Tektronix 465

[Grapsus]

Hello,

I'm starting to really like talking about my life on this forum. Therefore I will share with you my next repair project.

It's a Tek 465 oscilloscope that I got from local ads for 60 euros, sold for parts or repair.

Some knobs are damaged, the case is upside down, but most importantly there nothing on screen even after playing with focus, intensity and beam finder.

I will now try to troubleshoot it. I hope it's something with the HV PSU and the tube is not damaged somehow.

Any advice on testing the HV circuit ? I have decent DMMs but no HV probes.

See you !

[SoundTech-LG]

After you tested all of the Low Voltage supplies, what were the readings?

[Grapsus]

After you tested all of the Low Voltage supplies, what were the readings?

I haven't measured anything yet, I just received the scope and I will familiarize myself with the service manual before touching anything. But you're right, low voltage PSU will probably be the first step, I see humongous electrolytic caps.

I just opened the case and already found some scary things even if they probably aren't related to the fault. There is a cap with a dangling leg and a tiny transformer that has been ripped off the board.

Some info that I forgot to mention earlier: the scope has been made in 1973, there is a cal sticker from 1993 and when I turn it on, channel 1 and timebase have "uncal" lights on.

[tautech]

You MIGHT need to check the HV CRT supply, commonly minus ~2 kV and a Meter that can do this is useful. I keep an AVO just for this purpose.
As far as checks for the PDA voltage of 10 kV or more ....don't.
Quadruplers rarely fail.

If the 465 has an inverter style PSU for CRT HV you can quickly check for oscillation using a scope probe like a near field probe to search for EMI.
The inverter will have test points listed in the SM for accurate measurements.

Before you waste lots of time on "her", check for CRT heater operation. Beware some scopes have an elevated heater filament voltage. Visual check might be all thats needed.

[PaulAm]

The uncal light just means the center knob on that control has been turned off the detent.

The heater is tied to the cathode which is at -2450V, so you want to be careful with that..  I'd leave that until later.

If somebody's been mucking about, the crt bias control could have been altered, which will result in no visible trace.  You need a HV probe and be able to safely measure the -2450V on the cathode to set that correctly.

By far the most common fault is open power supply filter caps, bad bridge rectifiers and the odd tantalum failure. ALWAYS check the low voltage power supplies to see if they are in spec for V AND ripple.  Faults here can cause all sorts of weird problems.  Only then should you start poking around somewhere else.

It's possible the crt could be bad, but that's about the last thing I would expect.

Get the service/repair manual.  There are free ones on the net with very decent quality scans.  Just search for 465.pdf and you'll find something.

Nice scopes and they are very repairable.

[Grapsus]

I tested the low voltage PSU for DC voltage and ripple by following the service manual. Everything is well within spec. The ripple amplitudes are close to table 6-4 "typical low voltage power supply ripple" values.

According to the SM the next step is to check the CRT high voltage. I tried to pick up the 50 kHz inverter frequency with the probe loop trick but it's suspiciously quiet. I have an analog multimeter but its highest scale is 1000V with 20M impedance. Do you think I could measure the CRT voltage by connecting my AVO trough a 40M resistor ? I would connect everything with the scope powered off, then power it back on without touching anything.

[tautech]

I tested the low voltage PSU for DC voltage and ripple by following the service manual. Everything is well within spec. The ripple amplitudes are close to table 6-4 "typical low voltage power supply ripple" values.

According to the SM the next step is to check the CRT high voltage. I tried to pick up the 50 kHz inverter frequency with the probe loop trick but it's suspiciously quiet. I have an analog multimeter but its highest scale is 1000V with 20M impedance. Do you think I could measure the CRT voltage by connecting my AVO trough a 40M resistor ? I would connect everything with the scope powered off, then power it back on without touching anything.

The AVO's I keep for this have 2.5 or 3 kV terminals, yours not?
There should be test points for waveforms on the primary side of the HV inverter, and also make sure there is supply to the primary side.
It may be the inverter has shut down for an over current event, not familiar with the 465, post an image of that part of the schematic or provide a link.

