Altec 525 transformer secondary current

[Pat Maki]

I'm rebuilding a couple of 525 psu's and need to replace the transformers. Can anyone please confirm what current you're getting across the 250V secondaries (red/white leads)? I'm getting 1A....but this secondary is also putting out 305VAC instead of 250VAC, so no sure the current reading is exact enough.

Since I've gained a lot of space in the chassis after swapping the large canned elcaps to smaller modern ones, the plan is to simply add a second trafo in each psu to get the 250VAC.

 

[Khron]

I'm getting 1A

As in, you ARE applying some sort of dummy load to it? Or how are you getting that "1A"?

https://sound-au.com/xfmr2.htm#s113

 

[Pat Maki]

Thanks for the reference link. Will dive into that. Tube mic is attached with the capsule removed...not sure if the capsule being removed disqualifies it as the load.. my assumption is that the heater/cathode/plate/grids....even with no capsule, account for a vast majority of current drawn.

 

[Khron]

not sure if the capsule being removed disqualifies it as the load

It does not; the capsule is a capacitor, and as such, draws no DC current.

my assumption is that the heater/cathode/plate/grids....even with no capsule, account for a vast majority of current drawn.

That's correct.

I'm getting 1A

Still haven't gotten an answer to this, though...

And where are you measuring that "305VAC"? That's in case that is indeed measured anywhere...

 

[Pat Maki]

And where are you measuring that "305VAC"? That's in case that is indeed measured anywhere...

The 305VAC is across the red/white secondaries (where it shows 250V on the schemo). Measured across those leads and it's 305V both when they're in circuit and out of circuit. Not sure I fully understand your comment?

 

[Khron]

Measured across those leads and it's 305V both when they're in circuit and out of circuit.

Ok. What about the resulting DC voltages? Measurement points & values are also indicated on that schematic...

 

[Pat Maki]

The DC readings are all too high as well. Like the node between R8/C7 shows 315V, but I'm seeing more like 370VDC. I figured it was because the diodes I used in place of the order rectifiers (RS1 and RS2) weren't high enough using a 10k resistor after RS1 and 300ohm after RS2. I jacked those values up to 77K each and was able to tame the DC voltages, but also not getting any voltage at point 6 of the mic connector (shows 200V in schemo)....

 

[Khron]

Are you getting 6v between points 1 and 2?

 

[Pat Maki]

No, that's a strange thing. those lines go the + and - of C4. I'm only getting mv across there. But when measuring from ground, it's around 380VDC, which seems crazy high, considering those caps are rated at 15V (the replacements I picked are actually 35V). I haven't pulled those out to check capacitance, but their ESR readings look ok.

 

[Khron]

Umm... I promise i mean no offense, but it's kinda starting to sound like it might not be the worst idea if you went back to basics (including Ohm's law, transformers and how tubes work)...

 

[Pat Maki]

None taken, I'm passionate to deepen my understanding of how audio circuits work, so your comments are spot on. Actually been enrolled in an online electronics classes and have been reading a lot of material on how tubes and tube circuits work specifically. While I have a pretty good handle conceptually how all these components work, still early in my ability to calculate what voltage or current should be at any point in a circuit. The only part of this circuit that wasn't replaced 1:1 were the two rectifiers. My gut tells me this is the next place I need to do some studying.

 

[Khron]

Ok, so you're getting 305VAC where there "should" be 250VAC.

What about the other winding, where you should be seeing ~23VAC?

I'm rebuilding a couple of 525 psu's and need to replace the transformers.

Where did that "need" come from? And what did you replace the original(?) transformers with?

 

[Pat Maki]

Ok, so you're getting 305VAC where there "should" be 250VAC.

What about the other winding, where you should be seeing ~23VAC?

