Hello All,

Recently one of my customers gave me the opportunity to repair some 6021's that were shorting out when power was supplied. I believe they may have been damaged by an improper transformer connection to the output terminals, or just failed from old age, not exactly sure, but felt it would be worth a try to fix them. Thankfully I was able to get some guidance on how to get started from here: Reparaturen. It is short and in German, so I thought for the forum I could explain and expand on it in English for those that may be interested in reviving their 6021's.

The first step is to remove the external case screws from the heat sink and the plugs that hold the case together, the two screws are easy, the four plugs, a bit more complicated:




Drilling a small hole in the center of the plug allows a screw to be attached.

Once attached, a gentle pull on the screw with a set of pliers is all that is needed to remove it.



With the controller face down, the base may lifted off. The lower circuit board can then be litfed out and flipped over into the base to access the remaining heat sink screws and transistors.

There are two black screws in the heatsink that go into the case that need to be removed before the lower circuit board can be lifted out /BDNZ


Here the challenge is getting the transistor screws removed, the circuit board in front of them prevents easy access with a screwdriver, I used pliers to loosen them. Once they are removed there is one more screw holding the heatsink to the board that needs to be removed and heat sink is off.


Now the transistors may be desoldered, take note of the values and their position for the installation back on to the heat sink. the desoldering is probably the most difficult part, take care not to damage the board or any of the other surface components when removing the transistors and solder, the thru-holes must have all excess solder removed before reassembly.



Install the new transistors in the correct orientation on the heat sink.


Solder new transistors to the board and replace bottom heat sink screw.


Reassemble Control Unit and test...


All three 6021 units I received for repair had a least one bad transistor, I replaced all three in each with new ones and each worked well upon completion.

Parts used on each: 1 x BDW93CFP, 2 x BDW94CFP, STMicroelectronics brand. Other brands and types are compatible, just be sure they are of the correct values and insulated from the heat sink.

Tools used: 1.8 mm drill and pin vise, 2 mm self tapping screw, small serrated jaw pliers, phillips head screwdriver, solder station, solder wick, solder and flux.

Good desoldering and soldering skills plus knowledge of integrated circuits are required for this repair, please do not attempt if not proficient.

Cheers,
Gordon

Edited by moderator 29 January 2024 03:21:33(UTC)  | Reason: Topic made 'Sticky', added line breaks for clarity

David, I have no customer ID for Marklin but is it possible that you can order these pegs,
for me? Tell me the total costs included the amount to send it to holland. If possible I wil
give you my address in teh Netherlands. Or tell me how to get a marklin customer ID.
Ik have several controllers repaired but not the right pegs.
Thanks in advance for any answer.
Regards from Ton Schluter.

I drilled a 1.5mm hole and used C track screws as they are quite thin.

Here's a picture of the transistors in my 6021 just before they were removed.



As you can see

Left - BDW93CFP
Centre - BDW94CFP
Right - BDW94CFP

I'm told by an electrical engineer friend who loves this era of digital components that the left and centre transistor pair provide the track output and the 2nd BDW94 on the right provides power to the rest of the 6021. I received my 6021 in a completely dead state, and the used Marklin dealer who sold it to me will send another replacement unit when I next order, but I thought I would have a go at repairing the faulty unit. I have replaced all 3 output transistors with replacements obtained off eBay. Despite being of Chinese manufacture and not from ST Electronics, they were a direct like for like replacement with the new transistors fitting in the same orientation as the originals. They do not need an insulating washer or heatsink paste - the originals don't have this - just screw them as is to the heatsink with the screws you take out.

Replacing the transistors has bought my 6021 to life and it is working as normal. There were no signs of burnt components or scorching of any type. As clapcott has said to me peviously it is good that these transistors completely fail sacrificing themselves so that no further downstream damage is caused.

Source of transistors from Aliexpress - https://www.aliexpress.c...st_main.5.2a891802oianmy

I used a Pro'sKit desoldering pump, which does a good job of sucking most of the solder from both sides of the 6021's circuit board