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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th Mar 2007, 9:13 pm   #1
Roger Rabbit
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Default Roberts 707 - BF450/RA transistors

I've just repaired my first Roberts 707, it was dead on AM and very poor on FM. Ive replaced the Af115s and Af116 in the If module (what a labour of love that is!!) with Bf450/ra transistors supplied by Maplin. It fired straight up and works really well on all bands except SW. The band is a mass of hetrodynes when tunning across it. Its finding stations OK and lots of em but instead of audio all I hear is an oscillation. Sounds like something in the IF. Ive checked and double checked my wiring and the switches. Is it the BF450/ra substitution causing the problem or do I need to look elsewhere? Anyone else used these transistors in this set?
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Old 11th Mar 2007, 10:09 pm   #2
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Default Re: Roberts 707 - Bf450/ra transistors

I've got no Circuit of the module here, just a block which says IF Module. Helpful or what? Anyway, any Electrolytics in there that you could change. Also, try tweaking the cores one way or the other and see what happens. Remember to draw a diagram first and don't go too far.....

Cheers,

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Old 12th Mar 2007, 12:19 am   #3
Roger Rabbit
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Default Re: Roberts 707 - Bf450/ra transistors

I 'm loathe to open the thing up again, its a lot of work and the PCB is very delicate, I`ve no cct either there are only three transistors in there but a lot of coils, I`m assuming the SW IF frequency is the same as the Mw/lw IF. The instability is less at the lower end of Sw but gets serious above 6Mhz. I don't know whether the fault was there before the transistor swap so I may be barking up the wrong tree in blaming the module. Mixer maybe, but its tuning stations OK but is unstable, If hold the telescopic aerial the ocsillation damps and I can receive the occasional station. Could it be the AGC faultly?
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Old 12th Mar 2007, 1:07 am   #4
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Default Re: Roberts 707 - Bf450/ra transistors

This doesn't sound like an IF problem because, as you say, the IF strip works in the same way for all the AM bands. The mixer/oscillator stage is the most likely culprit - this is always the dodgiest transistor to substitute.

Paul
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Old 12th Mar 2007, 9:17 pm   #5
Darren-UK
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Default Re: Roberts 707 - Bf450/ra transistors

The matter of replacing Germanium transistors with Silicon jobs has arisen numerous times but the general opinion, in the past, has appeared to be that this procedure is fine when replacing the AF117 but can cause problems with others in the AF11x series.

Silicon replacement seems, for some reason, to be more successful with cheaper and more basic radios.....but less so with more elaborate models. Certain Hacker sets, for example, can be decidedly iffy when being ' Siliconised'. A degree of realignment is often necessary and I'd be tempted to try this first on the R707, but remember the alignment instructions given in the service data will probably now be a little inaccurate......it'll be a core tweaking job as Steve has suggested. Having said all that, I also doubt the problem stems from within the IF department and is more likely to be in the mixer/osc department.

Looking elsewhere, have you checked the SW wavechanger ? It's possible this may be where the problem lies; SW is not often used and the wavechanger may be dirty or clogged with dust.

It also occurred to me that your batteries may be a bit feeble but this won't be the problem if FM is working ok.
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Old 17th Mar 2007, 6:48 pm   #6
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Default Re: Roberts 707 - Bf450/ra transistors

Dear Darren.
Your right is is a huge job . You may find it helpfull to look at http://www.marie34.freeserve.co.uk/page5.htm By memory I think I used af125s still available from cpc. About 3 quid each . When I was an apprentice 40 odd years ago I was always told to suspect your own work first . I would look at trying the germainiums, then with the if unit on the long leads the IF unit can be worked on live, with the wave change in situ.
Regards .Martin.
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Old 17th Mar 2007, 8:10 pm   #7
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Default Re: Roberts 707 - Bf450/ra transistors

Hello,

Ron Bryan has fixed two of my R707s, one had a faulty switch board and a faulty LP1179/2 FM module. While the switch board was removed for repair, the original AF115 and AF116 transistors in the LP1164 IF module were replaced with AF125s, no problems at all. Here's the thread ...

https://www.vintage-radio.net/forum/...ad.php?t=13230

At least 30 AF114/AF115/AF116/AF117s have been replaced in my transistor sets so far by AF125s and no problems whatsoever have been experienced. I do keep BF450s but I have never used any and probably won't do so until I have exhausted my stock of NOS AF124-AF127s.

Howard
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Old 17th Mar 2007, 10:13 pm   #8
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Default Re: Roberts 707 - Bf450/ra transistors

I have done just one of these(not a job for the feint hearted, or some one with less than 3 hands!!) and have another in the "queue" which I am putting off for as long as possible.
I used some used CV1089(AF124?) which worked perfectly. If in doubt use the Germanium option where possible(we KNOW it works). After all, one wouldn't have to go through all that again!
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Old 18th Mar 2007, 2:41 pm   #9
Roger Rabbit
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Default Re: Roberts 707 - Bf450/ra transistors

I really don't think the problem lies with the BF450s. I used these as a "what the hell lets have a go" spur of the moment thing and on MW/LW the set performs excellently! I did a bit of osc/trimmer/ferrite realignement as detailed in the trader sheet and sensitivity improved even futher. I`ve servisoled the switches once again, they are the "open" type without the copper shield so the spray has penetrated, Ive checked for continuity as far as I can without removing the board again (The very last resort!), but the SW insabilty remains. Time to open the front end methinks.
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Old 18th Mar 2007, 10:02 pm   #10
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Default Re: Roberts 707 - Bf450/ra transistors

Yes. BF450's seem to work fine in MOST MW/LW sets, but I have only tried them in I FM set (Decca TP85--mixer). A MW/LW Hacker wasn't too happy with them ISTR.
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Old 23rd Mar 2007, 11:10 pm   #11
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Default Re: Roberts 707 - Bf450/ra transistors

I seem to remember one R707 where an extra earth connection onto the screening can of the LP1164 module cured some motorboating and instablilty problems, on SW. The set worked normally on the other bands.

Might be worth trying............

Phil.
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Old 20th Apr 2007, 7:09 pm   #12
Andy Day
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Default Re: Roberts 707 - Bf450/ra transistors

Hi Folks, I have just had two Roberts sets, an R600 and R707, force themselves upon me when looking in a local secondhand shop for furniture for my Daughter. Only a fiver each, which is just as well as they are both dead!

I guess it may be AFxxx syndrome, but I am a little confused by the idea of just sticking a Silicon replacement where a Germanium used to be. Surely the biasing will be all wrong, or did I not pay attention when doing my C&G?

I have to say I worked primarily as a digital electronics tech, so all this old analogue stuff is a bit of a re-learning curve!
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Old 20th Apr 2007, 7:47 pm   #13
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Default Re: Roberts 707 - Bf450/ra transistors

Just give it a whirl - no need to try to analyse the problem really - it just seems to work.....

i think the main reason is that the lack of correct bias removes the possibility of the whole set taking off in mad oscillation.....
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Old 21st Apr 2007, 1:33 pm   #14
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Default Re: Roberts 707 - Bf450/ra transistors

Quote:
Originally Posted by Andy Day View Post
I guess it may be AFxxx syndrome, but I am a little confused by the idea of just sticking a Silicon replacement where a Germanium used to be. Surely the biasing will be all wrong, or did I not pay attention when doing my C&G?
The bias characteristics of BF450s are closer to germanium transistors than most silicon types, hence their popularity as substitutes.

It's surprising how often a common or garden general purpose transistor like a BC327 will work as a drop in replacement though.

Paul
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