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SX-780 unfound question

Quick question. Just making sure I am doing this right.

I used my DMM and checked the 4 0.22 Ohm resistors just in front of the STKs. I did this with the resistors in circuit with no power on. The readings were:

Left Channel:
#1 = .2 Ohm
#2 = .2 Ohm

Right Channel:
#1 = .3 Ohm
#2 = .3 Ohm

I am assuming the left channel are ok. Is the .3 Ohm on the right channel too out of spec?

Below is a picture testing right channel:

IMG_3524.jpg

.
 
PS - If you do recommend changing out the 0.22 Ohm resistors, I will probably just swap all 4 out. Do you have a good lead on where to buy these?

Thank you in advance......
 
You need to use a lower Ω range if you have one on your meter. That reading is way down at the bottom of your measurement range, and is probably inaccurate. But being that low, they are probably OK.
 
If the meter leads are shorted, what is the resistance indication? Usually there is a tenth or so of resistance measure in the leads on the low resistance ranges.

My guess is that the resistors are O.K. but you can measure from what would be Pin 3 and Pin 8 of either STK module to verify a total resistance around 0.44Ω ± 5%, or essentially through both resistors simultaneously for a better measurement.
 
Unfortunately my DMM only goes down to tenths for Ohms. When I short the leads I get 0.0 Ohms. So I am guessing the shorted value must be less than 0.05 Ohms, or it would have rounded up to 0.1 if 0.06 or higher.
 
Ah, good idea. I see what you mean now.

I just measured resistance between Pin 3 and Pin 8 on both STK's. The resistance was 0.4 Ohm for both sides. So, I am assuming there is less than 0.45 Ohms on each . They should be good.
 
Well, they work!!! Got my replacement STK's, installed them, dim bulb power up and then full power up. Right channel now works and the sound is CRYSTAL CLEAR. Sounds wonderful!! Thank you!!

Now that it sounds great, I have notice one final issue:
  • Phono Input Not Clear - The AM, FM and AUX inputs sound crystal clear and simply heavenly. When I hook a source up to the Phono Input, it sounds very different. Very muffled sound with over powered bass. Any ideas on what to track down on the poor audio on the Phono input?
Here are a few pics of the installed replacement STK's:

IMG_3664.jpg

IMG_3663_Mark.jpg
 
Of course, the phono input is intended to be connected to a magnetic cartridge, with a typical peak output ranging around 2.5mV instead of the 1.5-2.0 V of a line input device. Over-driving the inputs by a factor of 200-800 times intended level the can result in a very muddy, garbled, and bass-heavy output from the amp.
 
It does look like it's black. That would make it a SX-780/W4. That was predominately a 1980 model. The SX-780 was a 1978/79 model.

It's kissing cousin (or closer relative) is the SX-790 or SX-790/W4 Made in the same time periods. The 790's have a black dial face vs. the silver of the 780's. Internally except for the dial face they are the same.


SX 790
ScreenHunter 694.jpg

SX-790/W4

ScreenHunter 695.jpg
 
Just checking back in as I am starting a different repair and saw you had some questions. No, it is the standard wood bonnet. I just used portrait mode when I took the picture and makes it look black for some reason.

I listen to it about everyday as I work from home. The repairs that this group helped me make brought back to life this beauty and still sounds great!
 
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