Aphex 141 transformer

[the_racker_reilly]

I saw Sisu MT's post the other day, and thought I'd ask here if anyone can help me figure out a replacement transformer for my Aphex 141? I'm a newbie, so apologies for any dumb questions and my general lack of understanding, and thank you for any patience you feel you can spare to help me!

The only info on the original transformer reads

100 / 120 V
TOPMARK
SEI:380-0300
E2HKWA03

and an internet search unfortunately doesn't turn up anything on that. I also emailed Aphex for info, but got a response saying they'd been acquired by Rode Microphones, and "unfortunately, no parts, part numbers or technical data was transferred to us for discontinued models".

Both the primary and secondary windings appear to be intact, it's just the thermal cutoff fuse that's gone, so I guess I could just try to replace that fuse.
However, I live in Ireland where we have 230V, so I'd need a step-down transformer to get me to 110V. So maybe it makes more sense to replace the original transformer with a 230V alternative, plus I might learn something along the way!

The two voltage regulators in the circuit are an LM1086 and an LM2991T, which specify max input voltages of +29V and -26V respectively. There are two large electrolytic caps between the bridge rectifier and the regulators that are rated at 35V. (There's also an LM2675 step-down regulator, but I'm not sure that helps us.)

Putting a multimeter across the secondary windings (photo below), I get roughly 7Ω and 7.9Ω between the central blue wire and each of the orange/green wires. I get 15Ω between the orange and green. So center-tapped secondary, if I'm not wrong?

Would I be correct in thinking that all this suggests I should be looking for a 24-0-24 V center-tapped secondary?

There's also the question of the power rating: written the rear of the unit is "AC LINE / 10 Watts" - does that translate to a rating of 10VAC transformer, or is that too simplistic of me?

Thank you for reading this far, and thank you for any help you can offer! (photos below)

 

[the_racker_reilly]

 

[CJ]

see if you can find the voltage rating on some caps downstream from the regulators.

i would guess it runs on +/-15 VDC or around there, so the caps might be 25 VDC.

input voltage max for the neg regulator says -26min and 0.3V max, wtf? i do not understand this,

i see no schematic online for the 141 nor a spec for the Taiwan made Topmark xfmr.

worse comes to worse you could unwind the bad transformer, count the turns and then we have the step down voltage ratio. Or maybe someone with the same unit can pop the lid and take some measurements.

 

[the_racker_reilly]

Thanks, CJ! I'll do some more investigating this evening on the board and see what I can find. Thank you for the help!

 

[the_racker_reilly]

O.k., so I removed the board from the enclosure and tried to learn something from the traces on the back:

It looks like there's 3 power rails: a bipolar one to feed the 8 NE5532A opamps, and single 5V rail powering the Cirrus Logic CS4385-CQZ DAC and Wavefront AL1402G ADAT chips.

The caps beside the NE5532A opamps are rated 16V. That rules out 24V anyway for the rails. The datasheet for the NE5532A recommends a supply voltage between +/- 5V and +/- 15V, so I guess the rails must be either 12V or 15V so.

Any ideas on how to figure out which, though? Would 12V be more likely, given that it's closer to the middle of the recommended range?

 

[CJ]

if you could find away to power the rectifiers with an AC source (variac, step down transformer
out of the junk drawer etc) you could energize the circuit read the DC volts on the rails.

16 volt cap on a 15 is possible but unlikely, i would guess 12 volts, so a transformer that would supply maybe 4 volts headroom for the regs would work,

so 12 + 4 = 16 vdc after rects, so 16.6 dc needed from xfmr (0.6 fwd v drop on diode)

16.6/1.37 = 12VAC, so a 120/240 : 12-0-12 VAC xfmr will work, 12.6 would be ok also.

1.37 is charging factor on caps, 1.414 (root 2)minus a bit for losses.

in other words, a 12.6 vac sec will charge caps to 1.41 times, = 17.7 minus DCR losses from the transformer,

if rails are 15 then 15 + 4 = 19 / 1.37 = 14 volt sec, which is uncommon, so a 120/240 : 15 VAC would cover you just in case regs are set for 15, but at the same time would not be excessive as to cause trouble as your filter caps before regs will handle it.

so go for a 120/240 : 15-0-15.

 

[the_racker_reilly]

Super stuff, CJ, thank you so much!!

 

[GM-GM]

Same situation here. Have you find the replacement?

 

[the_racker_reilly]

Finally got round to replacing the transformer on this today.


Short version: I used this 2x18V one from Mouser: https://www.mouser.ie/ProductDetail/Bel-Signal-Transformer/DST-4-36?qs=FESYatJ8odLYoam1fV70MQ==


Longer version:

I followed CJ's advice to power this up with an AC source, and used a 60VA 15-0-15 toroid from a GSSL build (measuring 36VAC rms across the secondary with a DMM).

These were the voltages I measured on the voltage regulators:

LM2991T:
-23.4V INPUT
-21.11V OUTPUT

LM1086
+20.38V ADJ/GND
+21.65V OUTPUT
+23.37V INPUT

LM2675:
Vin: +23.51V
FB: +4.98V

Current measured in series on one leg of the secondary winding: 59.5mA (200mA AC setting on DMM).




Output voltages with the DST-4-36 are measuring within 0.01V of those above. Input voltage is measuring 1V higher than above, around 24.4V or so. The regs don't have any heatsinks, so please tell me if I'm asking too much of them. I think 2x15V would have been too low, hence going for the 2x18V, and I wanted a transformer that could supply the 60mA well within its range, with the DST-4-36 rated at 170mA. Also, the physical dimensions of a replacement transformer could not exceed width 55mm, depth 48mm, height 37mm.


So far I haven't tested any audio through it. I'll add a note here once I do.

Hoping this is useful to someone somewhere somewhen!