Unequal resistance on output transformer primary

[bobloblaws]

I'm fixing a Beltone AP-22 amp which has a very low output. There are a couple of things that I suspect might be the problem and one is the output transformer. Checking resistance I get a reading of 157 ohms from primary center tap to one of the primary legs and 422 ohms between center tap and the other leg. Is this an indication that the OT is bad? If not, why the huge difference between the the two sides of the primary?

PV

[Mick Bailey]

Differences in resistance are common and expected, but not to the degree yours reads - the 422 ohms side looks bad. It could be partially open, where the winding burns but leaves a track of carbon and metal oxides - just enough to give a resistance reading on a DMM. I've also had transformers where the solder joint has been imperfect where the lead is soldered to the end of the winding. I can recall two, both of which were saved by slitting the wrapping to expose the joint and resoldering, then making good.

One check you can do is to run the amp with a constant input signal (from a signal generator, soundcard output or whatever). Check the AC voltage on the grids of your output tubes and they should be near to equal, allowing for PI imbalance. You only need a moderate signal to make sure the amp isn't clipping. Then measure the AC plate voltage on each side. If the difference is more than a few percent then it would confirm that the transformer is defective.

I always double-check any suspicions of OT failure, as an incorrect diagnosis is a costly mistake.

[Enzo]

Maybe. First make sure you are on the right wires. You have three wires on the primary, and you have given the resistance between two pairs, so is the resistance between the remaining pair around 265 ohms? Or closer to 579 ohms? If you get 265 ohms, then you were not on the right wire for center tap. If you measure 579, then you indeed had the wires right and something is probably wrong.

There are two ways to wind a transformer primary. You can use two pieces of wire, wrapped around together for the two sides of the primary, then joined as the center tap. This results in an even resistance diference. The other way is simpler, you wind the first half the proper number of tuns, connect a center tap, then wind the remaining number of turns. Now the turn numbers are even, but the later windings are longer wire, because they were wrapped around the earlier. Longer wire - more resistance, so the resistance is uneven. This is OK because transformers work on turns ratios, not resistances.

SO if you got my 265 ohms, that looks to me like a reasonable example of the latter: 157/265. If your numbers were correct, 157/422 SEEMS a bit more than I would expect.

There are tests. Push some AC into the secondary - the speaker connections. Maybe 6vAC from a small transformer. Now measure AC voltage from center tap to either plate wire. Do you see similar voltage ther? Or are they vastly different voltages.?

You can also test for shorted turns. GO to RG's Geofex web site, and find his tech tip of a transformer tester. It is a very simple thing - a battery and a neon bulb. And test your transformer with it.

[bobloblaws]

Maybe. First make sure you are on the right wires. You have three wires on the primary, and you have given the resistance between two pairs, so is the resistance between the remaining pair around 265 ohms? Or closer to 579 ohms? If you get 265 ohms, then you were not on the right wire for center tap. If you measure 579, then you indeed had the wires right and something is probably wrong.

Yes, I confirmed that the reading across the two legs (connected to output tube plates)was the sum of the two readings using the center tap (red wire connected to filter cap)

There are tests. Push some AC into the secondary - the speaker connections. Maybe 6vAC from a small transformer. Now measure AC voltage from center tap to either plate wire. Do you see similar voltage ther? Or are they vastly different voltages.?

I don't completely understand this test. Are you saying if I push 6vAC into the secondary (I can do this with my variac I assume) I should see a greatly reduced voltage on the primary and if I see somewhere close to 6vAC on the primary it confirms the OT is bad?

[R.G.]

I don't completely understand this test. Are you saying if I push 6vAC into the secondary (I can do this with my variac I assume) I should see a greatly reduced voltage on the primary and if I see somewhere close to 6vAC on the primary it confirms the OT is bad?

If you put 6Vac into the secondary, you should see a greatly ENLARGED voltage on the primary winding if the transformer is good. Moreover, you should see about-equal voltages on each half of the primary.

I agree with Enzo - use a heater voltage for the 6vac, not a variac. You want to not mess around with AC mains voltage any more than you have to, and a variac is not line isolated.

For a two-EL84 amp, the OT is probably about 8K to 8 ohms, impedance ratio of 1000. The voltage ratio would then be the square root of that, about 32. So 6Vac on the secondary would give about 192V across the whole primary, half that on each half.

If the transformer is bad, you could see other voltages. If the primary has a layer-to-layer short, you might see nearly nothing, as the short causes the volts per turn to go to nearly zero. If the speculation about a section or lead being high resistance is correct, you might see the nominally correct voltages, as there is no current flow in a voltage only test to drop voltage across a high resistance.

[bobloblaws]

If the speculation about a section or lead being high resistance is correct, you might see the nominally correct voltages, as there is no current flow in a voltage only test to drop voltage across a high resistance.

