 5v rail is generated on the mother pcb in the section around the power transistor Q111 which is a 2SD-213 NPN.

This power transistor is controlled by the transistor U133 RT1N241 which looks like a litte ic with three legs.
That transistor in turn is dependend on the same transistor type U135 which controls the NAND-Gates in U118 which is a TC4011BP CMOS type that then control U133.
Yeah, I saw that Q111 guy when I was looking at the schematic last night. I can get it, but it might take a couple weeks. It's weird that all the voltages on that header are standard values, but the two 12V's are the only ones using common fixed regulators.

I've got the spare TC4011BP here that I swapped from the transport.

Is there by chance a common US equivalent of that 1N241 transistor? It's scattered all over all the boards in this thing, so I figure it can't be anything that unique. The only ones I saw available were surface mount.

Thanks Again

PS- What do you think of my 15V rails? I'm getting +18.2 and -19.4 on that header. Is that far enough off that something there might have gotten damaged too, or should I leave it?
Old 2 weeks ago
  #17
rez
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rez's Avatar
 
1 Review written
🎧 5 years
Quote:
Originally Posted by Jidis ➑️
PS- What do you think of my 15V rails? I'm getting +18.2 and -19.4 on that header.
Hello!

If you look at the schematic and trace back from the header youΒ΄ll see that this measurement point is directly connected to raw transformer voltage just thru the diodes D113 and D114 and the bridge rectifier D111.
So at this point it is expected to see just the diode drops from raw transformer voltage on this windings especially if there is no load connected.

The +15/-15 are mostly used for the motors and that two voltages are only used on the control pcb all around the transistors Q501/Q502 and Q503/504 which are controlled by U507, which in turn is controlled from the main transport IC U501 thru a bunch of gates and the hex inverter U504.
The whole part there just makes sure that a transport button press on the front of the device is causing the two main motors doing what the button calls for and nothing else.

It is all pure analog logic control...

There is nothing in that device crunching numbers, except for the tape counter.
Everything is voltage driven and everything can be analyzed in small chunks only using ohms law and kirchhoffs laws if you break down the schematic to sections.
Everything that has to do with control is hi/low level logic where you can measure supply voltages and signal states quite easy.
Everything that has to do with sound is pure analog voltage driven circuits that is fairly easy to trace as well.
Everything that has to do with the power supply system is amazingly complex and admittedly a bit hard to understand.

What makes this device a bit frustrating to work on is the sheer complexity of it, but that is to be expected as i think of this particular deck as the pinacle of what tascam has built in that era. Some of its modularity is even great for maintenance and repair. The mkIII model then did cut some corners to save cost and was not as desireable.

peace, rez

Last edited by rez; 2 weeks ago at 10:08 PM.. Reason: added quote...
Old 2 weeks ago
  #18
Lives for gear
 
🎧 10 years
Much thanks again Rez!

I'll leave the 15V stuff alone and start working on getting that 5V line functioning properly.

I'm looking forward to seeing what this thing can do with all new caps, rubber and grease.

Take Care
Old 2 weeks ago
  #19
Lives for gear
 
🎧 10 years
Rez and all,

Looks like it was that TC4011BP chip you pointed to at U118. I did the two RT1N241 transistors first and still got 4V, so I did the gate chip. My 5 volt line is actually 5.8V now with everything connected, which I figure is what it started at.

I guess it's time to break out the test tapes!

Much thanks again for the help.
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