Dear all,
On the past weekend, having everything needed in hand, I made surgery on the patient.
- Replacement of BC328, which proved to be toasted by my component tester (as guessed by forum members !!)
- Replacement of C275 (220 uF), which 'leaked' a little too much> 4% but still within the correct spec: 221uF measured
- I soldered back in place the output cap (800 uF on the schematic but measured at 1000 uF
). Impossible to find a spare and with less than 1% leak measured, it doesn't seem to be the culprit for final stage death.
After checking everything, I put the item under tension (without L.S). I measured the common transmitter, to see if something has moved there: 8V (!!) which is a tad high (before surgery it was less than 6V). A little surprised, because everyone knows that we must have about U / 2, as I am on a battery around 6.1V
Loudspeker hooked, (vintage Grundig bookshelf circa 1973, 4 Ohm) in place, I turn on the knob, and the guy emits the characteristic hissing of the FM band between 2 stations! Wink
MP3 on the phone via the DIN input.
It's just bearable first, then better and better as you go.
Having put back into service some old stations, I know that it takes a little time for the items to sound 'right', especially when we swapped some components.
After 15 minutes it's really good Shocked. Many (too much) bass, which is not surprising knowing that the output capa is on the high side, but frankly a beautiful, dynamic yet clear sound. At this time, I was the happiest man on earth....
I tested several tracks, it 's out really well. And OUCHH, after 30 min, on a classic track, with violin ... big skid, no more amplitude, big distortion. I immediately thought that the final floor just blowed (once more?) I immediately checked voltage at the common emitter: 10.5 V ....
I turned off, the time to try to understand, and to make continuity measurements in search of a possible short circuit between the TO-9 boxes and the heatsink. I turned on and measured almost Ubat on the common emitter (and of course no more sound this time). Fingertip test tool... Everything is at room temperature, nothing overheating.
Put the radio in the freezer for 1 hour, no improvement, nearly Ubat on the common emitter.... RIP NOS transistors....
The next day as everything (power, loudspeker) was still in place, I decided to redo all the measurments to write a post on the forum. As I powered on ... the radio started to sing. Clear & Loud. ! scratch
I put the red lead on the common transmitter and I read 8V, as the day before ... but not for long. 8.1 ... 8.2 .. 8.3 ... The voltage climbed slowly but surely, whatever you do. Power knob position, input or not.
At 10.5V I stopped the experience, and turn the button to OFF.
Still no overheating or evidence something is going wrong.
In short, the station works, but what can explain this inexorable drift of the output stage ...?
- I imagine that it would be necessary to measure the idle current on the DC power line to the Ge transistors, to have a better idea of what's happening ...?
- I also did not measure the voltage at the bases of AD156/AD157, in operation
Any idea of what's happening ?
Thanks in advance for your contribution
Cdlt
Sebastian