Tannoy reveal active monitor

rotco

Active Member
Hi all
I have a pair of monitors. One of them is shorted (immeduetly blow fuses)

I cant find any schems.

I start dismantle the bridge rectifier and both filter caps. All looks good.

"Insted" of output transistors, its have two (and same) IC's.
PM93AF LM3886T.

Any advice what to check?

I tried to connect thru DBT and make some measurements, but i think because of the transformer, the loop is closed already before everything and the pcb barely get voltages (no voltages across seconday)

Thanks

I thought of comparing with the other orderly monitor, but better to wait with it and not damage it mistakenly also.
 

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Side question please,
in order to better understand transformers,
as we all know, while there is no currents in transformer, primary and secondary is a short circuit (separately)
what is the condition for a voltage drop along the secondary/primary?

i'm asking, because even when i disconnect the secondary from the PCB, still i've got short circuit and DBT was bright.
that's im assume, since the primary acted like a short circuit rather then maintain a voltage drop.
 
I had a pair of these - not able to answer your question sorry.

What I did do was repower them with some active LM3886 power modules from China - using the same power requirements. It ended up being a really cheap & simple fix and they sounded great (just in case you dont fix your issue).
 
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What I did do was repower them with some active LM3886 power modules from China - using the same power requirements.

Thanks for your respond.

Im not sure i got you right.
Did you bought a whole module (new pcb with all components already installed on?)
And what do you mean "using the same power requirements"?
Wouldnt it solve by replace both LM3886T i have in my pcb?
I rather stick with the original...

Thanks
 
Yea its just an option - if you want to save time. Ie: something like this - a complete amplifier module - it will be pretty much the same specs s your existing amplifier module. You just need to confirm the voltage requirements for it and compare this to the voltage coming from your existing power trafo.

These modules predominantly take an AC voltage input, and do the AC/DC conversion onboard, so you would be sorted. Onboard speaker protection and high power output from these chips.

https://www.aliexpress.com/item/Aiy...1e-4b82-a851-2fcb12bf98ed&transAbTest=ae803_5
 
i'm asking, because even when i disconnect the secondary from the PCB, still i've got short circuit and DBT was bright.
Does not sound good.
The transformer will have one primary winding, connected via a power switch with snubber capacitor and a fuse, connects to mains
power cord. The transformer may have more than one secondary winding, the yellow wires appear to be a secondary winding going
to a power supply board (rectifier, filter caps, volt reg...). Carefully check for other secondary windings, or post a hi-res pic of the
transformer showing all wires.

A serious fault on the primary would blow the fuse.

Disconnect power, measure resistance between one primary wire and chassis. Repeat for other primary wire and chassis
Then measure resistance between both primary wires.
 
Thanks @mbz
Im so glad to see you joined to help!
I will do and update back.

From input cable cord, it passes thru fuse and continue to power ON/OFF switch.

From there two ends of primary windig (blue&black) goes to transformer.
Secondary output from transformer is two yellows and middle zero point (white).

I already check for continuity between ends of black/blue and they are not open.
Repeat for both yellows and white, same, no open.
Then i checked for conituity between primary to secondary, open circuit.
Repeat for primary to chassis (ground), also, open circuit.

So, i guess i covered all transformer checks and it seems to be fine.

The secondary (two yellows) are plug&play.
Feed to the pcb.. Diode bridge & filter caps

With and withought secondary connect to pcb, the bulb fully bright.

Any advice?
Thanks

I will upload more photos later
Btw,according to datasheet of LM3886T it has many protections so i hoped it will keep it self from harm...
20190130_211504.jpg
 
Disconnect power, measure resistance between one primary wire and chassis. Repeat for other primary wire and chassis
Then measure resistance between both primary wires.

So, here is the update.

i started by open the orderly other pair, and disconnect the secondary, and powered ON thru dim bulb.
now i was sure that the issue is with the transformer itself (hopefully the only one).

i compared both transformers resistance, and found everything same, except of the primary.
orderly speaker, 18.6ohm between Blue&Black.
shorted speaker 5.5ohm between Blue&Black.
is it really so much matters? guess it is...
i looked closer to the primary until they get inside the torodial, and didn't see anything suspect/burn.

there is a black insulator at the entrance, maybe it covers any fuse or resistor, but i tried to feel it with my fingers, and i think nothing inside.

so, worse case is buy another transformer,
but, any thing else i can do?
maybe add in series 10-13ohm resistor? guess i can't... its actually an AC resisance rather then just normal ohmic resistance..

thanks!!
 
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Add some info in case others will need.

Secondary resistance measured 0.8ohm in both speakers (yellow-yellow).
Secondary to chassis 0.4ohm in both speakers (yellow-white)
Promary to groinding and primary to secondary was open loop (of course). In both channels.
And some more photos.
20190131_234652.jpg 20190131_234622.jpg 20190131_234540.jpg 20190131_234526.jpg 20190131_234121.jpg 20190131_234004.jpg
 
I'm concerned that the DBT glows brightly when the secondary (2 yellow wires)
are not connected, ie, under no load. Also you say

"i compared both transformers resistance, and found everything same, except of the
primary.
orderly speaker, 18.6ohm between Blue&Black.
shorted speaker 5.5ohm between Blue&Black."

