Thorn 3000/3500 Test Rig

I’ve received a number of contact requests for info on the test unit I use to diagnose faults on the BRC Thorn 3000/3500 module.

Sadly no it cannot be purchased and I don’t have a schematic. I may when free time permits, open it up and document what is inside. I managed to get it from the usual source of all things these days E-bay. Along with this there are Thorn panel extension leads for the various modules so you can run them connected but out of chassis. I also obtained the CPV  ( discussed elsewhere in this section) diode kit for correctly setting up the EHT on the Thorn family of sets see here for details.

Its Function

With the PSU module removed from the TV you place the test unit to the left of the power supply under test. Then straighten the power supply edge connector leads and plug it into the tester. You then insert SKt 3 interconnecting lead of the line time-base panel into Plg3 of the power supply panel in the normal way, connect the tester to the mains.

If the two panels are in working condition all three indicators for the supply rails will light. The EHT pulse output can be tested in the usual way either with an insulated screwdriver or a neon tester or you can user your scope and check the waveforms.

The 58-65V rail can be checked without connecting the line-timebase panel. All you need to do is connect the line pulse flying lead on the tester to the negative end of the diode W608 and the indicator will light.

The test unit has the standard 3000 cut-out and it will trip on the tester for the same fault reasons as if it were in the TV. Also the cut-out will trip id the 58-65V line is set too high when using the tester line pulse probe. You need to turn the SET EHT control clockwise to lower the volts and set for approx 61V on an AVO. This is approximately the correct setting and is recommended as when the PSU is reinstalled in the set it will run the Tripler a little more gently. See earlier link for set up.

In addition

I’ve built a test open-PSU-surgery platform for my Jig. I’ve found to work on these PSU’s and do a proper job, you really need to get to the underside of the unit and operate it in this fashion to take readings, let alone remove components. To keep soldering and unsoldering the four component lead-outs from the Chopper and diodes to gain access, is just not viable.

By opening it up and running it affords you a much easier time in fault finding and replacing parts. You can follow various exploits with PSU repairs here. The platform allows me to secure the module to the plinth, when the lead-outs are unsoldered. I have marked the foot-print of the module on the base which allows me to place it perfectly each time. I can then swing the PCB out and secure it to a central rest. All I do then is connect up the four unsoldered points between the two sections with the fly-leads, leaving me able to run the entire board ‘spatch-cock’ style.


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