PROBLEM.....Or Not?

Is this a mains frequency transformer, or a high frequency inverter? If the latter, a standard multimeter will read incorrectly, you need a true RMS meter to measure it.

Yikes! I wouldn't call that "slightly off-spec". It is more than DOUBLE what the service manual specifies. I would be concerend about the life expectancy of the display unit. It could also be the reason the original fusible resistor blew, etc.

I can think of several questions...

1) Is the manual correct? Are you sure that the service manual is for the model and serial number you are working on? For Sony stuff (with which I am more familiar), they sometimes had a specified range of serial numbers which the service manual applied to. Because they would make significant changes to the product in mid-stream and had to publish a different manual when they made the change, etc.

2) Is the transformer and mains wiring OK? Are the other voltages coming out of the transformer closer to what the manual specifies?

3) Is it the same display as in the manual? Can you compare the full part number to confirm that the display in your unit is the same one the manual thinks it is?

4) Is the measurement from a center-tap? Is there a grounded center-tap on the filament winding? Could the manual measurements be from ground to each side? I'm pretty sure that the circuit requires ground reference somewhere for the circuit to work properly, This is frequently done by grounding a center tap on the transformer winding, etc. Manuals frequently show voltage readings referenced to ground, etc.

5) Does it look like the filament wires in the display are being overdriven? IME, they should be glowing a very dull reddish color when viewed in a totaly dark room with your eyes accustomed to the darkness. If you can see them glowing under normal room lighting, it could mean that they are being overdriven. If you aren't sure what I mean by the filament wires, see the 2nd photo here.... >

I don't know. I was assuming that the lines in question were just one of the transformer secondaries.

The power transformer in this cassette deck has four (4) different secondary outputs:

18v AC 15v AC 5v AC 3.5v AC

Indeed. In my response, I had just assumed that the filament was being driven from a tap on the mains power transformer. If it is from a high-frequency inverter, your (unspecified) meter may not be reading properly.

Is it a big "heavy iron" transformer with the primary connected to the mains power cord? Or is it a smaller, lighter ferrite-core transformer driven from a power transistor (or two) Or perhaps a combination of those, i.e. a switching power supply?

It's a big transformer, with the primary connected directly to the mains power cord via the on/off power switch.

The service manual I am using is for this exact model. According to the service manual, there were NO revisions at all in the power supply (I double checked). The schematic and diagram apply to ALL Nakamichi CR-7 cassette decks.

Those things were famous for display problems. Wouldn't surprise me that the voltage doesn't reflect the service manual specs. If you had some low value 1 watt resistors you could experiment and see if you can drop the voltage to 3.25. Wouldn't hurt it and may add some longevity to the display.

Then your meter is most likely reading correctly (within its limits). Unless you are measureing across the windings when the manual is specifying from the center tap to each side, I would be VERY CONCERNED about feeding double the voltage into the filament of the display.

Does the manual show that that winding is center-tapped? Do the filament wires in the display look like they burning?

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Are you measuring the filamant voltage across the filament i.e from one terminal to the other and not one terminal to ground. The filament voltage is a differential voltage terminal to terminal. There is also a boas voltage above ground. If you want to correctly measure the FILAMENT voltage, you should place your meter probes on the two filamaent terminals, with NO connection to chassis ground.

Is that what you are doing?

Mark

RICHARD:

I don't know if the service manual shows that the winding in question is center-tapped. I have copied part of the schematic for the power supply, and put it online for you to look at.

Check it out:

The filament wires do NOT look like they are burning. Nothing at all is glowing except the display elements that should be glowing.

WHAT ABOUT THIS: Perhaps I am taking the voltage readings incorrectly? I put the meter's probes across BOTH of the blue wires coming from the transformer, thinking that the circuit had its own floating ground. That's when I get the reading that is more than double

3.25v AC. BUT, when I put just ONE probe on ONE blue wire and the other probe on chassis ground, I get a reading of 3.96 volts AC...much,MUCH closer to what I want to read.

What do you think?

If the voltage were really that far out of spec the filaments would be glowing like a lightbulb. If it looks ok then you're fine, leave it alone.

thats not an 'issue' thats a problem! ;-)

seriously, your only real chance is to experiment with series resistors if nothing else yields a result. ben

This clearly shows 3.5 volts either side of center tap. They add to seven across both hots. You're fine.

All good fortune,

Chris Hornbeck

The schematic diagram shows that there should be 3.5 VAC on either side of the center tap. The center tap is the line that goes up to the emitter of Q413. Sounds like everything is OK.

Thanks for setting me straight. I was laboring under the assumption that the 3.5v AC circuit was not tied to chassis ground but instead was its own circuit; therefore, I kept measuring both of the 3.5v lines at the same time. An embarrassing mistake, but one that I"m happy to finally recognize as indeed being a mistake!

It's not unknown for the specs of such items to change over the lifetime of the product and that info not to make it into the service manual.

If you could see the filament glowing red hot I'd be concerned but don't forget, this equipment has worked for years with that voltage being applied to it.

Graham

It's 3.5V - 0 - 3.5V. That's 7V centre tapped (with the centre tap being connected as it should be to a negative voltage).

It's meant to measure that way.

Graham

In your case the center-tap is actually connected to the

-14 VDC supply (IIRC?) this is the voltage that makes the selected (grounded) parts of the fluorescent display glow. But you likely didn't want to get that far into the circuit analysis. :-)

"EADGBE" wrote in message news: snipped-for-privacy@y42g2000hsy.googlegroups.com...

A mistake, yes, but not quite as you are imagining. For future reference, you should try to get straight and understand the whys and wherefores of this circuit.

As we pointed out to you when you first posted your question about this a couple of weeks back, this supply is *not* tied to ground, but is floated on a negative bias voltage equivalent to the display drive voltages, in order to reduce the stress between the filament and the display elements. This is usually done by just having a bleed resistor from the negative supply to one side of the filament supply. In the case of this item, it is done in a slightly different way, by having a centre tap on the filament winding, and then connecting that tap to to the emitter of Q413, which supplies -14.5v. So, you have an AC voltage, 'floating' on a negative DC voltage. This explains the reason that the designer saw fit to put a fusible resistor in each leg of the winding. The actual AC voltage across the display filament - ie the actual voltage that the filament 'sees' and which is being used to power the filament - is indeed the full 8v or whatever that your meter is recording. That is correct. The schematic clearly shows that the indicated voltage of 3.5v is from winding end to tap, as it is shown thus by the sides of both winding ends. With your meter set to DC, if you measure from ground to either end of the winding, you should see the negative bias voltage. Anyway, glad you got it going, and the group was able to help you with the diagnosis, and learning a bit more about how these things are done.

Arfa