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Old 22nd January 2013, 06:56 PM   #1
jake01 is offline jake01  United States
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Default Transformer specs for Dynaudio monitor

Hi all-

A Dynaudio active monitor is blowing fuses. Opened it up, disconnected the transformer output, fuse still blowing. Took out the transformer, reading a short on the primaries. Apparently this is a common issue on these, which is shameful, although I did pick the entire unit up for just $50.

Anyway, Dynaudio are a major buzz-kill in that they will not release ANY information. The supplier of the transformer is being equally vague, saying that it is a 50 VA part, 220~240 VAC primaries (not completely clear on how this works in the US, maybe jumper wires the primaries in parallel?), the secondaries are anywhere from 6 to 60 V, based on the code of the part A through Z, and this one is H.

So my problem is that the part is shorted, I can not think of a way to test the secondary voltage to see what replacement part to get. Do I open it up and count the turns?? Please help!
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Old 22nd January 2013, 07:06 PM   #2
Mooly is offline Mooly  United Kingdom
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If the tranny is blowing the fuse with secondaries disconnected then as you say... its duff.

(Be aware though that primaries do read very low DC resistance on a DVM and on a big tranny it can be surprisingly low... that's normal... at AC the impedance is much higher)

Identifying voltages can be difficult and more so if there are multiple secondary windings. A good clue can be to look at the circuit and try and work back (if thats possible). For example a winding feeding a bridge and 25 volt reservoir cap automatically puts an upper limit on things at around 17 volts AC. It may be lower, but no higher. Then you have to look at what it feeds and so on. If it fed a 12 volt reg then you would need at least 15 volts DC so that give a lower limit of around 11 volts AC. Very rough and ready but you get the idea
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Old 22nd January 2013, 07:32 PM   #3
jake01 is offline jake01  United States
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Quote:
Originally Posted by Mooly View Post
If the tranny is blowing the fuse with secondaries disconnected then as you say... its duff.

(Be aware though that primaries do read very low DC resistance on a DVM and on a big tranny it can be surprisingly low... that's normal... at AC the impedance is much higher)

Identifying voltages can be difficult and more so if there are multiple secondary windings. A good clue can be to look at the circuit and try and work back (if thats possible). For example a winding feeding a bridge and 25 volt reservoir cap automatically puts an upper limit on things at around 17 volts AC. It may be lower, but no higher. Then you have to look at what it feeds and so on. If it fed a 12 volt reg then you would need at least 15 volts DC so that give a lower limit of around 11 volts AC. Very rough and ready but you get the idea

Thanks for confirming, this is my first diagnosis, and I am surprised that I even got this far! I will look at and around the bridge to ID the parts. It sounds like I will have to unbolt the assembly after all to get to the chip amps. Identifying what chip amps are being used will hopefully help. Thanks.
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Old 23rd January 2013, 05:11 AM   #4
jake01 is offline jake01  United States
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After the bridge, the seconderies go to a bank of caps rated at 35 V, then to a pair of LM3886. I suppose I need to go with whatever works best for these amps, and hope its compatible with the built in active crossover and filters. Any suggestions would be greatly appreciated!

BTW it looks to have 3 sets of primaries in a config I have not seen before:
(Lead 1, 2 or 3 selectable via jumper)
Lead 1 + Lead 4 = 230 V
Lead 2 + Lead 4 = 120 V
Lead 3 + Lead 4 = 100 V

I wander if this design is what is responsible for the high rate of failure in these transformers..

Thanks!
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Old 23rd January 2013, 06:26 AM   #5
Mooly is offline Mooly  United Kingdom
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Multiple primary options are pretty common and shouldn't be a source of trouble.

It might help to see pictures of the tranny and the PCB. For example, is it running on a single "up to 35 volt rail"or a dual rail of "up to -/+35 volts). Also are there any other rails derived from other windings ?
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Old 23rd January 2013, 09:16 PM   #6
jake01 is offline jake01  United States
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Here are some pics of the situation. My best guess is that there are 2 identical rails total.

My only concern about the primaries configuration is that it indeed invalidates my test with a meter, since all leads are actually connected, and a reading of continuity is to be expected..
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File Type: jpg 2a.jpg (380.4 KB, 101 views)
File Type: jpg 3a.jpg (600.7 KB, 95 views)
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Old 24th January 2013, 07:43 AM   #7
Mooly is offline Mooly  United Kingdom
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It certainly looks that way. Can you confirm there is continuity here. Bit hard to be 100% sure looking at the pics. I think those eight caps are just paralled up in pairs. They must be I can see the outer caps link to the bridge and ground via top and bottom traces.

So each of those caps is 35 volts... yes ?

That means that 35/1.414 gives the theoretical max AC voltage per winding which would be 24 volts AC. That's to close for comfort with no safety margin. 18 volt is a common off the shelf value and would give -/+25 volts DC under load, higher than that off load, higher by the regulation factor of the tranny which could be 4 to 7% or so. Perhaps 27 to 28 volts DC.

So thats a real possibility, an 18-0-18

22 vac trannys are available but less common. That works out at -/+33 volts DC (ish) off load for a typical tranny. That's a bit near the limits of the caps.

Lets look at power output. 25-0-25 vdc would allow around a (guess) -/+21 or 22 volt swing into 8 ohms. Thats 21/1.414 which is around 15 volts RMS. Into 8 ohms thats 28 watts RMS. Does that fit in with any published specs ?

Next step is to determine the VA rating. If you measure the tranny dimensions you can compare with others,
Your Search Results | CPC | Results

look at the unencapsulated ones lower down the page.
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Old 24th January 2013, 03:40 PM   #8
jake01 is offline jake01  United States
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Yes the caps are 35 V, and from the transformer part number I know it is 50 VA, and 13% regulation factor, just don't know the secondaries voltage. I have been able to acquire a second, working monitor, so I am thinking to just open it up and poke around with a volt meter..

And yes, I can confirm that there is continuity between all 4 primary/input leads of the transformer, which makes sense after looking at the jumper trace leads (the jumper selects between 230 V, 120 V and 100 V), I think it just taps into the same primary winding only at different places, of course this is me guessing.

Last edited by jake01; 24th January 2013 at 03:42 PM.
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Old 24th January 2013, 06:05 PM   #9
Mooly is offline Mooly  United Kingdom
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The 50 va rating sounds small tbh but of course that depends on the units intended output. And your 13% regulation fits in with a small tranny. Small trannys have poor regulation compared to large.

I think the 18-0-18 is a real possibility here. The link I gave you shows a 50va one at 76 by 30 mm. How does that compare in size to the original ?

Worry about primary connections when you have decided what to fit. If you have another identical unit then of course have a measure and see what the voltages really are
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Old 24th January 2013, 06:33 PM   #10
jake01 is offline jake01  United States
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Here is an "X-Ray" of the circuit board. The output of the unit is 2 x 50W RMS, that is 50W from each of the LM3886. Of course, that's according to Dynaudio marketing literature, so who knows..

The transformer measures 8438.

Thanks for your help!
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