Looking for assistance with this unit that I'm fixing. The left channel volume is very weak. - right channel is 10 times as loud - no "scratching" sounds from volume control - volume adjusts properly on both channels - switching earphone sets did not resolve it, so it's not the earphones - happens with tapes or radio, so it's not the head positioning or tape path - happens on all radio bands including am - earphone jack seems to be ok - removed and put contact cleaner in volume knob I've already done the belt and found a broken motor wire. Maybe it's time to buy a service manual for it.
Given the problem is happening on both the radio and cassette, it has to be in the power amplifier section. Everything before this can be excluded. Assuming there is no problem with the headphone jack itself and the volume potentiometer, I would look into the 2 output coupling capacitors. They are 2 (usually 220uF) that are in series with the left and right channel, just before the jack. If you don't have the means to measure them, just replace. Some pictures with the PCB would be helpful as I'm not familiar with this unit nor do I have the service manual.
Thanks for the guidance, Valentin. I desoldered the jack and removed the contacts to carefully clean them. This had no effect on the problem so the jack can be eliminated as the problem. The potentiometer seems to be a very minimal thing. Unfortunately opening it up would destroy it. It's an Alps and a cursory online search found nothing like it. Below are pictures of the volume control, main circuit board and the attached small board. Everything looks fine to me. Nothing swollen or cooked. The earphone jack is the green thing at the lower right in the third photo.
I have not seen this type of potentiometer to go bad. And usually just spraying contact cleaner into it fixes any issues, you definetely don't need to open it (which will indeed destroy it). However I see something else that is a bit concerning: Nichicon capacitors. Nichicon caps in this small footprint usually tend to leak, seen many walkmans with such caps leaked. While I can't spot any signs of leakeage on the PCB, the fact it uses this particular brand is to be kept in mind. If you have the means, it would be a good idea to do some measurements in circuit to see at least the capacitance values. An LCR meter is the best tool for this, but a meter can give some idea also if it can measure capacitance. But before even doing this, I would do something else (which kind of needs to be done anyway): clean the 2 switches (marked in green). Given this is a recorder, I assume one of those 2 switches is a REC/PB switch. Spray some contact cleaner in them, move back and forth 20-30 times and re-check everything. If problem is still there, you need to start measuring around the TA7688F headphone amplifier IC: its supply voltage, input singals, output signals. I attached the datasheet of this IC. The output capacitors are marked in red on the attached picture.
The two switches are for Record and Reverse. I had already treated them with cleaner with no effect on the problem. I'll try contact cleaner in the potentiometer again, just in case that helps. I was thinking using a simple vom on its contacts should detect resistance changes as it is dialed, and resistance differences between pairs. But I don't know which of the 5 legs do what. And I have to buy a 12v battery for my vom. I'll replace the two capacitors. I note there are 2 other sets of the same value. I sure hope it isn't the ic. I don't have the tools to deal with it and I doubt I could find another circuit board. Edit: Huh, I found what looks like a source for the potentiometer: https://www.taobao.com/list/item/559268178847.htm
hi the audio signal before reaching the volume passes through the equalizer section and is connected via a flat circuit to the motherboard check the flat welds and clean the equalizer section
Thanks, and thanks for the diagram. I'll check the cable and equalizer. It does seem to work fine and not affect the weak channel.
I checked the equalizer ribbon cables with an ohmmeter, and the same with the potentiometer leads. The former test was inconclusive, as one might expect. Only two combinations of volume control, both involving the same pin, showed any life but did change resistance as the dial was rotated. This may make sense if the channels are split downstream of the volume control. I went to install the new capacitors but, because I didn't know any better, I had bought 16v ones instead of 4v. Back to Lees Electronics. They do not have 4v capacitors but said the 16v ones will work. However the other problem is that the 16's are twice the length of the originals and there isn't room for them even bent sideways.
Next update... I switched the original capacitors in question to see if the problem switched channels. The left channel was as weak as before. Which I believe should eliminate the capacitors as the problem. I sprayed some contact cleaner into the back of the potentiometer. Now, when I switch on the radio function I get a deafening hissing or rushing sound from both channels. The volume control does not reduce the noise. The left channel cuts out completely depending on the volume level dialed. Switching to the tape drive, the volume on both channels is very weak and does not increase as the volume is dialed up. And the balance between the channels changes from the left side to the right side as the volume goes from minimum to maximum, but without changing overall volume. In addition to my clumsy checks of the resistance through the potentiometer using a vom, this would seem to prove the potentiometer is the culprit. But the symptoms would seem to require that the potentiometer has separate circuits for the radio and the tape drive. I don't have wiring diagrams to check this.
There is not one volume for the radio and one for the tape and a single control that serves both radio and tape.. if you follow the black arrows that indicate the audio path of only one channel you can understand what it affects before the volume control and also after it
I'm going to take a break from working on it until I study the circuit diagrams. Obviously I need a better understanding of how it works. I just downloaded the service manual. I'll turn to working on the Sanyo M9998 in the meantime. If I need them for the Aiwa, where can I get 220uF 4v compatible capacitors that will fit the 5mm dia x 6mm form factor for the space available?
The capacitors can be found at the big electronics distributors: DigiKey, Mouser, Farnell, etc. You won't find such small capacitors at the local electronic component shops. But by the looks of it (since you swapped L & R ones with no change) capacitors are not your problem. Btw, you do not need to quote the previous message (which the "Reply" button automatically does) unless you only quote part of it. Quoting the entire message just makes the post longer by repeating the same info that was posted above it. Instead you can type into the box below and use the "Post Reply" button.
The capacitors all look clean and intact. As do the other components. With the vom set at 20k ohms, using the labels on the main board, and taking the first reading with the volume at minimum and the second reading at max volume, the resistance readings are: LO to E: 3.91 to 12.04 LO to RO: 0 to 1 LO to Rin: 19.32 to 1 LO to Lin: 1 to 0 E to RO: 3.1 to 11.7 E to Rin: 1 to 11.65 E to Lin: 1 to 12.1 RO to Rin: 19.34 to 0 RO to Lin: 1 to 1 Rin to Lin: 1 to 1 Hope this helps. (Just noticed the stray mylar washer.)
E - Rin and E - Lin are the total resistance of the carbon traces of the 2 potentiometers. These should be about the same and considering we have other resistances in parallel with the potentiometer in circuit, we would expect a lower value than 20k. E - Lo is the resistance between the wiper of L potentiometer and the common point. E - Ro is the resistance between the wiper of R potentiometer and the common point. Some of the readings are suspicious. It's unclear why they measure as they do, hence why I recommend to measure the pot out of circuit. Also the 0 to 1 variations are not looking good. From L to R you should have the highest value, when in practice you have a very low value.
