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I will agree with Roger and Harvey about the 1458 IC possibly being bad. They may have found the problem. The voltage James read make it look bad. I suspected this before the wrong voltage across the 3 ohm resistor was posted. The 1458 is (2) 741s in one IC. If James has or can get any, put in a new one to see what that does. I have used FET input ICs to test for a BJT type, both worked fine. I did put the original number back in.

Mark

As a second thought.? Grounding the anode of CR1724 will put the op-amp completely out of circuit.? The voltage at the supply will then be fixed at the lowest possible adjustable voltage, and I'd expect maybe about 60 volts giving the +65 volt supply a theoretical adjustment range of about +60 to +70.? The pot setting will not matter.? The voltage drop across R1734 indicates roughly how much current the supply is dumping into the load at the rate of 333 ma/volt drop across R1734.

Harvey

On 3/21/2022 10:43 PM, James55 wrote:

Great reply Harvey.

Made me smile. Thank you.

Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards

I take it you mean via the connectors?

and the chassis.

Do you mean disconnect any ground wires?

That's because you keep removing parts. Stop doing that unless you can justify removing them

Fair enough. The two diodes had a leg lifted in order to help find the last 'good point' with the voltage readings. All components are back in now other than the +15v rail, which still has a dead short lurking somewhere.

So, voltages...

The CR1721 +65v lines are reading 173.5v and 86 volts

In the hope that it may illuminate the situation, I just did a series of quick measurements 'across' the components of the +65v section, which are as follows;

R1722 - 128.3v - good since the input of U1724A should be high impedance
R1723 - 18.7v -
VR1722 - 20.2v - GOOD
VR1724 - 23.3v - normaly 56 volts,
C1724 - 23.3v - way low since the input of U1724A should be high impedance
CR1724 - 0.25v - GOOD
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v - way too low, possibly because +65 is low
VR1726 - 9.2v - GOOD
R1727 - 26v
R1725 - 5.8v - bad, since the input of U1724A should be high impedance
C1725 - 3.39v - bad, since the input of U1724A should be high impedance
R1726 - 16v - understandable since +65 is low
CR 1723 - 0.573v - GOOD
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v - way bad
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

As has been mentioned, 26 volts across 3 ohms would be a current drain of 8 amps.? It's not.? I think that (as has been mentioned), R1734 is open.

That leaves the only way to get any current to the +65 a path through VR1722 (allows 4 ma at worst), and then a possible path through CR1723 through Q1736.

I'd also suspect the op amp U1224A.

Harvey

+65v test point currently 25.32 volts (obviously)

There is lots I wish to add in response to the earlier post however it is very late here.

I'll get back on it tomorrow evening.


James

James,

There is a serious anomaly right at the beginning of your voltage tracing. Pin 8 of U1724 should be at +22V, regulated by the Zener VR1725. It looks very much like either VR1725 or U1724 is bad. The voltage on pin 2 of U1724 should be around 9V even with the low value of the +65 supply, you could confirm that the voltage on VR1726 is 9V. It could be that the op-amp is drawing a large input current due to the input voltage being more positive than the supply voltage so this isn't an immediate indication that U1724 is bad (somebody please correct me if I have this wrong!).

Keep at it and don't get too tired, you will probably save time in the long run.

Best wishes,

Roger

On 2022-03-22 10:37 a.m., James55 wrote:

Hi Mark,
... for sure I was tired last night, but hey!
If you have a 466 handy then that could well be a game-changer.

I have a 466 here but it died while I was testing it out. I haven't opened it up to investigate yet, but it needs to happen.

Don't mind helping out other 466 owners to the extent I can... Have never repaired one before, though. I follow threads like this one for hints. :)

--Toby

Thanks for your supportive words.
There has been some excellent advice throughout this thread, which has transformed my understanding of general fault-finding as well as certain things that I had read about but never really worked on in practice, such as op-amp feedback circuitry.
If I'm honest, there were times when I thought of giving up on this scope, however finding the crossed transformer wires was the missing part of the picture, and now I can see no real reason why this can't be repaired.
I'm very comfortable with electricity and electrical properties even if I don't yet have the necessary understanding of how certain components interact when placed in placed within particular circuitry. But that is coming.
I'm not looking to be a design engineer, however the experience and capability to recognise circuit weak-points is where I would like to get to, so projects such as this scope are actually a Godsend.
This scope will be repaired.
All the best.
James

Hi Mark,

... for sure I was tired last night, but hey!

If you have a 466 handy then that could well be a game-changer.

