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7603 Strange HV Need Help


Robert Simpson
 

In one of my 7603s the HV wasn't working. Found the 15V fuse blown and possible bad HV power transistors. Replaced the 2N3055 power transistors, the four electrolytic capacitors on the regulator board and the fuse. Now there is a 28V sine wave at about 42 KHz.

HV Initially Measures 2.4KV, but dropped to 1.9 then to zero. I also put an ammeter inline with the 15V unregulated feed and runs stable at 0.44 amps.


The oscillator does not change at all as the HV fades. Even an hour later it stays a 0.44 amps, 28V and 42KHz. The HV box is barely warm.


I tried searching through the messages, but didn't find anything like this. The HV briefly comes partly back after the scope has been turned off for awhile, but still fades to zero volts. What could cause the HV to fade without changing the Oscillator activity?
Bob


 

Hi,


The multiplier is the likely problem. You might try disconnecting the anode lead to see if you get the -2950 but I think you need to look at replacing the multiplier.


Jerry Massengale

-----Original Message-----
From: go_boating_fast@yahoo.com [TekScopes] <TekScopes@yahoogroups.com>
To: TekScopes <TekScopes@yahoogroups.com>
Sent: Wed, Dec 9, 2015 10:36 pm
Subject: [TekScopes] 7603 Strange HV Need Help






In one of my 7603s the HV wasn't working. Found the 15V fuse blown and possible bad HV power transistors. Replaced the 2N3055 power transistors, the four electrolytic capacitors on the regulator board and the fuse. Now there is a 28V sine wave at about 42 KHz.

HV Initially Measures 2.4KV, but dropped to 1.9 then to zero. I also put an ammeter inline with the 15V unregulated feed and runs stable at 0.44 amps.


The oscillator does not change at all as the HV fades. Even an hour later it stays a 0.44 amps, 28V and 42KHz. The HV box is barely warm.


I tried searching through the messages, but didn't find anything like this. The HV briefly comes partly back after the scope has been turned off for awhile, but still fades to zero volts. What could cause the HV to fade without changing the Oscillator activity?
Bob


 

I agree that the multiplier could be the problem but wouldn't the
input have to be disconnected to test this? Just disconnecting the
anode would still leave it in the circuit.

On Thu, 10 Dec 2015 08:29:59 -0500, you wrote:

Hi,

The multiplier is the likely problem. You might try disconnecting the anode lead to see if you get the -2950 but I think you need to look at replacing the multiplier.

Jerry Massengale

-----Original Message-----
From: go_boating_fast@yahoo.com [TekScopes] <TekScopes@yahoogroups.com>
To: TekScopes <TekScopes@yahoogroups.com>
Sent: Wed, Dec 9, 2015 10:36 pm
Subject: [TekScopes] 7603 Strange HV Need Help

In one of my 7603s the HV wasn't working. Found the 15V fuse blown and possible bad HV power transistors. Replaced the 2N3055 power transistors, the four electrolytic capacitors on the regulator board and the fuse. Now there is a 28V sine wave at about 42 KHz.

HV Initially Measures 2.4KV, but dropped to 1.9 then to zero. I also put an ammeter inline with the 15V unregulated feed and runs stable at 0.44 amps.

The oscillator does not change at all as the HV fades. Even an hour later it stays a 0.44 amps, 28V and 42KHz. The HV box is barely warm.

I tried searching through the messages, but didn't find anything like this. The HV briefly comes partly back after the scope has been turned off for awhile, but still fades to zero volts. What could cause the HV to fade without changing the Oscillator activity?

Bob


 

Besides disconnecting the multiplier to see if it is bad like Jerry
suggests, I would test CR1232 to see if it is open.

What kind of high voltage probe did you use to test the high voltage
at test point HVTP?

On 09 Dec 2015 20:36:15 -0800, you wrote:

In one of my 7603s the HV wasn't working. Found the 15V fuse blown and possible bad HV power transistors. Replaced the 2N3055 power transistors, the four electrolytic capacitors on the regulator board and the fuse. Now there is a 28V sine wave at about 42 KHz.

HV Initially Measures 2.4KV, but dropped to 1.9 then to zero. I also put an ammeter inline with the 15V unregulated feed and runs stable at 0.44 amps.

The oscillator does not change at all as the HV fades. Even an hour later it stays a 0.44 amps, 28V and 42KHz. The HV box is barely warm.

I tried searching through the messages, but didn't find anything like this. The HV briefly comes partly back after the scope has been turned off for awhile, but still fades to zero volts. What could cause the HV to fade without changing the Oscillator activity?

Bob


Robert Simpson
 

OK, I will disconnect the multiplier at HV diode CR1232, check the diode, reinstall and recheck with the multiplier still disconnected ( I think I still have some liquid insulation, 3M Scotchkote 43906). The Voltage probe is BK Precision PR-28 plugged into a Fluke 8024B Millimeter.
Bob


Robert Simpson
 

I disconnected the multiplier but left the HV diode connected and reassembled. Now I get nothing. Not even the oscillator. CR1232 measured about 250k ohms forward. I compared a working diode and it is about 150K ohms forward. I will replace the diode, although it will take awhile. Any special technique for getting that black stuff off without damaging the transformer wire?
Bob


 

Hi,


I am skeptical about resistance readings on HV diodes. There may be different numbers of internal junctions.



