I could use some help with a 1280 problem

[Jefferylu98]

Hello all, I have a 1280 with some issues. It goes into speaker protection mode. The amp boards dc balance is off the charts/ my fluke 117 goes right to OL and both amps idle current are in the negative/ turning the pot will not take them out of the negative readings. The speaker protect relay clicks about every 5 seconds and the front watt meters seem to respond to the clicking by rising to a few watts/what the heck lol. The power supply voltages are PIN 25 is -69.3 PIN 8 is 61.8 PIN 11 is 31.3 PIN 20 is -33.3 PIN 16 is 13 even. That's as far as I got with testing tonight. Any ideas or thoughts would be great. Then maybe I'll have a path to proceed. Thanks in advance

 

[zebulon1]

The +-68 regulated supply's are the fault.
Pin 8 +68v's is low causing the DC balance to be off.
The +32v at 11 is good enough so check R23 and Pin 8 for connection and R23 for value.
Also:
Something in one of the amps could be drawing down the voltage from the + side.
The way the amps +- supply's are connected it's hard to tell if its true. You would have to isolate the +-68 supplies from the amp(s). See if the +68v voltages comes back.
Can you disconnect the unregulated supply to the outputs? If you can - Do it. I never worked a 1280. I know the big caps are solder type. Any Molex plugs at the amp boards for the unregulated should be pulled.
The priority is to save the outputs!!!
Use your DBT while troubleshooting.
It could be getting ready to blow.

BTW, a very good post.

 

[Jefferylu98]

Ok, here we go. I did go ahead and unplug the 2 plugs on each amp boards. I am using a dbt for testing. I checked R23 out of circuit and it is 10.2 ohms. Checked R5 and it was double the rating/replaced with 4.7 resistor. I checked Q1 voltages, base=62.5/emitter=62.1/collector=91. I checked Q2 voltages for comparison, base=-70.2/emitter=-69.5/collector=-90.8. A couple of things from previous testing with amp boards plugged in (yesterday) dbt was pretty bright and I did find that the fuse was blown on the fuse board /PIN 6 to 11. The dbt is dim with testing today. Also just for kicks I replaced Q1 with a ksc2073tu to see if that would help, It actually dropped the voltages on all three pins as follows, base 0.2/emitter 0.3/collector 0.9. This is such a nice stereo, hopefully we can get it revived, because this one will go directly to my collection. I do have one already in working condition, but not as nice, cosmetically. Thanks for your help guys.

 

[zebulon1]

The fuse was for the 13 volt supply. Protection and the tuners.

 

[Jefferylu98]

Anybody have any ideas on which direction I should head??? Thanks in advance

 

[zebulon1]

What fuse blew?
Trying to back into the issue.

 

[Jefferylu98]

Fuse at Pin 6 and 11 on the fuse board

 

[zebulon1]

Did you install Q1 in correctly? The legs are ECB looking at the printing.
The fuse that blew is for the 13.5v regulated. Tuner and stuff.

If you removed the draws and the voltages are uneven, I would rebuild the power supply. New transistors and zeners and caps.
The blown fuse? If the voltages are good I would leave that alone. Maybe you caused it when checking voltages?

 

[Jefferylu98]

Thanks for your reply Zebulon 1/ I am looking at the Q1 transistor and I see it as base / collector / and emitter, left to right looking at the printing On the front of the transistor. Am I looking at it wrong or do I misunderstand your above post?

 

[Jefferylu98]

I did a recap and I have the transistors for the power supply, I'll have to look for a post that has the mouser part number for the zeners and get them ordered. I don't know if it means anything but with the amps unplugged the dbt goes nicely to super dim on power up.

 

[Jefferylu98]

Here are the readings after recap and all new transistors on the power supply board. Pin 25/-70.4. Pin 8/ 62.55. PIN 11/32.08. PIN 20/-33.59. Pin16/13.17. Still a big difference in the 25 and 8 pins.

 

[zebulon1]

Maybe I did give the wrong pin outs. Ops.
I'm all fresh from a good sleep.
The difference is the problem with the amps DC Balance (Offset)
The Zeners are 32 volts. I would use two 16v in series to make the 32v's.
Mouser P/N:
512-1N5246BTR (16v) D9 and D10. Good
The other is a 6.1v
512-1N5234B is a (6.2v) D13.Good
General run if the mill diode, works for almost everything but the rectifiers:
512-1N4148, D11 and D12

 

[Jefferylu98]

Ok thanks again / I ordered enough to replace all 11 diodes on the board and some extras they are cheep lol. Will be here Friday or Saturday. Will update readings after install. Thanks again for your help

 

[Jefferylu98]

Here is the results after replacing the suggested diods(not looking good). Pin 25 / -70.2. Pin 8/62.2. Pin11/31.6. Pin 20/-32.5. This is getting a bit frustrating. Lol

 

[zebulon1]

The EQ uses the +-32v's derived from the +-68v supply.
Maybe the EQ is drawing it down.
The best thing to do is isolate the feeds from the PS and see if the voltages even out.
It might be easier to remove the pins from the EQ than deal with the PS connectors.
You could desolder both headers on the EQ and isolate for any shorts, see if the voltages equalize.

 

[KingBubba]

Take Pictures of everything to help you find your way back, in case you run into delays and just in case.

 

[Jefferylu98]

Disconnected pin 2 and 6 from the EQ board and it made no difference in the power supply voltages. And for some reason I lost my 13 volts on pin 16 on the power supply board(realized this prior to disconnecting eq board.

 

[zebulon1]

Don't get frustrated. It's something that takes a little time to understand.
I'll have time to look at more tomorrow.

 

[Jefferylu98]

That's for all your help/ one thing I have found working on these old girls, is the harder the problem is to figure out , the more rewarding it is when it's up and running properly.

 

[zebulon1]

So whats the deal on the 13v supply? It has nothing to do with the +-68v supply.
Did the fuse blow again?
Check the AC voltage going into the board at pins 6 and 7.
The collectors of Q1 and Q2 on a previous post shows the voltage even. This is the rectified DC.
With all the feeds removed it boils down to the PS.
What is the voltage readings on the diodes D9 and D10?
Check every resistor in that +-68 volt circuit.
Check all the caps for correct polarity.