Unequal resistance on output transformer primary

[nosaj]

Even if the winding resistance was compromised by a few percent, you have no way to determine whether the resistance you're reading is good or bad. Given that most OT windings are not bifilar or comprising separate identical bobbins for the primary, then a considerable difference is to be expected. So, say you have a transformer that reads 38 Ohms one side, and 42 Ohms the other (so a roughly 10% difference), is that good or bad, in your opinion?

I've made a habit of carrying out a post-mortem on every failed OT for a number of years to better understand the failure modes. I can say that out of the ones that were shorted, the fault was usually hardly discernable and had localized damage involving adjacent turns. Some that had serious damage and were shorted had obvious signs of burning through a few layers, but even so the difference in DCR between the two halves of the primary was not remarkable and could not have been an aid to diagnosis.

A common complaint here is that people post to say "The OT primary reads X Ohms one side and Y the other, so is it bad?" The answer is always that you cannot use DCR to determine whether a transformer is faulty, and this is difference is entirely normal.

I'll post some pics of my test unit in operation when I get chance, and a link to the PCB artwork.

Here's the thread with artwork still want some op pics Post #11
Thanks,
nosaj
http://music-electronics-forum.com/t41004/

[Mick Bailey]

Here are a few pics of the tester in use. The first three are from a transformer with a partial shorted turn, which is not uncommon if you can catch it early enough, but usually rapidly progresses to a full short. What happens is a hotspot develops between (usually) two adjacent windings and carbonizes the insulation. This results in a conductive path of a few hundred ohms - enough to cause distortion under load and lower output. The first picture is the outer ends of the primary and on the face of it looks OK. the other two readings from centre show fewer 'rings'. The second pic - the side with the lower count - has the short.

The fourth one is the PT from an Orange OR120 with a dead short on one of the HT windings. With both transformers DCR reads fine.

[nosaj]

Here are a few pics of the tester in use. The first three are from a transformer with a partial shorted turn, which is not uncommon if you can catch it early enough, but usually rapidly progresses to a full short. What happens is a hotspot develops between (usually) two adjacent windings and carbonizes the insulation. This results in a conductive path of a few hundred ohms - enough to cause distortion under load and lower output. The first picture is the outer ends of the primary and on the face of it looks OK. the other two readings from centre show fewer 'rings'. The second pic - the side with the lower count - has the short.

The fourth one is the PT from an Orange OR120 with a dead short on one of the HT windings. With both transformers DCR reads fine.

[ATTACH=CONFIG]44007[/ATTACH][ATTACH=CONFIG]44008[/ATTACH][ATTACH=CONFIG]44009[/ATTACH][ATTACH=CONFIG]44010[/ATTACH][ATTACH=CONFIG]44011[/ATTACH]

Did you etch your own board?

What program did you use to print the PCB gif. I've opened it in several programs but it runs small. Any idea on correct size?
Thanks,
nosaj

[Mick Bailey]

Yes, I etch my own boards. Just tried the image and it opens fine on my laptop. I use Photoshop to open and print, but anything that will open a .GIF will work. The image is 4090x2765 pixels - plenty to get the necessary detail required to etch. I just re-size so that the pad spacing is correct for an IC. For etching you only need a 300 DPI image.

[nosaj]

Did you etch your own board?

What program did you use to print the PCB gif. I've opened it in several programs but it runs small. Any idea on correct size?
Thanks,
nosaj

Guess I was just trying to figure correct spacing for the image. Paint opens it to like 1.5in x 2 in.

What process do you use for etching?

Thanks,
Jason

[catalin gramada]

hope this help

[Mick Bailey]

I use the 'toner transfer' method. I print using an Epson AL-C900 Laser onto the cheapest inkjet photo paper and iron this onto the bare PCB. Soak the backing off then etch using 9% H2O2 + HCL. It clears the copper in under 90 seconds. Drill, then clean off the 'resist' with acetone. Or, wire wool + detergent.

You have to make sure the photo paper is heat-resistant. I test new batches of paper by cutting off a strip and testing it with a clothes iron set at the hottest (cotton/no steam). If it sticks then it's no good.

I've done elaborate double-sided boards and complete amps using this method.

