Newbie with Tek 465

[med6753]

Not shorted, but I found C1220 had been leaking (visual). It should be 1000 mfd @ 10V, but it measures around 455. I assume this is a Tantalum (never worked with them before)?

That will be specified in the manual.

If it is leaking then it is unlikely to be a tantalum. When very expensive milspec wet slug tants leak, PCB tracks completely disappear

I finally found it in the manual specifying which are tantalums and which are not. So old tantalums have a habit of shorting?

Yes, the beaded tantalums are notorious for shorting and the 400 series Tek scopes are loaded with them.

[tggzzz]

Not shorted, but I found C1220 had been leaking (visual). It should be 1000 mfd @ 10V, but it measures around 455. I assume this is a Tantalum (never worked with them before)?

That will be specified in the manual.

If it is leaking then it is unlikely to be a tantalum. When very expensive milspec wet slug tants leak, PCB tracks completely disappear

I finally found it in the manual specifying which are tantalums and which are not. So old tantalums have a habit of shorting?

Yes, tant beads do short circuit. There are many many references giving many examples of that.

We now know that it is sensible to derate the tant beads voltage so that the DC voltage aross them is less than, say, 2/3 of the rated voltage.

[Restorationnut]

Not shorted, but I found C1220 had been leaking (visual). It should be 1000 mfd @ 10V, but it measures around 455. I assume this is a Tantalum (never worked with them before)?

That will be specified in the manual.

If it is leaking then it is unlikely to be a tantalum. When very expensive milspec wet slug tants leak, PCB tracks completely disappear

I finally found it in the manual specifying which are tantalums and which are not. So old tantalums have a habit of shorting?

Yes, tant beads do short circuit. There are many many references giving many examples of that.

We now know that it is sensible to derate the tant beads voltage so that the DC voltage aross them is less than, say, 2/3 of the rated voltage.

Obviously, I'll need to check each tantalum. But should I just replace them all with aluminum electrolytics or film, or reduce the DC across them? And in the second case, how would I do that without calculating and adding a series resistor to each one?

[tggzzz]

Not shorted, but I found C1220 had been leaking (visual). It should be 1000 mfd @ 10V, but it measures around 455. I assume this is a Tantalum (never worked with them before)?

That will be specified in the manual.

If it is leaking then it is unlikely to be a tantalum. When very expensive milspec wet slug tants leak, PCB tracks completely disappear

I finally found it in the manual specifying which are tantalums and which are not. So old tantalums have a habit of shorting?

Yes, tant beads do short circuit. There are many many references giving many examples of that.

We now know that it is sensible to derate the tant beads voltage so that the DC voltage aross them is less than, say, 2/3 of the rated voltage.

Obviously, I'll need to check each tantalum. But should I just replace them all with aluminum electrolytics or film, or reduce the DC across them? And in the second case, how would I do that without calculating and adding a series resistor to each one?

There is no uniquely right/wrong solution, except that reducing the applied voltage is not an option, of course.

There are many discussion of the topic on this forum and elsewhere, e.g. TekScopes. There's no point in my repeating some of them poorly!

Start by replacing tant beads with tant beads with an appropriate voltage rating. Simply replacing a 15V tant on a 13V line, as in the 485, would not be wise

[med6753]

Not shorted, but I found C1220 had been leaking (visual). It should be 1000 mfd @ 10V, but it measures around 455. I assume this is a Tantalum (never worked with them before)?

That will be specified in the manual.

If it is leaking then it is unlikely to be a tantalum. When very expensive milspec wet slug tants leak, PCB tracks completely disappear

I finally found it in the manual specifying which are tantalums and which are not. So old tantalums have a habit of shorting?

Yes, tant beads do short circuit. There are many many references giving many examples of that.

We now know that it is sensible to derate the tant beads voltage so that the DC voltage aross them is less than, say, 2/3 of the rated voltage.

Obviously, I'll need to check each tantalum. But should I just replace them all with aluminum electrolytics or film, or reduce the DC across them? And in the second case, how would I do that without calculating and adding a series resistor to each one?

There is no uniquely right/wrong solution, except that reducing the applied voltage is not an option, of course.

There are many discussion of the topic on this forum and elsewhere, e.g. TekScopes. There's no point in my repeating some of them poorly!

Start by replacing tant beads with tant beads with an appropriate voltage rating. Simply replacing a 15V tant on a 13V line, as in the 485, would not be wise

Replacing them all in a 465 will be a quite expensive proposition. And if the design originally called for a tantalum in general you should replace with same. My experience with these little buggers is that the higher values, approx 15uf to 47uf, tend to be the most trouble prone. And 47uf the worst of them all. Start with those values if you decide you want to replace them. That will cover the vast majority of the magic smoke releases. 

