Tektronix 2235 CRO SMPS problem.

[Crash]

Hi Sarma, Les, Miguel,

Sorry it has been a while since I last replied. However, I have finally got the CRO going. It was the power transformer T948. When I finally un-soldered it from the circuit board and did a few measurements it was clear it had a shorted turn in one of the windings. Took a while to find a replacement but found one in Canada.
Thanks for all the help.

Peter

 

[piloteer]

Been trying to repair the 2235 got at an auction. Repaired a number of these many, many years ago when a bench tech in metrology dep't. Most had power supply issues.
This scope brings back the aargh I got back at that time only worse. Some parts can no longer be found. Found Sphere sold the last C906 in stock. Allied is a good source for semiconductors. Allied Electronics But back to this scope. I replaced R926, the 150k resistors that C925 charges through. Strange thing is that when I measure the replacement resistor it seems to discharge through the meter even before the scope is turned on. Measure in the meg ohms then after a few minutess settles in around 150k.
Found the 6.2 v zeners. out of tolerence or even open. The mosfet IRF710 has blown several times. Replaced Q930, 928, the 51 v zener in the crow bar and scr. Finally got the power led and the uncal'd leds to blink as if the 5 vdc as trying to turn on. Seems like it's trying to operate. Disconnected the links from the all the power supplies. They blink once about every 2 seconds.
Q928 sms8512 is not readily available. Subst. 2n2222, 3407, 4401 made no difference. Removed some of the caps. Ran them up to just under the breakdown voltages with an ammeter in series and found none with any leakage I'd be concerned about. Then measured them on a capacitance meter where they checked ok. What else can I do without putting the scope under the wheels of my truck.

Errors on the board include Q928 which is marked as Q944. The spot where Q944 really is is not marked at all. The serial # is B70418 option 1 and is the AN/ASM 448 mil version. Have the repair manual for the civilian model and found the mil version in pdf format. Both have the same errors.

 

[Nick_F]

Hi Sarma, Les, Miguel,

Sorry it has been a while since I last replied. However, I have finally got the CRO going. It was the power transformer T948. When I finally un-soldered it from the circuit board and did a few measurements it was clear it had a shorted turn in one of the windings. Took a while to find a replacement but found one in Canada.
Thanks for all the help.

Peter

Hi Peter & All,
My oscilloscope Tektronix 2236 got faulty and I also identified the T948 (P/N: 120-1348-03) defective. I also had a Zener Diode VR943 open circuit. The Power Supply around T906 was trying to start every few seconds, but the pulse on the current monitor resistor R907 was over the normal limits (approx 250mV instead of 100mV) and the power supply chip TL594 was self-protecting the circuit. Of course disconnecting the outputs of T948 did not help. I was also reading in the manual about a service circular that should be applied to improve the protection of the power supply in case of failure of some components. My oscilloscope did not have it done, maybe this is why the transformer got faulty.
I checked the transformer out of circuit using a LOPT Tester, designed by Bob Parker. All the leds were off when aplying the pulses to either of the primary windings (I would expect all eight leds to be on for a working transformer - this was the case with T906).
In case people wonder where the C908 is, it's located near C907, but it is not marked. There is some filter box that has to be unscrewed to have access to that capacitor. In my case, instead of 220pF, it only had 170pF. And the electrolytical caps are best tested with an ESR Meter, not with a capacimeter.
What I am not sure at the moment is if the first power supply (called power correction factor if I'm not wrong) can work with the T948 removed, at least that would point to either first power supply (around U930)or the second one (around Q938, Q939, Q944, Q947, Q946). I expect it should work and the output (pins 4-5 of T906) can be measured on C940 (but I am not sure what's the nominal voltage).

Regards,
Nick

 

[achimoto]

Hi All,

My oscilloscope Tektronix 2235 runs Ok but i have this issue: after 2 hours i check with digital termometer ( like auto RC 2 stroke engines ) the temperature of power supply area, lateral housing, and i mesured 50 degree celsius in the first 20 min and 71 degree after 2 hours, the main housing was very hot and overheating smell is present.... I see this power supply don't have cooler system maybe i should try to assembly one for good cooling capacity. Please tell me if you have the same operation mode and problems.

Sorry for my bad English


Best regard's Mike

 

[Externet]

Uncover and emove the switching power supply perforated top cover and keep it naked; turn it on for a while; unplug from the wall and finger check/thermometer check what component is the one overheating/smelling.
That oscilloscope should run cool without need for fans.

 

[achimoto]

Thank's for advice, i found the source of problems..... someone put serial rezistance in 220 V ~ 270 ohm value ( i have 50 V aprox. voltage drop for this reason ) please see pictures. I do not know the reason for this, maybe it was afraid to burnt this nice SMPS. Now i will buy this oscilloscope 100 euro, i think this price is ok for 29 years old scope isn't it?

Best regard's Mike

 

[Externet]

Never seen that resistor that looks factory installed in 120VAC oscilloscopes; perhaps is needed for 230VAC operation. Its presence explains the extra heat dissipated. Check if the schematic diagram shows it.
I would not remove it.

 

[achimoto]

Hi, i remove the rezistor and every think is OK, my oscilloscope runs without any problems for 6 hour's. This SMPS is from 70 to 250 AC volts, don't need resistive divider for 230 AC operated.
Now i'm happy, operational temperature was only 35-40 celsius.
100 euro is the best price, i will buy.

Regard's

 

[robertkendrick]

Hi
Very useful thread.

Does anybody know an equivalent for the SCR in the crowbar circuit Q935. Can find the listed part available anywhere.

Some of my problems are:
literally blown apart Q9070
blown (s/c) q908
Q935 s/c, 51v zener appears ok