Greetings,
I am trying to fix a Grundig Studio RPC 100 from circa 1978.
The main issue was with the FM section - the stations went in and out of tune. I traced the issue to the TCA 530 - it's output voltage was high and unstable. The pre-stabilisation part of the IC was not working, as the voltage on pin 14 was about 44V, instead of the 12.4V required by the schematic. I concluded that the internal transistor on pin 14 was not turning on.
Since the TCA 530 is very hard to come by and expensive, I could not justify changing it... Instead, I fitted a 5k resistor from pin 14 to GND. This dropped the voltage on pin 14 to about 17V. I would have needed something like a 3.3k resistor to bring the voltage closer to the schematic, but couldn't find one 🙄 Anyway, once I fitted the 5k resistor, the output voltage of the TCA 530 became dead stable at 30V. This sorted the frequency instability and FM has been working fine for the last couple of days.
However, I think there may also be an issue with the AM section. It is much quieter than FM. I am using about 1m of wire as antenna (the set does not have an internal ferrite antenna) and the tuning indicator shows 7 out of 10 on the strongest station, yet the output level is much lower than on FM... The is also a slight distortion - the sound has a bit of ringing to it 😀
I could not find anything obviously wrong with the AM section. Any ideas how to troubleshoot this further?
Thanks 🙂
I am trying to fix a Grundig Studio RPC 100 from circa 1978.
The main issue was with the FM section - the stations went in and out of tune. I traced the issue to the TCA 530 - it's output voltage was high and unstable. The pre-stabilisation part of the IC was not working, as the voltage on pin 14 was about 44V, instead of the 12.4V required by the schematic. I concluded that the internal transistor on pin 14 was not turning on.
Since the TCA 530 is very hard to come by and expensive, I could not justify changing it... Instead, I fitted a 5k resistor from pin 14 to GND. This dropped the voltage on pin 14 to about 17V. I would have needed something like a 3.3k resistor to bring the voltage closer to the schematic, but couldn't find one 🙄 Anyway, once I fitted the 5k resistor, the output voltage of the TCA 530 became dead stable at 30V. This sorted the frequency instability and FM has been working fine for the last couple of days.
However, I think there may also be an issue with the AM section. It is much quieter than FM. I am using about 1m of wire as antenna (the set does not have an internal ferrite antenna) and the tuning indicator shows 7 out of 10 on the strongest station, yet the output level is much lower than on FM... The is also a slight distortion - the sound has a bit of ringing to it 😀
I could not find anything obviously wrong with the AM section. Any ideas how to troubleshoot this further?
Thanks 🙂
Attachments
Check the currents/voltages on the pins as supplied here -
datasheet TCA530
This chip is associated with the varactor diodes check that overvoltage hasn't affected any one of them.
Also check the TCA440 voltages as this is the AM receiver chip-
datasheet TCA440
datasheet TCA530
This chip is associated with the varactor diodes check that overvoltage hasn't affected any one of them.
Also check the TCA440 voltages as this is the AM receiver chip-
datasheet TCA440
Expected, AM wire antennas need to be long, 20m or more ideally. That's why ferrite loop antennas are so handy as they have the same gain as a long wire in far less space. The length of the ferrite rod is important, the longer the better
for a strong signal.
For a typical AM station 20m of wire is only a fraction of a wavelength, 1m of wire is a few thousandths of a wavelength, essentially deaf to AM frequencies, more signal will be getting in via the speaker wires!