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Old 16th Sep 2020, 4:04 pm   #1
de_light
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Default National Panasonic RQ548S - learning project!

New member here! I'm a beginner electronics enthusiast and I'm really enjoying learning more through fixing things. I've managed to pick up this nice 'boombox' transistor radio for next-to-nothing and would really appreciate your guidance in helping me fix it! I've never repaired a transistor radio before but have played around with basic circuits, 555 timers, repaired a linear power supply etc. I've also had a read of some great books from the 60's and 70s on repairing transistor radios to give me a grounding.

I've found the closest model (the RQ548DS) schematic at https://elektrotanya.com/national_rq.../download.html

So, the unit powers on - great! No short on a dim lightbulb tester first. The AM signal is received but no FM - just static.

First of all, could someone walk me through a 'block diagram' in effect for this schematic?

So, left to right:

First set of switches and transformer coils to select band and boost signal
Is TR1 plus the L3/L4/L5 the mixer-oscillator?
Then going through the automatic gain control and IF amps to the detector D2. Does this detector handle both AM and FM? If not, where's the FM frontend?
What does AOC stand for in D3?

The signal then seems to go to the AUX input jack and then I start to get a bit confused. There seems to be a pre-amp/AGC section for what I presume is the tape player bit. I think the signal then goes to S2-3 switches and then onto the meter amp and the AF amp but could be wrong.

Thanks in advance!
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Old 20th Sep 2020, 12:01 am   #2
orbanp1
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Default Re: National Panasonic RQ548S - learning project!

Hi,

Welcome to the forum!

That radio that you gave a link to its schematics does not have FM!
Looking at the pictures of an RQ548S radio it looks to be quite a bit newer radio, so the schematics of the QS version probably does not apply.

The D3 diode in the QS is a low voltage stabilizer, it gives bias voltages to the various transistors in the IF and mixer stages.
I am not sure what does "AOC" stand for.

Regards, Peter
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Old 22nd Sep 2020, 6:05 pm   #3
tri-comp
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Default Re: National Panasonic RQ548S - learning project!

Hi,

I remember these radio-recorders from my time as an apprentice.
They became my headache because no-one else would touch them and I had no choice
There doesn't seem to be any free schematics available for download on the web, so that complicates things.
Obviously the radio has FM and there may be various reasons for it not working.
I would certainly try to locate the FM mixer/oscillator transistor and check it out. It's a candidate to be the cause of a problem like what you describe.
There are two other potential problems in this radio.
Lots of resistors have been layered-in on top of the printed circuit board.
This is NOT a longlasting and stable solution imho.
Also, there are numerous through-hole platings in the PCB and they suck as well.
W/o the schematic, testinstruments and the required skills it will be a challenge, but then again, what you learn banging your head against the wall, is what is not easily forgotten.

Rgds,

/Torben

Edit:
Forgot to say I got the pic's off a Japanese web-page here:
https://blog.xuite.net/shyu0610/blog...9F%B3%E6%A9%9F
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Old 15th Feb 2021, 11:13 am   #4
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Default Re: National Panasonic RQ548S - learning project!

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=176601
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