In you second image a few posts back there are a few orange Tant caps...... renown for failures, others might advise replacement but lets get "her" going first.
All I wish is to give you a "heads up".

[ModemHead]

Pull the "Chopped Blanking" cable out for testing.  The little toroid transformer is part of a blanking circuit that feeds the z-axis (intensity) section.  A fault there can result in the trace being permanently blanked.

[rqsall]

When the screen of my 465 died, it turned out to be the HV multiplier. Black box near the rear, you need to remove the vertical amplifier board to get to it. And also the tube to remove it. However, you may be able to desolder the ground lead of it, without removing the V-board and then turn the scope on. If the HV multiplier is shorted out, you will then see a trace on the screen, but not over the full width. If the HV multiplier is shorted, it will also load down transistor Q1404. Check the voltages on it, they should be

• Collector: +4.4V
• Base: +0.7V
• Emitter: +0.1V

A shorted out HV multiplier will load down the base of Q1404. With the HV multiplier taken out of the circuit, the screen will look like this:

In this picture, the V-board is also not present as you can see. If this is what you then see, replace the HV multiplier if you can find one, or make your own.

Good luck!

[Grapsus]

Thank you all for your replies !

The AVO's I keep for this have 2.5 or 3 kV terminals, yours not?
There should be test points for waveforms on the primary side of the HV inverter, and also make sure there is supply to the primary side.

Nope, I have a Metrix 462C and there is a separate socket for each range, the highest one is labeled 1000V. I should have thought about measuring 1-5 kV
 voltages before picking up an AVO.
 

In you second image a few posts back there are a few orange Tant caps...... renown for failures, others might advise replacement but lets get "her" going first.

Those are absolutely all over the place, if I start replacing them, there will be like 150 caps to change. I'm not sure I'm ready to do all this unless I see at least something on the screen.

In this picture, the V-board is also not present as you can see. If this is what you then see, replace the HV multiplier if you can find one, or make your own.

I'm not sure yet, so I don't think I will be touching the HV circuit unless there is strong evidence that it's at fault.

Pull the "Chopped Blanking" cable out for testing.  The little toroid transformer is part of a blanking circuit that feeds the z-axis (intensity) section.  A fault there can result in the trace being permanently blanked.

You were absolutely right ! With the damaged toroid, chopped blanking was doing something bad to the trigger circuit. When I pulled the chopped blanking cable like you said, the trigger panel suddenly became a lot less dumb. Now the TRIG light blinks periodically and "single sweep" does what it's supposed to. However this wasn't enough to get any visible trace. There must be another problem.

I noticed yesterday, with lights off, that the screen was slightly green, a sort of halo, so I assumed that the bias HV was somehow working (I don't have the equipment to properly measure it) and continued the troubleshooting with that hypothesis.

According to the troubleshooting guide, I have a problem inside the "Sweep Start logic circuit". I will continue working on it tonight. I only hope that the culprit is not U870, that would be a bummer for a scope with so few ICs !

Edit: I have a question: when they say "check QXXX" are you supposed to characterize it with a curve tracer ? I don't have one, so I just checked both junctions with a DMM in diode mode. If both junctions in a BJT are OK, can we say that the transistor is OK in general ? I suppose there may be some weird defects where the junctions are ok and the transistor still not doing its job, but I have no idea how frequent those are and what are the testing practices in the field.

[rqsall]

Thank you all for your replies !

The AVO's I keep for this have 2.5 or 3 kV terminals, yours not?
There should be test points for waveforms on the primary side of the HV inverter, and also make sure there is supply to the primary side.

Nope, I have a Metrix 462C and there is a separate socket for each range, the highest one is labeled 1000V. I should have thought about measuring 1-5 kV
 voltages before picking up an AVO.

You cannot check the HV circuit with an AVO. The input impedance of the AVO is way too low and it will load the circuit down giving you a completely faulty measurement. You need a high impedance (several gigaohms) voltmeter to measure the voltage in this circuit.