Thanks for your supportive words.
There has been some excellent advice throughout this thread, which has transformed my understanding of general fault-finding as well as certain things that I had read about but never really worked on in practice, such as op-amp feedback circuitry.
If I'm honest, there were times when I thought of giving up on this scope, however finding the crossed transformer wires was the missing part of the picture, and now I can see no real reason why this can't be repaired.
I'm very comfortable with electricity and electrical properties even if I don't yet have the necessary understanding of how certain components interact when placed in placed within particular circuitry. But that is coming.

I'm not looking to be a design engineer, however the experience and capability to recognise circuit weak-points is where I would like to get to, so projects such as this scope are actually a Godsend.

This scope will be repaired.


All the best.

James

James,

Your typo could have been when you were tired. The corrected voltage across the 3 ohm resistor is much closer to what it should be. It voltage across it seems low now. If you want, I can get my 466 open and help you if I can. All the great posts from others has been great to read. I know all of us would like to know what you find is wrong. Thanks for confirming about the other three parts I asked about yesterday.

Mark

Hi chaps.

Ozan,

Having a quick look, R1724 is 324mV

Q1732 did fail before but was replaced. It currently tests ok using diode mode on the DMM and when replaced just now with a 2N5551 the Base reads 25.90v with the Emitter at 25.48v.
Q1734 is a new TIP31C following my carelessness and Q1736 also tests fine in diode mode.

Mark,

R1734 is difficult to reach but it reads 4Ω in circuit and measuring again, reads 0.062v, NOT 25.25v (not sure what happened there?)
CR and VR 1718 don't show any obvious failings, whilst C1737 isn't shorted and measures correctly at 3.6?F


On a different tangent, and I know this is slightly off the page of study and not sure if this will help, however, the 140v TP reads around 98v, yet when lifting the cathode of VR1718 the Test Point goes up to 160v. The +65v remains the same at about 25 volts. VR1712 is not faulty as it's cathode reads 179.9v. Anyway, just thought I would add that.

I did grab a selection of diodes yesterday whilst in town, just in case...

Hi James,
See my comments below the measurements.

On Mon, Mar 21, 2022 at 07:43 PM, James55 wrote:

R1722 - 128.3v
R1723 - 18.7v
VR1722 - 20.2v
VR1724 - 23.3v
C1724 - 23.3v
CR1724 - 0.25v
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v
VR1726 - 9.2v
R1727 - 26v
R1725 - 5.8v
C1725 - 3.39v
R1726 - 16v
CR 1723 - 0.573v
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

+65v test point currently 25.32 volts (obviously)

-------
VR1722 will set bottom of R1722 at ~20V above TP+65. Your measurements of TP+65V=25.32V and bottom of R1722=173.5-128.3V=45.2V match so VR1722 is looking good and voltage at the bottom of R1722 is correct given TP+65V voltage.

Voltage drop across R1723 is 18.7V. This says 18.7V/15k=1.25mA is flowing somewhere. Unfortunately you didn’t measure voltage across R1724 but voltage across C1724 and voltage across VR1724 are same (23.3V) which leaves zero voltage for R1724 (please confirm). All other indications (CR1724 is not forward biased, VR1724 has less than 56V) also say current is not flowing into the opamp output. This doesn’t mean there is anything wrong with that branch, simply something else is pulling the current away. Other path is through diode CR1723.

From the voltages none of the devices Q1732, Q1736, CR1732 have gross failures. Perhaps they became leaky or damaged in a subtle way. I don’t like testing components without having a good guess of which one failed but this may be a case where testing Q1732, Q1736, and CR1732 may identify a bad component. My guess is Q1732 is bad, given its low Vbe but still passing an emitter current almost same as the lost current at the base node. You can start with Q1732 to minimize work.

Ozan

On Mon, Mar 21, 2022 at 08:20 PM, Mark Vincent wrote:

R1734 with 25,25V across it?! That much voltage across a 3 ohm 1/2W resistor
would blow it up.

---
That is what I thought too but C1737 voltage is 25.32V and Q1734E is at 25.39V. Voltage across R1734 is the difference between them, 25.25V must be a typo. James also noted +65V point is at +25.32V.
Ozan

James,

R1734 with 25,25V across it?! That much voltage across a 3 ohm 1/2W resistor would blow it up. That resistor being open would cause the low B+. It is possible C1737 is shorted. That would also open that resistor. You may have already replaced it. If so, then back to that 3 ohm resistor. If you do not have or can get a 3 ohm resistor, a 2,7 ohm will work. See about CR and VR1718.

Mark

Great reply Harvey.

Made me smile. Thank you.

Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards

I take it you mean via the connectors?

and the chassis.

Do you mean disconnect any ground wires?

