Acoustic 150 driver transformer

[TigerAmps]

I have an Acoustic 150 the someone blew the power board (and yes they had a 10A fuse in it to make sure it was cooked "well done".
The 6.8 ohm resistor and thermistor were incinerated and outputs shorted.

It looks like the secondaries of the driver transformer are toast

Resistance measurements:

GR - YL - 0.3 ohms
GR/B - YL/B - 0.9 ohms
GR - GR/B - 1.2 ohms (not good)
YL - YL/B - 0.6 ohms (not good)
GR - YL/B - 0.1 ohms (not good)

Transformer specs:

Triad Transformer
Model: TY-160X (Driver Transformer)
Turns Ratio: 6:1:1
Primary Current: 50ma DC
Primary: Red - Red/Blu - Blu (CT not used)
Secondary 1: Green - Yellow
Secondary 2: Yel/Blk - Grn/Blk

Anyone know of someone who can re-wind it?

Also, thinking if just changing out the power amp and dropping in a Velleman for some other kit type amp in it's place.
My concern with the Velleman is the heat and this is a small chassis.

Anyone have any ideas, suggestions, kit recommendations or done anything like this before?

[TigerAmps]

Also - thinking about how to handle a dual rail supply while keeping the single rail supply for the preamp boards ...

[J M Fahey]

Not sure about why or how you measured what you posted above.

1) transformer should be removed and standing alone on a table but at least: are transformer leads fully disconnected from anything else and "floating in the air"?
If still connected to board you have all kinds of possible shorted and burnt parts ther.

2) you donīt show primary measurements, yet you guess itīs "fine"

3) please measure DC resistance between winding ends and then between *one* winding end (any) and *one* in other.
I expect low resistance at each winding and open between them.

So post:
* Blu-Rd
* Gr-Yel
* GrB-YelB
* Blue-Gr
* Gr-GrB
* Blu-GrB

Replacing the full power amp is an option, the chassis is NOT small by any means by modern standards, but you have 2 problems:

1) heatsinking is weird, they used a flat or bent aluminum plate bottom venting through a window cut in the head tolexed case , no venting slots (so very poor convection) or something like that, itīs been many years since I repaired one.
Please post a couple gut pictures.

2) this amp is single supply only, while all modern ones are split supply, including the Velleman.

It may be adapted but takes some brain surgery in the process; so IF you can repair it as is, youīll save yourself some trouble.

[Enzo]

You MUST disconnect that transformer to take any meaningful readings. In fact the readings you present make me think your output transistors are shorted, and the parallel resistance is what you are reading.

I am not in the habit of replacing good old circuits wholesale. I fix things. I recommend that.

I would start with those 48-15 outputs, are they shorted?

[TigerAmps]

Just noting: I did remove the secondary wires with the above readings; I DID NOT disconnected the primary though.

So now that I unsoldered all the transformer wires from board, left them floating in the air and re-measured:

Blu-Rd = 42 ohms (primary wires)

Gr-Yel = 0.2 ohms (secondary 1)
Gr/B-Ye/lB = 0.4 Ohms (secondary 1)

Blue-Gr = infinite
Gr-Gr/B = 0.2 ohms
Blu-Gr/B = infinite

This was interesting:
Y-Y/B = 0.1 ohms
Y-G/B = 0.1 ohms

Upon initial inspection:
2 blow output
2 of the 0.5 ohm / 5W resistors were way off
I also noticed faulty solder joints in the 0.5 ohm resistors which could have played a role in the failure.
I was able to wiggle the 0.5 ohm resistor leads and see them move in the solder connection.
Everything (component wise) on the primary side of the driver transformer was fine. All the damage was on the secondary side.
I have seen outputs short and the 0.5 ohm resistors go, but this is the first time I saw a 6.8 resistor and thermistor burn up!

Where I am now:
I completely rebuilt the board output board:
new caps, new 0.5 ohm resistors, replaced all the transistors with other 48-15's I have that tested good, new 100 ohm thermistors and 6.8 ohm resistors and new 400 ohm 10W resistors
I also replaced the 1000uF/50V output cap with a new one as well.

I fired it up (with a 2 amp fuse) and it power-up - no shorts.
The sound was distorted and the output transistors were "temperature cool" where as they usually tend to warm up quickly.
The big thing i noticed is that the 400 ohm resistor on the +72V side was very very HOT and the ground side was cold (not even warm).
This sort of lead me down the road of the current going thru the driver transformer to ground.
If output were bad, the 2A fuse would have blown.

[Enzo]

So is the output bus sitting at about 37v as the schematic indicates?

[TigerAmps]

Sorry for the delay - I had to wire the transformer back into the amp ...

I measure 0.075 V from the + side of the 1000uF cap and the power ground. Way too low ...

So that explains why the 400 ohm resistor is HOT! It's seeing twice the voltage ....

[J M Fahey]

Ok, then transformer secondaries are shorted to each other.

Various alternative paths, none impossible but certainly annoying one way or the other.

1) best and Enzo might help: replace that power board with another Acoustic one.
A transformer driven one, with included (working) driver transformer would be a dream come true, but almost impossible.

