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Sx-1050 Power Issue

V

vmedia

Active Member
I am helping a friend with an sx-1050 he acquired as part of a trade. He was told that the unit was working until the previous owner "pushed a little too hard on phono inputs - which broke and entered into the chassis causing the unit to shut down."
Someone else has clearly worked on this before - a few wire wraps are soldered i place, mismatched outputs on one side. I doubt the phono input issue caused any of this to happen. On DBT no protection circuits click - bright glow...
I tested outputs - all appear good. DBT still glows when outputs are disconnected (no speakers at any time)
I found continuity across pins 3/4 on AWR-104 when red wires are connected.
I have discovered when I disconnect the two red leads (these were soldered in place, wire wraps removed by previous repair) from transformer on pins 2/4 of AWR-104 the unit powers up with two clicks.
I pulled and tested R1/R2 and C1/C4 all test good. There's continuity across the pins for C4 (with capacitor out of board) I'm suspecting a short in D1 - the enclosed high voltage diode unit? I will pull and test, but too tired to continue tonight and thought I would check here first.
Any help would be most appreciated
 
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The 24V and ±75V supplies are intact?

With loads disconnected from the power supply, does the DBT still have a bright glow?

What wattage lamp in the DBT? A 1050 may require a 100W or larger lamp to "get over the hump" when powering on.
 
Thank you for your response.
With the two red wires disconnected from AWR-104 the DBT does not glow bright (100 watt bulb)
Also, with the wires disconnected, here are the readings on awr-104 under DBT:
2) disconnected
3) +0.1 vdc
4) disconnected
5) -0.1 vdc
6) +57.8 vdc
7) -58.3 vdc
8) +17.6 vdc
9-14) 0
The two red wires from torodial measure 0 volts each
 
That was what I was worried about. Transformer bad?
There's also continuity between those two wires and the chassis ground.
Any other checks or possible cause?
 
It appears that you have already isolated the transformer winding from the rectifier bridge (and capacitors). Re-test the AC voltage from the transformer again. Make sure the meter is set for AC voltage and functions properly on that scale.

As for the continuity to ground, a quick look at the schematic shows the center tap of the transformer winding grounded. I'd be more concerned if you didn't read continuity, since that could indicate an open winding.
 
Re-checked and I'm getting 56.5 vac. on DBT.
Embarrassed to say that I probably forgot to switch from DC to AC. Wouldn't be the first time.
So...all voltages on that board appear to be about right.
 
Even the +-62 DC volts at pins 3 and 4?
Shorted big filter?
Disconnect the filters and DBT it.
 
No sorry - I was not as clear as I should have been. I meant the transformer appears to be doing its job.
When I reconnected pins 2/4 - got dim bulb bright again (of course) and those pins (2/4) register 0 vac when connected to the board.
0 volts on pins 3/4.
Disconnected big filter caps and still have bright DBT and still have 0 volts on pins 3/4
 
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One thing left.
The rectifier.
You can check the diodes in the circuit. Or is it a single rectifier? I have a 1050 here, I could look.
Edit: Single rectifier.
Scat Check:
Pins 2/4 are the AC in from the transformer.
Pins 3/4 are the DC output to the filters and output transistors.
 
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markthefixer said:
KBH25-04 25 amps 400v 1.125 inches square with terminals? not wires...
also check if the terminals are aligned with the faces, or if they are diagonal to them - some are...

mouser.com (I hope)

512-GBPC2504W $2.82 each with wires
512-GBPC2504 $2.82 each with terminals, one is at right angles to the other three.

www.allelectronics.com
FWB-254 $3.00 each

http://www.allelectronics.com/make-a-store/item/FWB-254/25-A-400-PIV-BRIDGE-RECTIFIER/-/1.html
Click to expand...

I found this old post, still relevant, except the pricing.
 
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Thanks for the guidance. Confirmed my initial suspicion and I learned a bit about transformers along the way. I have a replacement rectifier on order, as the KBH25-04 tests bad on one diode. I removed in order to be positive, not 100% sure of the readings I was getting when in circuit, but out of circuit there was a clear issue between two legs in both directions.
I'll report back once the new part has been installed (this weekend)
 
I've had a few single diode rectifiers shorted. Not that common but it's something to be aware of.
You and Watthour did a good job troubleshooting it.
 
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Just curious,
Can I bring up on DBT with rectifier out to see if bulb is still bright?
Always interested in multiple ways to confirm an issue.
Thanks
 
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