PSU problem B750 MKII

[stoN_Cold]

PSU 1. 178. 136 - 11 with :

UA78MG
UA79MG
UA78M20
UA79M20

Dead.

Can't find regulators above.

What to put into PCB to replace dead regulators.

Thanks in advance!!!

Regards stoN_Cold

[eckibear]

AW: PSU problem B750 MKII

UA78MG
UA79MG
UA78M20
UA79M20
...
Can't find regulators above.....What to put into PCB to replace dead regulators.

Hi there and welcome to this nice forum!

The regulators you mention are out of production since long time, particularly the ones with four leads. Unless you can find them on ebay or other surplus dealers you'll probably have to replace them with LM317 positive and LM337 negative adjustable regulators (3-pin cases). For this purpose you'll have to modify the circuitry a little by adding two correctly matched resistors of each of them. Unfortunately the different pin layout does not allow direct insertion to the PCB. With the "search" function (term= "ua78") you'll find other guys stories and experiences about the very same issue. Hope this helps.

Cheers

eckibear

[stoN_Cold]

Re: AW: PSU problem B750 MKII

Hi there and welcome to this nice forum!

The regulators you mention are out of production since long time, particularly the ones with four leads. Unless you can find them on ebay or other surplus dealers you'll probably have to replace them with LM317 positive and LM337 negative adjustable regulators (3-pin cases). For this purpose you'll have to modify the circuitry a little by adding two correctly matched resistors of each of them. Unfortunately the different pin layout does not allow direct insertion to the PCB. With the "search" function (term= "ua78") you'll find other guys stories and experiences about the very same issue. Hope this helps.

Cheers

eckibear

Thanks eckibear

Maybe schematic of replasment & repair will help!

& what about -10 V DC?

Regards stoN_Cold

[hugo habicht]

AW: PSU problem B750 MKII

hi,

you should be able to get the 7xm20; a lot of electronics parts dealers still stock them. i can send you schematics of a lm317/lm337 replacement (send me your email address per pn); following the search from eckibaer you should find pictures how to place them into the existing pcb.

regards,
hagen
.

[stoN_Cold]

Re: PSU problem B750 MKII

Can't find 78M20 & 79M20!

You have PN!

Regards stoN_Cold

[eckibear]

AW: PSU problem B750 MKII

The circuitry required to run LM317's or LM337's is very simple and can be found on any manufacturers datasheet of these regulators:

http://www.national.com/ds/LM/LM117.pdf
and
http://www.national.com/ds/LM/LM137.pdf.

Page 10 of the first datasheet shows top left the circuitry with additional protective means and formula for dimensioning. You may/should omit both D2 and C2 to make life easier. D1 is a very good idea though, BUT make sure you reverse its polarity for the other type regulator!!

Since your's is a MKII dimensions are (closest matching values):

IC1: positive regulation (LM317) +28V, R1=240 Ohm, R2=5.11 kOhm or 5.1 kOhm
IC4: negative regulation (LM337) -28V, R1=240 Ohm, R2=5.11 kOhm or 5.1 kOhm
IC2: negative regulation (LM337) -20V, R1=240 Ohm, R2=3.60 kOhm
IC3: positive regulation (LM317) +20V, R1=240 Ohm, R2=3.60 kOhm

[hugo habicht]

AW: PSU problem B750 MKII

hi eckibaer,

i always thought the negative 337 version requires a higher minimum current; hence 120 ohm as opposed to 240 on the 317 unless you can always guarantee the minimum current flowing externally.

regards,
hagen
.

[eckibear]

AW: PSU problem B750 MKII

I always thought the negative 337 version requires a higher minimum current; hence 120 ohm as opposed to 240 on the 317 unless you can always guarantee the minimum current flowing externally

The datasheet I attached above shows various application circuits with either 120 Ohm and 249 Ohm resistors. As long as precision requirements are "normal" the resistor values can be chosen as stated. Specified input current to the adjust pin is below 100uA which gives about 24mV output variation for the larger value, well bellow other tolerances.
If wanted, all values indicated may be halved without trouble, either by chosing the corresponding values from the E-series or paralleling two R's at each position. In case of a single low-ohm R2, the wattage of R2 on the 28V stabilizers should be increased slightly (0.6W) to avoid thermal drift and accelerated aging.

Using same resistor values on either +/- side of the PSU eases procurement a little.