Power Transformer Help

[smackoj]

I found an Acoustic Model 124 bass/guitar amp (100 watts into 4-10s)  The power tx had one side of the B+ shorted. I had trouble finding a replacement and ended up trying an import, (china no doubt) but I am stumped on the connections. This amp uses only the positive power it's not bi-polar like the newer ones. I need some help to hook this up correctly. The "instructions" are sketchy using terms like the "head" and "tail" to describe the 2 sets of secondary leads. Someone please xplain what I need to do? I am guessing they mean the blue leads are hot and the green leads go to ground? thanks amigos

[PRR]

I found an Acoustic Model 124 bass/guitar amp (100 watts into 4-10s)

Schematic?

The "instructions" are sketchy using terms like the "head" and "tail" to describe the 2 sets of secondary leads.

That is for the primary. And where are you ? 120V land or 240V land??

I also note that this is a 100VA transformer.
https://www.antekinc.com/as-1230-100va-30v-transformer/
A "100 Watt" audio amplifier will typically need a 200VA transformer, at least.

So back up. What is "short on one side.."? Did it smoke? The winding connects to the core? Have you peeled back the wrapping a bit to see? And that schematic, or a link to a LOT of desciption on this 124. (I know I have seen one but that was a looong time ago.)

[shooter]

here's one I used, the instructions are pretty easy to understand here.


I recommend testing before loading!
on mine to wire the 115 for 230 the "tail" of the top winding, gets shorted to the "head" of the bottom winding.
the "head" of the top winding and the "tail" of the bottom winding becomes primary IN.  The secondary should yield the 30VAC at each of the 2 secondary taps
30V is loaded, mine, even loaded is so big it's closer to 40VAC which is just enough to pop small transistors working hard

[smackoj]

Hi; Thanks for your reply.I am in AZ running 120vac. No the power transformer did not smoke. I have been trying to use the schemo from the Model 220 because I could not find the 124, however, I just now stumbled onto a schemo for the 124 (see link). The schemo file is too large to upload and I don't know how to reduce it. I don't think the 124 was a big seller. I found a little history that said it was only made in '79 and '80.


When I first opened the chassis there were problems with the power supply. Someone had installed one of the square, four pin rectifier chips that makes a bi-polar B+. They had the negative side running through the one large filter capacitor and then to chassis ground. I checked the 2 hot sides of the transformer and got about 30 vac on one side and 7 vac on the other. I changed the power tx for a vintage one I had laying around and it gives about 52 vdc. With that setup the amp is working and passing signal but I don't think it gets the 76 vdc it is designed to run on. I went searching for a replacement transformer.

I found this Antek toroidal that is suppose to make 30 plus 30 vac. It was cheap so I decided to give it a try. I thought it might not have the ability with 100va to work but I wanted to try and supply 76 vdc and didn't want to spend a lot of money on an amp that might not be salvageable and a vintage transformer that might get damaged. If this Antek is not useable, at least for experimental purposes, let me know and I will set it aside and keep looking for a proper replacement?

I will also do an inspection on the original transformer and see if I can find any questionable features.


Acoustic Model 124 schematic:  Acoustic Schematics - Music Electronics Forum (music-electronics-forum.com)

[shooter]

wire the antek like my stick figure sketch, test without a load.  have the meter connected to the secondary VAC, then bring it up on your variac.  When you get to ~~~~70VAC on the secondary measure your primary VAC.  NO smoke, NO heat??
IF all's good, i'd temp it in for a quick test, for the long term, that's your call

[smackoj]

Thanks Shooter;  I tested it the way you have drawn and measured 34vac on one lead and 12vac on the other lead. I must have something connected wrong.

[shooter]

test each section as stand-alone to verify the PT is working 1st. 

[sluckey]

Thanks Shooter;  I tested it the way you have drawn and measured 34vac on one lead and 12vac on the other lead. I must have something connected wrong.

I don't understand. There's only one measurement to make. Put one probe on the green wire and the other probe on the blue wire. One reading. Don't mess with the blue and green wires that are connected together.

[smackoj]

Thanks SL I thought I was looking for 30 volts on both leads to chassis ground.


OK that's better, 63vac looks real good.

[PRR]

Just the 124 power amp pages.

It is indeed single-ended. A 30+30V or 60VAC winding will rectifiy-out to 85V DC, which is significantly above the 76V design (which was already pretty bold). You want 53V or 27V+27V AC. And very nearly the weight of the original transformer.

[shooter]

^^^^^
Listen well 


The PT I  needed  8A 37vdc max.  The one I bought in the PDF "matched" closest - I thought
Reg_out UNDER load, is 43VDC per rail, NEVER gets warm, pops little stressed trannies transistors like it was fun!   
so the dave fix is in kit form on the bench, a variable DC Regulated PS module that's rated 8A
so the 10lb PT + 3lb filter AND now a 5LB regulator.....might...be a work around!!!
If none of that works I'll buy a 10:1 30A stepdown trannie and see if I can make a mini welder 

[smackoj]

Alright PRR and Shooter, thanks.