obsolete transistor replacement

[Dave]

Dave you wouldn't put the diodes from the 7915 output to ground but from
the output to your load in series......being a negative polarity output
you'd put the cathode towards the 7915 (and any additional diodes as well).

The trick to bumping up the voltage of a 3 terminal fixed regulator is
to insert a diode between the ground pin and ground (keep in mind to
insulate the tab if you do this!). The regulated voltage will now be
pushed up by the junction voltage of the diode or diodes (Diodes placed
as such in a 7912 would have their cathode face the ground pin.....anode
to ground).

Makes sense. One question: why do you say insulate the tab? If the tab is
already connected to ground, and you are just adding a diode, wouldn't you
only have the 1V of potential difference between ground and the pin with a
diode? Would I want to use a mica insulator between the tab and heatsink?

 

[Dave D]

Makes sense. One question: why do you say insulate the tab? If the tab
is
already connected to ground, and you are just adding a diode, wouldn't you
only have the 1V of potential difference between ground and the pin with a
diode?

Because the tab is connected to the ground pin internally. If you ground the
tab then the pin is also grounded, bypassing the diode.

Would I want to use a mica insulator between the tab and heatsink?

Yes. You'd need the whole heatsink 'kit', including the plastic insulating
collar which goes on the bolt, or a plastic bolt.

Dave

 

[Glen in Orlando]

We have original 2SB527 in stock for US$1.99 each.

http://www.acme-sales.net

Regards,

Glen Goodwin
ACME Enterprises of Orlando
1-800-575-9833

 

[Fred McKenzie]

Makes sense. One question: why do you say insulate the tab? If the tab is
already connected to ground, and you are just adding a diode, wouldn't you
only have the 1V of potential difference between ground and the pin with a
diode? Would I want to use a mica insulator between the tab and heatsink?

Dave-

I agree with G for the most part. I prefer the 7912 approach rather then
using the 7915. As someone else suggested, a capacitor from the output to
ground would be a good idea. I think you mentioned having one in the
original circuit.

Another alternative to the 7912 approach would be to use the negative
version of an LM317 with pair of resistors instead of the one diode. One
resistor would be connected from the regulator's output to its common
("adj") terminal. The other would be connected from the common terminal
to ground. Values would be chosen to produce the required output
voltage. The LM317 regulator actually has about 1.2 volts output plus the
voltage dropped by the resistor to ground. I assume there is a negative
version of the LM317, if you wanted to look further.

Of course an exact replacement transistor is the most sure-fire solution.
If you were to use an alternative part, I suggest using one rated for at
least the minimum hfe of the original part, or at least one having an
actual (measured) hfe of that value or higher.

Fred

 

[Dave]

We have original 2SB527 in stock for US$1.99 each.

http://www.acme-sales.net

Holy sh#t, this site is a GOLDMINE! You sell transistors that haven't been
made since 1975! Tons of them!

Thanks, it's in my bookmark file. I already bought a TIP42C (over-rated,
but at $1.99 the price was right) at my local electronics store. BTW, how
much is shipping to Canada?

 

[G]

Looks like you're getting lots of help Dave.
The comment about insulating the tab is so that you DON'T negate the
effect of adding the diode. The tab is common with the ground pin so
you'd be bypassing what you'd be trying to accomplish

Gord

 

[budgie]

(snip)

The trick to bumping up the voltage of a 3 terminal fixed regulator is
to insert a diode between the ground pin and ground (keep in mind to
insulate the tab if you do this!).

Sorry to be a spolier, but according to my data sheets the tab is connected to
pin 2 (whic is INPUT on a 7912 and presumably on other 79XX regulators). So the
O/P is going to have to go the insulated mounting route regardless of the diode.

 

[G]

Howdy Budgie....right you are.....thats not being a spoiler but
just being helpful . Must admit I based my experience has been with
the positive VR's (obvious eh!). Having said that......the tab of a
79xx wouldn't necessarily need to be insulated but it has to be kept in
mind for sure. (anyone here ever tried building their regulator into a
grounded side of a rectifier heheh......it works just fine - just looks
disconcerting when you look at the diagram).

 

[Dave]

I've identified a bad transistor in the power supply of an audio component,
circa 1981.

Well I went with a TIP42C, over-rated (6A vs. the 0.8A I am replacing AND
that's probably over-engineered) but available locally. I don't have any
mica insulators or plastic screws and they are an hour drive (or $15
shipping) away.