This suggests that you have a short between some of the primary transformer windings.
I would expect the transformer to make a buzz sound and also be slightly warm to touch.

For the working unit, the current is about 5.9Aac. For the short unit about 20Aac
which explains the blown main fuse and bright dbt.

Looks like the transformer needs to be replaced.

If you want, you can compare the secondary voltages of both units with DBT connected.
ie, measure AC voltage between 2 yellow wires or yellow and white wires
 
Is it worth a try to open the transformer and look for the short?
Maybe i can find something?

Your currents calculation should be multiply by 2. I live in 230VAC country.

The bridge rectifier rated at 4A.
The transformer rated at 2.1A (guess at the secondary)
And the main (first/entrance) fuse is 1A.

(4A at the diode bridge seems to me like a "too safe".. 4A sounds high current)
The bridge rectifier is KBL02
 
Is it worth a try to open the transformer and look for the short?
Maybe i can find something?
No, not in this case.

There is a short between windings typically caused by a breakdown in insulation.
While transformers are somewhat expensive, you are lucky that it's a simple configuration.
230VAC primary, 38VAC centre tap secondary, 2.1A secondary current so about 100VA (38x2.1) is needed. round up as always
so anywhere 100-200VA is ok.

The primary fuse of 1A seems over the top/useless(?) 500-750mA (slow blow) might do a better job of protecting the transformer
 
@mbz
Someone told me that trasformers (especially torodial) are really "strong", and if the primary got damage, it means something in the PCB is in seriously faulty, and couse the transformer damage.
if i will use the orderly other transformer in the dead speaker PCB, I most likely damage it also!

is that true?

thanks
 
Yes, I would agree in general that the transformer is very robust and it takes a lot to "kill" one.
However some budget manufacturers cut corners during manufacturing, so there are poor quality
transformers out there.

Since a serious fault may exist it the power amp, you should take some precautions like power up the new transformer on DBT.
I would also replace the primary fuse (1A), with maybe 100mA for initial power up and maybe 500mA for long term
 
Hi,

it lay on the shelf for half an year, and only now i've got back to it.

I've bought another transformer. (Antek AS-4438)
it is hugeeeee (heavy and bigger diameter. the transformer even bigger then the rear opening of the box. not to talk about mounting it to the rear panel)
the only way i can use it is by mounting it to the bottom of the box. but this will be a bit dangerous not to break the bottom wood.

it is an expensive transformer, very over then i need...much more then i needed!

is the purple wire in it is equal to my white wire in mine?
(is it acting like a center tap as well?)
@mbz

IMG_20190830_101119.jpg IMG_20190830_104320.jpg IMG_20190830_112033.jpg s-l1600.jpg
 
That's a (too) big transformer, hopefully not too expensive.
Diagram shows wiring options. On the primary you can connect the windings in parallel for extra current
or just use one winding (assume you have 110-120Vac mains). I'd try the one winding with such a big
transformer.

I'm a little uncomfortable with trying to replace the white CT wire with the purple wire, need to do some
reading...
 
@mbz
Someone told me that trasformers (especially torodial) are really "strong", and if the primary got damage, it means something in the PCB is in seriously faulty, and couse the transformer damage.
if i will use the orderly other transformer in the dead speaker PCB, I most likely damage it also!

is that true?

thanks
NO.

Those transformers are not "strong" by any meaning. I repair powered studio monitors down here, from expensive Genelec and Dynaudio to cheaper brands, and one of the most common fails are shorted transformers.

Regarding your new transformer, I see it has 2 x 38V windings, and the original had 2 x 19V windings. You don't have 19V windings in the new tranformer, just 15V windings or 38V windings.

Are you at 220V or 117V mains? I guess 220 since the original TR is 220V.

If so, you can: use the 15V windings, and see if 15V is enough to power the speaker, or use a step down transformer to reduce the 220V mains to 110V and use the 38V windings (they will output 19V with 110V, wired for 220V), or send back that transformer and buy the one that you need. That's what I'd do, send it back and buy the proper transformer.

You need a 19+19V 2.1A transformer (or "38V with center tap")

Now you have a "38+38V" (or 76V with center tap) and "15+15V" (30V with center tap)

I'd buy 2 transformers because the other speaker will fail soon. I bet before 2020 ends, it will be burned too.

This should work.Ask the seller anyway, they are experts in this regard:
http://www.antekinc.com/as-1218-100va-18v-transformer/
http://www.antekinc.com/as-1220-100va-20v-transformer/
 
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@mbz
@elnaldo

Thanks both of you!

So, elnaldo, i was concern about putting new transformer in the same speaker that was burn the transformer.
You said that it's not so much likely that there is a problem with the pcb, that might damage the new transformer as well??

After i realise i bought much more bigger (and expensive) transformer, i now know i need more "basic" one so i less concern about the the other transformer i will replace it as well...
 
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I think the transformer shorted by itself, the insulation melts and short the windings. Seems a cheap material or under sized transformer (so it heats up more than normal).

Anyway if the circuit had a problem, there is a fuse that should blow before the transformer heats to the point of melting the insulation.

I'm confident that nothing is wrong with your amplifier board. Just the transformer reached the end of it's life. I see this same problem 1 or 2 times a month. Just today I was checking a Dynaudio monitor and told the owner about the transformers, he said "yes I know, already happened, replaced the transformer some time ago"
 
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