Bush TR114 Major Struggles

[Nowhere-Man]

Hello to all!, The past 2 days I've been really struggling with a Bush transistor radio I picked up. I've even gotten a bit stressed and baffled by my experiences so far because I did actually have it going yesterday. Now, however, it's pretty dead but I'll explain:

For a start this radio has the dreaded AF117 transistors (3 of them). I didn't know that when I started (so it may be a factor). Anyway, when I started work on it I bypassed the switch and managed to get audio but no stations received. The switch was broken and had to come off for repair as a matter of course. I fixed the switch and finally ended up wiring the switch contacts the opposite side of the PCB. I did this to make life easier. I did try doing it the original way but struggled to get a tidy solder joint that would allow the pins to also fit back into the PCB holes without wires coming off under stress.

Anyway, yesterday, after much hard work, the radio fired up. It was picking up stations fine and I was pleased as punch. I've just repaired another Bush dated 1969 so it seemed I had the golden touch. Then somehow it all went wrong. I was finishing some soldering connections on the ear jack and soon the radio started to fade. The last station I picked up was a lot fainter. Then also I noticed the speaker cone had been getting very hot. And at the point I began to wonder if I'd blown the whole thing and at this point I tend to beat myself up a bit. I mean to be so close and then to come a proverbial cropper is kind of hard medicine to take.

At this point I must admit I sometimes get mental blocks with these radios. I know more about valves than I do transistors and I think especially the ear-jack had me confused. Wires to the volume pot and speakers all tend to radiate from the jack and outwards so I wasn't really sure where I was at (not having at this stage a workshop manual). Maybe I could do with by-passing the jack? Yes, I did take very careful notes when I unsoldered the pot wires but find that sometimes any old wire elsewhere can sometimes just come off. I did get the volume pot wires back 100 per cent accurate but a couple of wires elsewhere somehow just popped off. Sadly my camera pics don't help much there.

Anyway, as I see it, there are two possibilities? Either somehow the wiring wasn't quite right so the speaker heated up with a resulting burned out audio output stage or my AF1177 transistors had only managed a last flicker of life and finally gave up completely. I mean, the radio was actually working quite well so the only thing I can think of in terms of wiring could be a bias issue. And surely that would mean positive bias on a base (can't see how that could have happened).

Ohms resistance on my speaker seemed higher by far than on another 8 ohm speaker I tested.

No sign of much voltage around the AF117's, maybe 3.5 volts max. Seems they are kaput.

Looking on the brighter side, it seems that with AF117's  even if you do get them to work, you're on borrowed time. It now makes perfect sense to replace all three. Any thoughts? This may involve a trip to Maplins again and Maplins I find to be a touch expensive but perhaps there is some basic transistor someone can recommend? I could do with a larger type and also with the terminals easily identified. Also tips on how to remove the old AF117's and accurately insert the new transistors?

Sorry for the long post. I've been working hours trying to get this radio fixed and haven't seen the light of day.

[Yorkie]

I believe that BC557Bs are considered a suitable 'drop-in' replacement for AF117s, but I've not had cause to try them.

Nothing ventured - nothing gained, so you might wish to have a punt on them.  

They do have the merit of being very cheap - as in £1.78 'buy it now' for 20 post free from here:

http://www.ebay.co.uk/itm/20x-BC557B-PNP...ltDomain_3&hash=item2eb6b43585

Best of luck in your endeavours - we feel your pain!

[Refugee]

The overheating may well have run the battery down and caused it to fade out.
The most likely cause will be the large capacitors that filter the power as it goes through to the IF stages where the AF117s are.
These cause the audio amplifier to oscillate and overheat.
If the parts are anything like the ones in a TR82 the offending capacitors will be yellow and black with yellow tape on them.

[tiv48]

the transistors I use are BF150 please note the lead outs are completely different .
Rob T

[Nowhere-Man]

I believe that BC557Bs are considered a suitable 'drop-in' replacement for AF117s, but I've not had cause to try them.

Nothing ventured - nothing gained, so you might wish to have a punt on them.  

They do have the merit of being very cheap - as in £1.78 'buy it now' for 20 post free from here:

http://www.ebay.co.uk/itm/20x-BC557B-PNP...ltDomain_3&hash=item2eb6b43585

Best of luck in your endeavours - we feel your pain!

I'll try and upload a pic but may have to try and shrink it again.

[Nowhere-Man]

O.K., I've been putting a few hours into this project and I guess it's been a learning curve. The truth is this is hard radio for my actual level but nothing ventured, nothing gained.