Thanks for everyone's help, I learned a great deal. BTW, I threw the new
PNP into my DMM's transistor testor and it came up with an hFE of 390. I am
thinking that this is a fairly useless number other than to show that the
transistor is functional as the hFE measurement will vary greatly with input
current. Wonder what the old one would weigh in at? Interestingly checking
the old transistor with a multimeter showed perfectly normal behavior, B-E
conducted one way only, B-C conducted one way only, C-E did not conduct.
But it didn't work in my transistor tester, came up 000 for hFE.

Again, thanks for the assistance.

Dave

 

[GregS]

Well I went with a TIP42C, over-rated (6A vs. the 0.8A I am replacing AND
that's probably over-engineered) but available locally. I don't have any
mica insulators or plastic screws and they are an hour drive (or $15
shipping) away.

I still don't know where you got that spec. What was the manufacturer of that
obsolete unit.

Thanks for everyone's help, I learned a great deal. BTW, I threw the new
PNP into my DMM's transistor testor and it came up with an hFE of 390. I am
thinking that this is a fairly useless number other than to show that the
transistor is functional as the hFE measurement will vary greatly with input
current. Wonder what the old one would weigh in at? Interestingly checking
the old transistor with a multimeter showed perfectly normal behavior, B-E
conducted one way only, B-C conducted one way only, C-E did not conduct.
But it didn't work in my transistor tester, came up 000 for hFE.

Again, thanks for the assistance.

You really need sets of curves to get a real indication of things.
I have mostly tested them out with an analog ohmeter, my favorite.
Sometimes you will see extra leakage on CE or other terminals
when they are bad. I have not tested a bad unit with the diode
test function, which should indicate some voltage drop across CE.
greg

 

[Glen in Orlando]

Holy sh#t, this site is a GOLDMINE! You sell transistors that haven't been
made since 1975! Tons of them!

Thanks, it's in my bookmark file. I already bought a TIP42C (over-rated,
but at $1.99 the price was right) at my local electronics store. BTW, how
much is shipping to Canada?

Glad you like it, Dave! We carry a lot of "new old stock"
items, as well as the "latest and greatest."

Shipping to Canada is the same as anywhere else, and
shows up in the shopping cart as you add items. (BTW we ship
to Canada every day).

Orders over US$50.00 - FREE shipping and handling
Orders US$20.01 to US$50.00 - S & H = US$6.99
Orders US$10.00 to US$20.00 - S & H = US$8.49
Orders unders US$10.00 - S & H = US$11.99

For orders under US$50.00 a small additional charge
will be added for flybacks.

Typically we use US Postal Service Global Priority Mail
to Canada, which arrives in about 4-7 days.

If you need more information please call us toll-free at
1-800-575-9833 (USA and Canada) or 1-407-296-2333.

Glen Goodwin
ACME Enterprises of Orlando
http://www.acme-sales.net

 

[Dave]

I still don't know where you got that spec. What was the manufacturer of that
obsolete unit.

http://www.geocities.com/saphanlex/p/tr.htm
http://www.datasheets.org.uk/specsheet.php?part=2SB527
http://www.datasheets.org.uk/databooks/databook.php?q=100
translated directly from Japanese:
http://translate.google.com/transla...ient=firefox-a&rls=org.mozilla:en-US:official

I'll have a look at the part and get the manufacturer.

 

[GregS]

http://www.geocities.com/saphanlex/p/tr.htm
http://www.datasheets.org.uk/specsheet.php?part=2SB527
http://www.datasheets.org.uk/databooks/databook.php?q=100
translated directly from Japanese:
http://translate.google.com/translate?hl=en&sl=ja&u=http://homepage3.nifty.com/
ebina2540/data/2SB/2SB0501-0600.htm&prev=/search%3Fq%3D2sb527%2Brating%26hl%3De
n%26hs%3DQfp%26lr%3D%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-USfficial

I'll have a look at the part and get the manufacturer.

Taken from my Towers 1980 edition databook, the main
difference is the packge type, clearly a TO126 as
the replacement shows2sb631
http://www.ortodoxism.ro/datasheets/sanyo/ds_pdf_e/2SB631K.pdf
The max current spec does vary among the bunch.

I have about 8 different replacement numbers as I have gone through all this.

greg