Do you mean that if I get nominally correct voltages when doing that test the OT might still be bad?

[Enzo]

I could be wrong, but I think he is saying if there is an unusually large resistance in the winding, it couldn't provide current into a load as well.


Why not do the tests and see what results, it may make some questions moot. None are difficult.

[bobloblaws]

You can also test for shorted turns. GO to RG's Geofex web site, and find his tech tip of a transformer tester. It is a very simple thing - a battery and a neon bulb. And test your transformer with it.

Would an LED work in place of the neon bulb?

[Enzo]

A good transformer will inductively make a voltage spike that will turn on a neon lamp for a moment. A neon lamp needs a certain "threshold" voltage before it can light, so if a shunted transformer winding snubs the inductance, the lamp never lights. An LED doesn;t lend itself, though RG would have better insights.

In my head we could likely make up a detector to run and LED as an indicator, but that defeats the simple elegance of a neon bulb.

Google "NE2 bulb" and find a bunch of places selling them for about 50 cents. They are quite common, Radio Shack used to sell them, if you still have one of those around.

[bobloblaws]

OK, fair enough. I asked because Monday is a holiday for most stores since Canada fell on a Saturday. Didn't want to wait until Tuesday to try and track one down. I might be able to try the other tests in the meantime.

[Mick Bailey]

You can use LEDS, but in a much more elaborate circuit. I built a transformer tester that uses coloured leds in a bar graph display. It uses the same principle as the neon tester, but uses a comparator to count the number of 'rings' from a pulsed voltage across the winding. It pulses, then listens for the result and does this every few milliseconds so you get a constant display. It will tell you which actual winding is bad. The reason I suggested testing the transformer in the amp is that it's in its loaded condition (either with the speaker or using a dummy load). There are a range of tests you can carry out on a transformer, each of which will test for a specific fault. Running AC into the secondary and measuring the primary halves can eliminate an open circuit, or shorted turn. It may not necessarily show up a moderately high resistance so clearly. The neon test method is excellent, but can sometimes be inconclusive if the failure is marginal.

A scope will usually show up a faulty OT. If the tubes and sockets are good, and there's a clean signal on the grids and correct biasing and DC voltages, the scoped output will give a good indication of what's wrong. The amp is set to a clean setting with a sine wave on the input. As the amp is turned up look at the waveform. If it still looks sinusoidal, but is severely asymmetric with either one half missing or severely attenuated, then one side of the winding is likely to be open. Sometimes at low signal levels the waveform looks fine, but then one half disappears when the level is increased. This is a winding that's braking down under load and going open or high-resistance.

If the waveform stays sinusoidal at very low signal levels, then transitions quickly to a very distorted (and sometime slightly asymmetric shape), then that's a sign of a shorted turn. You may be getting (say) 3W clean signal out of a 30W amp before it suddenly gets really distorted. As the level is increased, the output doesn't increase, but the distortion does. A confirmation of this is usually the AC plate voltage is reduced, often dramatically on the shorted side.

[Enzo]

Cool, that is a much better answer than mine.

[bobloblaws]

A scope will usually show up a faulty OT. If the tubes and sockets are good, and there's a clean signal on the grids and correct biasing and DC voltages, the scoped output will give a good indication of what's wrong. The amp is set to a clean setting with a sine wave on the input. As the amp is turned up look at the waveform. If it still looks sinusoidal, but is severely asymmetric with either one half missing or severely attenuated, then one side of the winding is likely to be open. Sometimes at low signal levels the waveform looks fine, but then one half disappears when the level is increased. This is a winding that's braking down under load and going open or high-resistance.

If the waveform stays sinusoidal at very low signal levels, then transitions quickly to a very distorted (and sometime slightly asymmetric shape), then that's a sign of a shorted turn. You may be getting (say) 3W clean signal out of a 30W amp before it suddenly gets really distorted. As the level is increased, the output doesn't increase, but the distortion does. A confirmation of this is usually the AC plate voltage is reduced, often dramatically on the shorted side.

Thanks, this is something I can easily test as I do have an oscilloscope. The surplus store I had in mind was open today but didn't have the neon bulb.

[Mick Bailey]

Take a look around to see if you have anything with a neon in it - illuminated mains rocker switch, extension lead or similar. In the past I've liberated one from domestic servitude to serve a higher purpose. If you have a scope, then you're pretty much there anyhow.

[Gregg]

...157/265. If your numbers were correct, 157/422 SEEMS a bit more than I would expect.

Neither of those readings can be considered "good" or "normal". They either indicate a short in the primary or very bad quality output transformer.
The low output mentioned in the first post combined with those crappy reading indicates a short in the primary which means loss of primary inductance which and as a result the low output. The easiest way to check is to measure the primary inductance - end to end and from CT.