Hi guys. I do not have a lot of experience analyzing PCB's. As a project for myself I am trying to diagnose and fix a 1982 Toshiba RT-140S boombox that is drawing 0A when supplied with 9V at the battery terminals or plugged into mains AC supply voltage.

I have cleaned the ST3P (I think) switch where switches between modes Tape/AM/FM using WD-40 contact cleaner, then moved back and forth but have not had success. I have also tested tape deck mode and the motor does not seem to operate at all and in all cases the power supply says it is drawing no current.

I am after some insight as to where I could look next or possibly teach me which points I need to probe to test this particular SP3T type switch for the function (tape or radio modulations) as I'm not familiar with it.

I have posted the repair manual in attachment which is for the RT-120S but seems very similar. It includes a block diagram, circuit diagram and a schematic. All of which I'm having difficulty deciphering regarding finding a way to diagnose the 0 current situation Im facing.

Any help would be much appreciated!

Tom

 

Hi,
It's nice to see a circuit diagram.
The power socket may be faulty it's a common denominator with mains and battery power.
Looking at the switch arrangement, can you switch the motor on, (I know it's not working), then examine/probe around the power socket with the power on. With any luck, the motor will work if you can hit the right contacts.
I've chosen the motor because it seems the simplest option.
Try bypassing the power socket is another option..
Good luck.............

 

Thank you for your recommendation. On a boombox or appliance that accepts mains and battery I would assume they would be most likely be using a relay (powered by mains supply) that tells the circuit to either use the voltage from the mains or voltage from the battery. I cannot physically seem to be able to find this relay. It is possible they could also be delivering a higher power from the mains rectifier and using a diode configuration to deliver the power from the higher potential power source of the 2. That I cannot seem to find also.

Regarding the power sock did you mean the figure 8 female socket itself could be faulty? I assumed it wouldnt be the fault as when Im powering the device I tested both mains lead with AC mains supply and also a DC 9V power supply directly on the battery terminals.

Also, regarding the power switch (which is bassically the Tape (off) / AM / FM switch I am having difficulty figuring out the pin out and exactly how this switch works. It has 40 pins in total which seems like a lot to me as its only doing 3 different functions. How does this switch work and how would i go about testing with ohmmeter for instance which modes (eg. TAPE, AM, FM) are conductive at certain switch states and non conductive at other switch states. I have not been able to find a pin out online

Model number of the switch is: 22195944


It is the 40 pin part in the center of this schematic from PDF posted in topic.
When I look closer at the picture below I notice sections of the switch.
8 in total, labeled: 1-8, ?-?, 1-1, 1-4, 1-2, ?, 1-5, 1-3
I am also unclear on what this is referring to. Any enlightenment would be great!


Upon studying the rectifier circuit more I can clearly now see this part below is the switch that determines if the positive potential comes from the transformed and rectified mains or the battery. However I am unclear of exactly how this switch works. The conductive moveable through is attracted to the potential that is higher? (eg. the DC from power supply will be slightly higher than the 6*1.5V D batteries) Sub question, is this used with diodes or some other mechanism with a copper plate that just is attracted to the higher potential like sort of relay?

 

Update: Last night I noticed on the where the relay is located there was dry joints. I reflowed them all and now I at least have some current flow - 65mA.



- the radio circuit seems to be working as it is tuning in frequncies (at least AM, I didnt try FM yet)
- the right speaker is working but is very very quiet even with the volume turned full. (still only drawing 0.6W)
- the left speaker is quiet and is producing a static noise sound

I cannot seem to visually see any other dry joints so given that the radio tuning circuit seems to operate fine my next point of call would likely be the 2 power amp circuits?

 

There is no relay. It's just a switch operated by the power cord.

 

There is no relay. It's just a switch operated by the power cord.

Hi. But what sort switch would likely be? A relay is an option, a transister is an option even a mosfet maybe. I do not know. Its a 1982 radio. It needs to be some sort of electrically controlled switch.

Unless it uses a higher potential and uses diodes on the positive path of each supply like in this configuration from my old physics textbook (an alarm clock circuit)

 

This switch working?

 

This switch working?

Hi. I think S901 controls the tape deck motor and it seems to be working other than the broken belt. But for now Im trying hard to get the sound output right from the radio circuit.

Last night, I probed the inputs and outputs for the pre amp ic on bother channels and everything looked ok. I am not sure if the voltage values where accurate but they produced somewhat of a mirror image on each channel and the preamp out puts were significantly amplified - so I do not suspect the ic is faulty

 

Have you checked the switch which convert between tape and radio?
If this switch is broken you will not hear any sound or poor sound
Try clean it with some akai spray and see the result

 

Have you checked the switch which convert between tape and radio?
If this switch is broken you will not hear any sound or poor sound
Try clean it with some akai spray and see the result

Yep I cleaned it with contact cleaner and worked it several times. I did so with all inputs.

So currently the problem with the radio is:
1. The sound is too quiet on both speakers
2. Only the right speaker has clear sound and the other static noise

I have probed the pre-amp IC and both outputs have a clear signal coming out so I now suspect the power amp or something inbetween the preamp and power amp. I'll probe the power amp tonight

Thanks for the suggestions!