Hi,



Could anyone help me identify this capacitor. The pic has a 9011 Transistor and a capacitor (foil polystyrene? )
It's marked 5 0 and 20 on the body. Does that mean 50pF @ 20V or 20pF @ 50V ?

One leg of the Transistor (base) is broken and it needs to be replaced. In the process I would like to replace the cap
also.

The radio is a noname model from years ago - the cheapest chinese transistor radio from seventies, but I'm affectionetely
attached to it and would like to get it going again.

Regards

DenisM

Prolly neither, if the set is powered by a nine volt Transistor battery.

If it were me, I would simply re-attach the broken Transistor leg, and if the set then worked adequately, leave it at that.

...you're correct; that is a polystyrene cap. Unless you know there is something wrong with it, I'd leave it alone. They are not particularly failure-prone and are famous for their stability...

They're also famous for being very vulnerable to a wayward soldering-iron! Be very careful around those in tight quarters, it's easy to melt a big crater in the side with a single swipe of a hot iron tip.

--Bob

I'd guess 50 nF @ 20V.

Capacitor are Stiroflex 50 pF 20V . That was spectial for RF Stages.

Thanks for those comments. Very helpful.

I'll try the 'reattach the leg' option first.

My solder skills are a little shaky at this age, tho' I did assemble a Softrock receiver last year -
mostly SMD stuff.
I'm retired now and taking an interest in radio generally.
I did an amateur exam years ago but never got a licence.
Life etc intervened..... and technology diverted my interest into
computers /CPUs/ RAM/ROMs etc......

Analogue radio might be fun

Regards

DenisM

...a 50nF (.05µF) polystyrene would be a LOT bigger than that, even at 20V...and, IIRC, that's not actually 20V...the red band represents 160V (I think it was yellow=25V, black=63V, red=160V, blue=630V)...

Oh! Oh! Worst nightmare !
I re-attached the broken leg and it worked

but on re-assembly of the case a lead detached from the capacitor !!!!
I guess it had suffered too much

I've ordered some replacement capacitors and TRs . The set is not really worth too much,
but sentiment defies logic.

At least I now know it should work again when both are replaced.

Thanks again.

DenisM

God news

I replaced the 9011 with a BC635 (only possible match on hands) . I replaced the cap with a 50pF disc ceramic.
Wow ! It works -- better then ever. I guess the TR leg was fracturing over a period and affecting performance.

The BC635 has a different outline, but with a bit of leg twisting, it was possible to line it up.

Presumably it will throw the alignment out a little ? No matter - it works.

I would like to get one with a schematic, so that I could apply a little more logic to the process .
Looked at Ebay - Are old transistor radios made of gold ? Prices are horrendous. I guess luck is a big
factor in getting a good model for reworking ?

Regards
Thanks again

DenisM

Glad to hear that you got it working, and well at that.

If the Transistor is in the audio stage, it won't affect IF alignment.

Regardless, you can always try adjusting the two or three IF cores (not the red one though; that would be the oscillator), very slightly, as in less than 1/4 turn one way or the other, to see if gain can be improved. If you do, use a weak but non-fading station as your source.

It would help if i had a schematic - but no chance of that.

I reckon it is**(edit) the local oscillator TR. The base is connected to centre pole of the wave switch (seems to switch in /out the antenna coils), Collector to red Toko coil (two point side) , emitter connected to ground via 2.2K resistor.
The LW band is vary active and seems lined up ok. The MW band is hard to suss out. In Ireland we no longer use the MW band, so there are no local stations. I'm getting a no of French station at night. Seems normal .

I may trying a tweak as you suggest, but carefully.

**edit. -- I realise it can't be the osc TR ...... It is the first TR after the antenna. Is it's output mixed with the next stage TR osc ?
Struggling to learn here


DenisM

Hello DenisM & welcome to ARF !

Glad you got your radio going---could you post its pic ?

Could that be the converter transistor which functions as both the LO & mixer ?

John

How are we gonna get you a schematic on this, lol? We have nothing to go by.

First, how many Transistors does it have? If it's five or six, xrhonda91's suggestion would seem to be spot on, with one Transistor doing both oscillator and mixer functions.

Next, what exactly are you trying to accomplish? If they no longer use the MW band where you live, and you say the LW band works fine, then what are we trying to do here?

Dennis, Ireland huh? Guess that explains the use of a BC type transistor.

That polystyrene foil capacitor you mention is almost certainly part of a tuned circuit, as you said, part of the LO. If you can give us a really good photograph of it's guts, you never know

The usual design for a cheap transistor radio is for the three pin side of the red coil to be across one section of the VC, a tapping (the centre pin of the three pin side) connected to the emmiter of the transistor via a 50pf Cap, and then to ground via a 2k2 resistor. The collector of the transistor connects to one of the pins on the other side of the red coil can, the last pin then going off to the first IF (almost certainly the bottom left hand pin). This is called an "Autodyne" mixer/oscillator stage. There is often some switching involved here since for long wave reception, a capacitor is usually switched into the circuit and goes parallel to the LO section of the VC. This is known as a padding capacitor, its effect is to lower the frequency of the LO. This padding capacitor has to be pretty accurate, and it has to be a stable design - polystyrene, silver mica or NP zero ceramic. I suspect that this little polystyrene cap you've described is that padder. FWIW NPzero (or NP0) ceramic disc caps have a black painted band along the top of it and they are very stable, easily as good in my experience as polystyrene.

Give me a yell if what I've said is confusing.

edit:
VC = variable (tuning) capacitor
LO = Local Oscillator (red can)
Ist IF = yellow tuning core
2nd IF = white tuning core
3rd IF = usually black, but I've seen green. Doesn't matter, it'll be the one that isn't red, yellow or white

You'll probably find that almost any transistor radio circuit diagram will help in the repair of this radio, to be honest they're all pretty much the same - different board layout, different component values etc but a radio is a radio is a radio, understand what each section does and you won't need anything other than a generic diagram.

What we're doing here now is gossiping , since the radio is now working : )
Oh ! and educating Denis

1st photo is the pcb :

2nd photo was taken from ebay of a similar model with original box.
Mine is green - color of the vol control knob.
Like I said, it's the cheapest transistor radio made ever. I bought it because the colour/style reminded me of my first transistor given to me as a birthday present - back in the day ....

That one was a Standard/Waltham 8 transistor - more solid/robust than this one.

Dennis, no PCB pic mate.

I'm sure the moderators will tell us if we do wrong - in which case I will PM you my email or facebook and we'll carry on there.

However, as you've said, your radio is now working so the original thread subject is now solved.

Mods please advise?

The ban on gossiping applies mainly to the Classified section. Gossiping here about transistor radios is OK.

Dave

Thank you sir

Good news

Here's the photo :
reduced pixels W & H