Roberts R300 AF117 transplant ...

[Colin]

I've just repaired and refurbished a little red Roberts R300. A female friend of the family spotted it when she was in the workshop and announced that she'd always wanted 'a little red Roberts' ... As her birthday was coming up I took the hint and sorted it out for her.

There were the usual problems of a dry joint and a couple of duff AF117's. I didn't want to apply the time honoured tap of the screwdriver fix as they are prone to causing problems later on, so I thought I'd try replacing them with a more modern silicon based transistor - the BF450. I'd heard of this some time ago but have never tried it. I had been told the BF450 can be used as a direct replacement for the AF117 without any changes to the bias arrangements etc.

Dial Electronics in the North East suppl;ied the 450's for 50p each and in they went. They worked first time and the radio sounded great.

With a silicon front end and a full compliment of new caps (a mixed bag from the spares box) the little Roberts works well and should be good for a few more decades to come.

I wouldn't normally approach this kind of repair with the wholesale replacement strategy adopted here but the requirement was for maximum reliability. Although the look of the circuit board is substantially different the lady in question won't be opening the back until a new battery is needed and then she'll probably bring it back to me!

She's delighted with her new radio and I'm pleased to have found a cheap and reliable replacement for the AF117.

Apologies to those of you who already knew about the BF450 but I thought it was worth sharing.

I've attached a few 'before & after' pics.

Rgds
Colin

[Nickthedentist]

Originally Posted by stretch289

I thought I'd try replacing them with a more modern silicon based transistor - the BF450. I'd heard of this some time ago but have never tried it. I had been told the BF450 can be used as a direct replacement for the AF117 without any changes to the bias arrangements etc.

Apologies to those of you who already knew about the BF450 but I thought it was worth sharing.

Well, I hadn't heard of that, so thanks

Originally Posted by stretch289

I wouldn't normally approach this kind of repair with the wholesale replacement strategy adopted here but the requirement was for maximum reliability. Although the look of the circuit board is substantially different the lady in question won't be opening the back until a new battery is needed and then she'll probably bring it back to me!

This is a very valid approach if you won't be the radio's user. To some non-technical people, an old radio that ceases to work is thought of as rubbish, and may be treated as such. So by replacing a lot of components preventatively, you may ultimately be saving a piece of history from the bin. Obviously, if the radio is for your own use, a far more conservative approach is called for.

The cabinet looks really smart, by the way. Brand new knobs and badge?

I wish I'd bought more of these when they used to turn up at every jumble sale I went to for about 25p! (mid 1980s)

Nick.

[Colin]

Thanks Nick,

The cabinet's had a scrub with foam cleaner and a couple of coats of red Meltonian shoe wax. The Roberts badge is new and they're available from the Roberts spares dept. for £1.50. and the knobs are old ones from another set. Soaking them in biological washing powder brings the plastic up like new and the brass disks have been polished and had a coat of lacquer before being stuck back into place.

These little sets do come up nice. I've thought of re-colouring the rexine with fabric dye before finishing with polish. Have you ever seen ths done?

rgds
Colin

[Nickthedentist]

Interesting stuff, Colin.

I knew Roberts Badges were available, but didn't know they cost so little. Do you have to have an account, and is there a minimum order?

Re. the knobs: Did you polish and lacquer the knob inserts in situ, or did you remove them first? If so, did they just drop off, or did you have a cunning way of taking them off?

The shoe polish idea is nice. I haven't had any experience with dyeing Rexine, but I'm sure someone here has

Keep up the good work,
Nick.

[Darren-UK]

There's data on the BF450 at this link :

http://www.semiconductors.philips.com/pip/BF450.html

This transistor will also replace the AF127, OC169/170. The BF423 is also a usable substitute.

[Nickthedentist]

CPC list it at 50p for a pack of 5

[Colin]

Originally Posted by Nickthedentist

I knew Roberts Badges were available, but didn't know they cost so little. Do you have to have an account, and is there a minimum order?

The Roberts Scroll Badges are from the 'Revival' range but are the same as the old ones. They even fit in the same fixing holes on the set. The Roberts part number is 450.5914 - I've also had a 707 aerial - part # 450.1054 £6.95 - and a couple of the Revival handles in different colours at £5.75 each.

I don't think there's a minimum order although the packing & handling charge was £4.05 plus VAT and I don't have an account. They were very helpful.

The Spares Department can be contacted on 020 8560 6644 or email spares@rmtv.co.uk

With regard to the knobs I polished the brass disks before they were stuck back on - they seem to have a remarkable ability to fall off themselves so this isn't a problem. I've only ever taken one off and it was easily done by carefully drilling a small hole from the back (through the spindle hole) and giving the brass disk a gentle tap with the back of the drill bit.

rgds
Colin

[ppppenguin]

I'm slightly surprised that you can get away with replacing germanium transistors with silicon and not altering the bias. The Vbe drop is about 0.3V for Ge and 0.7V for Si. While a few circuit configurations will be inherently pretty stable with respect to this, most are not. Have you measured the before and after collector currents? Always assuming you can find a good AF117

The other worry is that the high fT of modern transistors, even very modest ones like the BF450, may lead to parasitic oscillations. Even the very humble BC108 makes a pretty good RF oscillator. Maybe the likely reduction in bias current caused by the higher Vbe tends to starve the transistor which will reduce any tendency to oscillate.

[Colin]

Your right to be sceptical - and so was I.

