Replacement of a dead IC in a portable player
Back to the blog overview page | Post date: 2025-08-16Objective
I have a Sanyo M1740F radio lying around. It offers rather convenient features:
- AM + FM radio and cassette playback
- Radio or microphone recording to cassette
There are two switches: a radio/cassette playback switch and a FM/AM switch. Radio recording would be done by first selecting radio playback and pressing the record button.
(Image source: crystalradio.cn)
Unfortunately, the device emits a squeaking sound when connecting it to mains. Both - radio and cassette playback are affected by the same problem. Let's try to fix this.
Reverse engineering
Friendly reminder: disconnect all power from a device that you want to open. Even better if you disconnect the power while it is still running, such that capacitors discharge more quickly.
After removing the volume and tuner knobs, and a few screws on the back side, there's more screws to loosen to access the whole circuit board underneath. The annotated numbers describe the screw length, in millimetres.
Here's the image without annotations: Image
Examining the one-layer circuit board reveals the following components:
- LA1260: AM/FM tuner IC
- This IC can be seen in the image above, to the bottom left of the variable capacitor (plastic case).
- UPC1213C (IC701): Power amplifier IC (by NEC)
- Located under the cassette player.
- (Loudspeaker: 3 W, 4 Ω)
Without further ado I connected a laboratory PSU (limited to 7.5 V and 100 mA) to the pins that would normally be powered by a five type C batteries (see bottom right space). By this I could confirm that the AC power input most likely still works correctly - because the squeaking sounds also appear with a clean power source.
By consulting the datasheets, following the traces on the PCB and measuring the voltages using a multimeter, I could slowly begin to understand the layout of the circuit board.
Signal checks
I did not have any oscilloscope at hand, thus connecting a 4.7 µF capacitor in series to cheap in-ear headphones made it possible to probe the audio signal lines across the circuit board. The AM/FM tuner IC does still work. Its weak audio outputs on pin 8 and 9 respectively do produce the expected radio background noise. Tuning in on a local FM station does work too.
There are a few more R-C filters and two (KS)C945 NPN transistors, most likely used as audio preamplifiers. So far so good. There is still a signal after this stage. Going by the rule of elimination, the culprit is most likely the power amplifier, IC701.
| Pin# | Voltage to GND | AC characteristic |
|---|---|---|
| 1 (+IN) | 4.4 | AM/FM audio (good) |
| 2 (n.c.) | 3.8 | amplified audio |
| 3 (Filter) | 7.3 | humming |
| 4 (Vcc) | 7.3 | humming |
| 5 (Feedback) | 7.3 | AM-like background noise, loud |
| 6 (OUT) | 1.55 | AM-like background noise, loud |
| 7 (??) | 4.4 | quiet humming |
| 8 (-IN) | 1.45 | humming |
Pin 8 looks strange. Its potential should be almost identical to pin 1. However, this does not happen because the amplifier output pin 6 stays at mere 1.55 V.
Component replacements
Hoping it would not be the IC, I swapped a few surrounding capacitors. Most of them were used to remove the DC offset from incoming and outgoing audio signals, or to stabilise the amplifier.
No success. The output stage of the IC is likely dead. What next? Whereas I do have a few amplifier ICs, none of them had a compatible pin mapping, let alone the special heatsink like the UPC1213C had. I could order a replacement part - but would this be any challenge? Would that IC survive longer? I do not know. I headed to the sketch book, and the breadboard.
The IC TDA7231 (that I have at hand) does fit inside the voltage range of the original amplifier. Its output power limit is only up to 1.6 W, thus not suitable as full replacement. Regardless, it turned out that < 1 W is already sufficient for my demands. The amplifier would also start oscillating anyway at higher gains that I could not fix on the breadboard. Perhaps its mechanical connections - or rather impedances - are to blame.
After a few successful test runs on the breadboard, I soldered an add-on circuit board and attached it to the pins that were used by the original amplifier.
Works for me. After reducing the cable length and insulating the backside of the PCB we get a well-functioning circuit board that is still flat enough to mount back into the case.
Final words
Tinkering is fun. If you have some spare time and do not know what to do - old electronics hardware is a great place to learn hardware reverse engineering, circuit debugging and to fix up issues.
This is a project from early May 2025. Only in August I have actually found the motivation to finalise the captured images into a comprehensive blog post. I hope you enjoyed it!
Until next time. See you.