Edit, and if the original cap was rated at 25V and the applied voltage is 15V or greater replace with a 35V rated part. 

[Restorationnut]

Understand. Do you have a specific source for the tantalums? My normal suppliers don't carry them.

[tggzzz]

Understand. Do you have a specific source for the tantalums? My normal suppliers don't carry them.

https://octopart.com/
https://www.findchips.com/

Note that in many cases a precise equivalent is not necessary. Classic example is PSU line decoupling caps.

[Restorationnut]

Understand. Do you have a specific source for the tantalums? My normal suppliers don't carry them.

https://octopart.com/
https://www.findchips.com/

Note that in many cases a precise equivalent is not necessary. Classic example is PSU line decoupling caps.

Thank you.

[med6753]

Understand. Do you have a specific source for the tantalums? My normal suppliers don't carry them.

Mouser has exact replacements.

[Restorationnut]

Well....not having much luck. It's not easy trying to get to the back side of the interface board.

Still no -8V. I followed procedures to isolate the CRT circuit. No luck. Then isolate the horiz. amp. No luck. Replaced all the electrolytics, and a lot of the tantalums. No luck. Checked all the rest of the tantalums: all check good.

Removed the bridge rectifier (CR1561). It tests good. However, with the rectifier in the circuit, I only get +3V AC from the transformer. Can't see any way to disconnect the bridge in order to retest the secondary winding.

All other voltages check O.K. When I move the intensity knob back and forth, I get a blip on the screen.

I'm now officially lost.

[med6753]

Well....not having much luck. It's not easy trying to get to the back side of the interface board.

Still no -8V. I followed procedures to isolate the CRT circuit. No luck. Then isolate the horiz. amp. No luck. Replaced all the electrolytics, and a lot of the tantalums. No luck. Checked all the rest of the tantalums: all check good.

Removed the bridge rectifier (CR1561). It tests good. However, with the rectifier in the circuit, I only get +3V AC from the transformer. Can't see any way to disconnect the bridge in order to retest the secondary winding.

All other voltages check O.K. When I move the intensity knob back and forth, I get a blip on the screen.

I'm now officially lost.

When CR1561 is removed what is the AC voltage from the transformer across pins 21 and 22?

[Restorationnut]

Well....not having much luck. It's not easy trying to get to the back side of the interface board.

Still no -8V. I followed procedures to isolate the CRT circuit. No luck. Then isolate the horiz. amp. No luck. Replaced all the electrolytics, and a lot of the tantalums. No luck. Checked all the rest of the tantalums: all check good.

Removed the bridge rectifier (CR1561). It tests good. However, with the rectifier in the circuit, I only get +3V AC from the transformer. Can't see any way to disconnect the bridge in order to retest the secondary winding.

All other voltages check O.K. When I move the intensity knob back and forth, I get a blip on the screen.

I'm now officially lost.

When CR1561 is removed what is the AC voltage from the transformer across pins 21 and 22?

That's the next problem. I can't seem to figure out how to remove CR1561 from the board without lifting the board up. And I can't lift the board up without removing the two adjacent boards. And that means removing all of the connecting wires. And at that point, I wouldn't dare apply power. Catch 22. It's only pins 21 and 22 on the schematic. The transformer is wired directly to the interface board at two points: 6 and 6. and have to remove the interface board in order to gain access to the bottom, where the wires are attached.

They certainly didn't make this simple. Am I missing something? Why would they hard-wire the xformer directly to the interface board?

[med6753]

OK, I just did a comparison between the -8V supply in a 465 vs a 465B and they are almost identical. The only difference is the 465B has a 5 Amp fuse between the transformer and the bridge rectifier which the 465 lacks and the value of the main filter cap is different. So tomorrow morning I'll pull the case on one of my functional 465B's and take some resistance readings at various points and pass them on to you. I still suspect the supply is shorted on the output and this will settle that.

[med6753]

OK, as promised I pulled a 465B out of the case and took some measurements.

-8 TP. Measures 47.1 ohms referenced to ground (chassis). Now your 465 might measure a little lower since it has a higher discrete vs integrated component count. But I would think it should measure no lower than 30 - 35 ohms. The voltage is right at -7.99V

Transformer secondary. Measures 11.4VAC at the rectifiers.

C1562. Measures +12.9VDC across it.

