SX-950 Relay Bypassed and Burnt Resistors

[petehall347]

i just figured out the transistor numbering scheme . the pairs of outputs are 4 digits +- . difference . had to do that as schematic is hard to read .
so i am thinking you haven't paired them up correctly ?
13 with 17 etc .

 

[rcs16]

it is the bias adj pot, you need the 10K for the offset adj, 531-PT10H-10K

measuring those voltages at the bases of the o/p bjts tells you that they are just starting to conduct.
You measure bias V across AWH-050 pins/terminals 12,13 & 27,28. that is 2* 0.50 ohms in series. You calculate the bias I (current) by V/1 ohms. be careful not to short anything out. i use lead grabbers, power off, attach clips, power on,measure, power off, remove the clips.
What voltages do you have on the 2 non-wiper terminals of VR1, VR2? with respect to ground ( -ve black lead of DMM grounded to chassis)
You measure/adj bias after a 30 min warm up period, in normal position with bottom plate on.

 

[petehall347]

@rcs16 do you understand the bias seemingly tied to the diff pair ? i haven't seen this done before . i cant figure it out .

 

[Pageplayer]

Im assuming the middle leg of the pot is the non-wiper terminals?
VR1 20.1mv
VR2 19.1mv
VR3 -4.01v
VR4 -4.3v

 

[Pageplayer]

Should I install the rest of the output transistors?

 

[petehall347]

Should I install the rest of the output transistors?

hold off for now .. i just need to clarify if they are installed as a npn pnp pair in the correct locations ..
q13 q17 for one channel
q14 q18 for the other channel

 

[Pageplayer]

I went off this diagram found on AK.. (Look at the top)

Logic tells me the rest..

So here's how I installed..

 

[petehall347]

appears wrong .. one pair is looking at the wires red goes with orange as a pair .. i would imagine the red is npn ..BUT better check on that with whats there ..
npn will will +50 on red wire wire .. pnp will be - 50v at orange wire .
p.s thats 850 not 950 .

 

[Pageplayer]

So, How about this configuration?

 

[petehall347]

to be certain you should see what voltages are red and orange wires to figure out positive npn or negative pnp ..

 

[Pageplayer]

Yellow is Base
Green is Emitter

 

[petehall347]

so is red positive voltage ? pull outputs first to check .. or work it out from the manual and the board .

 

[Pageplayer]

Red is +36v
Orange is -36v

 

[petehall347]

just as i suspected . red goes to an npn output . orange to a pnp to make up a pair ..

 

[rcs16]

@rcs16 do you understand the bias seemingly tied to the diff pair ? i haven't seen this done before . i cant figure it out .

I am not sure what you mean? Q5, 6 are the constant current generators that provide the current through the bias generator, D5,6,VR3,4
Q7,8 have to be sufficiently biased on for the CC current to flow.

 

[petehall347]

I am not sure what you mean? Q5, 6 are the constant current generators that provide the current through the bias generator, D5,6,VR3,4
Q7,8 have to be sufficiently biased on for the CC current to flow.

i am not sure what i meant now . all i can think is i had multiple schematics open and managed to look at the wrong one .
hoping it will all be good when a pair of outputs are installed . rather than 2 of the same polarity .

 

[Pageplayer]

Installed them like this on both channels...

NPN top right = (red wire)
PNP lower left = (orange wire)

When I flick on the power switch with 75watt DBT, the bulb goes bright and dims as normal, But goes bright again within a few seconds, at which point I kill the power

Removed all output transistors again and checked them and they check fine. Power up without the transistors and DBT stays dim.

 

[petehall347]

monitor idle bias at power on . ..
upper and lower slots for for outputs shouldn't matter . important bit is npn to red wire .pnp to orange wire .

 

[Pageplayer]

ok.. My fault..

From previous tinkering when getting those really low voltages, I forget to turn the bias pots back down!
As soon as I powered backon and gave it a few more seconds to settle into a voltage reading, It was bouncing around in the 50-70mV range

So I reset the pots and dialed them back up.
Running about 10mV on bias
DC offset is still no lower than .06v
Now. I can power up, Dim Bulb goes dim

 

[petehall347]

looking good then apart from the offsets .
are the input differential pairs matched ?