I had a hard time finding these diagrams. It's a nice Amplifier don't toss it fix it .
I have the rest of the manual should someone need something from it.
K301 goes bad most of the time the contacts get dirty.
Output from one or both of the channels will be intermittent and scratchy.
Also one of the output transistors went bad once.

 

I found this website when I was looking for a Heathkit power supply schematic:
http://www.vintage-radio.info/heathkit/

They didn't have AA-1205, so I sent them a copy.

 

Hey I don't mean to bump a super old thread but the thread starter seems pretty knowledgeable and I would like the other documentation that he has. I don't see any way to just send him a PM

Anyways I scored one of these units today at the thrift and I'm fairly amateur when it comes to this stuff but would like to start learning more.

The unit I got wasn't outputting in the left channel so I opened it up and saw there were two left channel fuses were blown. I replaced them and powered the unit back up without any speakers connected with a CD playing on the input and noticed now the left channel output meter was on but it was just pinned at like 5-6 bars where the right channel was bouncing around like it normally should be.

When I hooked up a speaker to the left channel the fuse located at f401 blew again.

So that's basically where I'm at. Kinda looking at some pointers as to what I should be looking for here. I ordered a soldering iron kit that's arriving today for a couple of other projects I have and would really like to learn how to bring an amplifier back from the dead like this.

Any help is greatly appreciated

 

Welcome to AAC!

When I hooked up a speaker to the left channel the fuse located at f401 blew again.

Check Q425. You should be able to check the junctions in-circuit with the diode check setting on a DVM.

I don't see any way to just send him a PM

I don't think you can send a PM until you have at least 10 posts. That member hasn't been on the site for over 2 years.

 

Yet another bump on an old thread...

I have one of these also. BTW, thanks loads for that schematic. Nice to have.

I was looking for the parts list. Specifically for technical specs of the power transformer... because i want to replace it for 230v.

I also have the "matching" AJ-1200 tuner. This one, conveniently came with a parallel/serial primary coil on the transformer, so switching it from 115v to 230v was just a matter of rewiring input to the primary coil. Works like a charm.

Going back to the AA-1205 however, this guy's primary only has the two wires going to the one winding. So i can't pull off the same trick as i did with the tuner. Here i have to replace the thing. I know the paired output voltages because i can turn the thing on and read the in-vivo RMS voltage with a volt meter, it's 27.5v on each (or 55v RMS across the two).

My though was to replace this with a toroidal transformer, going from 230v down to about 24v (because that is the closest reasonably priced thing i could find without going over voltage). I was also thinking that being about 10% low on voltage for an analog device would cause only a minor hit to performance while keeping all those other older parts a little happier.

Now for my problem... the original transformer is simply labeled "54-989" and "549-8206", those are internal HeathKit part numbers which i can't find anywhere. And if i'm going to replace this, i need to know the power (VA) requirements to size the thing. I also need to get the new toroid to fit into the available space. So too big is a problem. The bottom line: How can I figure out what is the smallest VA that would reasonably work??

The amp itself is listed as 30 Watts. So that would be 30 per channel (or 60) and it can handle speaker outputs of A, B, or A+B which would seem to indicate 120 Watts if everything is running full throttle (for music, not a constant single tone sign wave).

Any thought would be appreciated.

 

Now for my problem... the original transformer is simply labeled "54-989" and "549-8206", those are internal HeathKit part numbers which i can't find anywhere. And if i'm going to replace this, i need to know the power (VA) requirements to size the thing. I also need to get the new toroid to fit into the available space. So too big is a problem. The bottom line: How can I figure out what is the smallest VA that would reasonably work??

As a follow up...

I spoke with a very nice pre-sales engineer at Triad Magnetics. And he told me that a transformer's power rating is usually correlated to it's size within a given technology (i wouldn't have thought of that). So, i measured the existing EI transformer, and he looked through their offerings of EI transformers of a similar volume and concluded that the existing transformer seems like about 125VA, or so, as a rough guestimate. The other possibility, for another guesstimate, would be to take the whole thing out and weight it (i didn't do that - well not yet).

This lands me between their two 230v->24v+24v toroidal offerings of 100VA (87mm) and 160VA (103mm). The 100VA would fit better (with a 4mm overhang above the filters section of the amp), cost $7 less, and weighs only 2.2-lbs. But, the 160VA would give lots of power headroom, although it would hang over the filters section by 20mm and is 1.4-lb heavier, weighing 3.6-lbs.

The current transformer is not at all warm when the unit is on, so it probably has lots of power headroom already... so i'm leading toward the 100VA.

After i've made my decision and done the installation, i will come back with the results. Hopefully, none of the magic smoke will escape and the amp will continue to function.