That's because you keep removing parts. Stop doing that unless you can justify removing them

Fair enough. The two diodes had a leg lifted in order to help find the last 'good point' with the voltage readings. All components are back in now other than the +15v rail, which still has a dead short lurking somewhere.

So, voltages...

The CR1721 +65v lines are reading 173.5v and 86 volts

In the hope that it may illuminate the situation, I just did a series of quick measurements 'across' the components of the +65v section, which are as follows;

R1722 - 128.3v
R1723 - 18.7v
VR1722 - 20.2v
VR1724 - 23.3v
C1724 - 23.3v
CR1724 - 0.25v
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v
VR1726 - 9.2v
R1727 - 26v
R1725 - 5.8v
C1725 - 3.39v
R1726 - 16v
CR 1723 - 0.573v
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

+65v test point currently 25.32 volts (obviously)

There is lots I wish to add in response to the earlier post however it is very late here.

I'll get back on it tomorrow evening.


James

Interleaved:

On 3/20/2022 7:53 PM, James55 wrote:

A quick update.

It's been another week of long hours trying attempting to get the voltage rails in order.

It appears that those high voltages damaged several components, which has meant backtracking and testing previously tested stuff and replacing more transistors, rsitors and diodes... so far...

This is not surprising.? Resistors smoking is generally a Bad Sign (tm).

I am currently at focusing solely on the 65v and 140v rails which begin at the output of CR1721 bridge rectifier. According to the manual the voltages should be 173v and 83v respectively from the positives or C1722 and C1723, and whoopeedoo, we have 175 and 85 volts, which is close enough for me right now.

Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards and the chassis.

The 140 volt is piggybacked onto the 65 volt supply, and the 160 volts is piggybacked onto the 140 volt supply.? Ignore them for now

Following the 173v line, it supplies the collector of Q1716 after it feeds R1711 (33K), before splitting to feed R1723 (15k) and VR1722 (20v Zener), and here is where things start to get interesting, as according to the schematic, there should be 67.2v on the other side of R1723. This is only possible however, with both the VR1722 (zener) and CR1723 disconnected from the circuit.

As I said, ignore the 140 volt supply itself (that is the +140 appearing at the cathode of Cr1719).? If you have the bulk voltage (+173) withing tolerance, and you do, that's enough.

Please stop disconnecting things unless you can explain why (in terms of circuitry).? Not kidding, those parts could be useful.

For instance, VR1722 says that the bottom end of R1722 will be 20 volts higher than the output voltage.? Is it?? Then the zener is fine.? The voltages you are looking at, the 67 volts, are ONLY POSSIBLE WHEN THE SUPPLY IS WORKING OK.? We know it isn't.? Don't expect the 67 volts to be somehow "good" if the +65 isn't right.

VR 1722 is a 20v, 500mW device TEK 152-0304-00, which I believe can be replaced with a 1N938B. I could rig something up to test this, but the following diode won't be possible so it'll probably get changed.

NO, please don't unless the voltage across the zener is not 20 volts.? If it's less, then something is pulling down the 65 volt supply.

CR 1723 is a 175v, 100mA device TEK 152-0061-00 which I've been unable to find an equivalent. It said somewhere that it could it be 1N3718 but that is a completely different casing.

CR1723 needs to be about a 200 volt 100ma (or better) silicon diode.? Unless it doesn't have 0.7 volts drop across it when forward biased, then it's good.

This is all great practice but I have found myself going in circles at times.

That's because you keep removing parts.? Stop doing that unless you can justify removing them.? Realize that the circuit will generally not work the way you think with parts missing.

At the beginning of the week, I had measured and written down all of the voltages on either side of each component whilst using the dimbulb, locating the damaged components this way, only to be careless and connect the scope to the mains with a leg of R1715 lifted, which promptly blew something up (finally located the dead Q1734 TIP31C) but that is now history, it's just that it all consumes time.

Then you'll not be doing that mistake again, will you?? That's how we learn.

After years of working with transformers, motors, capacitors and the like, I thought I had a fair understanding of linear power supplies. I had thought that I understood how op-amps worked with the negative feedback loops, yet I am still not seeing how this section is working? It would have helped enormously to have every single correct voltage on both sides of every component, but hey!

Yeah, I'd like to see what those voltages are to analyze the circuit.

For instance, U1724A op-amp, CR1724, VR1724 and CR1723... Pin #1 of U1724 is an output (8.3v) which connects directly to the cathode of CR1724, which itself is back to back with VR1724 (56v zener), which in turn has 67.2v on its cathode. I had assumed that the op-amp was regulating the base of Q1732, yet am struggling to grasp how it's pin #1 is passing a signal through an already busy junction?