Check the base of Q1404. It will tell you if the multiplier is shorted. It's only 0.7 volt if the HV is not shorted and probably only 0.3 volt if the HV is shorted.

[PaulAm]

Sometimes the beam gets deflected way off to the side.  If you see any kind of green glow from the screen the HV is probably OK, or at least working well enough to track down other problems.

[Grapsus]

After I disconnected "CHOPT BLANK" cable like ModemHead said, everything changed. Now I have a proper z blanking signal on TP1486 exactly as described in SM. Still nothing on screen. I guess I will have to check the HV stuff. I will try to start with the lower voltages in the inverter and CRT circuits, maybe I'll find the problem in that way.

[Grapsus]

Still no success with this scope 

I focused on the CRT inverter. There is no debugging chart for this one, so I tried to understand it by myself which was not easy since it's a feedback loop with a lot of transistors and some inductive voodoo.
Now I'm sure that this circuit isn't working at all. On the primary side everything looks good. The primary driving circuit is formed by Q1404, Q1408, Q1413, Q1414, Q1416 and Q1418 which is the power stage. This circuit does try to start because at Q1418 collector there is a clean sine with the right frequency and half the amplitude in comparison with the screenshot 92 which shows the same signal at equilibrium. However, at the same time, there is garbage on the secondary of the transformer, the output signal looks like noise picked up by a dangling wire.

My guess is that the transformer T1420 has a broken connection or is burnt. The problem is that it is in a metal can under the main board and is not easy to access. Tomorrow I'll have to disconnect a lot of stuff in order to access this transformer.

[tautech]

Did you check C1419 & C1412 for correct values and ESR?

I seen the high value resistors fail or drift substantially in value on the secondary (HV) side of the inverter.
But I would eliminate anything dragging down the secondary voltage first, diodes, caps , stuff with a path to GND.

I'm lookin a a SM from here: http://elektrotanya.com/tektronix_465_oscilloscope_full_sm.pdf/download.html

It has some penciled in DC values that may help. (P228)

[rqsall]

Can you just measure the voltage on the base of Q1404? If it's substantially lower than what it specifies on the schematic (link amd page nr in previous post), that's a clear indication of a shorted HV multiplier as I've said earlier. A shorted HV multiplier is a very common failure point of these scopes.

[Grapsus]

Can you just measure the voltage on the base of Q1404? If it's substantially lower than what it specifies on the schematic (link amd page nr in previous post), that's a clear indication of a shorted HV multiplier as I've said earlier. A shorted HV multiplier is a very common failure point of these scopes.

Yep I've already checked Q1404 base. Looks like it's a very important point where the feedback from the secondary comes through R1431D and pulls down in order to provide regulation for the HV. In my case this point is at 3.7 Volts. The SM reference value is 0.7V, but I think you only get this value when the circuit is at equilibrium and some negative HV is present on the secondary. In my case, the circuit tries to start and there is nothing coming from the transformer, so Q1404 stays at its max value.

Did you check C1419 & C1412 for correct values and ESR?

I seen the high value resistors fail or drift substantially in value on the secondary (HV) side of the inverter.
But I would eliminate anything dragging down the secondary voltage first, diodes, caps , stuff with a path to GND.

I remember checking a few electrolytic caps yesterday, but I guess now I'll have to check every component attached to the secondary in order to understand why nothing comes out of it.

I'm lookin a a SM from here: http://elektrotanya.com/tektronix_465_oscilloscope_full_sm.pdf/download.html

It has some penciled in DC values that may help. (P228)

Yes I use the exact same PDF. The only drawback are the PCB component maps, almost impossible to read.

Those typical DC values are very nice but I think they are only true when the circuit has already started, before that, those voltages could be different without necessarily have bad components nearby.

[rqsall]

Can you just measure the voltage on the base of Q1404? If it's substantially lower than what it specifies on the schematic (link amd page nr in previous post), that's a clear indication of a shorted HV multiplier as I've said earlier. A shorted HV multiplier is a very common failure point of these scopes.

