Hameg Scope 203-4 / Disassembly and Repair / Help needed for replacement part

[Forsaken]

Hi everyone!

I've recently bought my first oscilliscpe (a Hameg 203-4) from ebay, which unfortunally has one defective channel. When powering it on it does not show any trace for the first channel, the second channel seems to be working fine. Here some resources for others working on that model:

Service Manual and Schematics: https://www.eserviceinfo.com/downloadsm/65162/Hameg_HM-203-4.html

After not seeing any obviously defective components I started by disconnecting the preamplifier (manual page 38) of the defective channel and after powering up, there is a trace!
So it seems like the problem is on the preamplifier board. To validate that I disassembled the scope, which turned out to be more challanging than I anticipated, and swapped the preamplifiers for channel 1 and 2. Afterwards a test showed the 1st channel working and the 2nd channel being defective, which seems to confirm my assumption.
I now measured various parts of the preamplifiers with a multimeter, without finding anything broken. Since there's quite a few socketed parts I decided to simply swap those to identify the broken component, which turned out to be T100/T150, a U441 dual JFET N-Channel Transistor e.g.: https://www.digikey.de/product-detail/en/vishay-siliconix/U441/U441-ND/4496477

Now here's my issue: I neither found any place where to order one of those, nor did I find any obvious alternative (e.g. using the according search function from digikey). I'm still far from an expert in electronics and honestly can't really tell why a JFET was used instead of a "regular" transistor or a MOSFET and what the exact differences are. I did boldly try to simply throw in two N-Channel MOSFETs I had available (IRF3205) in there matching the Gate/Source/Drain pins, without any success. (I did not expect that to work, but I thought it was worth a shot  )

Can someone help me figure out what kind of transistors I need to order to fix that issue and/or tell me how to figure that out by myself in the future?

Disassembly instructions
1. Remove the screws on the back and remove the back cover

2. & 3. Stand the scope on the back and pull out the electronics upwards (grab on the front panel) and put the cover away


4. & 5. Remove the caps of all small knobs and unscrew+remove them, then unscrew the big knobs and remove those as well. (This is what took me a while to figure out)

6. Remove the 6 screws at the edge of the front panel
7. Unsolder the ground connections and remove the front panel a bit, then unsolder the other cables and remove it completely


Best regards
Jens

[goaty]

You could maby check out two separate JFETS in TO92 like J113 and couple them tightly for thermal stability.
From U440 footprint this looks like it could work

[shakalnokturn]

As goaty suggested, although TO92's won't be the best for thermal resistance so they won't be the best for thermal coupling either.

In this case I'd use a couple of SOT23, on the smallest possible PCB, gates set opposite, sources and drains physically close on the layout, then cover them in a blob of insulating glue with best possible thermal conductivity.

You ought to consider getting a dozen and selecting a matched pair if you take either the TO92 or SOT32 solution. Thermal coupling is only part of point in having a dual package.

If you're prepared to pay 1/3rd of what your scope is worth on one component you could consider one of the following available at Mouser (I haven't looked into which would be the most appropriate):

U430
U431
IFN146
IFN5564
IFN5565
IFN5566
IFN5911
IFN5912

All manufactured by InterFET

Edit: JFET's are used here for their speed and high gate impedance.

[Forsaken]

Thanks a lot for the swift feedback! I decided to go for the J113 option and ordered those. I'll see in a few days if that will revive my 2nd channel

[goaty]

Hey Forsaken, I hope you checked the specs, this was just an example. I didn´t select the Vds etc.

[Forsaken]

Hey Forsaken, I hope you checked the specs, this was just an example. I didn´t select the Vds etc.

As far as I could tell it looked fine, I will see in a few days

[Forsaken]

The parts arrived and it seems to work!
Thanks for the help 

[shakalnokturn]

You ought to compare the repaired channel and the one with original components on a square wave with fast edges to see if HF compensation needs readjusting.
Then again if you're going to use it for audio work is should not matter anyway.

[Forsaken]

You ought to compare the repaired channel and the one with original components on a square wave with fast edges to see if HF compensation needs readjusting.
Then again if you're going to use it for audio work is should not matter anyway.

So far I only calibrated it using a 1kHz/10kHz square and sine wave, but I will keep that in mind. Thats why I said "seems to work" 
Thanks for the advice!