SX-780 Powers up, no relay

[jptl]

The unit powers up with all lights and movement on the tuner meter, but no movement whatsoever from the VU meters and no audible relay click.
I’ve read several threads about how to test the relay not coming out of protection mode but I'm not sure where to go from here. I have diode tested the power pack pins (original Darlingtons) with the following results:
Right:
2&9 - 1.128
2&3 - 1.128
8&9 - cont. .000

Left:
2&9 - 1.128
2&3 - 1.167
8&9 - 1.176

Does the continuity reading between 8&9 on the right Darlington point to a bad power pack without further testing needed, or do I need to get voltage readings?
With this cause the relay not to click/come out of protection mode?
Any help would be greatly appreciated!
JP

 

[EastPoint]

Does the continuity reading between 8&9 on the right Darlington point to a bad power pack without further testing needed

Yes. Desolder that power pack, then power on the receiver and see if the relay clicks.

 

[jptl]

Desoldered and removed the pack, powered it up and the relay clicked.
Thanks Eastpoint!
Is my next step replacing at least the bad pack and hooking up some speakers to see if everything works, or am I just getting started in the repair process?
I have yet to do research on replacement for this pack.
Are the original Darlington’s NLA?
If not what source is recommended for reliable replacements?
Should they be replaced in pairs?

 

[merlynski]

 

[EastPoint]

All the new packs on eBay are fake. There’s a kit on eBay that lets you build your own replacements. I would do that.

 

[jptl]

I did a little reading and I'm starting to think that my asking the question
"If not what source is recommended for reliable replacements?"
..is like asking “how often I should change my oil and what is the best oil to use?” on an automotive forum.
Is there a can of worms emoji?

 

[jptl]

Eastpoint, your reply combined with the wealth of information and expert advice on this forum has transformed this old Pioneer from a handsome dust gatherer, sitting on a dark shelf....to a work in progress that has a good chance of getting a new lease on life and seeing the light of day, on display in my stereo cabinet.
Thanks for taking the time to answer my question!
JP

 

[tsd71]

https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/

Great post by rcs build your own stay away from eBay the STK’s they have the China virus.

 

[jptl]

https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/

Great post by rcs build your own stay away from eBay the STK’s they have the China virus.

I had seen that in all of its 48 page glory. Clearly the bible regarding these power packs. I don’t doubt it’s the way to go, but I’m not sure I have the time and I’m pretty sure I don’t have the electrical expertise to take on a custom build like this.
I’m leaning toward giving an Audio Lab pack a shot and see what happens.

 

[Watthour]

Unless you can find a reliable source for a NOS or pulled/working module, the available optons are the unknown replacments of dubious quality, and the modern replacement boards. Those are about as bullet-proof as it gets, and are now well proven/well documented to dispel any questions. I think they bring the SX-780 up to the level where they should have been from the factory.

That said, I have used replacement modules from Electronix and Audio Labs of Georgia, and those have worked. I just don't push them hard. If you go that route, discipline yourself to use one pair of 8Ω speakers only, and make sure they are a true 8Ω.

 

[merlynski]

Ask and ye shall receive:


I have built the hbrown-rcs16 STK-0050 custom modules and installed them into an SX-780, they work great and are not difficult to build. If you can solder and turn a screwdriver you can do it easily. I started with bare boards, ordered my own parts and did all the assembly. My total out-of-pocket cost was less than $75 for both channels. And if you do blow them up they are easily repairable .

 

[ConchBoy66]

Checking them with a multimeter is limited.
But below is a test using the diode check with a DMM.
It's not a 100% accurate test, but should should give you
an indication whether or not it has failed, in much of the cases.
The pins will need to be out of circuit.

This will work for the STK0030, 40, 50, 60, 70 and 80 packs.
This is not for the “II” series packs.

Using the diode test, check between the following pins.

Check between pins 2 and 3, also between pins 8 and 9.
You should get no reading regardless which direction the probes
are connected. (this is just checking for shorts)

With the plus on 1 and negative on 10 you should get about 1.2 volts.
Reverse the leads between the two pins and it should read about 1.6 volts.

With the plus on 1 and negative on 2 you should get no reading.
Reverse the leads and it should read around .9 volts

With the plus on 1 and negative on 3 you should get no reading.
Reverse the leads and it should read around 1.3 volts

With the plus on 2 and negative on 8 you should get about 1.3 volts.
Reverse the leads and you should get no reading.

With the plus on 2 and negative on 9 you should get about 1.3 volts.
Reverse the leads and you should get no reading.

With the plus on 2 and negative on 10 you should get about 1.8 volts.
Reverse the leads and you should get no reading.

With the plus on 3 and negative on 8 you should get about 1.3 volts.
Reverse the leads and you should get no reading.

With the plus on 8 and negative on 10 you should get about 1.4 volts.
Reverse the leads and you should get no reading.
--------------

If the results vary much from this, then the pack is no good.

Trying to add to the conversation:
From an old thread I found:
I did the diode test on 8 stk-0050's that I had, six used originals with 1-0 printed on them, and 2 new cheapy charley ones that I bought to test and all but 1 tested out well as above. The voltages on all of them were slightly lower than listed and I noticed that with the last test it appeared that the instructions are backwards, with neg on 8 and plus on 10 giving the voltage and 0v reversed.
So maybe a question too? if they test out within these parameters they can still be bad?

 

[merlynski]

Testing junctions with a diode range on a meter can tell you if a junction is bad, but passing the diode test does not tell you they are good. They can still fail in circuit under real world conditions. Personally I don't trust testing the STK out of circuit. I test the amp with the two resistors in place of the STK. If the amp shows good there, I judge the STK is bad. If the amp has damaged components on the PCB connected to the STK pins I assume the STK died and took them out with it. I have not ever seen an amp that passed the 'two resistor' test fail to work when a good STK (original or custom) was installed afterwards. For me the choice of using the custom discrete component boards was a 'no-brainer', but YMMV.