PROBLEM: Nakamichi CR-7A Power Transformer

Download the service manual

Those things have a history of display problems.

Thanks for the link - I have already downloaded the service manual. That's how I know the voltages for the secondary outputs.

I should have made that more clear in my original post (sorry).

The 3.5V you've lost is almost certainly the filament supply for the vacuum fluourescent display. That explains why it's gone out.

Where does the other 3.5V go to ?

Graham

If you're not getting 3.5VAC from the winding there are a couple of possibilities. First is a fuse -- is there one between the transformer and the diode bridge? If so, look for a blown one. If you find one, figure out why it blew before replacing it -- possibilities include a shorted capacitor.

If no fuse, then your transformer's winding may have burnt out -- but usually you can smell that.

Try detaching the transformer winding from the board, if that's doable, and see if the voltage is still missing.

Peace, Paul

"Paul Stamler" wrote in message news:%R%Ki.608056$ snipped-for-privacy@bgtnsc04-news.ops.worldnet.att.net...

Where are you measuring the voltage ? As others have said, the 3.5v AC going to the display, is its filament supply. Assuming that it does go direct to the front panel as an AC supply (90% of manufacturers do it like this, but a few don't), then it will eventually arrive at either end of the VFD pin row, where you will find, a pair of pins separated from the rest of the row, by one pin gap. The trick is that sometimes, there is a safety (fuse) resistor or small wire-ended fuse in series with the supply to one end. Several of the Aiwa models have such items, and you can't see them unless the front panel PCB is removed. They go open for no good reason. I would try measuring across either end of the display, and where the winding arrives at the board. Don't trust that any such fuse will be shown on the schematics either. A further check for no display, just in case you are fooling yourself with the missing filament supply, is to measure from either one of the filament pin pairs, to ground. You shoud get a reading of between about minus 27v and minus 35 volts. You can measure it from filament to ground, as the filament is biased to this figure to reduce the inter-electrode stress in the VFD. It is very common in all manufacturer's kit for this supply to fail, as it is often derived from a voltage multiplier using ordinary electrolytic caps, which dry out and go open over the years, due to the continuous pounding they are taking from having high level AC across them.

Arfa

GRAHAM:

Both of the 3.5v AC supplies go directly to the flourescent display. They don't go anywhere else.

PAUL:

It's pretty much a direct path from the transformer to the flourescent display. They don't go to a diode bridge because they are never converted to DC voltage. The two 3.5v leads are soldered to the power supply board, where each of them runs into a 1-watt "r-fuse". (Both of the 3.5v leads have their own "r-fuse".) Then these lines go to a connector near the back of the deck that's mounted on the deck's logic board. That connector then has straight circuit board lines to a connector at the front of the deck which attaches to the flourescent display.

I am measuring voltage directly at the ends of the two 3.5vAC leads coming out of the transformer. No output at all.

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

Graham

Based on that, I would have to say that the 3.5v winding is open, but by the same token, I would also have to say that it is extremely unusual for a single secondary winding - particularly one that has to do little 'work' like this one - to fail without there being some kind of fault like a dead short on the end of it. If the windings come out to tags on the tranny, it would be worth checking that the soldering between the winding wire and the tag itself is good. I have known the laquer coating of the wire to never have been stripped properly, which has led to a bad joint at the tag which has managed to 'make' ok for years, before finally giving up. Also check that the winding wire has not fractured right at the tag. I have seen that as well, presumably caused by heat expansion and contraction over the years, working to break the wire.

As a final check, you could sub the supply. It doesn't have to be AC. You could just hook up a couple of 1.5v dry cells to give you 3v, and hang that across the two wires going back to the tranny, having first disconnected them, of course. The reason I say connect it there, is so that any bias supply that the filament supply is floated on, still gets applied.

Arfa

DC on the filament will actually cause a brightness variation across the display bit it's fine for fault finding.

Graham

ARFA:

There is one important factor that I have not mentioned.

I bought this deck on eBay, from an honest seller who clearly stated that the deck was NOT in working order. I bought it, figuring that I could fix it up.

