[rlaury]

This may be asking too much but, does anybody know the
values of R4, R5 and R6 on the board with the 2 2520's on it.
I have 2 550b's that have these resistors are burned to a crisp.
They seem to be part of the gain adjust circuit going to pin 11.
or If someone had the whole schematic that would be even better.
Any help appreciated.

Thanks all
RonL

[Walrus]

On the VERY bad photocopy I have of an August 1991 *preliminary* schematic, R4 is 56 Ohms, R6 47 Ohms and I cant find an R5, but there is an R8 which goes between R4 and the 2520 output, value of R8 is 200 Ohms.
You are welcome to a scan of the schematic, but I'm not sure if it would be readable.....

[sliebers]

R5 is 200R as well.  I have a 550b that R6 (47R) was burned up also.
I don't have a schematic, but the 550b schematic floating around on Dan Alexander's site is the same output section.  The designators are different numbers.
Scott

[sliebers]

Whoops, I meant to say the 550a schematic on Dan Alexander's web-site.

[rlaury]

Thanks Guy's
I'm going to replace them but I don't have a clue as to what
burned them out. It connects to pin 11 which is a gain trim
jumper in the card cage. Unless it got pluged into a non API
card cage where pin 11 connects to something else.

RonL

[sliebers]

I'm curious what burned them up too Ron.  I can't think of anything that would pull that much current through them under normal use, especially without causing serious damage to other components and the output transformer.  I got my 550b used, so I have no clue what caused it.  Perhaps there is a card-cage pinout out there that is not compatible?

Scott

[David Kulka]

Perhaps the fried resistors are protection R's in series with the V rails?  If so, I suspect that you have a shorted 2520.  I've seen this many times.  Check the 2520's for low ohms between V+ and V-.  (A low reading indicates trouble, but a high reading is inconclusive.)

You could replace the R's, then power the module up GRADUALLY on a bench supply while measuring rail current or just monitoring the temp. of the R's.  If excess current is drawn pull either or both 2520's and try again...

It seems that 550B schematics are non existent.  I phoned the owner once asking for a copy and got a big lecture instead.  But you can probably troubleshoot this without schematics.  Do keep in mind that testing with low voltage rails (a few volts) will prevent further damage as you sort things out.

Best wishes.

[sliebers]

In my case, the fried resistor was R6 (47R).  It's a series load resistor between the output of the final 2520 and the primary of the output transformer.  I don't know the history of my unit, so perhaps the 2520 fried as the output was shorted?  A jumper between pins 11 and 5 would short the output.  I bought my 550b used and it was repaired when I got it.  I only know about the fix because someone put a 1/2W carbon resistor in for R6 and the PCB is badly burned in that area.  I've put a 1/4W back in.  In extreme cases, R6 is a fuse, which is what it ended up doing.

[rlaury]

Thanks all:
I replaced the burned resistors and powered it up.
It works fine for about 2 min and then the positive portion
of a sine wave starts to collapse down, The current drain on the
-16V rails goes up to about 50mA. I'm going to borrow a pair from
another module and try again. Most likely a bad 2520.
So do you have to send API the bad 2520 to buy another one?
My customer will want the old one I'm sure.


Thanks Again

RonL

[sliebers]

Sounds like the 2520 is the culprit indeed.  You probably have to send the old one in as proof of ownership before they will sell you a new one.

[bruno2000]

Can anyone give me the value of C11 in the 550b?  It looks like 150, but 150 what?
Thanks!
Best,
Bruno2000

[QUEEF BAG]

so...you know about the schematic in the g-mail acct.

right?

if that resistor toasted, you might take a good look at the
output coupling cap too.

i agree about the 2520, and the lf411 chips are to keep the offset nulled.

don't have one here to look at right now
C-11 is 150, probably pf. unless it's a can, then it would be uf.
what does it look like?

[bruno2000]

so...you know about the schematic in the g-mail acct.

right?

if that resistor toasted, you might take a good look at the
output coupling cap too.

i agree about the 2520, and the lf411 chips are to keep the offset nulled.

don't have one here to look at right now
C-11 is 150, probably pf. unless it's a can, then it would be uf.
what does it look like?

No idea what it looks like.  Those things are difficult to open up!  I had one for about 15 minutes, and noticed the "packaged" voltage followers.  I "think" C11 has to be more than 150pF, but I'm not sure.  nF is not used anywhere in the schematic, and if it's 150uF, the polarity is not indicated.  Thus the question.
Thanks for your input!
Best,
Bruno2000

[jsteiger]

It's uF. In the one I have open, C11 is actually a 220uF, 25V bipolar.

[bruno2000]

It's uF. In the one I have open, C11 is actually a 220uF, 25V bipolar.

Thank you!
Best,
Bruno2000

[Nobru]

Hi! I have the same problem than you Rlaury, the 47 ohm resistor burned. When I mesure the resistance betwenn + and - voltage on my 2520 it reads 7.5 k ohm is it concidered low? both of the 2520 read this value. the coupling cap on my 550a is 500Uf it's pretty old so I'll change it for a 470U is 47 ohm resistor has to stay 47 ohm  if the cap changes to 470 U instead of 500U?? I might be good for 2 x 2520... Thx

[martthie_08]

sorry for reviving this old thread, but my two questions seem to fit into the discussion.

what is considered good practice on the "output fuse" from the second 2520 output to the input of the output transformer?
47R carbon comp resistor?
with 470uF electrolytic in parallel?
I see this on "newer" 550A schematics...

what's up with gain trim resistor connected to pin 11 on 550 EQ's?
Are these actually addressed (used) on current market API lunchboxes?
I have two REALLY old 550 (not A) and one has the resistor (56 ohms) and the other one doesn't... that made me curious...
the way I see it there is a series resistor from the HI input (pin 10 on card connector) to the non-inverting input of the first 2520 that might be another way to trim the gain?

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