Roberts R300: Do we really need Germanium Transistors in our repairs?

A cheap and cheerful Roberts R300 came my way so I decided to see if it would still work with the original Germanium PNP transistors replaced by Silicon options. I already knew the answer but let's have a look anyway.
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[@Codeeze]

Good video, and now in most respectable condition. Nice one Andrew. I have a very soft spot for the Roberts R300. Very possibly my favourite Roberts model ever. Replacing the AF117 transistors with silicon is now part of my standard servicing routine. I think I've zapped my last germanium. Thanks for the nod, and for showing my portfolio photos - taken just last week don' cha know...

[@Andrewausfa]

Yes nice radios. Enjoy those photos? :)

[@Steveuk405]

Sometimes heating them up can have the same effect. The Silicon trick is well known and the problem with with Tin Whisker Zapping is the only ones to be zapped are the ones that do actually conduct so when others are still growing inside and will make the transistor fail again in the future. You do sometimes have to change the biasing though in some cases.

[@Andrewausfa]

Good stuff Steve and yes a well known thing but good to actually show it as well

[@michaelstaley2241]

Excellent work..

[@andygozzo72]

you cant always just drop in silicons, but it depends on the circuit, if it seems to work as is, you may find the gain drops sharply as the battery voltage drops, if you dont alter the bias, ideally always increase the bottom bias resistor, not decrease top one, to get the dc collector current same as with germaniums,

[@andygozzo72]

i have a later mullard IF module that uses silicon pnps as from factory instead of AF11x types, i'll have to try and find it again and see what types are used, and draw out the circuit to see what resistor values and ratios are used

[@allthegearnoidea6752]

I’m not sure whether or not you were joking about the Mullard hammer. Great video and interesting on the Germanium transistors. I don’t think anyone has covered using alternative transistors so fantastic info.

[@Andrewausfa]

The Mullard hammer is all true, yours for only 5s 6d :) Thanks Chris.

[@Radiocruncher]

Interesting stuff Andrew. Germaniums certainly are getting very expensive now. I’ve heard of people using silicon transistors but never tried it myself. Cheers Graham

[@Andrewausfa]

You should try it Graham. About £4.50 a pop less than AF127s :)

[@SoddingaboutSi]

I like the R300, despite it being a Roberts. Interesting to see it working on Silicon without bias adjustment.

[@Andrewausfa]

Yes there is that caveat. They're much better to work on than the R200 even though they look the same

[@driverfilmsshortwavechannel]

Im starting to avoid radios with AF117s. Good work on The Finest of All Portables!

[@Andrewausfa]

No need Don, there are work arounds as shown! :)

[@berlewi]

Superb and really interesting video! Many thanks!!

[@Omegaman1969]

Germanium radios are far more efficient that silicon. You can run one for months on 1 battery. My modern Roberts eats batteries.

[@Andrewausfa]

Hi Simon, yes modern DAB sets use a lot of current and are very inefficient. Much better of with a vintage set :)

[@Thecando]

Does replacing the germanium transistors with silica effect the efficiency?

[@Omegaman1969]

@@Thecando It would but you would have to change a lot of components, you can’t simply swap a germanium transistor for a silicon one.

[@GeorgeChristofi]

A nice mix of geraniums. Maybe next time we can try petunias or nasturnums

[@Andrewausfa]

I like a nice Petunia!

[@zx8401ztv]

And i thought a bit more base current would be needed, perhaps different radios will be more picky. Looks much better now :-D

[@Andrewausfa]

Yes these sets are pretty tolerant, it is a relatively simple set and I wonder if the same would happen with more IF stages etc

[@zx8401ztv]

@@Andrewausfa Well the capacitance between the pins could de-tune the stage but that's easy to reset the i.f transformers :-D Getting the gain correct to boost up the low level signal could be tricky. if too high a gain the next stage would be overloaded and the filtering would widen. Also it could self oscillate (Annoying to stop it). If the gain was too low, you may only hear really strong stations and the filtering may become too narrow. Anyway i'm just saying the obvious that i'm sure you know way better than i do. My noggin isn't clever enough to cope with design calculations lol.

[@andygozzo72]

its not base 'current' that needs changing, but voltage, germanium junctions have a 'starting' voltage of about 0.25, silicon about 0.6 so if base voltage to emitter supply rail is about 0.7 or less, the silicon one wont work or only barely work, and stop if the battery voltage drop off with age...

