Tektronix 2230 PSU repair

[THM60]

As described here https://www.eevblog.com/forum/repair/tektronix-2230-scope-mechanical-parts/msg4529390/#msg4529390 I became owner of a poor Tek 2230 that undergo a moron's attempt to "repair" it. Its PSU was blown, not the tiniest sign of life in there. The numbskull who massacred it had installed a 5A fuse, adding  further damage to initial failure.

It took a fair amount of hours and a lot of head scratching but it ended in a successfull repair. Some pictures here : https://photos.app.goo.gl/8B4YyACh2ZtfrG8d9

I had to replace : CR90, CR907, Q9070, Q908, U930, CR908, R909, R908, C904. Given the dreadful access to those parts it has been the worst experiment I had since many years, but I'm very happy with the result. 

Something worries me with the way Tektronix designed the  PSU:  the value of the C906  (75µF) located after the full-wave rectification, is the image of the energy supplied by the power supply. The more the voltage supplied by the rectifier increases, the more the value of the capacitor decreases to supply the same energy and therefore the same power. Under 220V, it will provide four times more energy, which can cause Q9070 to explode in the event of a malfunction or a voltage spike, or a parasite on the mains network. 

Q = C x U in coulombs (Quantity of electricity)
E = 1/2 C x U²  in joules (Energy)
which gives :
 - with mains voltage = 110 V : E = 1/2 x 75 10 -6 x 110 ² = 0,45 J
 - with mains voltage = 220 V : E = 1/2 x 75 10 -6 x 220 ² = 1,8 J  that's enormous

I might be worrying for nothing though, but I'm tempted to power the scope through a 220/110V transformer.

I now have to find an elegant solution to fix what the moron did with his Dremel : https://photos.app.goo.gl/8B4YyACh2ZtfrG8d9

Thanks for your attention.

[David Hess]

Tektronix uprated the power supply in the 2235 series by changing the value of various parts so you might take a look at the parts list for the late 2232.

[THM60]

Without knowing I uprated or modified the following items :

- CR901,CR902,CR903,CR904  : HER208G (1000V/2A) instead of the original and obsolete MR5061 (600V/1,5A). Among other issues, scope was DOA and CR901 was shorted.
- CR907 : single 31DF4 (400V 3A 75ns) instead of the two paralleled BYD73G, one of which was shorted
- Q908 : 2N2907A SI-P 60V 0.6A 0.4W 200MHZ in TO18 metallic package. Q908 (TO92 2N2907A) had exploded.
- R908 : changed from 3KR to 2,2KR
- cut off Q9070 Molex connector and directly soldered wires to TO220

[Allen49]

Thanks for this post. It looks almost identical to the Tektronix 2221 I am trying to repair. I have a short to chassis ground on the 5 volt rail. Mine has started blowing the fuse.