Hi all, I've bought my Aiwa HS-PL55 new in 1990 and it's a beautifully made, awesome sounding player, still on its original belt. I recommissioned it a couple of years ago, replacing all of the electrolytic caps and all was fine until recently when I noticed that when set to Normal tape type, I hear very distorted audio in the headphones. Flicking the switch to Chrome and it sounds perfect. I've checked the schematic and the switch circuit is very simple, selecting Chrome pulls the tape type pin on the preamp chip to 0V, setting it to Normal essentially disconnects this pin and the schematic says it should measure 1.5V. It's very hard to operate the player with the PCB open as the tape sensor is fitted on the PCB, essentially I can't test to see if I have 1.5V on that pin with the switch set to Normal. The switch checks out fine with a continuity test. Have to admit I'm a bit stuck. I have cleaned the PCB thoroughly with flux remover, I checked for corrosion with a loupe and all looks pretty immaculate. The grounds measure ok too. Ill attach the relevant part of the schematic tomorrow, it's pretty simple. If I can't fix it it's not so bad, it still sounds outstanding, even compared with my Sony WM-33 and D6C which are far bigger beasts. It'd just be nice to have everything working the way it should. Any ideas? Thanks, Matt
Here's the PCB with replaced electrolytic caps (Nichicon UMW MW 4V) The caps in line with the audio outputs to the headphones were replaced with 2x 220uF in parallel tantalum caps. Fun bit of rework!
Thanks Raul. I did test the switch with a multimeter and it seems to function ok. The distortion is weird, louder on the left side than the right. Think the issue is internal to the pre amp IC or its associated components.
If you have small wires, you could solder them to the pin and then you will have it outside and you could then measure. I use that magnetic wires for laptop repair. The other thing, selecting chrome just set 0V on IC? Isn't there anything else on the Normal path?
No, the Normal path is shown as open circuit in the schematic. It shows NORMAL : 1.5 METAL : 0 which means there must be an internal pull-up resistor to 1.5V when the switch is open (Normal position).
Hi Matt Follow the reasoning if the signal enters on pin 22 and comes out of the preamplified 19, the only connection between output (Pin 19) and switch control (Pin 14) are these components on pin 23 if these are ok the problem is in the chip IC1... Try unplugging the 3.9K R8 and hear if it changes
Hi Silver965, thanks so much for taking the time to look into this for me, really appreciate it. What you say makes total sense. I'll have a look at those components, guess they're to do with the tape equalisation. I'll also check the IC1 pins for signs of corrosion as I did use a fair bit of flux when performing the recap. While I'm at it I'll solder a wire on pin 14 as suggested by Raul (thanks ) to check the switch operation.
I checked the components in each channel and they all measured ok with a multimeter. Pin 14 measures 1.5V when the switch is in the Normal position. I cleaned the PCB and IC1 again, all to no avail, same issue. I didn't remove the resistor as I don't have a good rework setup at home. I usually use two fine tip irons or solder tweezers at work.
ok ... have you checked if pin 23 or 2 is grounded? as in the diagram ... or there is a capacitor on the output or input of the preamp pin 19 and 20 that brings decompensation type c6 and just my curiosity
Looking at the schematic, the EQ is done by R8, C8, R6 (right channel) and R7,C7, R5 (left channel). Switching the EQ either keeps pins 2 and 23 high impedance or grounds them (grounding will bypass R6 and R5). The datasheet of the preamp/power amp IC gives a specification of switching transistor ON resistance: the typical value is 6 ohm, maximum value 18 ohms. I would check that, as those 2 transistors inside the IC are likely to be what's causing your problem. This would also explain why there is a difference between the left and right channel, as there is a transistor per channel, but a single switch control driving both.
I finally found time to have another look at the audio problem on my Aiwa HS-PL55, with playback issues in only Normal tape position. Under a microscope I could see some flux and debris between the pins of both the Dolby IC and the pre-amp IC. I think it was washed there when cleaning up after I did the recap. Using a few very fine tooth picks and some isopropyl I was able to clean up between the pins. I also found the PCB to chassis ground pad had tarnished a little, so I cleaned this up gently with a fibreglass pen. I'd like to protect this pad and was tempted to try some copper grease but maybe there a better product for this? Anyway, the end result is good, very good For the first time in many years I can enjoy the lovely sound of this player, it has a powerful and big sound compared with my Sony's. Dolby NR sounds a bit muffled so I tend to leave it turned off. This is the opposite of my Sony EX and DD Walkman's that all have a very effective Dolby circuit.
