Ensoniq VFX SD mainboard troubleshooting

[GiliRose]

I just checked the Vbb bus - it is OK all the way to J3 on the power board.

HOWEVER - Pin 21 on the power board has the WRONG voltage - 1.6v !!!
Wonder how I missed that the 1st time.

With a magnifying glass I could see some component burn around R2 & R4 on the power board.
Using the DMM set to resistance, I tested the whole column of components at this location on the board for continuity:

C21 - No continuity
R7 - OK
R1 - OK
R2 - dead and looks burnt
R3 - OK but dirty from smoke/fire
R4 - dead and burnt to the core
small silver diode 1 - No Continuity
small silver diode 2 - No Continuity
1st big black diode (4004) - OK
2nd big black diode (4004) - OK
3rd big black diode (4004) - OK
4th big black diode (4004) - OK
R5 - OK
R6 - OK

All 5 fuses are OK, F3 & F4 seem to have the fuse wire on the outside, not sure about that one

[Rasputin]

You just lost me. I don't know what you mean by saying the 50V rail is fine all the way to the PSU (or whatever you're trying to explain). The point of origin for the power is obviously the PSU. On the PSU, there's a header [J3] that jumps off and carries that voltage (via pin 3) to the display PCB. On the display PCB it gets distributed around to the multiple VFD driver chips.

According to your measurements, the driver chips are not getting 50V. So the question is if the 50V is even making it to the display board. This can be measured directly at the power header on the display PCB. And if it's present at that header then where does it go dead prior to not reaching the driver ICs?

If the 50V is not making it to the display PCB then further investigation is needed at the PSU to see if the 50V is even available there, and if not, why not?

Apparently you're measuring low voltage on the PSU pin 21, but pin 21 is VBB, so I'm unsure what you mean by saying it's "fine until" a certain point. Basically, if you've got bad voltage on PSU pin 21 then you need to fix the PSU. If you weren't measuring any voltage on the VBB rail on the display PCB then it seems you've got an additional problem beyond that though, as you should be picking up the bad voltage (however low) on the display PCB and not measuring nothing.

Can you take a picture of what you're talking about with the fuses and all that?

[GiliRose]

I just checked the Vbb bus - it is OK all the way to J3 on the power board

Sorry for the confusion. I meant the physical line of the Vbb starting from J3 on the power board,
via the cable to the display board and all the way to the last component being "supplied" -
is continuous, no breaks or funny wonky stuff.

The problem, as I stated, is that something bad and fiery happened to R2 and R4 on the Power Supply board.
Taking a look at the Power Board schematic you posted at the beginning of the thread, those are the exact two
resistors just before the Vbb line leaves to the display board via J3.

I will see how one uploads a picture, it is quite visible.

[Rasputin]

I just checked the Vbb bus - it is OK all the way to J3 on the power board

Sorry for the confusion. I meant the physical line of the Vbb starting from J3 on the power board,
via the cable to the display board and all the way to the last component being "supplied" -
is continuous, no breaks or funny wonky stuff.

The problem, as I stated, is that something bad and fiery happened to R2 and R4 on the Power Supply board.
Taking a look at the Power Board schematic you posted at the beginning of the thread, those are the exact two
resistors just before the Vbb line leaves to the display board via J3.

I will see how one uploads a picture, it is quite visible.

Okay, so you have continuity all the way from J3 pin 3 on the PSU to the VFD drivers on the display board, got it.

Now the question is if the PSU problem is a symptom or a cause. I don't know how you initially checked the PSU, so I can't tell you. If you are totally sure you measured 50VDC on J3 pin 3 (aka point 21 in the service manual) previously then perhaps there's a bad component on the display PCB which caused the PSU to go suicidal.

[GiliRose]

If you are totally sure you measured 50VDC on J3 pin 3 (aka point 21 in the service manual) previously ...

No - I am not totaly sure.
I am rather sure I overlooked this somehow.
I actually can't recall seeing a 50v reading previously, so this was the dead spot from day 1 that I got the synth.

The big question is what caused this, and otherwise,
what else needs to be tested for burn-ups before repair.

[Rasputin]

If you are totally sure you measured 50VDC on J3 pin 3 (aka point 21 in the service manual) previously ...

No - I am not totaly sure.
I am rather sure I overlooked this somehow.
I actually can't recall seeing a 50v reading previously, so this was the dead spot from day 1 that I got the synth.

The big question is what caused this, and otherwise,
what else needs to be tested for burn-ups before repair.

I think I would isolate the PSU by disconnecting the mainboard / floppy / display headers. Test Q5, Q6 and Q7 on the PSU to try to narrow down what might be fried, or anything else in the vicinity (schematically speaking). I'd probably check CR10 (4004 diode) and anything downstream.

