SX-780 project . Relay clicked but no audio

[Trompi63]

Guys,
A couple years ago, I picked up an Old SX-780 receiver . It worked fine until one channel started getting a lot of humm .. so I decided to upgrade this baby with newer caps transistors and probably change a bad STK.
Sure enough one stk was the cheap ones being sold on eBay. So I started my project by changing all caps and the main transistors starting with the infamous 25 which was about to get off the board.
Anyway, all tested good and relay clicked again.. great so far, but I have no audio… please help is really needed… one side note I have the .22 ohms resistors ordered because they are reading 0.4 ohms all four of them. A couple more are out of specs and they should arrive on Tuesday from mouser.

 

[calman46]

Your meter can probably only measure down to 0.4 ohms . having 4 resistors all exactly out of spec by the same value is unlikely as I see it. You need to check your work and check and clean all switches . Does the unit make any noise , even hum used headphones . What are the infamous 25?

 

[Trompi63]

I was referring to Q25, as I have seen many comments about this one getting too hot. I am using a fluke 179 so I have a reliable meter. I pulled the resistors out and still reading 0.4 ohms .
I also found that resistor R270 is supposed to read 10k and reading is 10 ohms.

 

[Trompi63]

I was referring to Q25, as I have seen many comments about this one getting too hot. I am using a fluke 179 so I have a reliable meter. I pulled the resistors out and still reading 0.4 ohms .
I also found that resistor R270 is supposed to read 10k and reading is 10 ohms.

Sorry this is ok. The schematics were not very clear but within specs.. 10 ohms and reading 10.04 ohms

 

[Trompi63]

This is maybe an stupid question, but I am new on the sx-780. I only have one STK installed, so I should get one channel on right? Or do I need both installed to get audio.
I am checking all test points now and the voltage is right as specs in every point.

 

[larryderouin]

You can run one channel at a time. Won't hurt anything. You do know to check your probe resistance 1st (touch both together and write down reading. Then subtract this from any reading during that session.) It helps keep you from chasing gremlins when you least expect it.

Make sure the relay is wired correctly. And the contacts are clean enough to pass current. Dirty, Pitted, contacts on the relay can/will cause this type of problem. Make some strips of paper and soak them in DeOxit. Pull each strip thru CLOSED CONTACTS, only once and in one direction only. Once a strip comes out clean, Power up and see if you have audio.

Also check that the TAPE MONITOR(s) are OFF(up), Speaker selector in in "A" along with the speakers. All other controls centered or in OFF position. Volume at full clockwise then advance to about 7:30.

 

[Trompi63]

Thanks!! I have a new Omron relay installed, when clicked, I got right voltage on that rail.
Let me go back and measure the voltages at the new STK. what is the probe resistance? Across R236 and R 238?
Really appreciate the help… I want to have this baby back in service soon…my wife loves it and keep asking… why did you try to fix it when it was working.. .. she was not able to hear the distortion..

 

[satellite65]

I was referring to Q25, as I have seen many comments about this one getting too hot. I am using a fluke 179 so I have a reliable meter. I pulled the resistors out and still reading 0.4 ohms .
I also found that resistor R270 is supposed to read 10k and reading is 10 ohms.

How many ohms does the Fluke read if you short the leads together? That reading should be subtracted from the readings that you get on low value resistors.

 

[Trompi63]

Ok, I missed the lead’s resistance in here, shorting the leads measured 0.2 ohms, therefore all 4 resistors are within range at 0.2 ohms.
What should be the next step?

 

[Trompi63]

I measured the newly installed STK, and this are the readings:
J1: -1.377 V
J2: - 42.78 V
J3: - 0.298 V

J8: - 0.296 V
J9: + 42.80 V
J10: + 0.835 V

I was curious and also read the pins on the other STK without the pack installed, I read :

J1: - 40.31 V
J2: - 42.78 V
J3: - 0001 V

J8: -0001 V
J9: + 42.79 V
J10: - 40.48 V

Q25:
E: + 13.58 V
B: + 14.20 V
C: + 28.68 V

one thing I noticed, while measuring STK pins the relay clicked once and the when I removed the lead returned to normal. I repeated and did not turn off again.

 

[Trompi63]

Ok, some updates, I was able to install the second STK, powered up and relay clicked.
But again, no sound. I checked for continuity between the relay pins and when clicking all 4 pins are showing continuity.
Q6 reading I don’t believe are ok. Base and collector are showing-54.45 V while the emitter is -41.40 V
Q5 is -5.4 V -41.2 V and 0 V
Any hints in here?

 

[Trompi63]

by the way second STK measured:
J1: + 1.21 V
J2: -42.64 V
J3: -0.19 V

J8: -0.19 V
J9: +42.64 V
J10: + 0.0925 V

is all this ok?

 

[comish4]

Where did you get the STK's from?

 

[Trompi63]

I’ve got it from Pyramid-Audio, apparently the best you can find nowadays.

 

[Trompi63]

Let me give an update about what I have found so far but first how all this started. I had a noisy channel and pre amp giving excessive hum. So I decided to upgrade all caps and some transistors, got the repair kit from EBay and started the job. First I replaced the 2 big caps C501 and C 502, as soon as I complete this I reassembled the capacitor assembly DeoxIT the board and powered up. No relay clicked, weird because it was working before, then I looked at the STKs modules and one of them was STK II generic the ones being sold on EBay for a few bucks. At this moment I decided to order a couple of new STKs from Pyramid Audio, at the meantime I continued replacing caps and a few transistors that came with the kit. When finished I shorted with 1k resistors the pins in the removed STK as I see in many threads to test the circuits and balance the DC offset. The relay clicked and i balanced the R and L channels to 0V .
Then I received one STK and installed, all good relay clicked BUT no audio.. I started checking the circuit and those are the values I have posted before.
I continue chasing the problem, but no luck so far. All I hear is a small pop on the speakers when I press speakers A switch On/Off .
Where should I look first..?

 

[Trompi63]

Let me give an update about what I have found so far but first how all this started. I had a noisy channel and pre amp giving excessive hum. So I decided to upgrade all caps and some transistors, got the repair kit from EBay and started the job. First I replaced the 2 big caps C501 and C 502, as soon as I complete this I reassembled the capacitor assembly DeoxIT the board and powered up. No relay clicked, weird because it was working before, then I looked at the STKs modules and one of them was STK II generic the ones being sold on EBay for a few bucks. At this moment I decided to order a couple of new STKs from Pyramid Audio, at the meantime I continued replacing caps and a few transistors that came with the kit. When finished I shorted with 1k resistors the pins in the removed STK as I see in many threads to test the circuits and balance the DC offset. The relay clicked and i balanced the R and L channels to 0V .
Then I received one STK and installed, all good relay clicked BUT no audio.. I started checking the circuit and those are the values I have posted before.
I continue chasing the problem, but no luck so far. All I hear is a small pop on the speakers when I press speakers A switch On/Off .
Where should I look first..?

 

[Trompi63]

I finally figured out the problem, a couple of transistors were completely out of specs. Q5 and Q6. I received those from mouser and boom .. back to life.. it sounds amazing now.