20.12.2019, 19:35
Ok, so I received my Loop antenna for 934mhz, looks to be well contructed but is pretty old, has never been assembled though.,
Also in the box was a brand new old stock stabo prs 933/934 radio, all in mint condition.
Looks to have been made around 1983 i think.
So After seeing what I had in the box, I assembled the power cord and microphone to the radio, checked the polarity to the power supply and switched on.
The first thing I noticed was I could not see the display, it didnt come on which isnt what should happen.
I could hear the static on receive but no display, the channel selection button did work as did the beeper.
I then thought I saw the vaguest wisp of smoke coming from the front of the set, immediately I switched off.
That was most concerning to me, so I decided to remove the covers and see if anything obvious stood out.
Its been probably 25 years or more since I had my nose inside anything electronic and these radios are fairly compact with a lot of smd components so I was a little concerned to say the least!
I couldnt see anything obvious on the pcb, no tracks burned, nothing shorted, the radio was keeping its secret.
Sniffing the pcb gave a clue, I could detect the smell of something that had become heated but not burned.
Hmmm, where to look?
Thankfully the circuit diagram I had sent from Stabo last week provided some help.
I had noticed that a small ceramic coated inductor appeared to be discoloured behind the front panel, so I looked at the location on the diagram.
Its connected to pin 1 of a transistor array which drives the display and from there it jumps off onto a 470 uf 10 volt electrolytic capacitor, the other side of which goes to ground.
Tracing backwards on the diagram, shows that the inductor is fed by an 8 volt line that comes directly from an AN 7808 voltage regulator.
Establishing that the regulator is the source of the power for the IC I turned my attention to the capacitor.
Using a needle type multimeter, set to ohms range of x10 I touched the leads and found a direct short. The diagram shows the capacitor in series to the power feed so it means the 8 volts is flowing directly to ground and turning the inductor into an electric heater.
A quick check of the capacitor in reverse showed the same result-shorted.
As you know, a quick and dirty check of electrolytics can be done using the charge/reverse charge method.
What I think has happened is over time the capacitor has broken down internally due to the electrolyte corroding the internal structure and allowing the short to occur.
Since the 8 volts has been directed to ground through the capaictor, it also means the transistor array has no voltage to drive the display, I do feel happy to have found the issue so quickly!
Im quite pleased with myself so far ( hopefully no other issues will be found) as its been a long time since I had anything to do with electronics of any kind.
I have ordered up a load of capacitors ( Rubycon from Japan) to replace all the other electrolytics inside the radio also. I shall also replace all the capaictors in the other Stabo radio too as its of a similar age.
I will report my findings when I have replaced the defective component!
Tomorrow, I may assemble the loop yagi and take a few pictures for you all.
Also in the box was a brand new old stock stabo prs 933/934 radio, all in mint condition.
Looks to have been made around 1983 i think.
So After seeing what I had in the box, I assembled the power cord and microphone to the radio, checked the polarity to the power supply and switched on.
The first thing I noticed was I could not see the display, it didnt come on which isnt what should happen.
I could hear the static on receive but no display, the channel selection button did work as did the beeper.
I then thought I saw the vaguest wisp of smoke coming from the front of the set, immediately I switched off.
That was most concerning to me, so I decided to remove the covers and see if anything obvious stood out.
Its been probably 25 years or more since I had my nose inside anything electronic and these radios are fairly compact with a lot of smd components so I was a little concerned to say the least!
I couldnt see anything obvious on the pcb, no tracks burned, nothing shorted, the radio was keeping its secret.
Sniffing the pcb gave a clue, I could detect the smell of something that had become heated but not burned.
Hmmm, where to look?
Thankfully the circuit diagram I had sent from Stabo last week provided some help.
I had noticed that a small ceramic coated inductor appeared to be discoloured behind the front panel, so I looked at the location on the diagram.
Its connected to pin 1 of a transistor array which drives the display and from there it jumps off onto a 470 uf 10 volt electrolytic capacitor, the other side of which goes to ground.
Tracing backwards on the diagram, shows that the inductor is fed by an 8 volt line that comes directly from an AN 7808 voltage regulator.
Establishing that the regulator is the source of the power for the IC I turned my attention to the capacitor.
Using a needle type multimeter, set to ohms range of x10 I touched the leads and found a direct short. The diagram shows the capacitor in series to the power feed so it means the 8 volts is flowing directly to ground and turning the inductor into an electric heater.
A quick check of the capacitor in reverse showed the same result-shorted.
As you know, a quick and dirty check of electrolytics can be done using the charge/reverse charge method.
What I think has happened is over time the capacitor has broken down internally due to the electrolyte corroding the internal structure and allowing the short to occur.
Since the 8 volts has been directed to ground through the capaictor, it also means the transistor array has no voltage to drive the display, I do feel happy to have found the issue so quickly!
Im quite pleased with myself so far ( hopefully no other issues will be found) as its been a long time since I had anything to do with electronics of any kind.
I have ordered up a load of capacitors ( Rubycon from Japan) to replace all the other electrolytics inside the radio also. I shall also replace all the capaictors in the other Stabo radio too as its of a similar age.
I will report my findings when I have replaced the defective component!
Tomorrow, I may assemble the loop yagi and take a few pictures for you all.