Author Topic: Help Understanding IC (UPC1237)  (Read 3362 times)

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Offline AngraMeloTopic starter

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Help Understanding IC (UPC1237)
« on: September 16, 2019, 12:49:21 am »
Hey guys,
I got an old Sony Stereo (LBTn355) for cheap and the speaker relay is not activating.
So it is controlled by the IC UPC1237. It appears to detect DC voltage and breaks off the relay to avoid damaging the speakers.
Now, I think this IC is bad, because if I understood it correctly (datasheet below) if pins 1 or 2 are greater than the threshold voltage (set by pin "8") then it wont activate the relay.
Now, this is the voltages I measured on the IC:
1 - 0.004V
2 - 0.011V
3 - 0V
4 - 2.52V (actually, I get a kind of a sawtooth wave that actually goes to positive and negative on respect to ground)
5 - 0V
6 - 36V
7 - 0.007V
8 - 3.49V

Am I understanding the IC correctly? Or those voltages represent something Im not understanding?

Here is the IC datasheet

https://pdf1.alldatasheet.com/datasheet-pdf/view/6669/NEC/UPC1237HA.html

(Schematic of the Stereo) https://www.electronica-pt.com/esquema/audio/sony-audio/sony-lbt-n355-1318/
Thank you guys!
« Last Edit: September 16, 2019, 03:30:37 am by AngraMelo »
 

Offline floobydust

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Re: Help Understanding IC (UPC1237)
« Reply #1 on: September 16, 2019, 02:43:56 am »
The IC is probably OK. I see a problem with pin 7 (Power-on delay timer input) if it goes above +2V, the relay is enabled (if no DC or overcurrent detected). You have nothing at pin 7, so something else is holding it down. 
Sony has the MCU pin 67 controlling that POWER_ON signal as the STANDBY switch. So the MCU thinks power is off and keeps Q1205 turned on.

If pin 2 (DC Detect input) goes over +/-0.6V, the IC goes into protect mode.
If pin 1 (Overcurrent Detect input) go over +0.7V, the IC goes into protect mode.
If pin 4 (AC mains Detect input) go under +3.5V, the IC goes into protect mode.
 

Offline AngraMeloTopic starter

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Re: Help Understanding IC (UPC1237)
« Reply #2 on: September 16, 2019, 03:29:25 am »
Thank you so much!
It is a pretty weird issue because the system is not reporting any errors, for example PROTECT or something alike.
It plays music but the relay wont engage.
Now pin "4" of the IC is fed by the taps on the transformer, the signal leaves the transformer goes to a resistor (R1303 and R1304), a diode (D1309 and D1310) and directly to pin 4.
That tap on the transformer reads 23Vac, but the anodes of the diodes read 2.5Vdc. I already changed the diodes and the resistor dont seem to be an issue. I have no idea how those 23Vac are becoming 2.5Vdc
 

Offline floobydust

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Re: Help Understanding IC (UPC1237)
« Reply #3 on: September 16, 2019, 05:02:18 am »
uPC1237 pin 4 has a zener inside, so it clamps to around 4V. Resistors R1303, R1304 4k7 drop the 12VAC down.

I would check base voltage on Q1205, +5V during standy/power off and goes to 0V when power is on (POWER-ON).
This (Q1205 base 0V) allows the relay to pull in. Or capacitor C1221 on pin 7 might be shorted, or Q1206 or IC504 shorted on the panel board which turns the LED's on.
Otherwise the MCU is thinking power is off and it is staying in standby.
 
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