Yep I've already checked Q1404 base. Looks like it's a very important point where the feedback from the secondary comes through R1431D and pulls down in order to provide regulation for the HV. In my case this point is at 3.7 Volts. The SM reference value is 0.7V, but I think you only get this value when the circuit is at equilibrium and some negative HV is present on the secondary. In my case, the circuit tries to start and there is nothing coming from the transformer, so Q1404 stays at its max value.

  seems like something failed open... hopefully it's just a HV cap somewhere on the main board, but could be the transformer or the HV multiplier 

[Grapsus]

This is by far my most scary repair ever 

I measured everything I could around the inverter transformer T1420, and the windings seem ok, around 0.5 Ohms on the primary and 5 Ohms on the secondary. However, the main rectifying diode CR1421 seems to conduct in both directions. It can't be measured directly because it's under the board with the transformer, but I measured it through a winding, between R1428 and C1421, there's no threshold voltage and only the winding resistance. Why the hell did the only part that can't be accessed decide to fail ?!

The only way to access that can with the transformer and the diode is to remove the main board which can't be removed without first taking out everything else. It's a nightmare, it took me an hour the disconnect the trigger pcb alone. There's still the input board. Then I'll have unsolder every wire that goes to the main board including the high voltage lines soldered with some special silver alloy. It will be a miracle if it can be put back and work again.

Did people take a month vacation back in the day to service those things ?

[tautech]

 
Be aware these HV DC restorer diodes have a high V_F.
Sometimes in excess of 10V, check a matching datasheet especially if your are to substitute it. 

[rqsall]

This is by far my most scary repair ever 

I measured everything I could around the inverter transformer T1420, and the windings seem ok, around 0.5 Ohms on the primary and 5 Ohms on the secondary. However, the main rectifying diode CR1421 seems to conduct in both directions. It can't be measured directly because it's under the board with the transformer, but I measured it through a winding, between R1428 and C1421, there's no threshold voltage and only the winding resistance. Why the hell did the only part that can't be accessed decide to fail ?!

You need to remove the vertical board, not the main board.

Good luck

[stephunk]

Do you really have ALL voltages from power supply OK? Check if the -8V rail is OK. The display function(high voltage) depends also on this. When i repaired a 465, the bridge rectifier CR1561 was bad.

[Grapsus]

Be aware these HV DC restorer diodes have a high V_F.
Sometimes in excess of 10V, check a matching datasheet especially if your are to substitute it. 

Yep I understand that I won't be able to replace CR1421 with a jellybean part. The service manual says "VG-12X; SILICON 1200V 5 mA". No public datasheet for "VG-12X". 1200 V must be the breakdown voltage right ? I hope there is at least something equivalent for this part that can still be purchased.

Do you really have ALL voltages from power supply OK? Check if the -8V rail is OK. The display function(high voltage) depends also on this. When i repaired a 465, the bridge rectifier CR1561 was bad.

I've already thoroughly checked the low voltage PSU and it complies with the service manual in terms of DC value and ripple for all rails.

You need to remove the vertical board, not the main board.

Are you sure you can access the transformer and the diode by removing only the vertical board ? Have you already done it ?

[tautech]

Be aware these HV DC restorer diodes have a high V_F.
Sometimes in excess of 10V, check a matching datasheet especially if your are to substitute it. 

Yep I understand that I won't be able to replace CR1421 with a jellybean part. The service manual says "VG-12X; SILICON 1200V 5 mA". No public datasheet for "VG-12X". 1200 V must be the breakdown voltage right ? I hope there is at least something equivalent for this part that can still be purchased.

Certainly not jellybean, however carefull substitution is very possible now you have a datasheet. 

http://www.datasheetarchive.com/dlmain/Datasheets-21/DSA-414399.pdf

[Grapsus]

Certainly not jellybean, however carefull substitution is very possible now you have a datasheet. 

http://www.datasheetarchive.com/dlmain/Datasheets-21/DSA-414399.pdf

Wow, well done ! We can see that there is a typo in the service manual: according to the datasheet VG-12X has 12kV maximum reverse voltage and not 1200V !!!