DOD SR 231 QX - Power Transformer

[TelRay]

thanks, I might try that.
in the meantime i have contacted mouser and asked them if the noise is normal
i am comparing it to a previously bought for another application and much bigger 370EX transformer... which is far quieter

[nevetslab]

Might even take cobbling small soft rubber grommets into the mounting tabs of the xfmr to 'shock-mount' the xfmr to the chassis. At first, I was thinking neoprene rubber sheet under the xfmr and mtg tabs, though the mtg hardware would be clamping it back down, so not as good of a vibration-isolation system. All depends on how different the mechanical noise is unmounted from the mounted noise.

[TelRay]

you're the man! that's a lot of info... let me analyze

[nevetslab]

This is acoustical hum from the newly installed transformer. That's one of the specs transformer mfgr's don't publish. Surprised you're finding that on such a small xfmr. You could try unmounting the xfmr, and temporarily sit it in some foam insulation, so it can be safely powered back up to then listen to it unmounted. That would be your 'baseline' for audible acoustic radiation. Is the chassis Aluminum or Steel? The magnetic field will couple to steel. You could try installing a neoprene rubber pad between the mounting tabs of the xfmr...1/16" thick would be sufficient. I assume the form factor of the xfmr is an EI core, open frame, with mounting tabs at each end. You'd cut the isolation pad and add mtg holes to align with the mtg holes in the chassis, and see if that helps quiet it down.

I've had to deal with this sort of thing in product development years ago when I was a product development engineer at BGW Systems (pro audio power amp mfgr), and have rejected many prototypes, as well as worked with the xfmr vendors to revise their design for lowering the radiated stray field and mechanical mounted resonance. I'm equipped with accelerometers and their mating preamps, as well as measurement microphones to quantify the acoustical noise.

Does the xfmr feel hot? In general, the closer you run the xfmr to it's rated VA rating, the higher the radiated stray field and acoustical noise. But, this is also where you get into selecting one mfgr's design over another, when you have that luxury during product development. Here, we've selected a 'best-fit' replacement for the product, and now finding an undesireable artifact. Tougher task, though might be able to minimize it. Now, fabricating an acoustical enclosure to fit over it is a lot more difficult. Easier to find a different suitable xfmr, which, of course, takes it out of the realm of being affordable in the first place, as I know you were looking for the least expensive solution in order to justify keeping the unit.

See what you can find out.....good hunting.

[TelRay]

hi! quick question

after I installed the transformer and checked the unit's functions everything seemed OK. Put some signal in and had some scope readings.
The only thing I do not like is a HUM coming out of the newly installed transformer. I do not see it going into the audio signal but I can hear it even coming through the housing with the cover closed.
is that something "normal" or i could raise a claim to MOUSER?
thanks!

[TelRay]

thanks a lot for the guidance!
i have learned a couple of things

[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!

[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!

[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!

[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]

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]

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]

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]

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]

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