GF-777 No power to drive motor

Hi,

Hoping someone can help. When you press 'play' nothing moves within the entire cassette mechanism. Power to the mechanism circuit board s via a 3pin connector, black, orange, red. I think this is what should happen:

1. Power switch off

• Red =17v, Orange=0v and Motors=0v

2. Power switch off and tape 'play'

• RED=17v, Orange=17v, Motors 16v

3. Power switch on

• RED=17v, Orange=17v, Motor=0v

4. Power switch on, tape 'play'

• Red=17v, Orange=17v, Motors=16v

Assuming the orange cable must be the feed, 16v is not detected going into either motor, suggesting it is something on the board? Have taken mechanism apart and performed usual maintenance, contact cleaner, belts etc, manually checked heads move freely.

Appreciate any suggestions

I'm sure someone with more knowledge than me will chime in soon, but while you have the boombox open and somewhat in pieces, please give it bath!

That’s not mine, belongs to my friend google. 😀😀

Is the motor siezied

With my limited electrical knowledge, I came to the conclusion that no power going to either motor, must be an issue with the source, as opposed to the motor itself?

What is the process for safely testing the motors?

9v,battery

Power only goes to motor when the micro switch on each deck works correctly

definitely?

baddboybill said:

Power only goes to motor when the micro switch on each deck works correctly

Click to expand...

Definitely? Took deck apart cleaned switches and tested them, all showing as working....

The PCB components and switches all seem to check out fine. Does anyone have or know where to find a circuit diagram for the deck itself, in case the fault is beyond the deck. Found schematics which des not include the deck and the GF777 RAR in manuals, is in french.

Appreciate any input.

First you need to verify if the motors are working or not, or you will be wasting yours and (everyone else's time). It is simple to do, just take a 9v battery and wire it up directly to either motor. If they spin, then you won't have to guess anymore. If you don't want to test and prefer to guess.... well, then good luck.

IF the motors work, then the next thing I will suggest is that you verify the voltages to the 3-pin connector. Please don't guess. You listed some voltages and scenarios. Is that just what you "think" should happen or are those test measurements? One of the wires is always hot. That's for the deck. If that tests good, and I presume that it does, then I suggest you next check the emitter of Q704. It should read about 9v. With the boombox powered up, put red lead of meter on Q704 emitter. Put black probe at good ground. If you aren't getting around 9v, then that regulator circuit needs examination for possible failure. Also, each deck has like 3 switches. They all work together so if you only test 1 of the 3 (for each deck), you haven't checked them all. Also, you said that you do not detect 16v going to either motor. First, the motors get power through the switches. For example, deck 2 power goes through one switch (main-a), which then goes through another switch (remote), before getting to the motor. However, the motor is not grounded. Ground is sourced through a transistor, which is triggered through switches (main-b) and (main-c). In other words, it's complicated... not just 1 switch and that's it.

So, in summary, both legs of a motor is switched by multiple switches. If any of them aren't working (including the solid state switches), then motor is not going to spin.

Superduper said:

First you need to verify if the motors are working or not, or you will be wasting yours and (everyone else's time). It is simple to do, just take a 9v battery and wire it up directly to either motor. If they spin, then you won't have to guess anymore. If you don't want to test and prefer to guess.... well, then good luck.

IF the motors work, then the next thing I will suggest is that you verify the voltages to the 3-pin connector. Please don't guess. You listed some voltages and scenarios. Is that just what you "think" should happen or are those test measurements? One of the wires is always hot. That's for the deck. If that tests good, and I presume that it does, then I suggest you next check the emitter of Q704. It should read about 9v. With the boombox powered up, put red lead of meter on Q704 emitter. Put black probe at good ground. If you aren't getting around 9v, then that regulator circuit needs examination for possible failure. Also, each deck has like 3 switches. They all work together so if you only test 1 of the 3 (for each deck), you haven't checked them all. Also, you said that you do not detect 16v going to either motor. First, the motors get power through the switches. For example, deck 2 power goes through one switch (main-a), which then goes through another switch (remote), before getting to the motor. However, the motor is not grounded. Ground is sourced through a transistor, which is triggered through switches (main-b) and (main-c). In other words, it's complicated... not just 1 switch and that's it.

So, in summary, both legs of a motor is switched by multiple switches. If any of them aren't working (including the solid state switches), then motor is not going to spin.

Click to expand...

Thanks for responding, here is the verified checklist:

• Motors tested with 9v battery and are working
• Circuit components checked by meter
• Voltages supplied are accurate
• 3-pin connector, RED is 17v and always on.
• Q704 - Power as follows:
  Pin 1 - 9v.
• Pin 2 - 16v.
• Pin 3 - 9v.

[*]There are 10 different switches attached to the deck PCB, (2 sit behind RW/FFD buttons) all cleaned and meter checked.
Is it possible these switches are testing okay via meter, but failing when put under load? (They look mechanical, so should either work or not work).

Welcome any other suggestions

Well, it sounds like you are reasonably comfortable taking measurements, so I'll provide you with the deck schematics. As you saw, there are a ton of switches that everything goes through, and lots of test points. It's really too many back/forth steps and handholding that I'm no longer in my age, have patience for, so with the schematics, I think you can probably follow the circuit flow and make sure there is connectivity along the way.