Ah, now here's what's going on there.

R1722 is trying to pull up the base of Q1732.? It does so through a diode.? If, for some reason, the voltage (the mythical 67.5 volts) goes to zero, then CR1723 is back biased.? What you have is a circuit where you have a series circuit with voltage drops.? At the output of U1724A, you have (we hope) 8.3 volts.? Current is flowing through R1723, through R1724, through VR1724, and through CR1724.? The voltage at the anode of CR1724 will be about 9 volts.? The voltage at the cathode of VR1724 will be 56 volts higher, or or about 65 volts.? The voltage across R1724 is proportional to the current through VR1724, which goes through the op amp output.? The op amp output is connected to only sink current because of Diode CR1724.? IIRC, I figured out the current to be about 13 ma, so 13 volts drop across R1724.

Do you see how that 67.2 volts is arrived at?

Now, Q1732 and Q1734 are a darlington (ignore Q1736 for the moment).? That explains why (Vbe drops), the voltage at the output is about 2.2 volts less (2 * vbe + 1x diode).

Now, because the load on the power supply is kinda small when nothing is connected, there's very little base current in Q1732. That doesn't pull down much current through CR1723.? Q1736 bypasses the regulator (a bit) and provides a minimum load to ground on the cathode of CR1723, which means that the diode will have enough current to conduct properly.

The 65 volts is divided by the combination of R1735, R1736, and R1737 so that the arm of the pot provides the voltage that the op amp compares the 9 volt reference to.? Changing the pot changes the division ratio, and then that tells the op amp which output voltage is "acceptable" (and produces the 9.0 volts).

So: do the voltages from pin 1 of U1724 up to the anode of CR1723 look good?? Ignore the voltage at pin 1, but add them all up. What are the voltage drops across each component?

Then is the voltage at pin 1 of U1724 right or wrong?? what is it?

Ignore the +160, Ignore the +140.? All that has to be right is the +173 and the +83 for the circuit to be supplied what it needs.

WE need to know the voltages at pins 1,2,3,4 and 8 of U1724A.

We need to know the voltage drops across CR1724, Vr1724, R1724 and CR1723.

What are the VBE drops on Q1732, Q1734, and Q1736.

Do you see what I'm doing?

Go through the circuit description.? Ask questions if you don't see how it works.

Current through R1723 goes two ways, one through Cr1723 to make the output section work, and then through R1724 to make the regulator do its job.

As mentioned at the beginning of this thread, I can no longer see the wood for the trees.

Please analyze the circuit.

Harvey

I'll see which components can be found tomorrow and see if we get any further?

Hope everyone is well.


James

A quick reply to all, as am in the middle of a jo, but...

...there is a 75v zener (VR1718) which I noticed this morning that has failed, so I'm gonna change that first before anything.
This is also why I was optimistic in my earlier post, however I shall be carefully considering all the suggestions.

I shan't be running the scope until the high voltages have been resolved.

Must go.

James

James,

Well done for discovering the error but you need to get the +65V fixed before something else gets too hot.

You need to (quickly ?) measure the voltage at each point along the chain from (pin 1 of U1724), CR1724, VR1724, R1724, CR1723, base of Q1732 and base of Q1734. The junction of R1724 and CR1723 should be at 67.3V to get the +65V at the output. Hopefully somewhere along this chain you will find the component that is malfunctioning. If pin 1 of U1724 is at more than 22V then check pin 8 to see if the positive supply to the op-amp is regulated correctly by VR1725.

If all the above components are working you shouldn't be able to get more than about (22V + 0.6V + 56V + around 2V IR drop across R1724) ~= 81V at the junction of R1724 and CR1723. So with +94V on the output there should be a problem somewhere in the above path.

Regards,

Roger

James,

Great to read you found your problem. I read the posts to see if I could help further. The others were masterful in helping! Stellar gentlemen who well deserve respect. I agree with Ozan about setting the input to the high position due to the mains voltages going up since these were made.

The 15V supply being dead, start to see if you have the B+ on the main filter. If the condenser is bad, the one that made the noise, a 871-B41505A7688M000 (Mouser part number) will work. The 10mfd output filter can be raised in value, if you want, and be 25V. Avoid using a 20V tantalum, too close to the tantalum's rating. a ULD 47mfd 25V will work fine. I did mention to see if VR1726 is open (likely) to get a 85-BZX85B9V1 (Mouser part number) as the replacement. Also check the devices in the 15V supply. The IC, transistor(s) or some combination could be bad.

Jim A. is right about a DMM not being good at measuring ripple at high frequencies. An analogue meter, VOM/VTVM, is better.

