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So a friend of mine had an old amplifier which he didn't want anymore so i told him to give me the transformer before throwing it away since i might need/can use it in a future project. But since it is a surface mount/PCB type transformer i have a hard time identifying the primary since i get continuity on most the pins. And i know that the transformer is fine since the amplifier worked perfectly before taking out the transformer.

I managed to find the schematic of the amplifier:

https://docuri.com/download/hcd-rg330_59c1dc7cf581710b2868a7b5_pdf

Here is a picture from that schematic which shows the transformer and then some pictures i took myself of the one i have:

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Any help is appriciated and if you need another picture of something specific please do tell. Thank you!

New picture of the pins:

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enter image description here

Figure 1. Close-up of the transformer connections.

Following the traces on the left out to the edge of the board we find the 120 - 240 switch legend so the left side is definitely the mains input or primary.

  • If the internal non-replaceable fuse is intact you should get a zero ohm reading between 5 and 6.
  • Read the resistance between 3 to 6 (120 V winding) and 2 to 6 (240 V winding) and you should find that the first reading is half that of the second.
  • Finally check that 1, 4 and 7 have no connections to other terminals.

It may be that the internal fuse has failed on thermal protection. You could power the transformer on 8 but that would not be recommended.

If there's a chance that the transformer has an internal short you can safely power it up with a mains light bulb in series with the primary. The bulb should not light - or very dimly at best / worst - if the transformer is OK.

Finally, note that the board uses very many standard length wire jumpers as bridges over the traces. In many cases several jumpers could have been replaced with one long one or better layout. It appears that the board was designed for pick and place population with standard "zero ohm resistors" (pre-formed wire links).

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  • \$\begingroup\$ Thank you very much! I will check the readings with a DMM and report back. \$\endgroup\$ – Xane Oct 21 '17 at 9:37
  • \$\begingroup\$ Picture is blurry, but isn't the 120V from 3 to 6? \$\endgroup\$ – Whit3rd Oct 21 '17 at 9:39
  • \$\begingroup\$ Fixed already. It must be 6 for numerical order. My mistake. \$\endgroup\$ – Transistor Oct 21 '17 at 9:40
  • \$\begingroup\$ So: between 5 -6 = no resistance 3 - 6 = infinite resistance since pin 3 doesn't have anything coming out of it (i guess it's because we only use 230v here) 2 - 6 = reads 9 - 10 ohms. 1, 4, 7 is not connected to anything else. \$\endgroup\$ – Xane Oct 21 '17 at 9:48
  • \$\begingroup\$ It's definitely dodgy. I'd bin it or dismantle it to see how it's wound and where the non-resetable fuse is. Lots of wire for electromagnet projects! \$\endgroup\$ – Transistor Oct 21 '17 at 9:51
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Generally, one row of pins will be the high voltage side, and the other row the low voltage side. This is to allow the two sides to isolated well on the PCB.

As it's a step down transformer, you can expect to measure a much higher resistance on the primary side (maybe a few to a 10 ohms), than the secondary side (usually sub one ohm, difficult to measure with a DMM). If you haven't got a DMM, get one now, you won't get far playing with transformers without one.

As you have another transformer (it looks like you're collecting these things), use it to give you a low voltage AC supply. This gives you isolation from the mains, and low voltage safety. Connect it to two of the pins on the mains side of your unknown transformer (not the low voltage side, that could give you dangerous voltages on the mains pins). Now use the DMM on AC volts to measure the voltages between all pins. This will allow you to identify all the windings.

When a primary has several taps, it's often to allow it to be strapped for 220v or 240v operation, or even a pair of 120v primaries.

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  • \$\begingroup\$ Thank you for the tip! And of course i have a DMM, i have 2 of them to be exact and a LOT of transformers.. Should i also use a resistor to be safe? For example i live in sweden so we have 230v mains. So can i use a small step down transformer which converts 230v ac to like 6v ac, and then i use a resistor to limit the current and that way i can "play around" to find the correct pin without getting a short circuit etc? Thanks. \$\endgroup\$ – Xane Oct 21 '17 at 9:30
  • \$\begingroup\$ if you're putting 6v into a mains winding, then you don't need a resistor. \$\endgroup\$ – Neil_UK Oct 21 '17 at 13:08
  • \$\begingroup\$ Hmm are you sure? You are obviously more experienced than me but if i "screw it all up" which i won't but just an example. If i touch the 6V ac without any current limiting, won't that like hurt a LOT? :P \$\endgroup\$ – Xane Oct 21 '17 at 14:46
  • \$\begingroup\$ if you touch 6v? I thought you were worried about short circuits. Up to about 40v is generally considered 'touch safe low voltage' by most authorities round the world. You won't draw enough current from that through skin to be a problem. However your tongue, or other mucous and/or innervated membranes, that's a different matter, even a 9v battery tastes really bad across the tongue. A small resistor won't do any significant current limiting, a big resistor would drop the voltage to your transformer too far to make good measurements. Anything less than 230v on a 230v winding is OK, 6v is OK \$\endgroup\$ – Neil_UK Oct 21 '17 at 15:08
  • \$\begingroup\$ That explained it! and yeah i was "worried" about short circuits but i just gave an example! Thank you :) \$\endgroup\$ – Xane Oct 21 '17 at 15:11
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The "sub power transformer" rather complicates the picture, but there are indications on the voltage range switch that 120VAC power goes to pins 1-2-3 and pins 4-5-6, and from there to the pins on the power transformer labeled "3" and "6". There is an internal fuse between pins "5" and "6", which usually means pin 5 connects to line (black wire) and pin 3 to neutral. There is a shield line, which should probably be grounded, at pin #17.

The metal bracket should also be grounded.

Those pins on the transformer have molded-into-plastic labels. Pin #6 is cut short, probably to force the connection through the internal fuse (but you might want to select your own fuse; it's easy to reconnect at position #6).

One can only guess at the output voltages, but it is usual for the center-tapped winding to deliver dozens of volts AC, so it might be possible to get a shock from pins 10 through 14. The two simple windings on the transformer secondary side are likely for instrument lights or logic power, and are likely to be low voltage.

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  • \$\begingroup\$ Forgot to say! "There is a shield line, which should probably be grounded, at pin #17." I just checked and you are correct.. At pin 17 there is a shield line. \$\endgroup\$ – Xane Oct 23 '17 at 11:43

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