Tek 2246 issues


nmi8080@...
 

Hello -- newbie here.

I've owned a third-hand Tek 2246 scope for about ten years, using it in two hobbies: restoring 1970s computers (currently fixing a Heathkit H8 used as a home for incontinent mice) and fiddling with tube guitar amps. I'm not really an electronics guy but have learned some basics and for my needs a single channel of the scope is perfect but every so often I need 2 or 3 channels to check computer timing issues.

A couple of days ago I tried to use three channels on a problematic and gate, and since then the scope has been acting weird: wandering display levels, signals vanish at higher speeds, and I see random display artifacts. Given it's age, it probably needs a power supply recap and other age-related updates. Does anyone know of a repair service for these units that won't cost more than the scope is worth?


Mark Vincent
 

nmi8080,

If you have enough skill and knowledge, recap it yourself. I do not know your skill level. The power supply should be redone. The electrolytics on the secondary side can be raised in value, e.g. 1000mfd to 1500mfd, 39mfd to 56mfd, etc.. The main input filter, 270mfd can be 390mfd. The electrolytics in the preregulator circuit need replacing as well. The 4,7mfd ones can be a 50V 5% film type. The types I have found to be the best in life, lowest ESR and are high temperature are Nichicon ULD, UHE, UCA/S/Y and LGR (this last one would be for the main filter >270mfd). The film types can be Kemet, Wima , etc.. After replacing the caps, adjust the 7,5V adj.. That should make the other voltages correct. I am looking at the schematic, I do not own this model.

Change ALL RIFA caps! These are usually in a clear amber colour case. Use X1 and Y1 types as replacements. The X1 and Y1 can be X2 and Y2 if you want to use a higher voltage rating for these. These are on the mains input and may be a few in the primary of the power oscillator.

If there is a battery in it for any type of backup, replace it. I do not know if an axial can be installed. If so, a larger capacity one is better, e.g. 2,4A/hr. If not, find the largest capacity that will work. The larger capacity will give a longer life before needing replacing. A weak/dead battery can cause weird problems. Refrain from replacing ICs in the logic circuits until a new battery is installed. You may need to use it a couple of times after replacing the battery for everything to work right. I have seen this in my 2440.

After replacing the above parts, clean contacts, make sure it is working then calibrate it, if needed. The beam finder switch should also be cleaned. That being dirty will cause display problems.

All these parts are available from Mouser.

Mark


Alex
 

Besides recapping, and while you are there (at the power supply board), one of the highest priority preventive work for any of the 224x scopes (2245, 2246/A, 2247A, and 2252), is to replace all the rectifier diodes on the LV power supply board (total 17 diodes) that have either ZM or ZS marked on them as they are well known to get leaky after some time and might fail. Original part number is 152-0400-00 (MB2501), and a good replacement would be to use MUR160 600V 1Amp 50nS diodes (Mouser PN 821-MUR160). Do not use regular 1N400x diodes.

Good luck.
Alex