JDR 3500 scope repair

[Radio Tech]

I never repaired test equipment till I found this site a while back. Dave and gang here really helped me to understand more about how to troubleshoot and repair various types of equipment. Over the past year I have repaired dozens of scopes, signal generators and power supplies.

8 months ago a guy brought me a Dick Smith model 1241 scope that powers up with no trace. Finding the half amp internal fused blown I traced it to the vertical deflection amp Q8 (D401A). He ordered the part and I installed it along with a new mica insulator and screw insulator. They used a plastic insulator on the screw that had completely melted. Scope has been working fine ever since.

Yesterday an old friend of mine came over with 2 JDR 3500 scopes. These are identical to the Dick Smith. Since the scopes were old I decided to check all the caps and replace where necessary. What I found was the exact problem that the Dick Smith scope had. Both scopes had blown fuses and Q8 shorted (D401A).

It looks to me that this standard package TO-220 transistor does not have adequate heat sink. It is mounted onto a 14 gauge rail about an inch wide with several other regulators mounted on it.

Would like to find a TO-3 replacement for this amp but nothing shows up on the search. I did not have replacements in stock to repair his scopes but he can order them and do it his self.  Maybe I missed something that is causing the heat up problem but I cannot find it.

Just wanted to share this repair and see if others had some comments about it since I seen 3 scopes with the same problem.

[tautech]

Thanks for sharing.

The primary side switcher is just one of quite a few components that are under various stresses in the HV of CRO.
Careful installation practices for heat dissipation and selection of quality replacements will reduce failures.

[Radio Tech]

Thanks for sharing.

The primary side switcher is just one of quite a few components that are under various stresses in the HV of CRO.
Careful installation practices for heat dissipation and selection of quality replacements will reduce failures.

Thank you very much for the comment.
I agree, there is just not enough heat dissipation for this part. And where this transistor is located it is less then 3 inches from the back wall of the scope. I told him to mount the transistor there with the proper hardware. That's was why I was thinking a TO-3 case replacement would be a better idea.

I would had loved to taken video footage of the repair or at least snapped a few pics but was pressed for time. He lives about 35 miles south east of me and is 74 years old. And still runs his electronic repair shop everyday.

[Radio Tech]

My buddy replaced the faulty transistor. He said he still did not have a trace on the scope and could not see a filament burning in the crt. Also said the transistor was very hot to the touch. I had him check from each pad where the transistor goes to and to ground to look for any low ohms readings. Nothing below 1.9k. But there must be something else in the circuit bad.

[tautech]

My buddy replaced the faulty transistor. He said he still did not have a trace on the scope and could not see a filament burning in the crt. Also said the transistor was very hot to the touch. I had him check from each pad where the transistor goes to and to ground to look for any low ohms readings. Nothing below 1.9k. But there must be something else in the circuit bad.

Might the 2SD401A not be oscillating?
These HV switchers normally operate in the 20-50 KHz range and can easily be checked for operation just by holding a scope probe near to pick up the EMI.
Your scematic is very fuzzy when enlarged but it seems the CRT filament is supplied from a winding on the HV transformer, correct?
So no switcher operation, no CRT filament supply.

[Radio Tech]

Might the 2SD401A not be oscillating?
These HV switchers normally operate in the 20-50 KHz range and can easily be checked for operation just by holding a scope probe near to pick up the EMI.
Your scematic is very fuzzy when enlarged but it seems the CRT filament is supplied from a winding on the HV transformer, correct?
So no switcher operation, no CRT filament supply.

Yep. I wished he did not live so far away, or had left one of the scopes with me to troubleshoot. I am not sure of the voltages that need to be present when it is operating correctly.

The schematic came from a PDF located here.
http://nebula.deanza.edu:16080/~norona/3500_oscilloscope.html
Too large to upload

[tautech]

It could be the secondary load is too large for it to oscillate correctly. You'l notice a FB loop from the Focus pot that controlls secondary voltage, All high value resistors should be checked as they drift in values. HV caps are suspect too, sometimes they just fall apart when touched.
DC restorer diodes sometimes fail but these normally have a high Vf and can be tricky to test.

IIRC a recent thread on a TEK 465 had a shorted PDA multiplier, disconnect yours and see if the HV fires up.
Hope these are a few clues that help you move forward.

[Radio Tech]

Well after all this time I finally got a chance to run down and pick this scope up.
Bout to pull my hair out. Found the new D401 transistor he installed was shorted.  Having no replacement I temporary installed a  NTE162 TO-3 package to see if this thing will do anything.
Seems the switcher will not start up. Checked about every component on the primary and secondary side of the transformer. When checking voltage I have minus 19 volts on base collector and emitter.
Really missing something here.

[tautech]

Well after all this time I finally got a chance to run down and pick this scope up.
Bout to pull my hair out. Found the new D401 transistor he installed was shorted.  Having no replacement I temporary installed a  NTE162 TO-3 package to see if this thing will do anything.
Seems the switcher will not start up. Checked about every component on the primary and secondary side of the transformer. When checking voltage I have minus 19 volts on base collector and emitter.
Really missing something here.

Check all polarised components for correct orientation just in case you have made an error, primary and secondary sides.
Check all windings on the transformer for shorts or open circuits. All windings should have ay least an Ohm or two resistance.

[Radio Tech]

Will Do. I checked the transformer the last time it was here.  But this time I will remove it and check it out. Good thing it is wired and not pcb mount terminals.
Thanks

[Radio Tech]

Got a chance to come out and test.
What I found this morning is the secondary side terminal 5 and 6 are open.  It cannot produce any heater voltage.
Not sure if I want to attempt to open up the transformer and repair it or siply add a second 6.3 volt filament transformer inside.

[Radio Tech]

Scratch that, enamel coating will get you every time. After removing the wires from the transformer they all check out fine
With the transformer disconnected I checked all nodes on the board to ground and they are up in the mega ohm with the acceptation of node 4 with checks 23k ohms to ground.