DOD SR 231 QX - Power Transformer

[nevetslab]

In the power supply of this DOD EQ unit, can you see what is being used in way of power supply regulation? I'd be selecting the 24V CT model, which is 12VAC which then is rectified/filtered to yield roughly 16.4VDC after the rectifier loss. From that, I'm guessing +/-12VDC regulation rather than +/-15VDC regulators. I'd want at least +/-20VDC for +/-15V regulators. For +/-20VDC, you'd need about 28VCT

Not sure if it's using TO-220 3-terminal IC regulators or just Zener diode regulation.

[nevetslab]

In the power supply of this DOD EQ unit, can you see what is being used in way of power supply regulation? I'd be selecting the 24V CT model, which is 12VAC which then is rectified/filtered to yield roughly 16.4VDC after the rectifier loss. From that, I'm guessing +/-12VDC regulation rather than +/-15VDC regulators. I'd want at least +/-20VDC for +/-15V regulators. For +/-20VDC, you'd need about 28VCT

[TelRay]

this is what I found,



I like the 2 marked in green because I could add a 115 / 230 V input selector
As I have found no schematics for this unit I am not sure of what is the precise voltage expected on the secondary. if I follow the 2x 21.9V CT I've seen while looking for a replacement transformer I am either 9% below (with the 20 CT) or 10% above (with the 24 CT). What do you recommend.
Both are above the 2x 184 mA = 368 mA so that should be OK.
The two individual transformers were able to handle 8 W each, is the 12VA listed here enough? I read 20 W at the back of the unit

thanks,

[TelRay]

thank you very much for the reply

I'm assuming the use of two power xfmrs was the current load of each 31 band EQ circuit was too much for one of their production xfmrs, so they used on per channel. Primaries wired in parallel. Now, it's odd that they've phased the primary wires of the two xfmrs as you stated. Maybe done to reduce the effective radiated stray field from the xfmrs?

That's what I thought, maybe trying to avoid some kind of electromagnetic interference. In any case the two transformers are isolated from the circuit boards by a metal divider

I also assume the other side of the AC mains connector is feeding the power switch. You're reading open circuit when you measure across the AC mains input connector, with the power switch set to ON? Do you get the same open circuit reading when reading between the one side of the AC input connector, and the xfmr feed side of the power switch? Very odd to find both power xfmrs with open primaries.

I did all that and then removed completely the two transformers from the circuit and separated all previously tied wires. Both transformers (completely separate entities now) have an open circuit primary.

I'd say yes, you could replace both power xmfrs with a single one having a higher VA rating, as suggested. There should be several mfgrs who would have one. You'll have to look at the mechanical space available in the chassis, as a larger single xfmr will no doubt require different mtg centers. You should be able to find something at Mouser, Digi-Key, Allied Electronics, Newark and other sources. Shipping cost is, as always, a consideration. Invariably at least $15.

I'll see what can I find and post a list here for validation

You could also disconnect the secondaries of the two xfmrs, and with an oscillator, drive the secondary and see what you get on the primary of each xfmr, just to confirm on voltage on the primary. It will be a step-up voltage in this test. Also can measure the DCR of each xfmr's primary separately,

too advanced for my current knowledge. but willing to learn more.I have a scope at home.

thanks!!!

[nevetslab]

I'm assuming the use of two power xfmrs was the current load of each 31 band EQ circuit was too much for one of their production xfmrs, so they used on per channel. Primaries wired in parallel. Now, it's odd that they've phased the primary wires of the two xfmrs as you stated. Maybe done to reduce the effective radiated stray field from the xfmrs?

I also assume the other side of the AC mains connector is feeding the power switch. You're reading open circuit when you measure across the AC mains input connector, with the power switch set to ON? Do you get the same open circuit reading when reading between the one side of the AC input connector, and the xfmr feed side of the power switch? Very odd to find both power xfmrs with open primaries.

I'd say yes, you could replace both power xmfrs with a single one having a higher VA rating, as suggested. There should be several mfgrs who would have one. You'll have to look at the mechanical space available in the chassis, as a larger single xfmr will no doubt require different mtg centers. You should be able to find something at Mouser, Digi-Key, Allied Electronics, Newark and other sources. Shipping cost is, as always, a consideration. Invariably at least $15.

You could also disconnect the secondaries of the two xfmrs, and with an oscillator, drive the secondary and see what you get on the primary of each xfmr, just to confirm on voltage on the primary. It will be a step-up voltage in this test. Also can measure the DCR of each xfmr's primary separately,

[TelRay]

Hello guys,

with all this "at home" time I am going through the "family museum" with the intention of fixing whatever is broken

In this case this 31 band per channel DOD rack EQ

I got this as a part of a bundle and it never worked, not even the ON light so I always suspected it was an easy fix: bad ON OFF switch, fuse, etc. But... the SWITCH is OK and... there's no FUSE
This is what I know:

- two transformers in the unit
- no signs of flames, smoke, etc
- the secondary of each transformers feeds one CHANNEL (either the L or the R)
- 3 wires coming out of the secondary (2 blue live wires and 1 white that goes to ground)
- 2 wires coming out of the primary (one WHITE and the other BLACK)
- the BLACK of one transformer is tied to the WHITE of the other transformer and hooked to the AC INPUT SOCKET while the other two WHITE and BLACK go to the SWITCH
- I can read AULT INC 26-0268-A on both transformers
- I get absolutely no VOLTAGE readings on the secondary on any of the two transformers
- putting a MMT across the primary it shows the circuit is open

i think they are dead...

I did some research and found what seems to be a replacement for this
https://www.fullcompass.com/prod/231...rmer-for-166xs
2x 21.9 V CT / 184 mA / 8 W
for $20

First question, can they be saved? I'm thinking there is some kind of thermal fuse in the transformer that could be bypassed and then add an external fuse

Second, I believe DOD used 2 transformers for any of the following reasons:
- it is what they had in stock for other rack units
- for some reason it is advisable that each channel has it's independent power supply

for the sake of the economy (i paid $20 for it and i do not believe it could be sold for more than $50) i wouldn't like to spend more than $20 in repairing it

can I look for a single transformer providing, say 400 mA? if yes, any idea on where to look for it?

thanks!