Repairing Monoprice 605500 5-in Powered Monitor Speakers - Need New Transformer Part

[phioschaos]

Hello all,
 
Hopefully this is in the right place.
If not, can a mod move it?
 
After about 20 months of usage, one of the Monoprice 605500 monitor speakers failed to power up.
No glowing LED, no sound, etc...
Given that people initially thought they were based on M-Audio BX5s, I looked up common power failures for that and found that caps were a common issue.
 
Unfortunately, after opening it up, the caps looked fine.
That would have been an easy fix. I might give it a shot any
 
A bit of probing revealed that there was no AC voltage coming out of the transformer block.
Monoprice labels there's BDT-57A794.
M-Audio BX5a are BDT-57501.
 
Both have the same specs:
Input: 120V / 60Hz
Output: 16*2V / 1.5A
 
Does anyone know where to find spare parts for transformers?
I found this but it is out of stock and appears more complicated for both 120V and 240V AC:
http://www.instrumentalparts.com/ma90054264600.html
 
FYI, for people interested in disassembly / repair - it is built similarly to the BX5a.
I mostly followed this guide:
http://mr-ives.blogspot.com/2011/09/mending-pair-of-m-audio-bx5a-monitors.html
 
The front tweeter and woofer need to be removed first because the wires connected to them are zip-tied shorter.

 

[kramer5150]

Have you checked all fuses?
Isn't it rare for step down transformers to just "die" a quiet death.  I thought usually bad things accompany their demise/failure....?

 

[kramer5150]

Heres one with a dual secondary, 15V - 2.1A
 
http://www.ebay.com/itm/R-Core-transformer-65W-Dual-15V-Sound-Designer-dedicated-power-transformer-/321671934175?pt=LH_DefaultDomain_0&hash=item4ae52430df

 

[phioschaos]

Thanks for the hints.
 
I suppose it may have been premature to jump to replace transformer just from no AC output on the transformer.
I've been slowly disassembling more and probing to verify if the transformer is the problem.
 
Regarding the fuse idea, I did swapped the outside fuses of the left and right speaker.
And the power cord and 1/4" TRS input cables. Still no power.
 
At this point, I have the transformer output cable disconnected from the board and the transformer's outer casing off.
Multimeter shows a low/zero impedance on the output pins (it's a coil of wire, so that's sensible).
But the input pins via the back panel power input have an open circuit between hot and neutral (power switch is ON position).
 
Currently, I'm thinking either a bad solder point on the input lines or the main coil is broken.....
After considering disassembling the transformer completely to get to the contact pins (http://www.instructables.com/id/Disassemble-an-E-block-transformer/), I decided it wasn't worth it as I doubt I could fully reassemble it correctly.
 
I snipped the input wires, stripped them a bit and probed for continuity. Open circuit. 

Either there's a break in the primary side coil or a bad solder connection at the contacts.
Either way, I think the best option is to replace it.
 
Thanks for the link to the R core transformer. But I'm looking for something that would be equivalent in size/mounting options as well.
The mounting points are 6.5cm apart. The original is a E core (no I from what I can tell).

 

[kramer5150]

Yeah I was just looking at the power requirements, that one clearly would not be drop in compatible.
 
Have you checked the Hammond www site?  I saw a bunch of 16V secondary coil transformers, but could not tell from the data sheets if they were dual secondary. 

 

[phioschaos]

Thanks for the suggestion on Hammond.
After looking through their various power transformers, I was not able to find anything that matched both power output closely enough and mounting size.
 
After more use of Google, I found that the OEM is likely Shenzhen Electronics:
http://www.custompcreview.com/reviews/m-audio-bx5-d2-studio-monitor-review/15601/3/
 
Turns out InstrumentalParts does have some stock so I'll check to make sure their part fits shortly.
 
Thanks again for the help.

 

[phioschaos]

Update:
 
I got the transformer from InstrumentalParts and following the Sept 2013 section of the blog post in my OP I got it wired up.
 
Good news: Outputs voltage - a bit higher at 19VAC but hopefully it'll work.
 
Bad news: Fuse burnt out immediately - twice. After the second fuse, I took a closer look.
 
Unfortunately, I forgot to take a before pic but here's the after:

 
Where there is an exposed part of the big electrolytic capacitor and behind the diodes was a tiny burnt ceramic capacitor that exploded at some point.
It was causing a short to ground - explaining the fuses burning out. The original transformer must have been less robust than the replacement and it broke before the fuse.
 
I'll be replacing it based on a ceramic cap value in similar part of the rectifier circuit and will mount it on the other side of the board.
The electrolytic cap with the partly melt plastic wrap seems too troublesome for now with all the glue and looks fine otherwise.
 
I'm trying to figure out why the tiny ceramic cap burnt up.
I'm thinking it was in a tight spot and smothered in glue and so it overheated.
 
Any alternative ideas on why or what to look for?
 
Thanks.