Non standard output transitors SX-1280

Lux Man

AK Subscriber
Subscriber
I recently picked up a non-working SX-1280.

My first discovery is that the AWX-127 boards have been replaced with some hand-made PCB's in order to accommodate some non-original output transistors. I knew it was a non-working unit but I did not know the extent of it's injuries. When I discovered this I was a bit set back and began to wonder if this is a worth while project. I have not looked to see what transistors were put in there in place of the originals.


It's present condition is that the left and right output amps have been disconnected. Other than that it does seem to not be molested to badly. I've not tried to power it up- obviously.

Is anyone aware of a hand-made board being used out there in place of the AWX-127 successfully? And are there replacement transistors that are worthy to use in this receiver?

What do you think the pit falls would be if I put it back together and brought it up on a variac just to see what it does and doesn't do and then look at voltages?
 
Last edited:
Pics ?

There are a couple of successful projects that involves replacing the original transistors in the 1980, but the only changes done is to the output transistors, the amp board remains the same. Search for 1980 on this board and username "Echowars"
 
Let's see some pictures.
There's one Amp board on the Bay.
Do you have the skills (desire) to make it better, faster, stronger?
 
Last edited:
Here are the pictures.
 

Attachments

  • DSCF8599.jpg
    DSCF8599.jpg
    76.6 KB · Views: 191
Last edited:
It doesn't look bad, not original but workable. EchoWars is much more organized and neater but check the components out for serviceability, clean up the harness and use it.
 
That LOOKS like my work, or a damn, damn close copy.

Spacer bars machining, and I used razored ruler cuts for the PCB.


edit:
nope, looking closer, the pcb line cuts and edge cuts are dremel, IIRC I did it on a table saw.

The hold downs are wider. Nice machining work on them tho...

The 4661 insulators are turned sideways from mine.

and I don't think I ever put thermal transfer compound between the transistor and the hold down.

But altogether, NOT BAD.... Your amps look like they were well cared for by a very skilled person.

SX-1280 MTF Modified Output Transistors.

attachment.php
 
Last edited:
Well- progress is being made- slowly. At first power up with a variac I could see there was a serious problem. Long story short- I could tell the issue was with both amps. After a whole bunch of time I found 3 transistors on each board that were incorrectly placed by the previous owner. One was the gotta form that base under and around and not touch the collector lead issue. The other two were swapped- the NPN was where the PNP was supposed to go and visa versa. (Q11 and Q13). I checked and re-check them all again because I will admit I thought I did it last night but I missed those two. Several sets of hands have been in this trying to get it to go.

So happy news- one channel is now working. And the other channel is a little healthier than it was but there is still a dangerous current rise if I leave it connected. So now it's back to finding what else may have happened either because of the mis-installed parts or something unrelated. I just pulled the amp again and will start re-testing and re-checking for bad or "otherwise" components.

I won't turn down advice and helpful thoughts. I am encouraged because it's been an unknown time since this has worked. The previous owner had it for at least a year and a half or so in this non-working condition.
 
FYI, It's acting like a near dead short. I've been thru all the diodes and transistors on the AMP board but no success yet gotta be here somewhere.
 
This is an odd one- I ahve taken both amps out and set them side by side and compared them for differences using an ohm meter but I can't seem to find any disparity. I did have a hunch it might be with the +and - 68v line on pins 5 and 6 so I separated them and the apparent fault goes away, but so far that hasn't helped me narrow it down any further. I am beginning to suspect Q11 and 13 which were swapped incorrectly earlier. I had just moved them where they should be but maybe they are breaking down. I do have the originals that came with the unit. I might put them back in just to eliminate that possibility. However they sure don't test in a suspicious way.
 
Do you mean the one power amp channel that isn't working powers up, but then starts to run away?

Check the non working power amp channel for the correct idle current pot, then pull r40 and r41 (4.7 ohm base drive for the outputs) , connect the bases of each output transistor to their individual emitter - this shuts down the outputs from passing current.

Then check that amp's operating voltages. The amp is (feedback) dc balanced by 100 ohm R28 & R29.

There shouldn't be much current draw through pins 6 and 5, check that the 100uf c8 & c9 are correctly installed. They do face in opposite directions. You could try removing them if they are pulling down the regulated 68 v rails.

Otherwise, interpreting what you posted leads me to believe that you disconnected the 68v regulated feeds which would have shut down the outputs by shutting down the amp itself, since the collectors of the outputs are permanently connected to the +/- 76v B+/- .

You could have a problem with the q4 current source, 1v across 100 ohms R15 is 10mA through the STV diodes, they should NOT get more current than that - they are specified at 7 mA. Make SURE that voltage does NOT climb (1v @ across R15)


The emitters of Q11 and Q13 (across 100 ohm R28 & R29) should be no more than ~ +/- 0.6 volts
 
Last edited:
What I meant was that one channel is now working, it appears, as it should, amplifies with a source attached and a speaker connected.

If the other amp is installed I can not increase the input voltage via the variac beyond 30vac. In fact 30vac is too much because it would pull 3a at that level.

I will follow your direction in pulling R41 and R40.

I had pulled one leg of C8 and 9 already just in case and they were not the issue and they were installed correctly as well.
 
I pulled r 40 and 41, tied the emitters and bases together for each output transistor. I brought it up slowly but at about 60vac R28 and 29 let their smoke out,but only some of it. I caught it maybe just in time. They still measure what they did before(about 120). I had also pulled Q11 and 13 and checked them but they look good using just a ohm meter.
 
Disconnect R17 and R18 (100 ohms) and D6 and D7.

That will cut the link between the bias section and the driver (smoking) section.

Then - we are EXTREMELY LUCKY, go to pins 10 and 16 on the board, follow them to the wire jumpers about 1/2 inch away from the pin, and pull the wire jumpers. That will cut power to the driver section and allow us to straighten out the bias in relative peace.

Finally, put a resistor (100 ohms to 10,000 ohms) between ground and pin 14 output to establish a ground input for the feedback system through the depowered section.

This also depends upon the previous disconnections and temporary connections.


It is the bias section that is commanding the overload
 
Last edited:
I am going to make a confession- I have no choice. I realized I have created all these last issues. I firmly believe that if it weren't for my screw up this unit would be 100% fixed right now. The only issues were all the incorrectly installed transistors- I caused the rest. Here is what happened- D4 wires broke off yesterday morning and I reversed the polarity-DANG DANG DANG! ....DANG! No surprise right- dang!

So here is where I am. I corrected that buggaboo, re-installed R40 and R41 disconnected all the base to emitter connections. Applied power slowly and the short/fault appears gone. There is a but however which I'm sure I created somewhere along the road. The second relay (protection relay) clicks in and then out after 5 seconds. Next confession which I had hoped to avoid was that I ruined the fuse link under R1 yesterday as well trying to take some voltage readings with 30 volts applied to the unit. R1 heated up and fuse link let go. 3rd confession, yes I know it is wrong but I jumped it so I could keep troubleshooting.

So I do need to order that fuse link and a R40, 41, 29, 29 and lastly a D4. The lead has broken off almost flush and will be a source for future issues if it is not replaced. What is the best source $17 form Pioneer?

So my last issue with the oscillating protection relay?

I guess it's appropriate to have egg on my face for Easter.
 
Not that it helped but I had a nagging feeling the issue was a glaring one and I wasn't seeing it. The D4 fiasco was only discovered when I was reading other posts about a possible replacement that I remembered what I had forgot- polarity- recheck the polarity. I wish that nagging feeling would have revealed itself in a more productive way but with all the issues with this I thought it natural that there was something more wrong- not me creating one.
 
Back
Top Bottom