Finding problem (with Amp circuit)

[irishape]

I have assembled this amplifer from a pcb I obtained on the internet. When I power up the R4 risistor gets very hot within 2 seconds. The instructions tell me that if the amp oscillates to change C2 from 220pf to 470pf to keep output transistors from getting hot. Is it reasonable to think that this may be my problem with the resistor which is a 100ohm resistor. I am doing this project just for learning but hate to remove something that is not the problem. In my inexperianced opinion I would suspect another reason. Any suggestions?

 

[House0Fwax]

You say it is a '100ohm resistor', the schematic calls for a 1K (1000 ohm) resistor.

 

[irishape]

You are right, it is a 1k resistor. I have been looking at too many numbers lately. That is to say it has a 1k resistor on the board.

 

[KeepItSimpleStupid]

Make sure Q13 and Q14 are installed correctly.

For power up purposes short the collector to Emitter on Q12. What DC offset do you measure at the speaker terminals?

 

[irishape]

OK, shorted Q12 but R4 resistor gets hot instantly so can not measure speaker output without damage to board.

 

[alec_t]

If R3, Q13 and Q14 are correctly wired and functional then the maximum current through R4 will be ~2.5mA and it won't even get warm. So, I agree with KISS. Double-check the pin-out and soldering of Q13 and Q14, and the value of R3. Check too that the pcb track layout around those components actually is consistent with the schematic (it's not unknown for pcbs to have layout errors).

 

[irishape]

OK guys, I have checked R3 Q13 Q14, even removed them to make sure of proper type. Every thing checks. Went ahead and checked all resistors and transistors. All I have now is to check all tracks in board to make sure they all go where they are supposed to. Being new at this it will take awhile. There are no solder strings or any thing like that. Whatever it is it acts like a dead short because R4 is hot instantly yet it never blows a fuse on either rail. Anyhow I appreciate your help very much so thank you until your better paid. IA

 

[ronv]

only a guess but the transistors may be in backwards. Google a datasheet and make sure which view of the pins you used.

 

[irishape]

Thanks ronv but I have already checked to make sure that npn and pnp are in proper place and the board has pattern as to which way they face. IA

 

[alec_t]

the board has pattern as to which way they face

Not all transistors face the same way. Unless the transistor code (type) of your transistor exactly matches the transistor code that the board was originally designed for you could have the transistor pins wrongly connected. Does the board have markings 'b', 'c', 'e' next to the pattern?

 

[unclejed613]

measure the voltage across R3, it should be about 0.7V. Q13 and Q14 are the current source for the diff amp (Q1, Q2), and the current source is set to about 3mA, which should give you about 3V across R4. also make sure your input ground is properly connected, as well as the ground for the current source. are you sure R3 is 220 ohms, and not 22 or 2.2? measure them, don't take the markings for granted. if they're color striped, read the colors under incandescent lighting, and not fluorescent.

 

[irishape]

No it doesn't have b,c, e but it has the shape of the transistor and also the part numbers of all the parts. However, I was not able to find the parts by those numbers and so went to NTE crossover page and crossed them all to their numbers. There could be the problem. In the instructions there is a drawing of the two types TO92 bodies, both A and B to show the way the leads are layed out. It shows the base is always in the center and that is the way I have checked them with the diode section of my MM. I am afraid if the crossover system messed me up I may have my work cut out as I am not experienced in electronic work. But I don't give up easy so on into the fray. Thanks, IA

 

[alec_t]

Tell us the codes on the transistors you've actually got and we may be able to help more.

 

[irishape]

I am going back to beginning and try to run down whatever I have probably done wrong. Thank you kindly for your help and interest. If I can't find the problem I will back hollering help. Thanks again.

 

[unclejed613]

there are 3 common ways that TO-92 transistors are pinned out, ECB (american, like 2N2222), CBE (european, like BC547), and ECB (japanese, like 2SC2240). the only way to tell the difference between an american pinout and a european pinout, other than a datasheet, is to do a zener breakdown test to identify where the emitter is.. a reverse biased E-B junction will act like a 5 or 6V zener. this can be tested with a 9V battery, a 100k resistor, and a multimeter. you are sure from multimeter tests that the center lead is the base. are you sure the transistors are the proper polarity?. if so take the 9V battery, and solder one end of the 100k resistor to the + terminal. from the - terminal. connect a wire that goes to the - probe of the DMM and an alligator clip. connect the open end of the 100k resistor to the + probe of the DMM and another alligator clip. the DMM should read about 9V. when connected across a forward biases B-E or B-C junction, the meter will read about 0.6 or 0.7V. swap the alligator clips between the transistor wires. a reverse biased B-C junction will read open circuit (9V), and a reverse biased B-E junction will read 5 or 6V. the BE should be ok after this test because the test current is only 30uA.

 

[irishape]

Thank you for the info, I will give it a try. Yes the center lead is the base on Q1-2-13-14 and they are the right polarity.

 

[unclejed613]

i always wonder why a transistor pinout identification funtion of some kind never found it's way into DMMs,or at least a low voltage zener (maybe limited to between 3 and 15V) testing function.... with modern DC-DC converters, and a constant current source, it's not that difficult, as it works the same as the diode check function, just with more voltage range and lower current...

 

[irishape]

I have removed the transistors and checked them and everything seems right but I am new so anything could be wrong. The housing I put this in is plastic so I installed three studs for grd, +v, -v to keep things neat. Well today I found that I had crossed the + and-. I thought OK dummy, now you know why it didn't work. Same thing, R4 gets instantly hot. Maybe the transistors are damaged now? Fuses still fine, nothing else hot I can find but then I don't leave it on long enough to check everything. The parts list calls for BC556 in all four locations which changes to NTE
123AP which I have used. Probably best to just start over. They are good boards and not that hard to remove pieces with out damage. On ward and upward.

 

[alec_t]

You can see the NTE123AP pin connections at
https://t1.gstatic.com/images?q=tbn:ANd9GcRiLP7UYk7OCbLK0z35nRUJuN2KTa1xK3TlI7jVGFUHWx5Ek32uhw

 

[unclejed613]

if you had the power supply leads reversed, you most likely have shorted diff amp transistors (among others). the NTE transistors have a different pinout than the BC556, and the NTEs probably cost about 4 times as much.