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466 +15 supply and U1724 (1458)

jbaydcoid@...
 

Just got a 466 that was described as 'working then just quit' several years ago. All voltage measure slightly low but the +15 is way down to 1.4V. Unregulated +15 is slightly high 25.9 vs 22.5 presumably because of no current through pass regulator Q1756.


Looking at U1724B which regulates +15 the +input is 4.5 vs spec 15. Output is 1.5 vs 16.8. - input is 1.4 vs 15.


Interesting the other half U1724A both inputs are 4.5 should be 8.9. Output 5.1 should be 8.3. And the Vcc is also 4.5. The reference voltage from VR1726 is correct enough at 9.3.


Is it possible that the 10uf cap to ground at the + input is bad pulling that input down?


How does that relate to the low Vcc?


Thanks. Obviously I'm jet learning op amps.


John

 

On 23 Jan 2016 18:04:53 -0800, you wrote:

Just got a 466 that was described as 'working then just quit' several years ago. All voltage measure slightly low but the +15 is way down to 1.4V. Unregulated +15 is slightly high 25.9 vs 22.5 presumably because of no current through pass regulator Q1756.
Are you sure about this? What about the +140 and +65 volt outputs?

The +65 volt output is used as a reference for the other supply
voltages so if it is low, then they will be low also?

Looking at U1724B which regulates +15 the +input is 4.5 vs spec 15. Output is 1.5 vs 16.8. - input is 1.4 vs 15.

Interesting the other half U1724A both inputs are 4.5 should be 8.9. Output 5.1 should be 8.3. And the Vcc is also 4.5. The reference voltage from VR1726 is correct enough at 9.3.
Vcc for U1724 being at +4.5 volts is an immediate problem and nothing
around U1724A and U1724B is going to work correctly.

What is the voltage at the +65 volt output?

Is it possible that the 10uf cap to ground at the + input is bad pulling that input down?
I am confused here. The schematic shows C1725 0.02uF at the + input
of U1724A and a C1757 10uF at the +15 volt regulated output.

How does that relate to the low Vcc?
Vcc comes from the +65 volt output so either the +65 volt output is
low making Vcc low or something is wrong with either R1727, VR1725, or
U1724. The connection to Vcc through any socket might be open. See
below.

Thanks. Obviously I'm jet learning op amps.

John
Is U1724 installed in a socket? If so those sockets were known to be
a problem and it may have a bad connection.

You said the output of U1725 is +5.1 volts while Vcc is +4.5 volts. I
am thinking that perhaps Vcc of U1724 is open at the socket and Vcc is
being powered by the output pin which is being pulled up by the +173
volt supply through R1722. That might not explain both inputs of
U1724A being +4.5 volts though.

Andreas Ragner
 

Please check the health of C1757 (10 mfd) at the +15V rail.

I remember having intermittent problems with my 466 3 years ago - at first the scope did shut down after half an hour of operation, later this tantalum did blow (horrible smell).

Due to lack of tantalum caps I replaced with an electrolytic - still running fine.

jbaydcoid@...
 

Thanks David for the input. Yes, I was thinking of the .02uf at the input but wrote the 10uf on the output. My bad.

The 65V is around 60V and adjustment is non-reponsive; but that is to be expected since U1724 is not working. No sockets on this board. So I'm going to check the supply to U1724 - R1727 and VR1725 and the U1724 itself.

jbaydcoid@...
 

Thanks Andreas. Will check this one too.

 

If the +65 volt supply is actually +60 volts, then that explains why
all of the other supply outputs are slightly low. They all directly
use the +65 volt supply as a reference so there is no mystery there.

Interesting the other half U1724A both inputs are 4.5 should be 8.9. Output 5.1 should be 8.3. And the Vcc is also 4.5. The reference voltage from VR1726 is correct enough at 9.3.
If the output of U1724B was driving Q1752 and Q1756 hard, like 8.8
milliamps hard which is within the output current limit of a 1458,
then Vcc would be pulled down causing problems with the +65 regulator
as well so I think we are looking at a problem with just the +15 volt
regulator. C1757 which you identified is the prime suspect. From the
Tektronix 466 parts list:

10uF 25V 20% 90201 TDC106M025FL Mallory

http://datasheet.octopart.com/TDC475M035NSF-CDE-datasheet-110389.pdf
http://www.cde.com/resources/catalogs/TDC.pdf

I was originally suspicious of a socket but now I think it was Colonel
Mustard, in the Conservatory, with the Candlestick. Err, I mean the
solid tantalum capacitor, in the +15 volt regulator, with a short.

There might be a short somewhere else on the +15 volt supply but
removing C1757 is the place to start. If you have a 10uF to 100uF
aluminum electrolytic capacitor laying around it can be used in its
place.

All voltage measure slightly low but the +15 is way down to 1.4V. Unregulated +15 is slightly high 25.9 vs 22.5 presumably because of no current through pass regulator Q1756.
Is it possible that the 10uf cap to ground at the + input [actually +15 volt output] is bad pulling that input down?
Given the above, this could be the whole problem. A shorted C1757
will pull the +15 volt output down, Q1754 will then limit the current
by pulling the output of U1724B down and that will cause Vcc of U1725B
to be pulled down.

On 25 Jan 2016 03:57:16 -0800, you wrote:

Thanks David for the input. Yes, I was thinking of the .02uf at the input but wrote the 10uf on the output. My bad.

The 65V is around 60V and adjustment is non-reponsive; but that is to be expected since U1724 is not working. No sockets on this board. So I'm going to check the supply to U1724 - R1727 and VR1725 and the U1724 itself.

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