Busted OScope, I think the transformer is fried!

[smeezekitty]

There is a discharge tube under the high voltage that is blinking twice a second.

That certainly doesn't sound right. The manual I have for the 561A doesn't include any mechanical drawings or photos so I have no idea of the physical location of components. If you could identify the tube concerned that would help, i.e. Vxyz. or tube type, 6xyz.

Steve A.

Not a valve, its a discharge tube (http://www.google.com/search?q=discharge+tube&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a)
It has no label.

[Steve Anderson]

Not a valve, its a discharge tube...It has no label.

Right, identified it from the parts list and the circuit diagram, it's an OG3 (V609). It's part of the -100V power supply. I have only seen this arrangement once before in a Harris 250kW TV transmitter, I would have expected to see something like a VR150. But I guess they had their reasons.

I haven't been able to find any data on this tube but analyzing the circuit I suspect the flashing is normal. One way to confirm this is to measure the -100V supply on pin 23 of both plug-in units. After all this time it might not be exactly -100V, but within 5% should be fine.

Interesting to note that the Philips datasheet for the two wire-ended 5642 EHT rectifiers (V822 & 832) rates their life at only 500 hours! They probably lasted far longer than this but for a professional piece of equipment left on eight hours a day that's less than three months! One of the reasons I suspect they changed to semiconductors for the 561B.

The reason for the dim display could be these two tubes are past it, and probably the arcing you had didn't help. I would very tempted to find a source for these and the 6CZ5 (V800). In the US I have found these two...they both have both tubes...

Antique Electronic Supply, (AZ), www.tubesandmore.com

Vacuum Tubes Inc. (FL), www.vacuumtubesinc.com

The 12BH7 (V814) used as part of this EHT inverter you should be able to get almost anywhere.

Steve A.

[smeezekitty]

Not a valve, its a discharge tube...It has no label.

Right, identified it from the parts list and the circuit diagram, it's an OG3 (V609). It's part of the -100V power supply. I have only seen this arrangement once before in a Harris 250kW TV transmitter, I would have expected to see something like a VR150. But I guess they had their reasons.

I haven't been able to find any data on this tube but analyzing the circuit I suspect the flashing is normal. One way to confirm this is to measure the -100V supply on pin 23 of both plug-in units. After all this time it might not be exactly -100V, but within 5% should be fine.

Interesting to note that the Philips datasheet for the two wire-ended 5642 EHT rectifiers (V822 & 832) rates their life at only 500 hours! They probably lasted far longer than this but for a professional piece of equipment left on eight hours a day that's less than three months! One of the reasons I suspect they changed to semiconductors for the 561B.

The reason for the dim display could be these two tubes are past it, and probably the arcing you had didn't help. I would very tempted to find a source for these and the 6CZ5 (V800). In the US I have found these two...they both have both tubes...

Antique Electronic Supply, (AZ), www.tubesandmore.com

Vacuum Tubes Inc. (FL), www.vacuumtubesinc.com

The 12BH7 (V814) used as part of this EHT inverter you should be able to get almost anywhere.

Steve A.

No, it is not an OG3!
It is a heaterless glass tube that has 3 elements in it and is underneth the box that is marked "Danger high voltage".

[Steve Anderson]

No, it is not an OG3! It is a heaterless glass tube that has 3 elements in it and is underneth the box that is marked "Danger high voltage".

Yep, that agrees exactly with what's in the manual. The same OG3 is used in the 561A & 561B. But whatever...is the -100V anywhere near -100V?

Unless the actual tube you have fitted is not an OG3...the OG3 might be a special Tektronix part (not unknown) and was replaced with a 'near equivalent'.

Steve A.

[smeezekitty]

No, it is not an OG3! It is a heaterless glass tube that has 3 elements in it and is underneth the box that is marked "Danger high voltage".

Yep, that agrees exactly with what's in the manual. The same OG3 is used in the 561A & 561B. But whatever...is the -100V anywhere near -100V?

Unless the actual tube you have fitted is not an OG3...the OG3 might be a special Tektronix part (not unknown) and was replaced with a 'near equivalent'.

Steve A.

No, the OG-3 is on the other side of the board.

[Steve Anderson]

So unless you can tell me which tube this is I cannot help you further.

You said it was a discharge tube, the only one fitted is the OG3 in the -100V PSU. Unless some idiot has moved tubes around.

Steve A.

[smeezekitty]

I attached some pictures.

[Steve Anderson]

They are the two wire-ended EHT rectifiers type 5642 I referred to earlier. They are not gas-discharge at all. This is your problem. They are the only wire-ended tubes in the device. You need two new ones of those. Unless that arcing has also fried the EHT transformer. But the only way you're gonna find out is to replace the two 5642s.

At the same time I would replace the 6CZ5.

Steve A.

[smeezekitty]

What should the input voltage to the ETH transformer be?

[Steve Anderson]

What should the input voltage to the ETH transformer be?

The AC part is going to be some gross AC waveform (not a sine-wave) at several kHz, and the manual doesn't specify it anyway. The DC element on terminal 5 of the EHT transformer I would expect to be between 350-400V, the supply is an unregulated 420V. Terminal 5 has a 0.02uF capacitor (C801) from it to ground, and is fed from the unregulated supply by a 680 Ohm, 2W resistor (R801)...worth checking that.

But your problem is on the output side of the EHT transformer, not the input, but it's worth checking anyway.

Steve A.

[smeezekitty]

What should the input voltage to the ETH transformer be?

The AC part is going to be some gross AC waveform (not a sine-wave) at several kHz, and the manual doesn't specify it anyway. The DC element on terminal 5 of the EHT transformer I would expect to be between 350-400V, the supply is an unregulated 420V. Terminal 5 has a 0.02uF capacitor (C801) from it to ground, and is fed from the unregulated supply by a 680 Ohm, 2W resistor (R801)...worth checking that.

But your problem is on the output side of the EHT transformer, not the input, but it's worth checking anyway.

Steve A.

I registered slightly over 300v going in.
When the scope is first warming up, the rectifer is constant blue, as it warms up some more, it starts blinking.

[Steve Anderson]

I registered slightly over 300v going in. When the scope is first warming up, the rectifer is constant blue, as it warms up some more, it starts blinking.

Yep, that confirms there is too much current draw from the secondary of the transformer. I would expect that 470 Ohm resistor is getting very hot!! Did you measure its resistance? Measure the voltage at the other end of it (the raw supply) normally it should be 420V, but now I guess somewhat less.

Steve A.

[smeezekitty]

I registered slightly over 300v going in. When the scope is first warming up, the rectifer is constant blue, as it warms up some more, it starts blinking.

Yep, that confirms there is too much current draw from the secondary of the transformer. I would expect that 470 Ohm resistor is getting very hot!! Did you measure its resistance? Measure the voltage at the other end of it (the raw supply) normally it should be 420V, but now I guess somewhat less.

Steve A.

What is the phscycle location of that resistor?

[Steve Anderson]

What is the phscycle location of that resistor?

As I said before there are no mechanical drawings or photos in the manual I have, but follow the wire from terminal 5 back and you'll come to a resistor...that's the only one connected to terminal 5.

Steve A.

[smeezekitty]

Schematic attached.
I am not sure which one is terminal 5.