Hi All, I have been able to replace the caps in this Aiwa HS PC202 MK2. Sound is restored in both channels. However, I have a problem of Hmmm sound in right channel in all modes of Play, Rew & Fwd. I'm unable to find the service manual of this model online. Need help to trace the signal of R channel to find the culprit of Humm sound. There maybe a problem of ground circuit near volume pot is what I have been told by an expert. I have never used Oscilloscope before to trace signals. Will anyone please guide me...
Hello @Valentin I need your help with this small problem with my Aiwa PC202. The Humm sound is always present even when volume is at Zero or Full. Tape sound is prefect in left channel. The sound in right channel is slightly radio like and slight unclear along with the Humm sound. I rechecked all caps and they are correctly installed.
@rahul_unde If you could record a sample of the problem, would be ideal. Signal path on the schematic is marked with arrows, I highlighted it with red lines on the attached pictures. If you have oscilloscope you can start tracing it: output of head preamp, input/output of Dolby amp, input/output of DSL amp, input/output of power amp. Volume potentiometer has a common ground, so a floating ground on the potentiometer can only be explained by an internal fault. As stated already, be sure to thoroughly clean the potentiometer with contact cleaner.
@Valentin, thank you for sharing the signal path. I have purchased a small Oscilloscope of FNIRSI DSO 153 with signal generator capability. I have never used oscilloscope before. Will you please help me know with an example what settings to use on the device and start probing. I was searching for demonstration videos online about how to probe and check signals but couldn't find it.
There are 2 main settings that you're mostly concerned with on a scope: vertical sensitivity and timebase. Another aspect to be mentioned is the probe. Most probes are a selectable x1/x10 attenuation. x10 offers higher bandwidth, while x1 offers no attenuation. This scope given it has only 1Mhz of bandwith doesn't even need the x10 assuming it is there and you're also interested in seeing low level signals anyway. Normally you would want a test tape with a sinewave recorded on it to test the entire signal path, but given you're interested in a parasitic signal which is present without cassette you can proceed with the measurement. Note that not having the PCB ground connected to the mechanism chassis (note there's a spring doing that when PCB installed) you will have motor hum. So either disconnect the motor entirely or solder a wire to solve this problem. Then proceed checking the signal at pin 5 & 20 (output of preamp), 8 & 17 (input of power amp). Start with 20-50mV/DIV and adjust accordingly to what you see on the screen. Same goes with timebase. Normally you would also need to set the trigger point (which is that arrow on the right of the screen) to be within the signal boundaries, but that's already in the middle.
Hi @Valentin thank you for sharing the detailed steps. I was able to trace the problem to this capacitor. It needed resoldering and that solved the problem. Regarding sound improvement, the device sounds just as it should. However, I prefer a bit bass since this WM is missing DSL. So, I needed to know which 220uf caps can be changed to 330uf to improve the bass frequency on this device.
Out of curiosity what headphones are you using ? Low-end response is pretty good on these to begin with, never felt the need for boost even on the model with DSL. The output caps you mention are C15 and C16. However, changing these from 220uF to 330uF will make very little audible difference unless the impedance of your headphones is very low (like <= 16ohms). If you want a significant improvement, would rather change C5 and C6 from 47uF to 100uF. But note you need to re-check/adjust Dolby levels if you do this.
I'm using Sennheiser IE 300 IEMs. It's impedance is 16 ohms. Will first attempt upgrading C15 & C16 to 330uf. Considering the impedance on my gear it should hopefully suffice. I'm attaching the specs for your reference.
