Tektronix 2215 Blowing Fuse No Shorts Found - Page 1
Forum section: electronics / testgear

author: Romsk
author_rank: Newbie
timestamp: March 06, 2022, 10:25:39 pm
content: I just picked up a Tektronix 2215 (not an A). I downloaded the Service Manual which is excellent. I followed the diagnostic flow chart and I am at the point where (unpowered of course) I don't see any shorts across the 45V zener, the big filter cap in parallel with it, the 4 full wave bridge rectifier diodes (2 on the Main A10 board and 2 on the Current Limiter A19 board) and the inverter pair transistors that drive the supply voltage transformer. The 2A fast blow fuse blows in 2 seconds. I am going to try the 40W incandescent bulb in series with the main 120VAC power input as a current limitrr to see if can keep it powered up for more than 2 seconds, tried it 3 times. What I noticed is on the supply voltage transformer (the one that creates 5V, 100V, 8.5V 30V, and such) the input windings are as follows:The Input windings are isolated from the Inverter Transistors by opening up the Isolation pads.One side of the winding is pin 2, Ceter Tap pins are pins 3&4 on same node, and other side of winding is pin 5.I read 0.3 Ohms between either side of the winding to the Center Tap, and 0.6 Ohms between both sides of the winding.This seems very very low, I was expecting something around 40 Ohms (just a SWAG). Does anyone have a model of this O-Scope that they could isolate the input windings (the Isolation Pads are just adjacent to the pins on the solder side) and tell me what resistance they read among these pins?To check out the diodes on the Current Limiter board I had to remove the board, to do that I had to unscrew the clamp around the FET that holds it to the chassis as a heat sink. To isolate the Drain, they used a 3mm tick piece of ceramic but not thermal conducting compound! Is that right?I also found the 0.5A Slow Blow fuse was blown (those are on order so powerd up troubleshooting is on hold for a few days.I read in one of these forums that someone bypassed all the PSU circuitry and drove the transfomer input at 40 VAC (not even using the inverter), but with such a low impedance I see on those windings that would have to be a beefy supply 47A = (40 x 0.707) / 0.6. That makes no sense and you can see why I think the input winding impedance seems very low (and maybe why the fuse blows as the supply ramps up in 2 seconds)If that transformer is the problem, I may have to call this scope a brick. At least the price was right - free.

author: bob91343
author_rank: Super Contributor
timestamp: March 07, 2022, 12:16:44 am
content: I think that is one of the scopes that has a switching supply, the output of which is 43 Volts. So you can disconnect the power supply and run the scope from a dc supply at 43 V. It doesn't draw a lot of current; a lab supply is what I used. If the scope works that way, then the problem is in the power supply.

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 03:07:06 am
content: That transformer runs at ~20kHz so you need to take that into account when you are measuring its DCR and L. The way to bypass the first stage of the PSU is to supply it with 43VDC right at filter capacitor C937 and let the rest of the PSU do its job normally. The whole scope should then work except for the line trigger feature. Did you test the triac Q925?