[RGallahue]

Hello to all,

Working on the next project, a 40-150. Recapped fully, replaced all out of spec resistors, ohmed out the power and output transformers and voice coil, all good. Went to fire it up, no output (ie reception or static) and one of the 41 output tubes overheated. Although I had replaced the coupling cap (#52), I do not have the ability to check for leakage, so I re-replaced with a new cap. #46 to the 7C6 was also replaced.

Pulled the 41s, put in the rectifier tube and went to check voltages. Measure 330V across the 12 mfd electrolytic (#61) as opposed to 270V, and I understand that the new meters do not load a circuit to the degree that the old VTVMs did. Checked across the 16mfd electrolytic and read 286V as opposed to 185V and plate voltages at the 41s was 288V as opposed to 180V. Now, I would presume that the voltages would be higher due to the fact that there is no load, but does this high a voltage seems out of the ordinary?

Wanted to get opinions prior to the next phase of troubleshooting as I really don't want to fry a 41 while I am checking the voltages with the tubes in place. Have already checked all connections I reworked in replacing components against the schematic and my work checks out, so no incorrect connections or cold solder joints.

Thanks for any assistance you may render.

Richard

Schematic here:http://www.nostalgiaair.org/Resources/331/M0013331.htm

[morzh]

If you read the voltages with the 41s out they will be higher.
Now, 1) check the field coil of the sleaker, see if it is 1k or it is in fact lower, much lower or downright shorted. 2) Cap 61 is 12uF as required, not 20uF or so. 3) Xfmr input windings read 210 and 240 Ohms or close, no much lower, or open. 4) caps 55A/B are ok and not shorted.

[RGallahue]

Field coil is 1149 ohms, Cap 61 in indeed 12mfd, Caps 55 A/B are OK, xfmr input windings are lower than spec; 166 vs. 240 and 150 vs. 210ohms.

I did not think this would be an issue; apparently I am mistaken?

Richard

[morzh]

Well, it is not nesessarily an issue, depends.

Is it the original xfmr or a replacement?
If iot is a new replacement, it my be OK, as what truly matter is the impedance, and not just the DC resistance.
if however it os the original, there is a good chance you see the lowered DC resistance due to a large section short, then this may explain the overload. Again at this point it is a shot in the dark

[RGallahue]

Well, it is an original, but I was not sure that the resistance wasn't too far out of spec, and I was relieved that nothing was open. However, is this is too large a variance to expect?

[Radioroslyn]

One of the 41's overheated? What where the indications?
Terry

[morzh]

And which one overheated? was it on the side with the lowest resistance?

[RGallahue]

Well, we might be getting somewhere now...Tube became very very hot, much more than the other, smoked the glass, and it was the one on the low resistance side (150 ohms).

So it would appear that a new output transformer is in order?

[morzh]

First you want to test the transformer. You need to inject it with audio signal, say 1Vp-p, on the output side, and knowing the turns ratio (you do know it, don't you), say 100, measure the voltage at the input side (should be 100Vp-p). Using scope is the best, or if the frequency is low enough, a DMM will do. You have to measure the voltage right across the winding when injecting signal, and not at the generator's output.

Alternatively you could use a variac for the same purpose, BUT !!! inject the voltage in the INPUT side and measure the output at the output (speaker) side.
Use an isolation transformer before the variac lest you be zapped. It is not required for the experiment but is good for your safety. Use low voltage first, like 10-20V.

[RGallahue]

No scope, so that is out. No, I do not know the turns ratio and the Philco Repair Bench does not have the 32-8053N listed. Ron likely has this memorized (if you are out there). Great idea for a testing process.

Once I am able to find the data, I will try a test voltage as noted above. But in looking at the circuit, It would appear that we have likely identified the problem as all else has been pretty much eliminated. However, I am open to other ideas or opinions.

And THANKS for all of the help so far!

Richard

[morzh]

Remember: you have not idntified the problem until you have identified the problem
Now roughly, 41 tube has about 8k load at this voltage, and say the voice coil is 4 ohm, so the ratio is 2000 and square root of that is roughly 45.

So, roughly speaking, if you apply 45 V from the variac between the centertap and eityer end you shoud see 1V output. Bring it up slowly, listen to buzz and look for heat and smoke.

If the voltage is higher or lower it is OK as long as it is sensible.


Another way is, connect the full primary (ends) in series with 110V light bulb (table lamp, low wattage, like 10-20 watts) and plug it in isolated variac at 110V. Keep the seconday open. The lamp should not glow. Then short the secondary. The lamp should start glowing.
If the lamp glows with the secondary open, there is a turn(s) short, no matter where. Then shorting the secondary should have no effect.

Be careful and observe safety precautions. Do all cconnecting and disonnecting after first bringing the variac voltage to zero.

[RGallahue]

I was looking at the last test you spoke of, that being connecting the lamp in series with the primary ends of the transformer. However, is there not continuity between the primary ends? In other words, wouldn't the lamp glow regardless of the state of the secondary (open or shorted)? If I measure resistance across both halves, it is the additive resistance of the two sections above (316 ohms).

Sorry for all of the questions, I just want to understand the testing procedure. If I indeed had the turns ratio, this probably would be simpler as I could test for the ratio of voltages.

And I really do appreciate all of the help.

Richard

[morzh]

Rich

Transformers are there (that is, step- up or step-down, not pulse types) to transfer power.
They consist of two windings, primary and secondary. The tranfer is through magnetic flux.

Now, if you have no load in the secondary, the primary will present itself as almost pure inductance. This is why the lamp won't glow, or will glow very little, because the reactive impedance is way higher than active one (210 and 240 Ohm).
Now, if you introduce a load, the current in the secondary creates the flux opposite to that created by the primary (Lenz law). This reduces the inductance, and the impedance becomes more active than reactive.
In the ultimate case of a short in the secondary (connected leads, or simply a turn short), the flux of the sec. will prctically consate the one from primary, and the inductance will disappear, leaving us with pure resistance of the primary. That is those 210 and 240 Oms in series.

So, if your reactive impedance is, say, at 1H is 360 ohms, and your active is 450, the total is roughly 550. Short the output and it will drop back to 450. This calculation is for illustration only, typically the inductive is a larger portion.

So, even if the lamp glows with the output open, it will glow more with it shorted. If there is a short in any winding, itwill glow at the maximum no matter whether the secondary is shorted or not.

Of course it is better to test with a generator and a scope at higher frequencies, but this should work too. Better to test with a smaller lamp as the active impedance will limit the current and a large bulb may not glow noticeably, whereas a 5-10 W one will.

[RGallahue]

Thank you, Sir. It has been many years since my ham radio days, and my theory is a bit rusty. Apparently forgot all that I once knew about the properties of inductance.

I will test and let you know the result.

Thanks again,
Richard

[morzh]

Speaking of lightbulb, you can use a DMM in series with lightbulb in series with primary, set on Amps. This way you don't have to rely on visual perception an will see the actual change in current. You can even ditch the lamp and leave the ammeter only, just use low voltage Variac setting and Amps and not milliamps on the DMM.