Jerry Massengale

-----Original Message-----
From: go_boating_fast@yahoo.com [TekScopes] <TekScopes@yahoogroups.com>
To: TekScopes <TekScopes@yahoogroups.com>
Sent: Thu, Dec 10, 2015 4:14 pm
Subject: [TekScopes] Re: 7603 Strange HV Need Help






I disconnected the multiplier but left the HV diode connected and reassembled. Now I get nothing. Not even the oscillator. CR1232 measured about 250k ohms forward. I compared a working diode and it is about 150K ohms forward. I will replace the diode, although it will take awhile. Any special technique for getting that black stuff off without damaging the transformer wire?
Bob

[Non-text portions of this message have been removed]









[Non-text portions of this message have been removed]


 

On 10 Dec 2015 08:01:51 -0800, you wrote:

OK, I will disconnect the multiplier at HV diode CR1232, check the diode, reinstall and recheck with the multiplier still disconnected ( I think I still have some liquid insulation, 3M Scotchkote 43906). The Voltage probe is BK Precision PR-28 plugged into a Fluke 8024B Millimeter.

Bob
So you will not be able to use the probe to measure the high frequency
AC output of the transformer.

On 10 Dec 2015 14:14:12 -0800, you wrote:

I disconnected the multiplier but left the HV diode connected and reassembled. Now I get nothing. Not even the oscillator. CR1232 measured about 250k ohms forward. I compared a working diode and it is about 150K ohms forward. I will replace the diode, although it will take awhile. Any special technique for getting that black stuff off without damaging the transformer wire?

Bob
I was thinking that the diode could be open since a short somewhere
would kill operation of the inverter. Now that you get nothing, I do
not know what to think.

Everything should operate without the high voltage multiplier
connected or even present so I would start looking for the real
problem. The CRT trace should still be present without the high
voltage multiplier but much dimmer.

A safe solvent like naphtha (lighter fluid) might work on the black
stuff.


 

Hi Bob,
A good curve tracer is your best bet for measuring a HV diode.
Unfortunately the 576 only goes up to 1500V, and the 576 only goes up to 1600V.
Forward drop on a curve tracer will tell you just how many junctions are in the HV diode.
Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: Thursday, December 10, 2015 5:08 PM
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] Re: 7603 Strange HV Need Help

Hi,
I am skeptical about resistance readings on HV diodes. There may be different numbers of internal junctions.
Jerry Massengale

-----Original Message-----
From: go_boating_fast@yahoo.com [TekScopes] <TekScopes@yahoogroups.com>
To: TekScopes <TekScopes@yahoogroups.com>
Sent: Thu, Dec 10, 2015 4:14 pm
Subject: [TekScopes] Re: 7603 Strange HV Need Help

I disconnected the multiplier but left the HV diode connected and reassembled. Now I get nothing. Not even the oscillator. CR1232 measured about 250k ohms forward. I compared a working diode and it is about 150K ohms forward. I will replace the diode, although it will take awhile. Any special technique for getting that black stuff off without damaging the transformer wire?
Bob

------------------------------------
Posted by: jerry massengale <j_massengale@aol.com>
------------------------------------


Robert Simpson
 

The HV diode has been replaced. The symptoms have changed. Now the 15V supply went up about 20 ma to 0.46 amps. The sine wave is 1-2 volts larger, but is clipping. Also the HV is now steady at 1.1 KV

As for looking at the Transformer output, I do have a P6015 probe, but how would I access the HV signal?

Bob


 

On 10 Dec 2015 21:14:12 -0800, you wrote:

The HV diode has been replaced. The symptoms have changed. Now the 15V supply went up about 20 ma to 0.46 amps. The sine wave is 1-2 volts larger, but is clipping. Also the HV is now steady at 1.1 KV
The high voltage might be steady at 1.1 kilovolts but that is the
wrong value. Now it looks like there is a large amount of leakage
from the high voltage DC cathode supply to ground. Under these
conditions, the regulator is going to force the oscillator to full
power.

As for looking at the Transformer output, I do have a P6015 probe, but how would I access the HV signal?
I was thinking that you could look at the high voltage high frequency
AC output from the transformer itself to see if it even existed.


 

The HV test point is in the top rear corner; it's a hole abt 1/2 inch diameter.It may be obstructed by some nearby wiring.
 HankC, Boston WA1HOS


Stefan
 

Hi Bob,

this is not solved yet, right ?.
Some years ago I had the same problems with the same symptoms
(some remaining HV, excessive currents in the 3055, blown fuse,
no cure with other transistors).

After long time of investigation it turned that the problem was the
cascade.

