Hi everyone, and thanks for reading this post. I'm doing my best to learn some electronics repair, and lately I've been working on a vintage Sears Model 10656 clock radio. It's nothing special but I like the look of it and I was hoping to fix it up. Sorry for the long post, I'm trying to include as much relevant information as I can.
Although the unit worked, it had scratchy pots and poor reception in the FM band. I opened it up to start working on it, and I noticed that there were a couple of burnt resistors on the clock board. I worked through estimating the resistance of the burnt resistors, and looked and the circuit to determine that they appear to be related to the Bright/Dim option for the LED display.
The main clock chip is a TI TMS1952NL, a 40-pin DIP, and I cannot find a datasheet for it, but after probing around it seems like it might be pin-compatible with the MM5316 which I could find a datasheet for.
The main question is, it seems to me like the resistors that burnt are being made to dissipate quite a bit of power, and yet the originals where only 1/4W each. To me this suggests either something else isn't working right in the circuit, or it was a bad design from the get-go. Problem is I'm too new to know which the answer might be so I'm hoping for some guidance here.
I measured the voltage between the points just before and after where the resistors were, and I am seeing 12V DC. The burnt resistors were in series, and I believe they were in the range of 10 ohms to 100 ohms. I've since temporarily connected two, 100 ohm 1W resistors in parallel to give me a resistance roughly in the middle of the expected range, and with a bit more ability to dissipate power. This provides a decent brightness but using some power calculators to see dissipation requirements for resistances that seem to be in line with the old resistors, and it looks like they would have been required to dissipate anywhere from 2-4W, well over the 1/4W rating of the original resistors.
Any guidance on what I should check next would be appreciated. Below are some of the things I've done so far:
A few details:
1. The clock board is powered by a transformer which has two red wires and one black coming out. When checking voltage between the red wires and black, both red wires are putting out 10.5V. I confirmed no shorting between secondary and primary.
2. If the TMS1952NL is in fact pin-compatible with the MM5316 clock chip, then the VSS pin is seeing just short of 9V DC, and VDD is -16V DC. Both pins also seem to have about 4 VAC on them. I'm checking the voltage by connecting the DMM to the incoming black transformer lead, and then probing the VSS and VDD pins with the red lead. Hopefully not making a rookie mistake there.
3. There were two electrolytic capacitors on the board which I pulled, measured and replaced. They were still border-line in spec., but since they were out for testing anyway I've changed them. The larger capacitor was replaced with one rated significantly higher for voltage (from 35V to 250V) because it's all I had in the right capacity. I've looked over each of the remaining resistors, and compared their marked values to measured values, and they are all within tolerance. Measurement was done in circuit, but I figured since the values were close to their marked values this is likely OK. Maybe I'm setting myself up for failure here?
4. I've done basic diode checks for the diodes on the boards to see that they are still only conducting in one direction, and that the drop seems reasonable.
5. I've pulled the two transistors from the board, checked them, and tried to match them up with datasheets to see if their gain was in-line with expectations and it seems to be.
Thanks again for taking the time to read my blabbering....
Although the unit worked, it had scratchy pots and poor reception in the FM band. I opened it up to start working on it, and I noticed that there were a couple of burnt resistors on the clock board. I worked through estimating the resistance of the burnt resistors, and looked and the circuit to determine that they appear to be related to the Bright/Dim option for the LED display.
The main clock chip is a TI TMS1952NL, a 40-pin DIP, and I cannot find a datasheet for it, but after probing around it seems like it might be pin-compatible with the MM5316 which I could find a datasheet for.
The main question is, it seems to me like the resistors that burnt are being made to dissipate quite a bit of power, and yet the originals where only 1/4W each. To me this suggests either something else isn't working right in the circuit, or it was a bad design from the get-go. Problem is I'm too new to know which the answer might be so I'm hoping for some guidance here.
I measured the voltage between the points just before and after where the resistors were, and I am seeing 12V DC. The burnt resistors were in series, and I believe they were in the range of 10 ohms to 100 ohms. I've since temporarily connected two, 100 ohm 1W resistors in parallel to give me a resistance roughly in the middle of the expected range, and with a bit more ability to dissipate power. This provides a decent brightness but using some power calculators to see dissipation requirements for resistances that seem to be in line with the old resistors, and it looks like they would have been required to dissipate anywhere from 2-4W, well over the 1/4W rating of the original resistors.
Any guidance on what I should check next would be appreciated. Below are some of the things I've done so far:
A few details:
1. The clock board is powered by a transformer which has two red wires and one black coming out. When checking voltage between the red wires and black, both red wires are putting out 10.5V. I confirmed no shorting between secondary and primary.
2. If the TMS1952NL is in fact pin-compatible with the MM5316 clock chip, then the VSS pin is seeing just short of 9V DC, and VDD is -16V DC. Both pins also seem to have about 4 VAC on them. I'm checking the voltage by connecting the DMM to the incoming black transformer lead, and then probing the VSS and VDD pins with the red lead. Hopefully not making a rookie mistake there.
3. There were two electrolytic capacitors on the board which I pulled, measured and replaced. They were still border-line in spec., but since they were out for testing anyway I've changed them. The larger capacitor was replaced with one rated significantly higher for voltage (from 35V to 250V) because it's all I had in the right capacity. I've looked over each of the remaining resistors, and compared their marked values to measured values, and they are all within tolerance. Measurement was done in circuit, but I figured since the values were close to their marked values this is likely OK. Maybe I'm setting myself up for failure here?
4. I've done basic diode checks for the diodes on the boards to see that they are still only conducting in one direction, and that the drop seems reasonable.
5. I've pulled the two transistors from the board, checked them, and tried to match them up with datasheets to see if their gain was in-line with expectations and it seems to be.
Thanks again for taking the time to read my blabbering....