Can I use a LM741CN or TL082 opamp in place of a C1458C?

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
I am trying to fix my Tek scope power -8 and -15 power supply. I suspect that the op amp in the circuit is bad. It is a C1485C.

I do not have one on hand but the Shack has LM741CN and TL082 op amps. I really do not care if one of these is the best choice. Would one of these work for now?

I am not great at reading datasheets. But would want to run ti by the folks here anyway.

I have included the PS schematic below.

Here are my voltage readings for the -8 section.

U1464A
Pin 1 = .26 (should be 2.1)
Pin 2 = 0
Pin 3 = 7.5 (should be 0)
Pin 4 = .8


voltages for -15 section

U1464B
Pin 5 = 12 V(should be 0)
Pin 6 = 0
Pin 7 = .51
 

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iONic

Joined Nov 16, 2007
1,650
Maybe I'm blind, but I don't see a C1468C IC on the schematic.
Bottom line, compare the datasheets of the op amps for compatability.

iONic
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Maybe I'm blind, but I don't see a C1468C IC on the schematic.
Bottom line, compare the datasheets of the op amps for compatability.

iONic
That is not the part ID, it is the number on the Opamp itself. The chip number is not listed on the schematic.

And it is a C1485C not a C1486C as you posted. I posted the correct chip part #.
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Hello,

Is the +50 Volts OK?
The - 8 Volts and - 15 Volts depend on the +50 Volts.

Bertus

Yes it is normal. Now. Before it was 13V high and +15V was about 3V high. I do not understand why it is back to normal.

I inspected the +50V circuit carefully (for what I can see w/o removing the board) and saw nothing unusual. I tapped the board looking for intermittent connection but +50 stayed rock solid. I do not have freeze spray or I would try that.

I was told that since those transistors are in sockets, it is easy to bump them taking off the cover. Perhaps I reseated one of the transistors checking the circuit.

The other other thing it could be is that I toggled the line voltage select switch (with power off) and then checked the voltage. I made the mistake of not measuring the +50V again right before doing this so I have no way of knowing if this is what fixed it. Though I can't understand how it could.
 

SgtWookie

Joined Jul 17, 2007
22,210
A 1458 dual 741 opamp would plug right in; so would a TL072/TL082/LF353.

I can't seem to find a datasheet for a C1485C, but the pinouts on the schematic agree with the above opamps.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
A 1458 dual 741 opamp would plug right in; so would a TL072/TL082/LF353.

I can't seem to find a datasheet for a C1485C, but the pinouts on the schematic agree with the above opamps.
Thanks Sarge for the confirmation.

Yeah I knew the pinout of the TL082 was similar to the schematic but I was unsure of the other operating characteristics.

The C1485C is a Nec chip. The other number on it is N0202E.

But thanks for confirming that it would work. I am going out for a bike ride to enjoy this beautiful day but later I will run out to the shack a pick a few of them up. Hopefully getting this scope working.
 

SgtWookie

Joined Jul 17, 2007
22,210
You might find that one or more of the transistors connected to the output is shorted.

If the opamp is in a socket, why not try testing it on the bench?
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
You might find that one or more of the transistors connected to the output is shorted.

If the opamp is in a socket, why not try testing it on the bench?
Sorry as you remember I am a newbie. Can you suggest a circuit to test? And what I should expect in the test?

The reason I suspected the op amp is because of the weird voltages there and the fact that both -8 and -15 are out. That op amp is in common with both. Could a shorted transistor cause both op amps to fail? I suppose it is not beyond the realm of possibility I have a shorted transistor in -15 too.
 

hgmjr

Joined Jan 28, 2005
9,029
Actually the voltage measurement I would like you to make is the voltage across the terminals of the capacitor. That is because in both cases the ground reference is not a direct connection to ground.

hgmjr
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Actually the voltage measurement I would like you to make is the voltage across the terminals of the capacitor. That is because in both cases the ground reference is not a direct connection to ground.
hgmjr
I did. I gave you both measurements on both caps. Across and from + terminal to ground. - to ground is of course 0 because I have 0 out.
 

hgmjr

Joined Jan 28, 2005
9,029
Just so that I am clear. You placed your DVM probes on the negative and positive terminals of the capacitor and the voltages you posted are the ones you read.

hgmjr
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Just so that I am clear. You placed your DVM probes on the negative and positive terminals of the capacitor and the voltages you posted are the ones you read.

hgmjr

Correct. That the across.

And to be clear on what is positive terminal on those big cans with 4 pins in a triangle (3/1). Positive is the center one on the 3 pin in a line side correct?
 

hgmjr

Joined Jan 28, 2005
9,029
I would have to know the part number to answer your question with certainty. In the meantime, I will use the values you have measured.


hgmjr
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
I started reviewing the lessons on op amps on this site because I think the problem with my Tek scope power supply is a bad op amp.

I went out a purchased a couple of op amps from Radio Shack and built a comparator like the first one on this page. I used +5 VDC as my supply and my inverting input.

I have it working. Is this a good enough test for U1464 A and B in this schematic?

I could probably ramp the voltages up to 15 VDC if that is recommended.
 

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hgmjr

Joined Jan 28, 2005
9,029
First off it was probably not necessary to start a new thread since members are going to be a bit confused between the two. That can be remedied.

As for the power supply, the opamp is actually being operated as an opamp rather than a comparator. The voltage you measured across C1462 was around 15 volts. The trick to the -8volt regulator stage is that Q1468 is acting to control the positive-most end of the 15V secondary transformer winding so that it is exactly 7 volts positive when measured with respect to ground. If you imagine that the load on the -8V output increases then the -8volt output would drop slightly toward 0v. As the -8volts drops the positive input to the opamp begins to go positive. This causes the output of the opamp to go positive which drives the base of transistor Q1466 which in turns drives the base of Q1468 positive. With an increase in the base drive applied to Q1468, it begins to turn on harder. This action pulls the positive end of the transformer secondary supplying the 15V DC down slightly. This results in a restoration of the -8volt output so that the droop due to the increased load is cancelled.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Spinaker,

You will notice that I have merged the newer post with the older post since the second thread was a continuation of the first. This will help prevent confusion since replying members will be less likely to be confused.

hgmjr
 
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