Roberts R500 Tin Can

[Bigears]

Hi Guys, here we have a Roberts R500 Tin Can which I am trying to check and test it, does anyone know the best way to go about this, as I am still in my very early days of electronics, maybe I should have said it is inside the Tin Can.

The 12v cap seems to be leaking apart from that it looks ok. any help would be appreciated.
Regards Ken

['LIVEWIRE?']

The most likely fault in that I.F. module is that one or more of the AF117 transistors are leaky - see the sticky thread about 'tin whisker' problems - these affect all AF11x and the earlier OC17x series transistors. In the past I've replaced AF11x devices with Russian transistors, whose type no. escapes me at the moment, but which are usually available from eBay sellers at low cost. What makes you think the Electrolytic Capacitor is leaky, b.t.w.?

[Bigears]

It shows signs of leaking like a dark yellow wax leaking

[Andrewausfa]

That's solder flux

[Graham G3ZVT]

Originally Posted by Bigears

It shows signs of leaking like a dark yellow wax leaking

In this context, we are talking about electrical leakage from the transistor germanium "chip" to the can of the transistor which is typically connected to the common supply rail by a fourth lead on the AF11x device.
Please read this thread

[Herald1360]

Originally Posted by 'LIVEWIRE?'

In the past I've replaced AF11x devices with Russian transistors, whose type no. escapes me at the moment, but which are usually available from eBay sellers at low cost.

GT322b in English.

Prices vary from about £1-20 each in small quantities to about 35-40p in 100 up lots.

Beware of the (very) different pinout!

Some more info: https://www.vintage-radio.net/forum/...d.php?t=121961

['LIVEWIRE?']

I'd forgotten they were GT322b's, Chris as it's quite a while since I used them. They have the advantage of long (25mm)lead-outs, which means the leads, sleeved if necessary, can be crossed over one another to reach the required points on a PCB.

[poppydog]

All my gt322b leads are short, I've never seen these types with long leads. Can anybody post a link?

poppydog

[Davewantsone]

I have repaired quite a few of these "IF can" Roberts Radios. If I have a problem I replace all the AF series transistors with BC557. I have even used them in the R200. I stick the AF transistor can over the BC557. Just leave the screen transistor lead unconnected. I have taken out good AF series transistors so I can use them as RF amp/ mixer osc. for faulty VHF radios.

[Herald1360]

Originally Posted by poppydog

All my gt322b leads are short, I've never seen these types with long leads. Can anybody post a link?

poppydog

No help with 25mm leads. My stash have leads of 14mm. Long enough for sleeved lead crossing exercises on PCBs where no advantage has been taken of the AF11x longer leads. Easy enough to extend them for the likes of TR82s, the join can be hidden in the sleeving.

[Radiocruncher]

Hi Ken

Usually changing the 2 AF117 transistors brings these back to life. I use either AF127 or the Russian GT322B. They are tricky to change as the pinout is different and it’s a tight squeeze working between the IF transformers. Also the board traces are not very well bonded so it’s easy to damage them.

Good luck

Graham

[Bigears]

Zapping the short circuit, I read by joining 3 leads of the AF117 resistor you can Zap it using a Cap, it also mentions naming the lead a b c I have removed the AF117 and one has a red sleeve and one white and two without so how can I tell which of the 3 I should join?

Regard Ken

[AC/HL]

https://www.radiomuseum.org/tubes/tube_af117.html
It's E B C that are to be joined together securely, and connect the capacitor between here and S.
Note the wide spacing between C and S.

[Bigears]

Thanks, Bill very helpful

['LIVEWIRE?']

Personally I never bother zapping AF11x devices, but replace them with GT322 as noted in posts 2 - 6 (Thanks for jogging mu memory, Herald 1360).

[Graham G3ZVT]

Hi Ken before you zap them check if there is any resistance reading between screen and any of the other wires. Tap the can with a screwdriver as you watch the meter because mechanical vibration can change the leakage.

A few weeks ago I zapped the mix/osc AF117 in my Bush TR2C. It I had a reading of about 50k ohms and it went to infinity after treatment with 40v from a 220uF cap.

If you already have an infinite reading, there is no point zapping.

This was the secondary phase of the tin-whisker disease. 15 years ago I fixed the same transistor by disconnecting the screen lead, but now the whiskers have engulfed more than one electrode, needing more radical treatment.

Incidentally, I zapped it in darkness expecting to see a flash from the base, I didn't see one.

Livewire, I take your point about replacing these transistors, and when they are sealed in a tin can out of sight I would probably do that, but in a TR82C they are all too visible, and in some perverse way, I find them worth saving, because they had the audacity to try to fail!

['LIVEWIRE?']

Fair comment, Graham, but I have my metaphorical 'service engineer's hat' on when doing these jobs, which, in these circumstances, means that I replace faulty components with the nearest available eqiuivalents, and do not worry if the replacements have different type numbers or look different from the originals -sorry if that sounds sacreligious!