[@zx8401ztv]

@@andygozzo72 You are right but base current will change with voltage. Not in a linear way though. i use 0.65 to 0.68 volts as a crude check for ordinary stuff. Germanium transistors are horrible leaky devices, not my cup of tea. Just my personal opinion of cause :-D

[@andygozzo72]

@@zx8401ztv the circuits used in radios mostly feed the base with a voltage from a potential divider across the supply, the base 'current' in these cases is irrelevant, its the voltage that matters, if base sits at say 0.4v to the emitter or emitter supply, a silicon transistor simply wont work, if pure current fed such as single resistor from base to supply line, the base turn on voltage is irrelevant in this case and silicon should work as is drop in , yes, germaniums can be problematic, especially the ones with metal can, metal inner (glass type like oc44 less a problem but can still fail, i've had one go resistive in a r200) but that was all that was made and available at the time and circuits usually took 'parasitic' dc leakage into account, resistor values being lower that what can be used with silicon , it should be possible to convert any germanium circuit to use silicon, but bias alterations will be needed in many cases... output stages may be more an issue as the higher turn on voltage also means higher losses and output wattage may be reduced, especially if the supply voltage is fairly low, output stages will definitely need bias altering or you will have severe cross over distortion

[@davidroberts965]

Great video Andrew I have used BC557 transistors with no issues I your not bothered about authenticity why not use cheaper alternatives

[@Andrewausfa]

Hi David, exactly my thoughts. They would have no cared a jot about 'originality' back in the radio reapir shop days, the imperative was to get it working. I maybe wouldn't try this on a really rare or mint set however!

[@mikepasko7493]

GREAT JOB

[@martincaldera386]

I´ve a nice Hitachi radio it´s a Wh-859D with Germanium transistors that is failing, sometimes there is no sound at all. It was constructed using these transistors numbers: 2A A 131 (Freq. Mixer) , 2AS A130 (Local Oscillator), 2S A12 (.F. Amp.) , 2 S B75 (A.F. Amp.) , 2 S B77 (A.F. Amp) , and 2S B156 (Power Amp.). I´d love to change them to Silicon transistors. Which easy to find and equivallent transistors would you suggest me to find here in North America?, Thanks in advance from México.

[@Theoobovril]

Looks very nice, Andrew, did you have to do a realignment ?

[@Andrewausfa]

Hi Theo, nope!! All good afterwards

[@andygozzo72]

that metal shield is original, i repaired a r300 for someone many years ago and it had one

[@jormamannikko4304]

what silicon transistors did you use to replacing the germanium ones?

[@cymap]

Hi, Which silicon ones did you use in the end ? Thanks and great vid btw

[@Andrewausfa]

I used BC559 I think. Most important thing is to get the LO running, some circuits may be a little twitchy.

[@cymap]

@@Andrewausfa Thanks for letting me know. I am only a beginner so could youy explain what LO means. You don't happen to have done a video on this particular change have you? Just bought my first radio (R300) and it needs an AF117 change. Thanks again

[@Andrewausfa]

@@cymap Hi, LO = Local Oscillator. No not done a how to change an AF117 in a R300 video but plenty of my videos show changing components. Read up about transistor superhets www.vintage-radio.com/repair-restore-information/index.html

[@cymap]

@@Andrewausfa Thank you, will take a look now.

[@johnb6723]

It’s a pity so few germanium transistors are made these days. Is it due to the Earth’s supply of germanium being almost used up?

[@aramboodakian9554]

Can you swap “geranium” transistors with petunia transistors?

[@Andrewausfa]

Only if they can handle 40 volts I'm afraid.

[@toddblanks]

Mullard lockfit resistors, hunts mouldseal capacitors, mullard Ge AF11 series transistor problems. The Ge transistors & diodes are a major part of the radio's battery efficiency. Im not 100% on this but i think it is only the mullard/philips AF's that suffer with the tinned whisker problem. I don't think the european and japanese TR's do in the same way, but i do know that they can become noisy. Of course this is all because these radios etc have been during some time in life stored in a cold damp environment for yrs. With germanium transistors the metal can absorbs air born moister like a sponge and becomes badly tarnished. Hence the tinned whisker syndrome! The same happens to plastic cased silicons where the leg terminals enter the case, they go black and become very noisy. Hence why some of the silicon TR's have gold plated leg terminals to junction to help pro long its life.

[@Andrewausfa]

Erie resistors actually, the transistor cans are hermetically sealed, it's the tin wash inside that causes the issue.

[@s.szorokin2001]

MP423 is ok instead of AF117.

[@CASHSEC]

Hi Andrew. I was given an R300L by my late brother and so it is precious to me. It too has the dreaded whisker issue s and with tapping got it running again but it only lasted a day and now it is silent. I so want to get it working again so you state yiu have used the BC 559. I assume it must be PNP and the connections are the same as for the germanium. Collector to Collector etc. Please can you kindly confirm.

[@gordongecko9405]

It can't be otherwise

[@Theoobovril]

Not seen a new video from Phil at Steel City, he has not put one out for nearly 12 months now, seems a bit odd, Andrew.

[@kd6tas]

The simple answer is no. Germanium transistors can be replaced with silicon and re-biased. If you're trying to make a living doing low-cost repairs in volume, it's not practical.

[@frankpitochelli6786]

You spelled and called them " geranium "...lol...germanium 👍 .

[@Andrewausfa]

Thanks Frank...it was intentional :)

[@frankpitochelli6786]

@Andrewausfa ...all good....sometimes I do it with certain words too..🙂