PROBLEM: Nakamichi CR-7A Power Transformer

I have discovered a fault on the power supply board - in fact, it's the only possible fault there could be, other than the transformer.

And it looks like my initial panic about a defective transformer MIGHT have been an overreaction.

First of all, I must apologize for my hasty (and incorrect) assumption that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are ONE complete AC supply.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service manual, I am measuring a steady 7.97v AC.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires of the 3.5v supply (which is actually measuring at 7.97v) are each connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to be regular dull blue 1-ohm metal film resistors. But the schematic symbol shows a small rectangle with symbols for a fuse and a resistor inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED AS FUSES as well as being a voltage drop for the "3.5v" supply which is actually 7.97v?

Whoops, I forgot to specifically mention what the fault was.

One of the "R-FUSES" has become open circuit.

So the transformer voltage cannot reach the flourescent display.

Fuse resistors are pretty common. They are not meant to be replaced in the same way as a normal fuse as they do not normally burn unless there has been a serious problem. (Fuses often just get tired and blow when they haven't been told to). You have to correct the problem, then replace the fuse resistor. If yours are in series with the filament supply and one has blown, it is because FAR TOO MUCH CURRENT HAS GONE THROUGH THE FILAMENTS AT SOME POINT.

AND EXCUSE ME FOR SHOUTING AS LOUD AS I CAN.

So it does look as though the unit has suffered some kind of over voltage thingy, and the filamant may now be a piece of curly toast. Unless of course the problem is just that the cabling and circuit board from transformer to display has a fault instead. Go and check the resistance of the filament right now, as I suggested earlier on. This may all be academic if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame proof so they wont make a lot of alarming smoke and a cringeworthy smell, and have been designed as a fuse, not a resistor.

Gareth.

You need to get the service manual, which will detail how the transformer is wired.

This is because it is completely unloaded. Inexpensive transformers will often have poor regulation, and will shoot up to fairly high voltages when unloaded.

They are fusible resistors; 1 ohm resistors that have controlled burnout characteristics for use as fuses. MCM Electronics carries replacements. You can substitute a stock 1 ohm resistor if you are adventurous, or replace it with a picofuse.

Does an ohmmeter show them as open or not?

--scott

--
"C'est un Nagra.  C'est suisse, et tres, tres precis."

GARETH:

I have just checked the resistance of the filament, as you asked (sorry I didn't do it sooner).

It seems to be fine. There is a solid reading of 7.2 ohms.

This is an accurate reading because I disconnected the filament from the rest of the display window circuitry before measuring it.

It's interesting to note that the filament circuit consists entirely of just these items:

1) AC voltage coming from the transformer - dedicated to just the filament and nothing else. 2) Two (2) inline fuse resistors - 1 ohm, 1/4 watt each 3) The filament itself 4) A jumper wire connecting the whole thing together.

Well you may be lucky and find the fuse resistor blew before the filament then.

I have no idea what the filament resistance should read, but calculating 7.2 ohms + 2 x 1 ohm resistors at 3.5v gives about 380 mA max, thats 0.14 watts per fusible resistor. I shuld imagine that as the filament heats up its resistance will increase dramatically, reducing the current, so it looks like you may be in the right ball park.

Gareth.

Again, where are you measuring exactly ? If you are measuring between ground and either of the two wires, you will not get a reading, because as Graham says, the filament winding is completely floating. You must measure *across* the two wires, with your meter set to its AC 10v range. If you have a reading of 1.6 ohms across the winding, and the rest of the transformer is working, it is highly unlikely that there is anything wrong to be causing the 3.5v winding to have no output. Also, when the winding is connected where it's supposed to be, you need to measure across the two sets of double and slightly spaced pins at either end of the VFD pin row to read the AC filament voltage. If you read between ground and those pins, you will not read filament AC voltage, but with the meter set to DC you should read a negative voltage of 25 to 35v. If you do not, then it is the negative supply which is missing, not the filament supply. This is the much more common fault condition in my experience.

Arfa

Whilst in theory I would agree that fuse resistors often fail because of a following problem, I would have to disagree with it as an electronics 'generalisation'. I have been repairing kit for as long as fuse resistors have been in common usage ( and a good many years before that ), and I find that as many go open circuit for no detectable reason, with no physical signs of distress, or having been stressed, as fail for a valid reason. This seems to be particularly true of ones that are positioned in AC feeds, so maybe they don't like AC across them too much. As I said in an earlier post in this thread, several models of Aiwa hifi employ a 1 ohm fuse resistor in series with the VFD filament supply, and it is very common for them to fail. A replacement always restores completely normal operation of the display, and you don't get a 2 week bounce from them either.

Arfa

ARFA:

I measured the resistance between the two blue 3.5v wires of the transformer, with the jumper that leads to the flourescent display DISCONNECTED.

I got the resistance reading of 1.6 ohms.

I measured the voltage the same way.

I took NO readings involving the ground...only between the two 3.5v wires coming out of the transformer.

SCOTT:

The fuse resistor that didn't blow reads exactly 1.04 ohms (that's an in-circuit reading).

The fuse resistor that blew reads open circuit....and that is with the resistor TAKEN OUT of circuit completely.

I figured I could replace the fuse resistor with a regular 1-ohm resistor and get things working again, but I also realize that that is NOT safe, and so the deck will not be powered up again until it has a new 1-ohm fuse resistor in place...and I might even replace the other

1-ohm fuse resistor whilst I'm at it.

Are you measuring with the display not connected? The transformer's output without a load will be higher than with a load.

Quite possible. These are called "fusible resistors", and they're designed to protect the circuit by blowing up if too much current flows through them. If you combine loading on the transformer and two 1-ohm resistors in series with the filament, that might give you the 3.5V you want.

Next step: put the ohmmeter across each of the 1-ohm resistors and see if they're blown.

Peace, Paul

It's not uncommon for the filament on a VFD to be fed from a centertapped transformer secondary winding. The center tap is grounded, and the two ends of the winding feed the two ends of the filament. The same effect can be created using a dual secondary winding, of course.

This approach has the effect of balancing the filament voltage around the system-ground voltage, and helps ensure a consistent anode-to-cathode voltage (and consistent brightness) across the width of the VFD.

If you feed the filament from a single secondary winding, and ground one end or the other, the display brightness will vary somewhat from one side to the other. DC drive is even worse in this respect.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

you

That suggests the 3.5V winding is 'floating' at zero volts and somehow the negative supply to the VFD has failed. This would give the same symptoms but is probably much easier to fix.

One now needs to check out the circuitry driving the VFD. It'll be a case of examining the schematics.

Graham

And when the equipment is working correctly, both blue wires will show a negative DC voltage too (from the VFD supply).

The filament btw acts as a directly heated cathode in a VFD. The segments that illuminate are the anodes and the remaining connections are control grids.

It's a big multipin triode.

Graham

Plus, if it's working correctly, they'll be at some negative DC potential too.

It would make sense to measure the resistance of the filament. If there's no continuity, then the display would need replacement.

Graham

The filament needs to be 'tied' to some negative DC voltage for the display to work.

Check for this DC voltage. If not present you won't get a display either.

Graham

I agree. That's certainly the right kind of value.

Graham

I wouldn't excessively fret over not fitting a genuine fusible resistor. The only difference between them and normal resistors is that they are (a) designed to fail open and (b) are flameproof. There's still another one in circuit !

If one of those fusibles had failed (as it seems) that's why you have no display.

Graham

Actually, the centre tap of such a winding for a VFD would usually be at some negative potential, NOT grounded.

However he doesn't have a centre tap so please don't confuse the issue.

Graham