Tek 2335 Vertical HV Board Problem


Joseph Strickland
 

Since my last post wherein I had managed to replace the LV bridge rectifier and then was able to adjust the voltages to the recommended levels, I received a HV multiplier that I had ordered to replace the one in this scope.

The scope had no measured HV at the feed to the anode of the CRT.

Today I finally got around to removing the vertical Amp/HV board A15 in order to replace the HV multiplier block. While carefully disconnecting the various plugs to/from the board, I noticed that there was a burned 1/4W resistor on the board. It is R113, a 1.6K ohm resistor in the emitter circuit of Q114. I found that the resistor is open. Resistance checks of Q114 seem to indicate that the transistor is OK. One end of R113 connects to the +102V line through R122, a 100 ohm 1/4W resistor which is bypassed by C122, a .022mFd @ 200V to ground. The collector of Q114 connects to the collector of Q115, whose base is grounded and whose emitter connects to the emitter of Q116 through R115, a 20 ohm 1/4W resistor.

Q93 is a Z-axis amplifier. Signal from the collector of Q93 is applied through CR100 to the input of a high-speed feedback amplifier at the base of Q100. The feedback amplifier consists of Q100, Q107, Q114, Q115 and Q116. The feedback path includes gain-controlling resistors R101, R102 and R128, connected between the amplifier output and input at the base of Q100.

The junction of Q114 and Q115 collectors is connected to pin 5 of the HV multiplier circuit through R127, a 100 ohm resistor. The AC voltage present at pin 5 of the HV multiplier is referred to as AC grid bias TP130. Internal to the HV multiplier the signal is coupled to two rectifiers along with another AC pulse coming from the flyback T167 through CR130 and R130 a 1K resistor. CR130 checks OK on my ohmmeter. The rectifiers inside the HV multiplier couple their negative output to Grid No. 1 of the CRT which also have two neon lamps, DS196 and DS197 which connect to the CRT cathode.

If there is no negative bias on the CRT, it could conduct so heavily that there might not be any HV at the anode of the CRT.

So far I do not see any other burned resistors and Q114 and Q115 seem to be OK. I do not have a second scope to use in checking waveforms. I do have my multimeter and a VTVM. Should I replace the burned resistor and try the unit without replacing the HV multiplier? Any other ideas?


I have added some of the circuit explanation pages from the SM to my photobucket site at:
http://s1213.photobucket.com/user/rubal182jhn/library/Tektronix%202335?sort=3&page=1 http://s1213.photobucket.com/user/rubal182jhn/library/Tektronix%202335?sort=3&page=1


Joe
KC5LY


Joseph Strickland
 

Not having received any suggestions, here is what I propose to do for now:
I will replace R113, the 1.6K ohm 1/4W 5% carbon film resistor with a new one (ordered a set of them).
I have ordered a replacement for Q114, a 2N5451, and Q115, a 2N5551 in case I need to replace those.

I will install R113 first and reconnect everything and install the board on the chassis. I will apply power and see if R113 burns again. It is possible that I might have caused the burned resistor because at one point I had one interconnect cable plugged in 1 position off.

If R113 holds, I can then take some AC and DC voltage measurements on the Vertical & High Voltage board to see where that leads.

If R113 burns again, I will open the resistor and measure other voltages on the board to see if that produces any other indications of trouble.

Joe
KC5LY


Gary Robert Bosworth
 

Joe:

Be very careful about opening resistors. Often, the resistor is part of a
divider chain and it sets the output of a voltage power source. Opening the
resistor can send the voltage source to catastrophic levels and cause
severe damage to entire driven circuits.

Gary
On Dec 24, 2014 8:16 AM, "strijw426@... [TekScopes]" <
TekScopes@...> wrote:



Not having received any suggestions, here is what I propose to do for now:
I will replace R113, the 1.6K ohm 1/4W 5% carbon film resistor with a new
one (ordered a set of them).
I have ordered a replacement for Q114, a 2N5451, and Q115, a 2N5551 in
case I need to replace those.

I will install R113 first and reconnect everything and install the board
on the chassis. I will apply power and see if R113 burns again. It is
possible that I might have caused the burned resistor because at one point
I had one interconnect cable plugged in 1 position off.

If R113 holds, I can then take some AC and DC voltage measurements on the
Vertical & High Voltage board to see where that leads.

If R113 burns again, I will open the resistor and measure other voltages
on the board to see if that produces any other indications of trouble.

Joe
KC5LY

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



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


Joseph Strickland
 

Gary;

Yes I understand that an open resistor can lead to other problems. Of course it has already been operated that way while I was troubleshooting other areas of the unit. Apparently nothing else that I can determine at this time was damaged - no fuses or other parts have been found to be burned etc.

I will admit that I did not discover the burned resistor right away. I had done a fairly detailed visual inspection in the early stages of troubleshooting, but it turned out that when this resistor burned open the color band that indicated the value 6 had changed color to appear orange and I thought I was looking at a 1.3K ohm resistor. The body color (light brown) had changed so little that I did not notice it. It just goes to show how easy it is to misinterpret something while examining a piece of gear.

Thanks for responding!

Joe
KC5LY


Joseph Strickland
 

As a follow up to my last post on this area of the 2335 Vertical Output and HV Multiplier board, I finally received all the parts I had ordered. R113 had already burned. I replaced it with another 1.6K ohm 1/4 watt resistor and tried the unit - R113 burned again. Then I replaced Q114 and Q115 and R113 again together. R113 began to smoke so I immediately removed power. I did some further checking and discovered that C108 and C140 were both shorted. I think the original problem was due to C108, but I managed to damage C140 by touching it with the hot barrel of my pencil iron while working on some of the other parts. I had a .047mFd mylar film rated at 630V that I used to replace it. I used a 10% tolerance part for it. The original C140 was a styrene capacitor of .047mFd 20% rated at 200V.

I was mislead by the low impedance circuits around Q114 and Q115 and missed the shorted capacitor on my early ohm meter checks. I lifted one end of both C108 and C140 and confirmed that they both were shorted.

However, for C108 there is an issue with getting the correct value and tolerance. That capacitor was also a styrene capacitor of .005mFd of 0.1% tolerance (very tight) and rated at 200V. Are the original capacitors, P/N 502F02PP460 still available? Or is there a suitable replacement? I haven't started searching yet, but I am wondering if a metalized film polypropylene capacitor would be OK in this application instead of using a styrene. Or would a suitable hand selected 500V rate silver mica be OK. It appears that this may be part of a wave shaping circuit.

Any Ideas?

Joe
KC5LY