Here is what I have found so far:

The AF117 transistors have a screen leak. I'm not quite sure how significant this will be as I figure there are other issues. The screen as far as I'm aware is colour coded black. I was getting some Ohms reading between the screen and the blue collector lead. Not an infinity resistance. And the same applies to the other AF117's. I was soon locking into vintage radio discussions on the AF117's and a lot of the guys were suggesting to cut the screen lead off but many experts are advising it's far better to remove the transistor and subject to a high voltage capacitor "blast".

Tests between the P/N junctions are reading fine. Between base and collector/emitter any reading only happens in a single direction. The other is infinity. So, my best is the transistors ought to be removed and the capacitor "blast" used to hopefully cure the screen leak. I don't have a capacitor but am wondering if any chance of using a battery in series to get 50 volts D.C. or around.

Sure I could ditch the transistors but it would really devalue the radio.

Something else very weird. I was using a PSU at 9 volts supply and the radio is now humming. However, I noticed the PSU voltage will just drop off and the neon light goes out. At that point it's output is a few millivolts. So, something very wrong there. I ask myself could the speaker coil be cancelling the voltage from the PSU or is it an internal short of the main power caps?

Anyway, I've still been feeling a bit discouraged over all of this as I ask myself have I wrecked this radio somehow? I think the truth is at my present level I'm not good enough yet to understand all the ins and outs of a circuit so have had to relay on notes and blindly put ear-jack wires back to where my sketches indicate. Perhaps there was a simple mistake somewhere? Or maybe what happened was it just briefly gave a last gasp.

[Nowhere-Man]

The overheating may well have run the battery down and caused it to fade out.
The most likely cause will be the large capacitors that filter the power as it goes through to the IF stages where the AF117s are.
These cause the audio amplifier to oscillate and overheat.
If the parts are anything like the ones in a TR82 the offending capacitors will be yellow and black with yellow tape on them.

The only yellow components you can see in the pic although they have 3 legs. No markings at all on them. However, I don't mind changing the caps anyway but would like to try and save the AF transistors if I can.

[tiv48]

try cutting the screen lead on the af117's
also change the blue Philips electrolytic
Rob T

[Refugee]

The yellow cans will be OC81D and two OC81 transistors with sleeves over them like the red ones on the AF117s.
What is the component that is slightly obscured by the aerial between the coil connections? If this is a capacitor replace it before you do any other work as it is likely to be the power decoupling capacitor for the audio driver (OC81D) and will be causing the oscillation and overheating.

The method I use to deal with AF117s is to disconnect the screen lead by heating the solder and lifting the wire and cleaning the solder off the board so that there is no connection and then measure it again. Your meter will be reading the biasing resistors if the screen is connected to the earth track.
If there is now a reading when the isolated screen is measured it can be treated using a 12 volt power supply and a 100 ohm resistor and three silicone rectifier diodes.
First connect the diodes in series and connect the banded ends the same way round with the end one connected to the negative of the power supply and the resistor to the other end and connect it to the positive side of the power supply. You can if you want monitor the junction between the diodes and the resistor with a moving coil meter.
Now connect the junction between the diodes and the resistor to the screen and touch the negative of the meter and the banded end of the diodes to the electrode with the lowest reading when you tested with the meter. If you have the moving coil meter connected you should see a very short dip in the reading from a little under 2 volts for a fraction of a second. You should now find that the fault is gone.
In the data sheet for the AF117 the maximum applied voltage between any two pins is about 3 volts or so so the silicone diode shunt regulator is enough for you not to need to take the transistor out of the board.

[Old Sparky]

Ah! The dreaded 'Tin Whisker' problem with AF117s and their ilk. BTW I always replace AF11x transistors, rather than fiddle about trying to cure them. Recently I have found, courtesy of a fellow meber of UKVRRR, that theree is a russian transistor, type no GT322, which can replace AF12x & AF11x types. It is a TO-72, presumably alloy-diffused, device like the AF126/127, etc, however it's base & emitter connections are reversed compared to those of the AF12x series -i.e. the same as tose of an AF139, 239 etc. Specification, however, is similar to that of an AF127. The GT322 is available from several eBay sellers at around £1 each in quantities of 15, so is a more economical buy that AF127s.
As to the problem with the overheating, I can only second the suggestions that smoothing capacitors may be the culprit(s), or possibly faulty output transistors, though it's just possible that the fault may have been caused by soldering connections to the earphone jack with the radio switched on. Incidentally, if i may mention it here, the service info. for the TR114 is available from 'Vintage radio Service Data' ( part of UKVRRR), but you have to register as a member before you can purchase this.