U67 PSU help

Hi All,

So I'm looking at a U67 PSU that's got some voltages that are too high. http://www.danalexanderaudio.com/Micpics/Neumann/NU67uPSU.jpg

One thing I'm not sure about is the value of GR3, but as it is I'm getting -15V across it, and the variable resistor R4 is not affecting the voltage drop. I'm getting the same -15V everywhere to the right of R2, all the way to the heater output.

The caps in here are original, but it doesn't seem like out of tolerance caps would be giving me this sort of issue, would it? My main suspect is GR3 at the moment.

I'm also getting between 35V and 40V too much along the B+ supply, all the way between GR1 and the B+ output. The supply is jumpered for 117VAC input and I'm getting very close to that at the input.

Any suggestions?

Oh, I probably need a load across my heater supply, don't I. I was testing this all with nothing connected. What's a normal value for heater resistance?

Still learning about practical application of all the electronics stuff I've been reading for so long, this place is really valuable. Thanks!

bobschwenkler said:

Oh, I probably need a load across my heater supply, don't I. I was testing this all with nothing connected. What's a normal value for heater resistance?

Still learning about practical application of all the electronics stuff I've been reading for so long, this place is really valuable. Thanks!

Click to expand...

Not only a load, but the proper load from the microphone with the tube in place.

You must measure the voltages at the microphone itself because the cable will effect the readings you get.

Plate under load: 210v
Heater under load: 6.3v

http://www.neumann.com/download.php?download=copi0039.PDF

Mark

out of curiosity, was/is there actually a problem?

A couple leads on the bridge rectifier had broken to open. I fixed them and was concerned about the high supply volts and then realized that no load probably was throwing them off.

bobschwenkler said:

A couple leads on the bridge rectifier had broken to open. I fixed them and was concerned about the high supply volts and then realized that no load probably was throwing them off.

Click to expand...

I've done that myself... I know that feeling...

> What's a normal value for heater resistance?

Depends on the tube, don't it?

Anywhere 1 ohms (2A3) to 600 ohm (60FX5).

What does it say on the plan?

Looks like 6.5V and something about 0.2A. Assuming a bit more resistance on the line or mike, that's reasonable for one small 6.3V heater. Or look how it drops the trimmed 12V to 6.5V through 22+5.1 ohms.... 5.5V/27.1 is 0.203A.

So 6.5V at 0.2A is 32.5 ohms. Oh, and 6.5V*0.2A is 1.3 Watts. So put four 33r 0.5W resistors in series-parallel.

But if all you did was fix broken wires, there's no reason to think it is wrong. And the "high voltage" is utterly normal since they use a massively wasteful (33% efficient) R-C-R-C filter to get clean with the small 50uFd caps available in the day. (The HV is not quite so saggy.)

So I tested the PSU with the mic on, heater voltage is fine. I'm getting 227 on the B+. How big of a deal is that? Having the modulation leads connected to a load or not (which they currently aren't) shouldn't make a difference on this rail, right?

bobschwenkler said:

So I tested the PSU with the mic on, heater voltage is fine. I'm getting 227 on the B+. How big of a deal is that? Having the modulation leads connected to a load or not (which they currently aren't) shouldn't make a difference on this rail, right?

Click to expand...

Good stuff, the manual says that the plate voltage should be 210VDC +/-2%.

Is the power supply set for 115v or 127v?

If you've got it set for 115v you could set the primary voltage to 127volts instead of 115volts to drop the plate voltage down. You would then have to adjust the heater voltage (R4) to correspond to the new primary voltage.

Other than that it looks like you could fine tune the plate voltage by changing the value of the R3 resistor (82K).



Mark

Aah, the manual. Hadn't printed that out yet. It's jumpered to 115 right now. I'll give the next setting a shot. Thanks!