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B4 Problems

P

Paul3

pfm Member
Whoops
Switched on my B4 for the first time. Got 8 leds, then 4 outside died, followed by the inner 4.
Q7 and 8 were hot.

The value for Q7 + 8 printed on the board, are not the same as BOM specified.

Looks like ebc are reversed on BC547(bom) vs BC546(board)

What do Q7+8 do? and what will have been damaged by their reversal?

Should I just turn them round, or replace them? How do I test them?

Paul
 
The BC546 and 547 are complements of each other; one npn the other pnp so you can't just reverse them. Usually there would be one in the top half of the circuit and the other in the same place on the bottom half. Ie you would need one of each. This may be where the confusion arises on board vs BOM.

The chances are they have blown but they can be found nearly anywhere. Try them in their correct positions if you like but it will nevertheless be prudent to get new ones since some non-terminal damage can happen.
 
the leads do vary on these transistors and on similar types. they are still pnp or npn but the emitter base and collector wires may not be obvious.....
 
The transistors are part of the VBE's or gyrators (or cap multiplier) in front of the regulator circuits. 547 is the positive VBE and 557 is the negative VBE. They are the reason why the B4 has such excellent PSRR.

Paul i have spares if you just need a couple. Its probably best to replace them but the B4 will actually work without them, if you link them out you can still test the board.
 
LoBo said:
The transistors are part of the VBE's or gyrators (or cap multiplier) in front of the regulator circuits. 547 is the positive VBE and 557 is the negative VBE. They are the reason why the B4 has such excellent PSRR.

Paul i have spares if you just need a couple. Its probably best to replace them but the B4 will actually work without them, if you link them out you can still test the board.
Click to expand...

Our posts crossed.
I have another pair, as I am building 2 B4s, but I will need 2 later.

Your first paragraph is way above me, but I guess they are desirable. On the other hand, I seem to remember Neil saying he had or would miss them out.

How would I link them out? connect e and c? (Just guessing)

Suppose I could try both ways and compare.

Many thanks
Paul
 
Hi Paul

You know the PFM flea? well the flea has the exact same principle built into it's circuit. A transistor, 3.3uF film cap and a resistor in front of the regulator. You may also have heard of reg boosters here on the forum, again exactly the same trick. A most excellent way to cut down noise before passing current to a regulator.

Yes, just link across the three pins c to e and remove the Wima 2.2uF and 10k resistor. If you look carefully at the bottom of the board you will see the emitter (e) pins of the transistors joined to the input pins of the respective regulators. This is where the VBE finishes if you like so get power to these points.
 
Hi Lo
I know nothing. LOL. well very little.....just starting out in this hobby.

Over the last month, I've built 2 headphone amps, cmoy and pimeta. Both worked first time....beginners luck.

I have bought and fitted Teddy Regs, they similar? I don't know how anything works, but I'm keen to learn.

Paul
 
lostintheozone said:
As I am about to start on this please can someone clarify is the BOM wrong or the board wrongly marked?

Guy
Click to expand...
Hi Guy
There are different part nos on the board (BC547 and 557), to the nos in the BOM (BC546 and 556) , and it seems that the pin out on the components is different. Someone more experienced, would probable been more careful.

It's ironic because I queried the pin out on the diodes, but already had it correct.

I'll let you know if I manage to get mine working.

Paul
 
lostintheozone said:
As I am about to start on this please can someone clarify is the BOM wrong or the board wrongly marked?

Guy
Click to expand...

The BOM specifies BC547 / BC557 whereas the board marking indicates BC546 / BC556. Each pair of transistors are NPN (positive) and PNP (negative) respectively.

They are the same part family except 546 / 556 has slightly higher gain and breakdown voltage. Either type will do, they all have the same pin arrangement. Just make sure you have a pair that end with '7' or '6'.

Download or search for the manufacturers datasheet of the type that you bought and check the pin arrangement:

http://www.datasheetcatalog.com/datasheets_pdf/B/C/5/4/BC547.shtml

http://www.datasheetcatalog.com/datasheets_pdf/B/C/5/5/BC557.shtml
 
The pin out is the same for both, so I must have do something else wrong.

Checked my ps output....... +20.4v / -20.4v and I'm sure that I connected them the right way round.

??????????
 
Strange Paul, it sounds as if you are giving them too much current possibly?

The PS voltage sounds fine. Give details of your power supply - transformer size (VA rating) caps and resistors. Are you using a CAP6 type rectifier board?

If you got the transistors from a reliable source then they shouldn't be dodgy.

If possible take some pics?
 
I built the ps using parts from RS as per PDs post

A 10VA tranny would probably do, something like http://uk.rs-online.com/web/search/s...duct&R=2239272

Panasonic FC caps are cheap but good - 72p each for 1,000uF. They come in packs of 5, which will leave a few to put in the parts jar. http://uk.rs-online.com/web/search/s...duct&R=0571127

Current draw is tiny, so http://uk.rs-online.com/web/search/s...duct&R=6882221 would do nicely - again the pack will give some spares.

Transistors are from Mouser

Pictures
https://picasaweb.google.com/paul31154/B4?authkey=Gv1sRgCJWf2f7p4uq7SA#

I don't know how to post them on here.
 
This little discussion has alerted me to the fact that the Farnell sheet of the BOM (which I shopped from) doesn't have prereg components. Lines 3,4 and 17 differences also.
 
I have found the problem!

Some of the caps C8,9,11,12 etc were wrong way round - didn't see the polarisation mark

Only 7 from 8 were wrong

Will they be OK? How can I test them.

Thanks All
Paul
 
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