Unequal resistance on output transformer primary

[bobloblaws]

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.

In a later post I mentioned that I fixed another problem with the amp and now I am getting plenty of output. Here's the schematic.

[bobloblaws]

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.

Yes, I did want to know if the tranny was bad, and when the low output was no longer a factor it still left the question of whether those wildly different resistance readings indicated a problem with the OT. Now the plot thickens because as I said, when I re-checked last night the high side had gone down from 422 to 207 and today back up to 320 as opposed to 157 for the other side.

[Mick Bailey]

Are your probes scratching through any surface oxide or contamination? Also, make sure your battery is good in your DMM.

[SoulFetish]

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.

The heat transfer method seems to work well for some people. I don't know if you've ever tried the acetone transfer method, but I've seen some great results. An improvement over heat transfer in my opinion. But the DIY best etching quality I've seen yet was shown to me by an old timer I know, who build and repairs organs. He found an old sign maker's plotting machine and uses a sharpie. I asked him if it was a fine point or specialized marker, and he said the best results he got was using a new regular type sharpie. I took a picture of the board as a reference because I was so impressed. What sucks is that my mother used to own her own sign shop before I was into electronics and her plotters are long gone.

[bobloblaws]

Are your probes scratching through any surface oxide or contamination?

No, I'm getting good contact.

Also, make sure your battery is good in your DMM.

Batteries are good and double checked with second DMM.

[bobloblaws]

So this is indeed very strange. The one side on the OT is stable at 157 ohms. Nothing wrong with my DMM, for that matter there is nothing wrong with both of my meters or the batteries in them. The problematic side shows 340 ohms as of this writing. So having checked the resistance at various times it has been 420 ohms, 207 ohms, 320 ohms and now 340. Also strange is the fact that if I sit and watch the meter for a few minutes I see that the reading jumps around constantly on that side. It bounces around within a few tenths of an ohm, up and down constantly but seems to trend downwards by a couple of ohms per minute or so at times and then upwards again at other times. Something is definitely wonky with this thing. Any other insight would be appreciated.

[Tom Phillips]

It seems to act like there is a bad splice in the problematic winding inside the transformer. Have you removed the end bells to look around a bit? Does it act differently if you vibrate /heat /cool the transformer or pull on the wire lead of the problematic side?

[Enzo]

Agree^^^

Look inside to see if the solder connection between the end of the winding and the insulated wire that comes outside is loose.

[bobloblaws]

Hey guys, can you help source an ideal replacement for the transformer? Again, the amp has a pair of EL84 output tubes. Same output tubes as a Blues Junior. Would this one do the trick?

https://www.tubesandmore.com/product...es-junior-15-w

[nosaj]

Hey guys, can you help source an ideal replacement for the transformer? Again, the amp has a pair of EL84 output tubes. Same output tubes as a Blues Junior. Would this one do the trick?

https://www.tubesandmore.com/product...es-junior-15-w

no that is an 8ohm tap. This schematic you used in another post shows a 4 ohm output. Now if your speakers at 8 ohm then it would be ok.

This would be a better option as you have a choice of 4 or 8 ohm taps. Not much more and they're transformers sound good too.
Fender Princeton Push Pull Style Output Transformer, 125A10B & 022913

nosaj

[Tom Phillips]

Hey guys, can you help source an ideal replacement for the transformer?...

I'm curious to know if you looked under the end bell as suggested in the recent posts? Any comment on that?

[bobloblaws]

I'm curious to know if you looked under the end bell as suggested in the recent posts? Any comment on that?

I did, I didn't see anything that looked like a bad connection except for a bit of fraying on the lead for the center tap which I mended with a bit of solder and I tested the three primary leads for continuity. At one point the leg that appeared stable all along at roughly 157 ohms started reading infinity and I wondered if I had made it go from bad to worse, but later it returned to showing 157 and the other side still moving around constantly.

[nosaj]

Thanks, nosaj. According to this TDPRI thread (linked below) it is not an official schematic. I read somewhere else that the schematic is for a different brand but is nearly identical. In fact I have identified a couple of things different between the schematic and the amp itself, the wiring coming off of the PT and filter caps is different (for example in the amp there are 2 - 20uF filter caps as opposed to 3 on the schematic) and there is an additional 3uF electrolytic cap in the amp that appears to be original factory installed. So I don't think the 4 ohm designation on the schematic can be taken as gospel despite the fact that the OT appears to be original. I admit that I didn't even notice that marking on the schematic and assumed the output was probably 8 ohm. Is there some way to take a measurement to determine what the OT is looking for?

Voltage Troubleshooting | Telecaster Guitar Forum

Your going to be looking for an OT that has anywhere from 6 to 8k on one side and the other side depends on your speaker if it's 4, 8, or 16 ohms. The 6 to 8k depends on the value of your cathode resistor. http://www.r-type.org/pdfs/el84.pdf
nosaj

[bobloblaws]

no that is an 8ohm tap. This schematic you used in another post shows a 4 ohm output. Now if your speakers at 8 ohm then it would be ok.[ATTACH=CONFIG]44114[/ATTACH]

This would be a better option as you have a choice of 4 or 8 ohm taps. Not much more and they're transformers sound good too.
Fender Princeton Push Pull Style Output Transformer, 125A10B & 022913

nosaj

Thanks, nosaj. According to this TDPRI thread (linked below) it is not an official schematic. I read somewhere else that the schematic is for a different brand but is nearly identical. In fact I have identified a couple of things different between the schematic and the amp itself, the wiring coming off of the PT and filter caps is different (for example in the amp there are 2 - 20uF filter caps as opposed to 3 on the schematic) and there is an additional 3uF electrolytic cap in the amp that appears to be original factory installed. So I don't think the 4 ohm designation on the schematic can be taken as gospel despite the fact that the OT appears to be original. I admit that I didn't even notice that marking on the schematic and assumed the output was probably 8 ohm. Is there some way to take a measurement to determine what the OT is looking for?

[nosaj]

Thanks, nosaj. According to this TDPRI thread (linked below) it is not an official schematic. I read somewhere else that the schematic is for a different brand but is nearly identical. In fact I have identified a couple of things different between the schematic and the amp itself, the wiring coming off of the PT and filter caps is different (for example in the amp there are 2 - 20uF filter caps as opposed to 3 on the schematic) and there is an additional 3uF electrolytic cap in the amp that appears to be original factory installed. So I don't think the 4 ohm designation on the schematic can be taken as gospel despite the fact that the OT appears to be original. I admit that I didn't even notice that marking on the schematic and assumed the output was probably 8 ohm. Is there some way to take a measurement to determine what the OT is looking for?

Measure your speaker.Your going to be looking for an OT that has anywhere from 6 to 8k on one side and the other side depends on your speaker if it's 4, 8, or 16 ohms. The 6 to 8k depends on the value of your cathode resistor. http://www.r-type.org/pdfs/el84.pdf

nosaj