[bobloblaws]

So I did some testing.

Ran input signal from sound card and checked AC voltage on power tube grids. Result: left -> 230mVAC, right -> 237mVAC.

Pushed AC into secondary and measured AC voltage from center tap to both primary legs. Result: left -> 84.4, right -> 67.2

Tried the neon bulb test, bulb did not light up. However, I couldn't get the bulb to light up under any circumstances so I might not being doing this correctly or may have the wrong type bulb, although I'm pretty sure it is NE-2.

Looked at the output on my scope, it appears normal at low level and higher levels. This doesn't surprise me greatly as the sound of the amp seems pretty normal as well now that I have the preamp working properly.

Now here's the kicker. When I started this thread I reported that the resistance measurements I got on the two sides of the primary were 157 and 422. Last night after doing all these tests I decided to check the resistance again out of curiosity. To my surprise it read 157 one one side and only 207 on the other side. But just now before typing this reply up I decided to double check and lo and behold it now reads 156 and 320! In all three cases I verified the resistances on another DMM. So I'm inclined to play through the amp for a couple of days and see whether the tone is consistently good and if it is leave well enough alone, if not go ahead and replace it. Thoughts anyone, given the data I've provided?

[Enzo]

Do the NE2 test on a transformer you know is good. Any time you get a new piece of test equipment, whether a fancy digital scope or something as simple as the neon bulb tester, it pays to try it out on something working to get a baseline.

AC voltage on power tube grids. Result: left -> 11mA, right -> 35 mA.

Um... AC voltage would be volts, not milliamps. What exactly did you measure?

[bobloblaws]

Do the NE2 test on a transformer you know is good. Any time you get a new piece of test equipment, whether a fancy digital scope or something as simple as the neon bulb tester, it pays to try it out on something working to get a baseline.

Exactly, that's why I'm disregarding that test at the moment, I couldn't establish a baseline. I got the same result with another transformer that is in perfect working order to the best of knowledge. I couldn't get that lamp to light up even by feeding it 6V directly.

Um... AC voltage would be volts, not milliamps. What exactly did you measure?

Oops, my bad. Just tested it again for voltage, not current. Left -> 230mVAC, right -> 237 mVAC. I'll edit my previous reply.

[Tom Phillips]

So I did some testing. Ran input signal from sound card and checked AC voltage on power tube grids. Result: left -> 230mVAC, right -> 237mVAC...

That is a very low drive voltage. Based on that measurement I would expect the amp to have low output and would then be investigating the pre-amp and phase inverter circuits for the cause of the low drive voltage. Please explain the conditions under which you made those measurements.

[Enzo]

6v is nowhere near enough to light a neon lamp. And the bulb won't just come on, it will make a brief flash. Put a bit of shade over the bulb so the room light is not so bright. makes it easier to see. I forget now what lights an NE2, seems to me it is like 60-90v. SO when you touch a battery to the transformer leads, it is when you break the circuit that an inductive kick occurs.

[bobloblaws]

That is a very low drive voltage. Based on that measurement I would expect the amp to have low output and would then be investigating the pre-amp and phase inverter circuits for the cause of the low drive voltage. Please explain the conditions under which you made those measurements.

Output of soundcard, sine wave. I guess I was being ultra conservative with the input level. So what I did just now is I observed the voltage while play a fat low A note on the guitar. Looked like it was going up to 2.5 VAC or thereabouts. So I switched back to the sine wave input and pushed it up until I got 2.5 volts on the grid, then checked the other side, virtually the same, 2.6 VAC.

[Tom Phillips]

Can you post a schematic of the Beltone AP-22 amp you are working on? You went down the "bad OT" path right from the start but now the low drive voltage has my attention as a suspect clue to the problem of "very low output" that you reported in your first post.

[bobloblaws]

SO when you touch a battery to the transformer leads, it is when you break the circuit that an inductive kick occurs.

I could try it again if the other information is still inconclusive. The other transformer I tried it with is single ended, is that OK? And do the primary leads need to be un-soldered from the plate pins? Also, to be clear, the diagram on that tech tip shows a DC voltage source, that's what I want to use for the bulb test, correct?