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SX-580 120hz hum

looking safe to replace the modules .. maybe try one at a time and measure offset at speaker terminal again . power on dbt until they both read about zero volts with both modules installed . then try on full mains .
 
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Hey, sorry, I made an error and didn't open up the speakers throughout that initial testing. Didn't realize my mistake until I was almost done.

So I've now taken a few steps back and replaced STK packs with the resistors again and remeasured (with speaker circuit engaged) got +12.5 mVdc on the left and -16mVdc on the right. Still good to proceed? Or should I halt moving forward?

I did at least power everything up with both STK's in before realizing my error and the bulb remained dim. So maybe I got something out of it before moving back to resistors only.
 
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J A said:
Ok powered up. bulb lit up pretty bright off the bat, then dimmed to barely visible. <1mV at each terminal .5-.7 ish. I did end up taking the reading just red to red black to black at the terminal.
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That is exactly what it should do!

Measure dcv on pins 1 and 0 (black on ground) on each STK connector. They should be about equal in voltage but opposite in polarity, pin 1 + and pin 0 - . With 1kΩ resistors the will be around +/-7vdc, the exact voltage is not critical, but they should be balanced around 0vdc. Often they are not perfectly balanced but close, within 100mV. Measure dcv on pin 3 (or 8), sb very close to0vdc, like the speaker outputs were. DC offset in not adjustable on the unit, but is controlled by the diff amp (Q301, Q302). If that is good, remove the resistors and replace the STK in one channel. Power up on the DBT and check dc offset on that channel. It should still be close to 0vdc. Then remove resistors and install the other STK. Power up again on DBT, check DC offset on that channel. If it passes DBT, take it off the DBT and power up, rechecking offset. If it is still good, power down, connect speakers and an input, then power up and try some music.
:thumbsup:
If dc offset is bad with an STK installed, but good with the resistors, that STK is bad.
 
Ok, did some looking around and looked like those previous dc offset numbers were at least safe so Im back to having them both plugged in (both STK's) but still on the DBT. Measurement is now .5mVdc on the left and -125mVdc on the right
 
125mv is not dangerous. Swap channels with STK and see if the offset moves to the other channel. If it does (not what I expect) the STK has a problem. If it does not move with the STK the usual fix is to replace the diff amp dual transistor for that channel. Replace with a matched pair of KSA992 transistors.
 
Swapped the STKs and removed the DBT box now to plug into main power.

-109 mVdc on the right
-177 mVdc on the left
 
Reswapping them to validate the previous reading. I'll use main power this time unless you recommend otherwise.
 
That is not a dangerous voltage. May need to do both diff amp transistors. But first make sure the power supplies have correct voltages on main power. Do you still have the hum?
 
In the interest of validating the accuracy of what I'm giving you I went ahead and swapped the STKs back to the original configuration.

-83mVdc on the left
-188mVdc on the right

Going to hook up speakers now and check for hum.
 
let it warm up some and check the dc speaker voltages again . it might drop enough ..it might not but here is hoping .
 
I thought the same but when I checked it after a while I edited my post with the numbers. Still pretty rough but, super cool that it's doing anything. Really neat bringing this thing back. Now I'm going to browse around and see about the transistors that were mentioned. I would like to replace them now if it's another inexpensive item that might be fun.
 
QTY 10 KSA992FTA on the way to me from Mouser. After shipping ~$1 per. Obviously I don't need 10 so if anyone needs some in/around the Boston area let me know.
 
KSA992s come in tomorrow and I'll be ready to go. Anything I can do in the interim to prep? I suppose remove the existing ones?

Just to confirm, is that Q301 & Q302?

Also, I've read around on Google a bit to try and understand the comment about matching vbe > hFE. Can you elaborate? Do you mean when matching transistors?

Also, my multimeter doesn't have an hFE section. I see some formulas for calculating vbe and it seems to be (is it the same as bias?). Is this the same as where I get the Vdc at the emitter points?

Edit: Correct me if I'm wrong, but I see the Q301/302 voltage callouts on the foil drawing. I'll take V readings there after replacing? Any concerns about power with transistors? Machine has been off for a couple of days.
 
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merlynski said:
May need to do both diff amp transistors. But first make sure the power supplies have correct voltages on main power.
Click to expand...
How do I validate the voltages on the power supply? I don't see any voltage callouts in the power supply section to check (assuming this is 7b of the drawing location)? Should I also validate the voltages that are called out around Q301/302 prior to replacing?

Edit: Ok, obviously can't be q301/302 because those have 5 "legs" and these ksa992FTA have 3 "legs". So these must be for Q303/304?
 
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