National Panasonic RF-100L - FM problem

[Skywave]

Mark:- Japanese transistor prefixes: my understanding is same as yours - which I know to be correct: 2SA, 2SB = germanium; 2SC, 2SD = silicon.


Mod Edit: This is not true - the scheme does not differentiate between germanium and silicon.

2SA: high-frequency p–n–p BJTs
2SB: audio-frequency p–n–p BJTs
2SC: high-frequency n–p–n BJTs
2SD: audio-frequency n–p–n BJTs

From https://en.wikipedia.org/wiki/Transistor..._.28JIS.29

The link in post #7 originally pointed to that data, but the link no longer takes the reader straight to the scheme because the page has changed since then.


Anyway, to your Panasonic radio problem: I've read what you have written. If I understand that correctly, with it you can receive BBC R2, albeit a bit weak, but cannot receive other stations on VHF FM from your (roof + amp.?) aerial. Yes, obviously you could have a low gain 'front end' transistor or a biassing problem with it. But have you considered RF misalignment of the RF stage - misalignment either on account of someone twiddling or a component fault that has upset that alignment? You'll know what to do to check that out and you have the necessary kit to do it, (I don't believe in teaching my granny how to suck eggs ), so . . . .
OTOH, perhaps I've misunderstood / misread something (not unusual, plus it is now late at night - I really should be )

Al.

[Mark Hennessy]

OK - I finally got the chance to get back to this one. Sorry it's a bit of a long post...

I decided to try a BF184 - on grounds that I had one, and have seen it used in this position on similar sets. Of course, it's electrically equivalent to the Lockfit BF194/5 transistors seen in UK FM tuners throughout the '70s...

It was fun working out how to install it, given that the pin-out was completely different. And "fun" because I was only half-sure I'd worked out where the connections should go, based on my reverse-engineered schematic and my assumption it was working in common-base mode.

Once fitted, the reassembly began. It took nearly an hour before I was ready to test it - honestly, I just can't describe how bad this set is in this regard. The PCB fits into half of a metal box, and this section covers the front (tuning capacitor end), sides and top of the PCB. The top of the box has a number of feed-throughs that connect to flying components or lengths of TCW. It's not 100% clear where each of these should go, and annoyingly I hadn't taken a good picture before I took it all apart.

Then the next section of the box has to go on. This clips and screws back in place. You might think that one could leave this off for testing purposes, but there are several lengths of braided wire that you must connect back up before you can test it, and one of two of these are soldered to this lid.

Finally, the wires from the IF PCB need to be reattached. This was easier because my standard practice is to snip them close to the terminal, leaving just a bit of the insulation in place so that you know which wire goes where - assuming the colours are all unique.

At this point, if it didn't work, it was going straight back to the owner. Fortunately, all was well! The sense of relief was indescribable

So, the final reassembly can begin. The FM tuner is secured to a bracket which also holds the AM capacitor - the tuning pulley has a gear which drives both tuning capacitors, and it has to be removed before one can remove the FM tuner. But once you've done this, you'll find that the tuning capacitor still won't budge! That's because the bracket needs to be removed from the chassis - there are 3 screws that need to be undone from the front. Helpfully, one of them is hidden under a layer of black felt that is glued to the front of the chassis (these sets have a "pointer" that indicates on the tuning scale which band is in use).

Each tuning capacitor has a metal anti-backlash gear on it, and both these need to be tensioned before the plastic pulley can be put back on. Once you've found the tiny springs, which seem to leap out at every opportunity! Of course, despite my careful efforts, the dial cord fell off at this point, so it had to be restrung later.

Once I was happy that both the tuning capacitors were being driven correctly (it's easy to out by a tooth or two), I was able to fully test the chassis. Fortunately, there were no other faults anywhere else, apart from a noisy volume control (switch cleaner did the trick). The chassis is lowered into the case, and 7 self-tapping screws secure it back in place. It's interesting that Panasonic were using red screws that early - I'm only really familiar with their stuff from the 1990s, which is where I'd first seen this...

Re-attaching the dial cord to the pointer is fun. Almost unbelievably, as the pointer stays in the case; you have to unhook this before the chassis can come out. There is almost no space to do this, and ideally you need a hook or similar - perhaps something that a dentist might have? Achieving any sort of accuracy is tough, but I think I did OK in the end. The cord is secured with a dab of paint.

If batteries leak in these sets, a metal bracket at the bottom of the rear panel bears the brunt of this. It wouldn't come out (someone before me had tried to drill out a screw with no success), so I cleaned it up as best I could, and used an M3 tap to clean up the threads on the 3 holes that take screws from the bottom. The original screws that held the back on were either lost or corroded, so I found some modern screws that were the right length for the bottom and back. Despite the age, they were of course metric, which makes that much easier.

All in all, one of the worst sets I've ever worked on. I used to think that the Roberts R707 was bad, but this is just awful. Worse than some of those complicated German sets. Normally, Japanese stuff is tricky at first, but gets easier once you've worked out how they "tick", but this one just kept on fighting. If it had been any more involved electrically, it wouldn't have been fixed.

The sound quality isn't great either. These sets have two loudspeakers wired in parallel with no crossover. The "woofer" is 4", and the "tweeter" is 3" with very stiff suspension. The former doesn't produce any bass, and the latter doesn't produce any HF to speak of. In the available space, a 7" by 4" elliptical unit - such as what the aforementioned R707 uses - would have done a much better job.

So, I hope this tale of woe will help anyone who might be considering working on one of these. It's certainly a challenge. One for a collector, but not one to take on as a paying job. What should I charge the customer? On the face of it, I only changed one transistor. But I reckon that I probably spent something like 8 hours on it, all told. Even if I was more familiar with the set, I don't think I could have done it in much less time. Having the service data would have helped with my confidence in changing the transistor - OK, my diagnosis was pointing pretty firmly to it, but I don't like changing things "blind". Knowing that the passive components in Japanese sets tend to be reliable did help me to tip the balance in favour of the transistor, but even so...

Big thanks to Refugee (on UKVRRR) and Lawrence here for the "2SC" tip - I still find it incredible that it's silicon in a set of this age...

All the best,

Mark

[pwdrive]

Hats off for getting there Mark, if it was on my bench it would have been put under the hammer assuming it made it that far.

Lawrence.

[Skywave]

Mark H., quote: "All in all, one of the worst sets I've ever worked on. . . . Worse than some of those complicated German sets." Unquote.

Indeed: I know exactly where you're coming from with that comparison!
Recently I had a real uphill struggle with a Sony reel-to-reel tape recorder (see another thread here) - but your tale seems much worse. Although I consider that I can be tenacious and determined with obstinate, uncooperative bits of electronics, I honestly feel that that Panasonic would have beaten me - so well done.

If another one ever comes your way, ("Would you have a quick look at this, please?") I think we can rest assured of what your response will be!

Al.