Mark

On Mon, Mar 14, 2022 at 04:48 AM, James55 wrote:

What had happened is that when the bridge rectifiers were renewed, one of the
CR1721 AC wires had inadvertently been swapped with one of the neighbouring
wires which had originally connected to the centre of C1723, whch then links
to the transformer centre tap.

This explains why center tap wasn't behaving like center tap.

+140v ... 201v
+65 ... 94v
+15 ... 0.8v (kinda expected following the exploding part last night)
+5v ... 7.21v
-15v ... -20.1v
-8v ... -10.2v

These voltages are still wrong and if the scope is powered like this it will cause more damage. Most of them are wrong because +65V is wrong. R1736 adjusts +65V but your measurement of 94V is beyond its adjustment range.

Repair is moving in the right direction. Given the scope was exposed earlier (and now) to much higher voltages than expected there could be several damaged parts in several sections and this could be a long project. If you want to continue the debug it is best to disconnect regulators from rest of the scope if possible and measure following voltages:
What are the new voltages for C1723 and C1722?
What are the voltages at pins 1, 2 and 3 of U1724A?
What is the voltage at the base of Q1732?

Ozan

On Sun, Mar 13, 2022 at 07:37 PM, James55 wrote:

Having never tested for ripple with a DMM I'm not sure for how long the
ripples would stay visible, however, when probed on the AC voltage range,
C1723 flashes 53v, then 11v, then zero, 4.9v, 0.9v then stops on zero. The
entire process lasts maybe three seconds and the same voltages repeat each
time the probes are contacted.

When you touch the probe tip to the contact, the tip jumps from zero to whatever the contact is at. Your DVM sees this as a large AC voltage, which it is. As time passes, the only AC the probe sees is the AC ripple on top of the DC, so what you described above is normal, and the zero AC that your DVM settles on indicates that there's little or no ripple at this point in your circuit.

BTW, using your DVM to measure ripple usually only works when you're working with 50-60 Hz line voltages. If you're looking at the output of a switching power supply, the frequency is likely to be too high for your DVM to measure. Most DVMs have a rather low limit to the AC frequencies they can measure.

On the question of the dim bulb problem:

In case it's not obvious by now, the series connection of the dim bulb, reduced the voltage supplied to the input of your scope. So, while you have the scope input set to your normal line voltage, the scope is actually getting less. You can measure what it's actually getting by probing where the power enters the scope. Let's suppose that you find that voltage to be 20% low. Now you can check the AC that's coming out the secondary side of the power transformer and verify that those voltages are also 20% low. (You'll need to figure your own percentages.)

You'll be comparing transformer input and output AC voltages, so these measurements need to be taken BEFORE any rectifiers or resistors on the outputs.

If you do this with the dim bulb connected, you may be able to convince yourself that the transformer inputs and outputs have been wired correctly, or not, while also avoiding overheating that one resistor, as well as any other downstream components.

I have found the problem.

As mentioned at the beginning of this thread, the scope had been working after the initial repair, and it was only after upgrading the two bridge rectifiers that the problems started.

Following the strange readings last night across CR1721 and its transformer centre tap, I decided to remove the two bridge AC wires and take readings. These were 14Ω across the wires, yet 29Ω across one of the wires and transformer middle, with 14Ω from the other AC wire.

The schematic didn't give any clues....

Fortunately, however, when I bought the scope, I took detailed photos covering every area of each board, detailing all components and connections.

What had happened is that when the bridge rectifiers were renewed, one of the CR1721 AC wires had inadvertently been swapped with one of the neighbouring wires which had originally connected to the centre of C1723, whch then links to the transformer centre tap.


Thank God I took those photos, but I owe you all an apology for leading everyone on a wild goose chase. my only hope now is that there has not been any serious damage done.

A quick check on the rails reads;

+140v ... 201v
+65 ... 94v
+15 ... 0.8v (kinda expected following the exploding part last night)
+5v ... 7.21v
-15v ... -20.1v
-8v ... -10.2v

After everything that has been tested on this scope during this marathon thread, I shall never again feel intimidated by the prospect of deep-diving into a vintage Tek scope.
Nothing compensates for experience and certain things which I had previously studied yet never really grasped their practical application, have now become standard to by knowledge bank.

I would like to sincerely thank everyone who has contributed, with particular thanks to Ozan, Harvey, Jim and Roger, with bonus points going to Harvey for his patience and homework given.


Much respect,

James

You probably do want to up the wattage on that bulb. I've got a board with 5 bulb holders on wired as a dim-bulb tester, so I can play tunes (up to 5 * 200W bulbs if I can find any).

D.