I had never had the opportunity or the inclination to test this out before even though I had heard the substitution was a straight swap without any other complications. On trying it I was surprised and pleased that it seemed to work!

I needed to get this radio sorted fairly sharpish as I'd left it until a day or so before the birthday. Because of this I didn't have a chance to really take a look at what was going on. It did work and run perfectly for the two hours or so that I had it on before wrapping it up. There were no unwanted side effects.

It may be worth checking this out in more detail but the substitution seems to work fine.

rgds
Colin

[mikej56]

Darren's link is another source of useful data!
But don't try to sub a BF450 for an AC127 as the latter is an NPN.

This transistor will also replace the AF127, OC169/170. The BF423 is also a usable substitute.

Although it would sub an AC128.

michael

[Sam]

This all sounds like a useful thread for my! My girlfriend has taken a shine to my R300, which is looking a bit falorn in my room! Reliability will be the key here again, so I may give the Ge to Si change a go - or I could just use the last few AF127s I have! Best give it some thought. Any opinions on reliability of both transistor types?

Didnt know the badges were so cheap. I was just going to re-spray mine with chrome/silver paint, like I did on my RIC-1)

Now just to work out how to add FM to it! (In an ENTIRELY reversable way, I hasten to add! )

Sam

[mikej56]

Thanks Sam,

Remind me to go to the optician this is probably the consequence of sitting in front of a monitor for too long

michael

[zak]

Originally Posted by stretch289

I've just repaired and refurbished a little red Roberts R300. A female friend of the family spotted it when she was in the workshop and announced that she'd always wanted 'a little red Roberts' ... As her birthday was coming up I took the hint and sorted it out for her.

There were the usual problems of a dry joint and a couple of duff AF117's. I didn't want to apply the time honoured tap of the screwdriver fix as they are prone to causing problems later on, so I thought I'd try replacing them with a more modern silicon based transistor - the BF450. I'd heard of this some time ago but have never tried it. I had been told the BF450 can be used as a direct replacement for the AF117 without any changes to the bias arrangements etc.

Dial Electronics in the North East suppl;ied the 450's for 50p each and in they went. They worked first time and the radio sounded great.

With a silicon front end and a full compliment of new caps (a mixed bag from the spares box) the little Roberts works well and should be good for a few more decades to come.

I wouldn't normally approach this kind of repair with the wholesale replacement strategy adopted here but the requirement was for maximum reliability. Although the look of the circuit board is substantially different the lady in question won't be opening the back until a new battery is needed and then she'll probably bring it back to me!

She's delighted with her new radio and I'm pleased to have found a cheap and reliable replacement for the AF117.

Apologies to those of you who already knew about the BF450 but I thought it was worth sharing.

I've attached a few 'before & after' pics.

Rgds
Colin

Why bother with silicon transistors when you can still fairly easily get germanium ie AF280 etc,these will then be a direct replacement,no need to alter biasing.

[mikelect]

Hi Guys, Just for interest MODE CMPONENTS stock AF117's @£1.95 each
and BF 423's @ £0.25 each and usually post at cost, I know Chris Tredwell who runs Mode Components and he will try extremely hard to source any other semiconductor you want
( no I'm not on commision, wish I was8^))his web site is www.modecomponents.co.uk

Mike

[howard]

Hello Mike,

Well I Emailed Chris Tredwell at Mode Components enquiring about acquiring a dozen AF117 transistors, he said that he'd get a quote for me on Monday as his supplier had a stock of 600 or more, but despite a reminder I've heard no more, not even an acknowledgment.

Howard

[radiogammon]

Is there any point in replacing a dodgy AF117 with another unreliable AF117 which may fail at anytime (they are all old stock)? I have now got a stock of BF450's although I have not needed to try them yet (10p each from http://www.dalbani.co.uk/electronic_...hp?offset=6825, although there is a minimum order of 50)

[Sean Williams]

Sycom Components stock the AF116 which works well as a 117 replacement....

Cheers

[howard]

Hello,

Now I'm totally confused .... I chose to use replacement AF117s in my collection of Bush TR82s (several have had their ground wires cut already) for the sake of originality .... now I have two more options for replacements Some of the earlier TR82s have OC series transistors, would they be suitable replacements as well if I could get some ?

Howard

[radiogammon]

The AF116s' will suffer from the same problem as the AF117s'. The OC series were more reliable.

[Darren-UK]

Within this thread are comments about the + and -'s of changing 117s with other 117s or using a substitute.

My own ground rules are :-

1. If a radio is for my own collection I will retain AF117s or replace with the same for the sake of originality.

2. If I'm asked to repair a radio, or get a dead radio working again, and am being paid to do so, I will get shot of the 117s and replace with a more reliable substitute.

3. If I am flogging an AF117-fitted radio on, say, Ebay, I will leave the 117s in situ so long as they are working ok at the time of sale. In this circumstance I would normally mention in the item description that the radio is 117 fitted.

To get back on the specific topic; Howard, you mentioned swapping AF117s for OC series in a TR82. If you meant OC44/5's then forget it - it won't work without a lot of other work changing components, biasing etc etc. If you want to stick to Germanium, you can use the OC169/170 in place of the AF117. These are rather more reliable and at least look the part, but my view is if you wish to be rid of the germanium AF117 nuisance then stick with the BF450 or BF423. For some strange reason, as others have said, this works without the need to alter anything else.