I'll offer the following suggestions and advice:

1. Some switches carry significant current so yes, it's possible that with little to no current flow, they test good, and yet, with large current, immediately go N/C or high Ω. However, in most such cases, they will usually allow current to pass for a brief time while the connection heats up from the current flow.
2. Many of the other switches however, carry voltage but not much current -- they are for the motor drivers (signal level current for the transistors).
3. I don't have a 777 in front of me, and it's been many years since I've last had one apart. There may be switches not on the main board, or are hidden in the mechanism. Look closely in case there's some you don't see.
4. If you are testing switches by manually actuating them, you may be applying greater force than that exerted naturally by the operation of the switch buttons. For example, if you directly push on leaf switches, you might input 1/4" of action whereas depressing the actual switch buttons themselves might only result in 1/8" of action. That discrepancy could provide you a false positive on switch connectivity.
5. The motors get almost full system voltage. However, they operate at lower voltage. The rest of the voltage is dropped by way of the transistors which sources ground for the motors. In other words, the motors need voltage to operate, but also need a ground, which goes through the drivers. The 2 decks use separate drivers (one NPN for motor 2, and a NPN/PNP pair for motor 1). As I didn't think it would be likely for both systems to go kaput at the same time, I've not considered their failure, however, unlikely does not = never happens.
6. Q704 is the main regulator voltage source for the signal that powers the 2 motor drivers. As it is in common with both motors, it's why I had you test it. You didn't tell me which reading is for B/C/E and 1/2/3 is not a good indicator of which transistor lead corresponds to which junction. However, I presume that the readings are correct if the higher voltage is the collector, although I would expect the base to have slightly more voltage than the emitter (about .7v difference). Not sure if your 9v readings are just rounded but the fractional numbers are important.
7. If the motor drivers are suspect, you can take them out of circuit temporarily by providing an artificial ground to the motors. This way, you can test the power supply circuit to see if they are working correct. Because the motors may burn out at 17V, you should not tether the motor neg directly to ground. Instead, do it through a resistor, 125 ~ 200 Ω. If the motors operate normally now, you can conclude that there is a problem in the motor driver circuits.

Walking through each point,1,2,3,4 relating to switches, there is no way to test them in situ.

Point 6: Full readings are Collector = 9.46v Base =16.22v Emitter =9.94v.

Point 7: I think this is where the issue lies, through process of elimination everything else checks out under load, apart from those switches.

I doubt switches are available to buy?

Look, firstly you have the voltages and transistor lead assignments wrong. There's no way I can see that those measurements are correct with regards to Q704. It would only make sense if you have the Collector at 16.22v, Base at 9.94v and Emitter at 9.46v, in which case, everything looks perfect. If I can't trust your measurements or descriptions, or have confidence that you know what you are doing, I won't be able to help you.

Secondly, like I said, both legs of each motor is switched by separate switches. It's easy enough to verify if the first leg (+) is switched properly. Look at SW711. When it is activated, both motors should receive +V at the positive terminal. Put meter probe at that location and operate SW711. This is true if the power switch is OFF. However, when the power switch in ON, both motors will receive +V at the positive terminal regardless of SW711 position. Motor (2) + signal also travels first through the remote jack, which adds another potential source for N/C, but Motor (1) does not so that motors (+) terminal should receive +V if either SW711 or Power Switch is working properly.

Now, moving onto the 2nd leg, which switches the ground through transistor drivers... Motor 2 gets its ground source through Q703. Motor 1 gets it's ground source through Q701 which is triggered by Q702. Again, it's highly unlikely that all of these drivers fail at the same time, so I am doubtful of this scenario. On the other hand, the signal that triggers the drivers involve Switches 701, 702, 703, 704, and possibly 709 all can have some affect on the function. For testing, I recommend that SW709 but set to single (default) position. The switches mentioned here are all low current switches.

So, you have all the information you need here to fix your deck. Can't see how I can be of much more assistance. Perhaps someone else here can chime in.

As to your question about buying switches... I personally don't think they are bad or can't be rehabilitated if contacts are merely oxidized. Factory switches are almost surely obsolete now.

So, if you want to do some direct tests on the driver circuits to verify if they are working properly or not, you can do that by injecting a +9v signal to the B of driver Q703 or Q701. But I don't recommend that you do that until you can properly identify the B/C/E of the transistors. Injecting at the wrong lead can cause some damage.

Oh BTW, the driver signals travel through several diodes, and in the case of motor 1, travels through many diodes. If these failed, then operation can be affected and if you like, you can check them with your meter, although I'd suggest that you lift a leg before testing to ensure that some parallel circuit is not giving you false readings.

Hmmm...It's entirely possible I got my wires crossed and reading Q704 from L-R as C/B/E, as opposed to B/C/E. Back to the drawing board, I'll go over it all again.

Chz

To be clear, C/B/E or B/C/E, etc are generic expressions intended to suggest/request that the different junctions/leads of a transistor should have their measurements expressed separately. It does not in any way suggest the actual lead order or position, something that can only be determined by the components data sheet. A schematic diagram almost never shows the actual component layout & lead order. For that, you’ll need to view a PCB wiring diagram that identifies such leads but the best way is to simply find a datasheet for the actual component under test, as the datasheet will show how to identify each lead.

Hi got your message on eBay yes I have the board
The mech is from a box and motors were working wwhen i removed it
But i canot promise this is your only problem
PM me here not eBay if you want the board

Could this be the issue...looks like a piece of the board missing?

See I said to check the micro switches. You can see the post has poor contact with solder joint

That soldering doesn't look like factory work ,it looks like its been replaced .sharp would never send out any work like that.