When I looked it over, I found problems that clearly indicate that this deck had encountered some sort of power surge or maybe even a lightning strike. All three fuses on the power supply board were blown (two for 15v AC; the other one is for 5v AC), PLUS two voltage- regulating transistors on the power supply board were blown (one of them burnt to a crisp), PLUS a couple of diodes had shorted out. Oh, and a buffering capacitor in another part of the deck had shorted completely.

After fixing those problems, the power supply is stable and gives good voltage levels throughout the deck, and the deck runs perfectly.

Perhaps the failure of the 3.5v tap on the power transformer is the result of some stress on the transformer from the power surge?????????????????

QUESTION: Could it be possible that the 3.5v tap had been damaged by the power surge, and was "tired"?

ANOTHER QUESTION: Is it possible that such a low-voltage tap could have its own thermal fuse inside the transformer?

NOTE: The 3.5v AC is the ONLY voltage line that comes directly from the transformer and is not "handled" or "modified" in any way by the power supply circuitry.

Thanks for the information on what to look for on the transformer itself. I'll be sure to check it out.

GRAHAM:

I must admit that I was assuming that the two 3.5v wires were two supplies. They could easily be two parts of ONE 3.5v supply.

I was assuming (I know--a dangerous practice) that they were two supplies because the transformer seems to have a common ground wire (black wire) soldered to the ground circuit on the power supply board. Or maybe the black wire isn't a common ground for the entire transformer....read below:

Besides the outputs I have already mentioned, there is a black wire soldered to the ground circuit of the power supply board, and this wire is labeled "CT GND". There is also a grey wire soldered to the power supply board in another spot, and it is labeled "CT". What does "CT" stand for?

The filament supply will most certainly NOT share a comon ground. It has to 'float' at a negative voltage provided by the rest of the psu, so it seems you have one winding only. Have you measured it with a DVM ? I would expect it to show 'infinite' ohms indicating it's 'burnt out'.

Centre tap. Probably just for the main audio +/- supply.

So, I'd suggest you follow Arfa's advice and connect 2 1.5 batteries in series to the display filament. That should bring it back to life.

What one does about it in the long term is another matter. Is there room to fit another small transformer inside ?

Graham

GRAHAM:

I'm a happily married man, but nevertheless, I could kiss you! With your latest suggestion, you have given me some hope.

Up to this point, I had always assumed that the two blue 3.5v wires coming out of the transformer were TWO 3.5v supplies. It never occured to me that it was a single winding.

I disconnected the jumper wire carrying the 3.5v charge to the deck's logic board and connected my DVM up to the two wires, where they are soldered to the power supply board.

It measured resistance of 1.6 ohms! It is NOT infinite, and therefore NOT open circuit!

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED ON???

Thank you VERY much for your help so far, Graham! I at least have a glimmer of light at the end of the tunnel.

Maybe yes, maybe no, definately irrelevent.

Almost certainly not, almost certainly irrelevent since you won't be able to fix it.

I would go back and double check with an ohm meter, not a voltmeter, that the 3.5 volt winding is definately open circuit - likewise check that the display filament isn't open circuit, again with an ohm meter. There is no point wasting time with the transformer until you know the display is OK.

Gareth.

Are you measuring between the two wires and is your meter on AC volts or DC volts?

Gareth.

Have you checked for voltage *across* the blue wires? If it's an isolated heater supply, either wire on its own will show zero with respect to chassis ground, but will show 3.5 volts with respect to the other blue wire.

Which is not at all out of the realm of what such a winding might be.

Because if that winding is NOT connected with the common ground, and you are trying to measure the voltage of either one in reference to that common ground, you won't get any meaningful reading.

Instead, you need to measure the AC voltage between these two wires, NOT between each of them and ground.

I'd bet when you do so, you get the voltage you're looking for, which probably indicated the display is shot, among other things.

Disconnect the transformer from the board. Is there still no output across it?

Either the transformer is bad, or something on the board has failed into a short.

You can either spend big money to have the old one rewound, or you could retrofit a second transformer in there to replace the 3.5V winding that is bad, or you could call Steven Sank and have him send you a new transformer from his large stock of Nak parts.

--scott