But a slightly more modern one, with driver transistors, might be available, even one pulled from a dead Acoustic amp.

Those are backwards compatible, including single supply and output capacitor, so it would be easy to integrate.
Maybe Enzo or somebody else has one in his junkyard.

2) maybe those diehard Acoustic fans reissued a compatible board, empty (better) or populated, ask there.

3) search around, including DIY Audio, somebody may sell a single supply kit or board.
If you dare, you may etch your own (maybe even cloning an Acoustic one if somebody published the artwork)

4) mod the amp for a split supply amplifier.
Advantages: you wonīt need those pesky expensive "computer grade" supply capacitors but a couple of way cheaper lower voltage ones.
Disadvantages: youīll need a new power transformer, although you may well already have one in your parts junkyard: any pulled from an 80 to 120W Peavey (say a Bandit), Crate (anything 80 to 120) , Laney, Fender, you-name-it, including unknown generic brands.
Anything supplying around 30+30VAC

5) you keep the single supply preamp, it does not actually need +75V or whatever, but can really be happy with +35 to +40V which will be available even in a split supply amp, so ....

Worse problems have been solved, somebody here (the Dude, G1?) shoehorned a modern Class D amplifier bought from EBay inside an Acoustic 120 or 200W Bass amp.

[Enzo]

Enzo does not have any of those specific parts.

[g1]

Worse problems have been solved, somebody here (the Dude, G1?) shoehorned a modern Class D amplifier bought from EBay inside an Acoustic 120 or 200W Bass amp.

Not me, but The Dude, on a more modern Acoustic amp I think, starting around post #21 here: https://music-electronics-forum.com/...l=1#post466623

[J M Fahey]

Thanks.
I watched a video showing Acoustic 150 guts, and must somewhat correct my earlier assumptions: although the chassis *is* quite large, they assigned very cramped space for the power amp and heatsink at one end of it:

from 0:15 to 0:33

No real space for a new amp (which to boot has to be single supply, or heavily modded or PT replaced) so annoying as it looks, real option is to rewind OT so "everything remains the same" so to speak.

Maybe you are lucky and they wound primary first (closest to bobbin/core) so you "should" not need to mess with it, and only need to rewind secondaries, which means less turns of thicker wire (easier).

I am quite certain it was wound "bifilar", meaning you use 2 identical wire spools (for the second one you may prewind enough wire around a plastic bottle or piece of broom handle, no need to buy 2 actual separate spools, we are talking a couple ounces wire here, you can get it from EBay) held in left hand and wound "together as if they were one" while right hand turns improvised winder handle or hold plain battery powered screwdriver, the "poor manīs winder".

Bifilar winding would also explain why burnt enamel shorted secondaries together, they are touching side by side all the way.

If you dare, RG has a nice page explaining how to recycle and repurpose small transformers, his example is called "making a transformer rewinding kit" or similar.

If you donīt want to get involved in that, maybe some local electric motor or transformer rewinder can do the job for you.

Again, the usual "best" option of just installing a new power amp there and calling it a day is not, because of the very limited space available and the unusual supply.

[TigerAmps]

"Maybe you are lucky and they wound primary first (closest to bobbin/core) so you "should" not need to mess with it, and only need to rewind secondaries, which means less turns of thicker wire (easier)."

I looked at RG's PDF for rewinding the VOX transformer and he suggests dual primaries (wind 1/2 the primary - the secondaries - then 1/2 the primary).
I thought - why not all primary then all secondary?
But then I looked at the Triad transform in the Accoustic 150 -> the primary was center tapped but they cut the center tap wire off at the edge if the windings.
I can see the the small CT wire "stub" under the outer paper wrap. This observation does confirm that they used a dual primary setup in the transformer.

I'm going to follow RG's PDF, order materials and try to wind one from scratch ...

Thanks to all for the input! I will post results after I give it a try using a donor transformer ...

[J M Fahey]

Ok.
Then carefully disassemble it, take lots of notes and pictures.
First separate (the way he explains it) laminations one by one, notice which way they point which is important, being a "Class A" transformer , I expect all E pointing one way (except maybe the very last one on each end pointing the other way to help keep them together) and a pack of I ones closing the magnetic path.
They might use a thin piece of paper separating E block from I stack to create a small gap.

Then cut outer wrapping paper or tape until you see the enamel to plastic covered wire twisted/soldered joint, more pictures, and then you unwind the outer layer, keeping track of turns, layers, and any insulation between layers.

I expect secondaries being wound bifilar (2 wires together wound "as if they were one") but I might be surprised.

Then measure each wire diameter with precision, you may have to burn enamel with a lighter and remove ash with very fine emery paper or steel wool to measure actual copper diameter.

Wire does not have "any" random diameter but jumps in definite steps and must be one of those in AWG wire tables, that will help you pick the right one.

Make a wooden core to hold the empty bobbin for rewinding, drill its center and use a suitable bolt or threaded rod to mount it to a cordless drill or similar which you clamp to a table.
Makes a pretty decent winder.