23-04-2015, 06:02 PM
(This post was last modified: 23-04-2015, 06:03 PM by Nowhere-Man.)
(22-04-2015, 11:09 AM)Yorkie Wrote: I don't want to appear critical, discouraging or dismissive of your efforts, but think that most of us, before embarking on any work on a radio - especially a transistor radio with a high component density on a PCB, would first obtain the service data, and would go about diagnosing any faults in a methodical way before making assumptions about what might or might not be faulty based for example of folklore about AF117s. True, the problems they can pose are well documented but we shouldn't jump to conclusions - often they're not faulty at all.
The most productive approach is methodical diagnosis. The first thing to do is to measure the voltages as compared to the circuit to highlight any variances which might pinpoint the cause of the fault.
In the case of the Bush TR114 the Bush service data runs to 7 pages, and though it shows component placements, isn't too descriptive as to the PCB track layout. I'm not normally a fan of Trader Sheets, but in the case of the TR114, the two-page Trader Sheet shows an X-ray of the track layout so you can identify key components to enable you to test, and if need be, unsolder and remove them. If the PCB is SRBP rather than fibreglass, as it most likely is, the adhesion of the tracks will be poor, so you need to be quick at unsoldering component leads and use a pencil tip iron and de-solder pump. Never be tempted to 'wiggle' a component when de-soldering, to free it up or sure as heck, you'll lift or break the track.
If you don't adopt a methodical approach to fault diagnosis, you'll inevitably disturb, and possibly damage components that aren't faulty, and may introduce faults that weren't there in the first place.
You can do a quick check if the oscillator is running by placing a set with the same IF frequency alongside the TR114 and turning the Tr114 tuning cap across the same waveband as the other set to see if you hear 'whistles'. If you do, it's an indication that the mixer/oscillator in the TR114 is working.
On a dead set which has an earphone socket, open circuit contacts are often the culprit and bridging out the socket can check that out, but in your case, the overheating could possibly be caused by C31 being leaky (in the electrical sense). That can only be checked by taking it out of circuit. The audio stages in that set are a reliable design and pretty standard in many sets of that era. Everyone has their own approach to servicing, but personally, I use a signal injector/tracer to pinpoint faulty stages. If you inject a signal into the slider of the volume control and hear a loud tone from the speaker, that's an indication that the audio stage is working. (Even scratching the slider tag with a screwdriver should be audible in the speaker). If you get no tone, and apply the tracer (just a small amplifier) to the slider of the volume control and can tune stations, that means the RF/IF stages are working. If you don't hear stations, you can work your way with the tracer to the front end of the set - for example checking at the collector/base of each transistor to isolate the faulty stage.
In the TR114 - as in the Bush TR82 - if the audio stages are working, but no stations can be heard, a likely culprit is a faulty detector diode (referred to as 'MR1' in the TR114 circuit), which is an OA90 germanium diode, hidden in IFT3. It's quite a palaver to get to that diode, but there's no need to dismantle anything. Leave the duff diode in place, and a replacement diode can be tried externally across pics 1 & 2 of IFT3 (the right way round!) and if that brings a dead set back to life, it can be permanently soldered externally between pins 1 & 2.
You can get an excellent Velleman kit to make a signal injector tracer from ESR Electronics for about a tenner inc P&P:
https://www.esr.co.uk/velleman/k7000.htm
You just need to add a little speaker, on-of switch and some sockets, put it in a box and make a probe, then you can use it on any valve or transistor radio. There's a pic of one I made below.
As I said at the outset, the last thing I want to do is to dampen your enthusiasm, so the above comments are meant to be helpful - not critical, because it's evident from your comments that you are putting in as great deal of effort, with this set and your Ultra valve radio, though without a structured approach, and without at least some basic test gear and service data, which is reducing your chance of success. You're treading a well-worn path, down which we've all trodden, you can rest assured about that!
Every good wish in your endeavours.
Thanks for that. Yes. it's time to get my act in gear and I think the plan now is to sort out the Ultra first and hopefully that will give me a bit of a boost to return to the Bush tranny.
I think I just got caught hopping as I know far less about transistor radios and, as you said, not having a schematic doesn't help. However, there is hope. I did notice one capacitor did seem a bit leaky and the AF117 transistor is still fit to be used. Plus, I made very detailed diagrams so should be able to get everything back as it should be.
Now, if I somehow cobble up the Ultra, that will not be good for self confidence. It is looking fairly good, though, now and I only have 3 more capacitors to insert (bought them today). Only one I'm having a struggle to find is a 0.005 mFD, rated at 1000 Volts DC which Maplins didn't have (not even AC film cap). Odd that such a low value would be awax paper cap, normally it would be Mica but the original cap looks a bit suspect.
I will indeed be using a lamp dimmer with the ultra - basic and simple design. I was also curious about using a 300 milliamp inline fuse which I think I can get. I seem to recall most ACDC sets average about 170 milliamps current so maybe a fuse would worth looking into.
So, fingers crossed.