It's very close...just under 24VAC. Replaced the selenium rectifier with two 1N4001's. Why I need to study more on rectifiers is because I'm reading over 300VDC where the cathodes of those new diodes meet. I was under the impression from my reading that the resulting DC from a rectifier should be close to the inputted AC....which it's clearly not in the same universe in this case. But I don't know enough to know if this result is an issue with the circuit, or just me not understanding diode rectification well enough yet. I've look at the DC on scope and it's definitely rectifying...just a very high DC level. I understand the the heater voltage should be about 6VDC and on this schemo it shows 6 between the heater leads (looks like 60 which is wrong), so this is where I need to study, because in my understanding, the 6VDC would be ground reference, but according to the schemo, the 6VDC is measured across points 1/2, so I'm guessing that one of these lines should be 6VDC higher than the other, but again, this is me wandering into assumption land....so I need to study how the heater in the 5840 tube is powered to fully understand why the 6VDC is being shown this way. Anyway, thank-you for talking me though this....I really do appreciate your time and expertise. I'll get this figured out. Back to the books!

Where did that "need" come from? And what did you replace the original(?) transformers with?

Well the trafo on this psu is pretty close, but I was hoping to have it read according to the schemo. BUT on the other psu, the trafo was clearly sitting in water at some point and the outer paper insulation fell off and all the paper is moldy AND just reads about 120V between the red/white secondaries (vs 250V in schemo).

 

[Khron]

I was under the impression from my reading that the resulting DC from a rectifier should be close to the inputted AC....which it's clearly not in the same universe in this case.

Well yeah, but where are you measuring that AC voltage, versus where are you measuring the resulting DC?

because in my understanding, the 6VDC would be ground reference,

Says who? The heater is just a coil of wire (like the tungsten filament in lightbulbs), it doesn't know, nor does it care, where and how it gets is power supplied from, as long as it's within its ratings.

If things worked as you (incorrectly) assume, you might've just discovered over-unity energy production Getting hundreds of volts out of a 23VAC winding...

The "ground" of that heater supply alone, is actually on the negative terminals of C3 and C4. What DC voltages are you reading across C3 and C4 (as in, between the terminals of C3 and C4, respectively)? Try to ignore where the overall "circuit ground" is...

 

[scott2000]

I'll just put Ian's article here in case it helps.....
maybe knowing some values of things like .3mA etc... can help sort stuff.....

 

[Pat Maki]

Well yeah, but where are you measuring that AC voltage, versus where are you measuring the resulting DC?

24VAC between Red/Orange leads, 12VAC from CT to Orange or CT or Green. DC measured from black DMM lead on chassis and red DMM lead on cathodes of RS1 diodes.

Says who? The heater is just a coil of wire (like the tungsten filament in lightbulbs), it doesn't know, nor does it care, where and how it gets is power supplied from, as long as it's within its ratings.

Ha, wise words!

If things worked as you (incorrectly) assume, you might've just discovered over-unity energy production Getting hundreds of volts out of a 23VAC winding...

I've heard these mics are special, but wow, that would be really special! . I noticed that the RS1 diodes also go reverse biased when I get that 200VDC reading on the diode cathodes. My sense was that another part of the circuit may be 'injecting' that 300V since a few wires come together at that node...that's where I got to this morning before starting the 'day job'. Eager to keep reading and chasing!

The "ground" of that heater supply alone, is actually on the negative terminals of C3 and C4. What DC voltages are you reading across C3 and C4 (as in, between the terminals of C3 and C4, respectively)? Try to ignore where the overall "circuit ground" is...

Interesting...thanks for pointing that out. Reading across C3 is -.5VDC (red DMM probe on + and black probe on -). on C4 is drops to near 0...like 5-10mv). The readings were jumping around...on C4 when lifting and replacing the DMM probe, it would briefly show close to 300VDC...then quickly drop to near 0.

 

[Khron]

Interesting...thanks for pointing that out. Reading across C3 is -.5VDC (red DMM probe on + and black probe on -). on C4 is drops to near 0...like 5-10mv). The readings were jumping around...on C4 when lifting and replacing the DMM probe, it would briefly show close to 300VDC...then quickly drop to near 0.

Is all the wiring still in one piece? Solder joints all solid, not intermittent? Are you positive you installed the new rectifier diodes the right way around?

If I didn't know any better, i'd start thinking they might be both reversed or something, so you're still reading AC before them, but no DC on the other side.

After leaving it powered off for a while, you could measure the resistance across C3 and/or C4.