Pay special attention to R1569 (1.1 ohm) on the emitter of Q1566. I've seen it burn up before. Make sure CR1564 is OK. Verify all the resistors around U1554B.

Hope this helps. I will leave this 465B out of the case for the rest of the day if you need additional measurements.

[Restorationnut]

OK, as promised I pulled a 465B out of the case and took some measurements.

-8 TP. Measures 47.1 ohms referenced to ground (chassis). Now your 465 might measure a little lower since it has a higher discrete vs integrated component count. But I would think it should measure no lower than 30 - 35 ohms. The voltage is right at -7.99V

Transformer secondary. Measures 11.4VAC at the rectifiers.

C1562. Measures +12.9VDC across it.

Pay special attention to R1569 (1.1 ohm) on the emitter of Q1566. I've seen it burn up before. Make sure CR1564 is OK. Verify all the resistors around U1554B.

Hope this helps. I will leave this 465B out of the case for the rest of the day if you need additional measurements.

You are going above and beyond, and I really appreciate it. I'll reply once I make the measurements.

[Restorationnut]

O.K. Got my 12VAC back at the transformer. Broken wire. -8 to ground is 50Ω. Still nothing across C1562. R1569 is 1.2Ω. CR1564 is good. All resistors around U1554B are within tolerance. Even though I get 50Ω between -8 and ground, CR1569 shows short both directions on my diode tester.

[med6753]

Lift one side of CR1569 and check it again. If it is indeed shorted that would explain no output. But it doesn't explain how you measure 50 ohms at the TP to gnd. It should read short also. Perhaps that diode only shows short in the presence of a higher voltage than your ohm meter can provide. If true that would be an unusual event.

 Note: you can pull and install components from the front side of that double sided board. It has fully plated feed thru's.

[Restorationnut]

Lift one side of CR1569 and check it again. If it is indeed shorted that would explain no output. But it doesn't explain how you measure 50 ohms at the TP to gnd. It should read short also. Perhaps that diode only shows short in the presence of a higher voltage than your ohm meter can provide. If true that would be an unusual event.

 Note: you can pull and install components from the front side of that double sided board. It has fully plated feed thru's.

Diode's good.

[med6753]

Yea, I thought so. So, if you have good transformer secondary (12VAC) but nothing across C1562 that basically leaves 2 things. Either there is an open somewhere or bridge assembly CR1561 is bad. 

[Restorationnut]

Yea, I thought so. So, if you have good transformer secondary (12VAC) but nothing across C1562 that basically leaves 2 things. Either there is an open somewhere or bridge assembly CR1561 is bad.

I've already run continuity at every point on that part of the circuit, and everything checks out. I'll retest CR1561, but it did test O.K. once. Then I'll retest all components.

If I still can't find anything, I guess it'll go on the shelf for a while.

[Restorationnut]

Now CR1561 shows open on one side! All I have are 1N4007's. I know it's an overkill at 1000V, but can I build a bridge with those, or should I try an find something else?

[med6753]

Now CR1561 shows open on one side! All I have are 1N4007's. I know it's an overkill at 1000V, but can I build a bridge with those, or should I try an find something else?

They will work just fine. Or any bridge 50PIV/3A and above will be fine.

Report back if you finally have -8V.   

[Restorationnut]

Now CR1561 shows open on one side! All I have are 1N4007's. I know it's an overkill at 1000V, but can I build a bridge with those, or should I try an find something else?

They will work just fine. Or any bridge 50PIV/3A and above will be fine.

Report back if you finally have -8V. 

Tried it twice. Double checked polarity. With it out of the circuit, I get 12VAC going into the bridge, and 11VDC coming out. But as soon as I insert it into the circuit, the lower diode smokes. Both times.

[med6753]

From the symptoms you describe it sounds like Q1566 is shorted itself OR the collector is shorted to ground via the mounting. The mica insulator might have given way.

The transistor itself is a MJE2801 which crosses to a readily available NTE182 on Amazon.

Edit, when I measure resistance from the plus side of C1562 to Gnd I get almost 12MEG.

[Restorationnut]

From the symptoms you describe it sounds like Q1566 is shorted itself OR the collector is shorted to ground via the mounting. The mica insulator might have given way.

The transistor itself is a MJE2801 which crosses to a readily available NTE182 on Amazon.

Edit, when I measure resistance from the plus side of C1562 to Gnd I get almost 12MEG.

First, I want to say thanks for all the assistance. But I've spent entirely too much time on something that I don't use, anyway. So it's going on a shelf. Perhaps in the future I'll re-visit it, and all the help you have given. But for now, it's back to radios.

Thanks again.