In fact the potting material is known to incorporate water resulting in
leakage. I replaced the (formerly black )cascade block with another one
(now green, same part number) and the problem was solved.
In fact I had 2 replacement cascades where the first one I used was also
leaking. I heard some drizzling noise from the HV compartment not
knowing that this would be the end of the (new) cascade one week after.

If you have a replacement cascade (or a long time stored 7603) you
might consider baking it to remove the water in the potting material.

Best regards,
Stefan


 

If the cascade is dead and you can't easily get a new one, it's not difficult to build one yourself, using the threaded parts to affix it and the existing HV lead from the block

I made a mold from flexible silicone rubber, using the existing block. Next, I built the circuit and connections "floating" in the silicone mold and filled that with polyurethane resin. Has been working without a problem for two years or so.

Encapsulating is necessary, because the space is too small to allow air-only insulation.
Make sure there are no sharp points from components or solder. Cut or file them away where necessary.

Raymond


Robert Simpson
 

Fixed, partly

After initial HV repairs, turns out my HV probe was failing. I checked the probe against a working 7603 and the HV showed way low. So I purchased a Fluke 27 DMM kit with 6KV probe. Now both my working 7603 and the problem 7603 show HV in the 2900 range.

Note. I paralleled a 10 meg resistor with the 600 meg failed probe resistor and essentially no effect. I figure the 600 Mohm HV resistor in the probe has failed.

Still no trace, I suspect the multiplier.
Thanks for the tip on building a multiplier. I have another bad one I will try and disassemble and use the case.

Bob


 

Thanks for the tip on building a multiplier. I have another bad one I will try and disassemble > and use the case.
>
I don't know if you're referring to my post where I wrote that it's not difficult to build your own multiplier for a 7603.
Anyway, I didn't really disassemble the original case, just used some mounting points and the original HV cable:
I cut away the HV cable and using silicone rubber, created a mould around the HV block, leaving free the surface where the resin had been poured into the original multiplier housing. After the rubber set, I took out the original HV block.
Next, I removed the metal parts from the bottom of the original housing and put them into their corresponding places in the rubber mould.
Then I built up a new circuit in the cavity in the mould, avoiding sharp points and creating appropriate distances between components. I connected the HV cable through a hole in the mould in the original place.
I used polyurethane resin to fill the mould and after the resin set, lifted out a complete multiplier block, complete with metal fastening points and all connections.
It has been working fine for quite a while now (I think two years or so).


Raymond


 

On 13 Jan 2016 14:33:26 -0800, you wrote:

Fixed, partly

After initial HV repairs, turns out my HV probe was failing. I checked the probe against a working 7603 and the HV showed way low. So I purchased a Fluke 27 DMM kit with 6KV probe. Now both my working 7603 and the problem 7603 show HV in the 2900 range.
Doh!

Note. I paralleled a 10 meg resistor with the 600 meg failed probe resistor and essentially no effect. I figure the 600 Mohm HV resistor in the probe has failed.

Still no trace, I suspect the multiplier.
Thanks for the tip on building a multiplier. I have another bad one I will try and disassemble and use the case.

Bob
I ran a test not long ago by disconnecting the PDA (Post Deflection
Acceleration) connector between the multiplier and the CRT on my 7603.
There was still a legible display on the CRT but it was not focused
quite as well and was about half size and slightly distorted so if you
have the correct negative cathode voltage, I do not think that just a
bad multiplier is the only problem on your 7603.


Robert Simpson
 

Thanks for the tip, I'll check that.


Mark Wendt
 

On 01/13/2016 06:58 PM, hewpatek@gmail.com [TekScopes] wrote:
I don't know if you're referring to my post where I wrote that it's not difficult to build your own multiplier for a 7603.
Anyway, I didn't really disassemble the original case, just used some mounting points and the original HV cable:
I cut away the HV cable and using silicone rubber, created a mould around the HV block, leaving free the surface where the resin had been poured into the original multiplier housing. After the rubber set, I took out the original HV block.
Next, I removed the metal parts from the bottom of the original housing and put them into their corresponding places in the rubber mould.
Then I built up a new circuit in the cavity in the mould, avoiding sharp points and creating appropriate distances between components. I connected the HV cable through a hole in the mould in the original place.
I used polyurethane resin to fill the mould and after the resin set, lifted out a complete multiplier block, complete with metal fastening points and all connections.
It has been working fine for quite a while now (I think two years or so).

Raymond
Raymond,

You didn't happen to take any pictures of that process by chance?

Mrk


 

You didn't happen to take any pictures of that process by chance?
I added a few photos in album "7603 DIY HVM https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1307960540";

The in-air functional test setup didn't show any corona, acoustic nor visual. I only tested for about 20 minutes.

I didn't mind (accepted the electrical risk of) the bubbles and dimples, because of the published (optimum) dielectric strength of 22 kV/mm of the epoxy and the minimum 5 mm intercomponent distance and distance to side wall.
Still, I promise to do better next time.

Raymond