You've reached a point where it is no longer Ensoniq specific, so you'll have to depend mostly on basic electrical system testing. At least until you get your 50V back on the PSU when it's unloaded. Replace anything bad and hopefully VBB will be correct again.

Then you'll need to test the display PCB for shorts between VBB and ground, so get out the resistance setting on the DMM.

If it looks like connecting the display won't short anything and the PSU has correct voltages all around then cross your fingers and power it up.

[GiliRose]

I found the culprit! Thanks Rasputin, your the man!

It seems the previous owner suffered a short that burnt out Fuses F3 & F4 (the display board fuses).
Wanting to get back to his music ASAP, he "repaired" the burnt fuses by wrapping a short length of
wire on their exterior and stuck them back in.
Next time he suffered a spike or power surge, there was no fuse to protect the components down the line,
resulting with blown resistors R2, R4 and perhaps others. Pop went the Vbb bus.

If someone could explain how to upload images, I have a whole set I took that could illustrate the situation.

Otherwise, I disassembled the power board and display board and will be replacing any suspicious
components this coming week. Fingers crossed.

[Rasputin]

Wanting to get back to his music ASAP, he "repaired" the burnt fuses by wrapping a short length of
wire on their exterior and stuck them back in.

Next time he suffered a spike or power surge, there was no fuse to protect the components down the line,
resulting with blown resistors R2, R4 and perhaps others. Pop went the Vbb bus.

It's a classic tale, unfortunately. Glad you've got it narrowed down!

[GiliRose]

You've reached a point where it is no longer Ensoniq specific, so you'll have to depend mostly on basic electrical system testing. At least until you get your 50V back on the PSU when it's unloaded. Replace anything bad and hopefully VBB will be correct again.

If I had any hairs left, I would be pulling them out just about now.
Over the last two weeks I checked and double checked anything I could that made sense to me, having to do the with the Vbb part of the circuit. I will break this down into parts to make it easier to follow:

1. Tested ALL voltages coming into the PSU board and leaving it.
All voltages were within spec, only the Vbb is at 3.62V.
Also, +VU and Display Offset are at 12.1V, a wee bit over the 8V-11V spec.

2. My area of focus starts from capacitors C17 and C13 on towards the Vbb pin.
Both are receiving the correct voltage from right after fuses F2 & F4.

3. I don't have a Fluke or Cap tester, so pulled out all the caps (C13, C15, C17, C20).
They look OK and behave correctly with a simple Multimeter resistance test. Put them back in
(replacing C13 and C15 with new ones. Couldn't get hold of 63V 470uF).

4. Replaced the burnt resistors R2 & R4. ALL resistors are OK and test with correct values.
(R1, R2, R3, R4, R5, R6).

5. ALL Diodes test OK. Even so, replaced CR11 & CR12 as they were just next to the blast zone.

6. Replaced all three Power Transistors (Q5-Q7 NPN MJE182 from TI, Q6 PNP MJE172G from ON).
Damned manufacturers list their pins in revers order from each other - took me a while to figure.
Tested all three Transistors for behavior and pinout - all are fine.

Something which is NOT obvious is going on.
I looked for breaks in the paths, cold solder and cracked joints, made sure I bridged two pads that came loose
(board tech has come a way since the 80's) and double tested.

All I can think of is a voltage test at various junctions in the path, but I have never done that and wouldn't know
what values would be the correct ones.

[Rasputin]

So the VBB voltage has gone up a couple volts from what it was previously?

I have a feeling it is one of the caps, especially since you didn't have a particularly valid way of testing them. If the voltage is correct after the fuses then you know it's not the transformer. And since VBB is too low when the PSU is not loaded by any other boards (display or main) then the fault is a component between the fuses and the PSU output header -- specifically the area where you've been looking.

That says to me that there's a flaw in your testing or something else you feel you've already checked, examined, or replaced.

Could easily be a defective cap, especially one that you didn't replace.

1. Tested ALL voltages coming into the PSU board and leaving it.
All voltages were within spec, only the Vbb is at 3.62V.
Also, +VU and Display Offset are at 12.1V, a wee bit over the 8V-11V spec.

2. My area of focus starts from capacitors C17 and C13 on towards the Vbb pin.
Both are receiving the correct voltage from right after fuses F2 & F4.

3. I don't have a Fluke or Cap tester, so pulled out all the caps (C13, C15, C17, C20).
They look OK and behave correctly with a simple Multimeter resistance test. Put them back in
(replacing C13 and C15 with new ones. Couldn't get hold of 63V 470uF).

