DOD SR 231 QX - Power Transformer

[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!

[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]

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!!!

[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,

[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

[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.

[TelRay]

I believe it's only diodes (photo of AC IN connector + caps and diodes)




in the rest of the circuit the only ICs I see are a bunch of OPs. 4560Ds (about 20 of them on each board) and 1x LM324N (full panoramic in case i have missed anything)

on the 4560D's datasheet I read: operating voltage ±4V~±18V and then INPUT VOLTAGE ±15 V

[nevetslab]

I see the full wave diode bridge rectifier D15-D18, the unregulated filter caps C39 & C40, followed by IC Voltage Regulators mounted to the two small heat sinks to the left of the filter caps. I'm guessing they have this set up for +/- 15V regulated supplies, with the IC regulators being 7815T and 7915T. You'll have to unmount the main PCB and lift it out, enough to read what the first one is along the back edge. Not sure if the front panel PCB will let you do that without having to also unmount it first. If they are 7815T and 7915T, then you'll need a power xfmr capable of 28VCT to 36VCT @ around 500mA.

If instead, those IC's are 7812T and 7912T, then using the 24VCT 500mA power xfmr would work, though I still find the unregulated voltage on the low side to prevent regulation drop-out under low line conditions.

[TelRay]

right you are
one of them is a 7915A but I will need to desolder the heatsinks as I had to "open" one of them to see what was written on the IC, will be more prudent with the other one

[nevetslab]

right you are
one of them is a 7915A but I will need to desolder the heatsinks as I had to "open" one of them to see what was written on the IC, will be more prudent with the other one

The other one will be a 7815T (they are TO-220 devices, as those heat sinks are for that type of semiconductor package. So, you WILL want to select a higher voltage secondary. 40VCT is a bit too high, while in the 32-36V CT, you'll have adequate headroom while not overheating the regulators with the excess unregulated voltages. I think the voltage limit on the input to the regulators is +/-30VDC, which that high up is burning up heat. 6-8VDC headroom (above the 15V regulation output) is what you typically want.

[TelRay]

thank you very much nevetslab,
I've been doing some research and think the the HAMMOND 187 D36 that is a Single Primary 115V 30.6VA, 36V CT @.85A is going to be the right choice at $13.39
here the link if you want to check it out https://www.mouser.com/ProductDetail...%2F4rr6 Apw%3D
thx!

PS: ... why the hell don't these things come with a FUSE? is just for cutting costs? blowing transformers is not fun
i'm not sure if i am going to keep this one or sell it and even if i do keep it it won't have a super extensive use in number of hours of continuous operation (will use it with a HiFi receiver) but... does it make sense to add a fuse? how do calculate what the right fuse value should be?

[nevetslab]

thank you very much nevetslab,
I've been doing some research and think the the HAMMOND 187 D36 that is a Single Primary 115V 30.6VA, 36V CT @.85A is going to be the right choice at $13.39
here the link if you want to check it out https://www.mouser.com/ProductDetail...%2F4rr6 Apw%3D
thx!

PS: ... why the hell don't these things come with a FUSE? is just for cutting costs? blowing transformers is not fun
i'm not sure if i am going to keep this one or sell it and even if i do keep it it won't have a super extensive use in number of hours of continuous operation (will use it with a HiFi receiver) but... does it make sense to add a fuse? how do calculate what the right fuse value should be?

Considering what you paid for this (wasn't it $20?), and finding this Hammond 187 D36 xfmr for $13.39 + shpg, I still see this as quite a bargain. I haven't looked to see what they cost new, but it's way more than what you will have invested, even after adding a rear panel Fuse Holder & fuse. A 1/4A slo blo fuse should be more than adequate. I see 19 dual Op Amp IC's and a single Quad Op Amp (or comparator) IC, so figuring 8mA per IC, that's 160mA. There's maybe 10mA running the two voltage regulators, so that's 170mA secondary DC load. I forget the calculations for computing the Primary current that would have in terms of AC current. There's also the excitation current of the transformer itself. But, from seeing the typical fuse values on products like this, I'd think a 1/4A Slo Blo would be adequate. Easy to check once it's powered up if you have a power analyzer. If not, you could temporarily wire a 1 ohm resistor in series with the AC mains and measure the AC Voltage drop across it. That would be a one-to-one relationship (100mV drop = 100mA AC mains current), for example.

[TelRay]

Considering what you paid for this (wasn't it $20?), and finding this Hammond 187 D36 xfmr for $13.39 + shpg, I still see this as quite a bargain...

"me" like the bargains
Thanks a lot for all the help!

[TelRay]

got the transformer today, have just installed it and took the first measurements.
First... the device lights up!!!
Second, with the Variable Transformer I was bringing the INPUT VOLTAGE slowly up and monitoring the DC Voltage at the output of the regulator and the diodes.
When I got to 85 VAC I already had +15 VDC at the regulator output and +17 VDC at the diode
When I got to 120 VAC the +15 VDC stayed the same (that's what regulators are for, right?) and I had +25 VDC at the diode.
According to what was suggested before I am 2-3 VDC above the optimum input voltage, I guess it should be fine.
Does that make sense or is the voltage regulator going to be burnnig up too much heat? I've checked a couple of datasheets (TI and ONSEMI) and the MAX Input Voltage is 35 VDC, so in theory it should be OK.
Thanks!

[nevetslab]

got the transformer today, have just installed it and took the first measurements.
First... the device lights up!!!
Second, with the Variable Transformer I was bringing the INPUT VOLTAGE slowly up and monitoring the DC Voltage at the output of the regulator and the diodes.
When I got to 85 VAC I already had +15 VDC at the regulator output and +17 VDC at the diode
When I got to 120 VAC the +15 VDC stayed the same (that's what regulators are for, right?) and I had +25 VDC at the diode
Does that make sense or is the voltage regulator going to be burnnig up too much heat? I've checked a couple of datasheets (TI and ONSEMI) and the MAX Input Voltage is 35 VDC, so in theory it should be OK.
Thanks!

YUP.....that's what they should do, and you have sufficient headroom to keep the supplies in regulation. +/- 25V unregulated supplies is typical....might be a touch high. You could take a temperature measurement of the regulators mounted in their heat sinks to see how hot they're running. Probably enough to not allow you to leave your fingers on them, which would indicate over 50 deg C. I think you'll be ok. Nice work!