R600 Transistors.

[Joe]

Just a quick (hopefully) question.

I'm about to order some components and while I'm at it may as well lay in some BF450s for if I'm ever brave enough to tackle properly sorting out my R600.

What I don't however know is how many I need, I intend replacing the lot while I've got the module open.

Thanks
- Joe

[tiv48]

is this for the af114/ af115. if so you will need 3 itgood fun taking the modual apart even more fun taking the inside out of the old trasistors good luck
rob t

[Joe]

Thanks Rob. Like I said in the OP it'll be a job for a day when I'm feeling brave!

[Joe]

I've just been fitting the transistors and have managed to maintain my 100% (lack of) success rate. Of course I don't KNOW that the AF115s were the problem in the first place. I bet I've connected one or more of the BF450s wrongly as the insides of those modules is far more miniaturised than I'd expected and I couldn't really tell how the originals had been wired.

The only reason I'm posting this is to report an interesting effect. As I was scanning through the wave-bands I discovered a narrow point at 100Mc/s (on the scale) which blocked Radio Two on my main set.

If anybody wants any R600 bits then give me shout as I'll probably end up ripping the 'radio' parts out and just keeping it as an amplified speaker as for all that they don't work they're still pulling current and reducing battery life.

Cheers
- The Wireless Wrecker of Olde Sheffield Town

[Mark Hennessy]

Hi Joe,

Don't give up just yet

I've not done an R600, but recently summoned enough courage to tackle an R707 (now that was a job!). I know what you mean about those modules - I've still got pretty good close-up vision, but keep a x7 eyeglass to hand for those sorts of things!

You ought to be able to find a schematic for the module. It might not have component values on, but you should be able to work out enough from the diagram to double-check your work. It's not easy, but removing the module is much easier than in an R707

If the worst comes to the worst, I might have a spare module - or be able to fix yours for you...

As for the effect you noticed at ~100MHz, you've now proven that the local oscillator is working (R2 + 10.7MHz = ~100MHz)

All the best,

Mark

[Joe]

If we ever do get the chance to meet up I may well let you have a pop at it. They've got an evil reputation as regards servicing and I can see why. A big part of the trouble is the old problem of PCB tracks that start lifting as soon as they see a soldering iron approaching so repeatedly taking it to bits to try and sort it out isn't really an option as it'd destroy it in the process.

It's more use to me as am amplified speaker if it comes to a point but I was ordering some stuff anyway so added the transistors mainly out of curiosity as to whether I could do it or not.

I think it'll be a good while before I dare touch that Ekco I got a few weeks ago.

- Joe

[Steve_P]

You said in an earlier post that you didn't know it was them. And you also add that you noticed an effect which shows that one of these little beasties in that can was OK.

I have wondered in the past if this was some sort of batch problem, as I have a few sets here with the original transistors in and they are OK! Just a thought.

Have you injected a signal at the volume control (Middle pin) and see if you can hear it. Either a signal generator or an I-Pod type thingy will do? The output transistors do tend to fail on these. Easier to change as well.

Prove a fault before you start swapping bits. Don't just rely on hearsay or other people, check for yourself. Oh, and try heating the transistor up. Sometimes works. Or cut the screen lead...

Cheers,

Steve P.

[Mark Hennessy]

Hi Steve,

The effect he noticed was caused by the FM local oscillator, which is separate to the IF module with the 3 dreaded AF11x transistors...

Cheers,

Mark

[Joe]

Thanks for the replies which I've only just noticed.

It was after changing the transistors that the interference effect was noted. Given the amount of time I've fiddled about with it in the past I can't believe that it was doing it before.

The audio stages are fine as I've been using it as an amplified speaker for months. I finally set about changing the AF115s partly out of interest and also because something was guzzling current - 85mA quiescent.

As for proving a fault before tackling it there didn't seem anything else left to try; it seemed no less logical to change transistors which have a bad reputation than to rip out Hunt's capacitors on sight. Getting to the module was much easier than I'd expected but I wouldn't have touched it had I realised how tightly packed the inside was; there was no chance of getting at the screening leads to cut them and the PCB tracks were so delicate that it seemed to make more sense to hit it hard and hopefully sort it in one go than constantly keep dis-assembling and re-assembling it.

I've got a set of schematics for those Mullard modules but as the one in the radio hasn't got a model number on it's a bit difficult knowing which it is as none of the pin-outs seem to match.

There's nothing really lost, apart from a few bob on the BF450s, as it still works as an amplified speaker and the quiescent current is down by about half. It's not in great cosmetic condition either so isn't a disaster to our national heritage.

My current plan is to remove the chassis and put an LM386 based amplifier in the case using the original speaker. All connections and controls will be on a new top panel so this modification could be reversed. This would certainly be more use to me as I've got perfectly good 3-band radios standing spare but a battery operated active speaker is something I don't have.

I'm inclined to regard the chassis as a source of spares either for my own use or anybody else's. Besides whatever it is that's stopping it from receiving anything the aerial is broken, the handle tatty and the main switch bust - I've got 'round that by utilising a pair of spare contacts on the Tone control. That said my natural bloody-mindedness will mean I'll probably carry on fiddling and either kill or cure it.

As for the Ekco, Jamie, that's dead on VHF and the chassis looks horribly congested and unfortunately confidence is waning instead of growing. That said fears about the 'high' voltages involved seem to have left me and I'm now no more nervous in that regard than with any mains powered-equipment.

I've had some success as I've already prototyped the amplifier alluded to above and I did fix my mate's Vidor. I tried to buy a DAC90A earlier thinking that if I couldn't fix that I really should hang my soldering iron up but didn't manage it - it was the old story of it going for a price I'd pay but not when the postage is added on. This was probably for the best.

Sorry for rambling.
- Joe

[Mark Hennessy]

Hi Joe,

Just seen this; can't say I blame you for being a bit fed up with this set by now! The R600 was hardly Roberts' finest hour (the R606 performs much better), and they are pretty plentiful, as you say. I've restored one of them in the past for a friend, and didn't enjoy it one bit - the mechanical construction is pretty horrible. But then, I've been spoiled by another UK brand of portables

An active speaker is really very useful, and it's worth putting in a bit of effort to make it sound nice - I built one some 10 years back from free/scrap parts and ended up using it as a bedside radio in conjunction with a walkman-type radio. I don't know if you'd be able to get some ideas from it? http://www.mhennessy1.f9.co.uk/activespeaker/index.htm

The only change I'd make would be to have a little more gain - indeed next time I take it apart, I plan to add a switch to add 20dB or so to the input amplifier. Obviously mine isn't battery powered, but the amplifier is basically floating, so it doesn't cause hum loops.

Look forward to seeing a write-up of the project soon,

Mark