Hacker Hunter FM volume R4

[Findlay]

Looking at the BC549 B I have in front of me I'm confused about the pin out. Looking at it from the flat side, which has the number and E47 marked on it, the diagram I Googled came up with EBC from left to right. Yet data sheets for the BC549 indicate the reverse, CBE. Help!

[julie_m]

A quick search shows images with the leads both ways around .....

I just tested a BC547 -- which is from the same family, and has the same pinout, as the BC549 -- and the emitter was at the bottom of the D shape of the package, holding it with the leads pointing away from me. It also had a gain of about 400.

If you have a multimeter with a transistor test socket, set the range to "hFE" and insert the transistor into the NPN side of the socket, with the middle pin in the hole marked B and the others in C and one of the E's. If the reading shown is less than 20, swap over C and E. If the reading is more than 100, C and E are the right way around. Don't worry if the reading varies about a bit.

[ms660]

Flat towards you, legs pointing up....EBC.

Lawrence.

[paulsherwin]

It's good practice to always check the pinout using a tester, if only to remind yourself. It's not unknown for some transistors to be made with a different pinout to the 'official' one.

[Findlay]

Thanks guys - I don't have a transistor tester so am a bit stuck. Maybe I should just be patient and order a 2N3904 or one that might be even better suited and has a reliable pin out? This is why me and transistors don't get on....!

[julie_m]

One advantage of the "base in the middle" configuration is that the only way you can insert it wrongly without a deliberate effort is also the one least likely to do any lasting damage, either to the transistor or the rest of the circuit.

If you swap over the collector and emitter of a transistor, it will just exhibit an unusually low gain; but everything probably will work fine once you get them the right way round.

Apropos of something, maybe:

• At least one factory has sent out at least one batch of circuit boards with a BC547 transistor reversed. They actually worked fine, on the end-of-line test set and in the field, even with the gain in single digits!
• Multi-component analysers, once a device has already determined by earlier tests to be a transistor, test it for gain in both likely configurations, and report the one which gave the higher gain. They sometimes get it spectacularly wrong with old power transistors, which don't have much gain even with the collector and emitter the right way round; and display either two diodes, or a transistor with the metal can as the emitter.

[Findlay]

Thanks Julie - very interesting! What with the BF224 that needs replacing having a weird pin arrangement - CEB - I've sort of lost that relaxed feeling I had an hour ago when I was about to get started. Hoping to build up courage again soon. Is there a better transistor though that I can order? Blood pressure may have come down by then.

[paulsherwin]

You will almost certainly have an hfe test function on your meter. That's all you need to confirm the pinout. You're just as likely to find an 'odd' leadout with a supposed 2N3904 as you are with anything else.

Adjust the replacement leadout to conform to the original while the original is still in place, and recheck the leadout with your meter to make sure you've got it right. A bit of PVC insulation stripped from some wire will help prevent short circuits where the leads cross, though this isn't strictly necessary if you take care.

I don't understand why you're so nervous about this. Nothing will go bang if you get the leadout wrong, it just won't work.

If you really don't want to try the BC549, an MPSH10 is a closer substitute electrically and has the same leadout as the original.

https://www.ebay.co.uk/itm/10x-MPSH1...9/140992887505
https://www.datasheets360.com/part/d...0907630689128/

[julie_m]

Most cheap multimeters have a transistor test function -- annoyingly, this seems to be in place of the continuity beeper of more sophisticated models. Or you can buy component analysers from the usual places online -- they cost a pound or so more ready-built than as a kit -- that can take almost any two or three leaded device and tell you what it thinks it is.

If you can order an exact matching replacement part for a sensible price, then go with that. If you're nervous soldering transistors, just get some cheap ones and practice soldering them onto Veroboard. If you can solder and unsolder the same transistor a few times without its gain changing much (assuming you also acquire some means of testing transistors. Be sure to let the transistor cool down fully to room temperature each time, for a fair test; transistors are temperature-sensitive devices) and with the copper tracks staying in place, then you can be confident that you can replace the one in your set without damage.

Re blood pressure, it's been scientifically proved (by me, with a simple home B.P. / heart rate monitor) that stroking a purring cat can lower your blood pressure. I managed to reduce both systolic and diastolic readings by 8mm. Hg. between the beginning of the experiment and my cat jumping off my lap ..... Now look at members' avatars .....

[Findlay]

Thanks Julie and Paul. My meter has a continuity tester which is also marked with a diode symbol - will this do the job?

I guess I'm just a bit nervous because it is my Dad's old set and I don't want to damage it....

A cat around the house would be nice!

[paulsherwin]

Typical cheap DMM with a transistor test function:

https://www.ebay.co.uk/itm/1-Digital...k/183659641259

Note the small blue transistor socket.

[Findlay]

Oh dear - I just made a huge mistake. I unscrewed the FM tuner board thinking it would just lift off and have broken a few connections from the tuning capacitor - one of them to the small little coil on the board which seems to have sheared off. I guess I'm in for a really bit repair job now...

[Nuvistor]

Take pictures of what you have now to refer to when reassembling, tuners are not the easiest things to work on.

[Findlay]

Thanks Frank. I am worried that I have broken the small inductor.

[julie_m]

It's more likely that the solder has just broken away from one of the two surfaces, and the inductor itself probably is fine. If in doubt, try to get a close-up picture of the broken bits, post it here and watch this space. It probably will take several goes, but one of them will be clear eventually!

[Findlay]

Thanks Julie. There is one more contact to the tuning capacitor I need to unsolder before getting the board off. Think I will leave that until tomorrow then post with pics. Thanks for the encouragement. I am pretty sure it was you that helped me so much repairing a 1960 Sony set a few years back which is still going!

[Findlay]

Well I've managed to lift the board off. Thinking I'd at least try to get the tuner working before changing the transistor I hooked up a few wires to connect up the four posts on the tuning capacitor to which the connections had sheared. One of the small mica capacitors to the post top left has broken so I quickly substituted one I had handy. I'm not sure of the value needed as I can't understand the circuit diagram. But the tuner isn't working - could this capacitor be critical? I'm not sure if it is allowed to post the circuit diagram.

[paulsherwin]

You can post part of the circuit diagram to illustrate a problem under discussion, but not the complete diagram.

[Nuvistor]

Tuner circuit in post #30.

[Findlay]

Thanks Paul and Frank. I can't work out from the circuit where the 4 connections are to the air capacitor! Tr1 is the one with the brown sheathing around it at the top of photo 1 , if it helps, near that small broken capacitor.