4. Replaced the burnt resistors R2 & R4. ALL resistors are OK and test with correct values.
(R1, R2, R3, R4, R5, R6).

5. ALL Diodes test OK. Even so, replaced CR11 & CR12 as they were just next to the blast zone.

6. Replaced all three Power Transistors (Q5-Q7 NPN MJE182 from TI, Q6 PNP MJE172G from ON).
Damned manufacturers list their pins in revers order from each other - took me a while to figure.
Tested all three Transistors for behavior and pinout - all are fine.

Something which is NOT obvious is going on.
I looked for breaks in the paths, cold solder and cracked joints, made sure I bridged two pads that came loose
(board tech has come a way since the 80's) and double tested.

All I can think of is a voltage test at various junctions in the path, but I have never done that and wouldn't know
what values would be the correct ones.

[GiliRose]

Could easily be a defective cap, especially one that you didn't replace.

I have a pair of "under-spec" 50V 470uF caps I can replace (originals are 63V).
I didn't replace them yet because of their lower rating. That might not be a problem,
given that they need to handle 14-24 volt in the scheme of things, as I understand it.
I'll give it a shot, lets see what happens.

Thanks!

[GiliRose]

The Power Board is back in order!

I aught to kick myself real hard too! Apparently I was sold the wrong R2 & R4 resistors,
and out of natural trust, forgot to check. They were 22K and 360K - rather than the specified
22ohm and 360ohm.


OK, Vbb is back and chirping at 54.3 volts. The problem now moves on to the display board.

Ahead of things, I replaced the three 10uF axial caps just to make sure they are clean and fresh.

Now, when I connect the display board and power up, I immediately blow fuse F5, which normally
feeds 9.7 VAC rms to the Filament. I replaced the fuse and powered up again - exact same behavior.
I also checked to see whats up with the Vbb on the display board end of the connector -
Nada, nothing.

I disconnect the display and go back to check the Vbb on the Power board - thank god - it is still there, at 54.3V,
sans Fuse F5.

So - something on the display board is blowing F5.

[Rasputin]

The Power Board is back in order!

OK, Vbb is back and chirping at 54.3 volts. The problem now moves on to the display board.
Ahead of things, I replaced the three 10uF axial caps just to make sure they are clean and fresh.

Now, when I connect the display board and power up, I immediately blow fuse F5, which normally
feeds 9.7 VAC rms to the Filament. I replaced the fuse and powered up again - exact same behavior.
I also checked to see whats up with the Vbb on the display board end of the connector -
Nada, nothing.

I disconnect the display and go back to check the Vbb on the Power board - thank god - it is still there, at 54.3V,
sans Fuse F5.

So - something on the display board is blowing F5.

Well, firstly, I applaud your persistence!

Secondly, I would systematically test the display related voltages by disconnecting the display board power connector from the PSU and replacing the fuse. Make sure everything is fine and no fuses pop and the voltages are as they should be again. Then connect just the display power connector cable WITHOUT connecting it to the display board. In other words, unplug the power connector from the display board but connect the other end as it should be. Make sure everything is still as it should be, no blown fuses and that the correct voltages are present at what would be the display board end of the harness. Wiggle/twist the display power cable around some (generally just fiddle with it) and If everything is still good then you know the problem actually lies on the display PCB. Hopefully there's just a short in that wire harness or some kind of bad ground wire (pin 1), etc.

It seems as if there's potential for the display power harness to be bad if you're also not getting VBB on the display. If it's still there on the PSU then it should be there on the display as well, assuming the wiring harness is good. There's something really, really suspicious about that, so check for both the filament and VBB voltage at the display end of the power connector, just without the display actually connected to the cable.

Assuming the problem is on the display PCB, test for resistance between pin 5 and 6 on the display PCB power header. Clearly if there's low to no resistance then there's a short somewhere. If you wanted to be brute about it, you could cut the filament traces that go to the actual VFD and then see if the fuse pops again. If the fuse doesn't pop with the traces cut (or pin 41 & 74 on the VFD disconnected) then there's a strong chance the VFD is messed up.

To summarize: Take the filament power out of the equation by preventing the PSU from sending it to the display PCB and then figure out why VBB isn't making it to the display.

[GiliRose]

OK - I follow the concept. Will test and report back.
I love the internet - thanks again Rasputin!!!

[GiliRose]

The forum login timed out on me so I will re-write this shortly:

1. The Cable is OK, all voltages are correct at the display end - while it is not connected.

2. Vbb is shorting out at on the display board. Resistance between the Vbb and GND pins is 0.00 Ohm.

3. Very possible short with the filament too - Resistance between pins 5 & 6 of the PCB connector is 4.8 Ohm.

Cable OK, Shorts on the display board.

TIP:
No need to go brutal. using the point of a nail file, you can unlatch the pins from the connector housing
and pull them out. they click back to place when you push them back in.