Hi All!
I have an old 2235 'scope which has died on me. It looks like the resistor R909 and Q908 are fried . It also looks like the IRF820 ( on heatsink) is blown. Can anyone help with an excerpt from a service manual ( diag) or part numbers/values ?
Does this happen often with these babies ?
Regards
John
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Hi All!
I have an old 2235 'scope which has died on me. It looks like the resistor R909 and Q908 are fried . It also looks like the IRF820 ( on heatsink) is blown. Can anyone help with an excerpt from a service manual ( diag) or part numbers/values ?
Does this happen often with these babies ?
Regards
John
-- Using Opera's revolutionary e-mail client: http://www.opera.com/m2/
"john" wrote
Oh Heck! - I hope not, always fearful for mine; but I did manage to get the manual - will mail you the "Power Supply & CRT" schematic. Does anyone else have experience of this fault?
- Julian
john wrote in news: snipped-for-privacy@funkmeister.agent-j.com:
here's some data I compiled while at TEK;
TO-220 FET SPECS
The Molex connector to the pre-reg FET(IRF-820) should be removed and the leads soldered direct to the FET,that's a TEK 'mandatory' mod(meaning it shoulda been done when in for any service).The Molex would discolor and burn up.
R909 is a 1 ohm,ww,fusible 2W resistor,p/n 308-0677-00 I believe Q909 is a TIP31A,and you should replace both of the pair when one blows.(TEK p/n 151-0476-02
You should also check the PS electrolytics for bad ESR. The rectifier diodes also may short.
Watch out for poor focus/astig,the focus string resistors increse in value and eventually open and should be replaced with carbon composition types instead of the carbon film types in there now.
The way this PS works is that the line V is rectified and filtered,about
170VDC,and then a switching pre-regulator(TL494/594 based) drops it to ~43VDC,then the power oscillator converts that to the secondary voltages and HV.-- Jim Yanik jyanik-at-kua.net
Hi Jim, thanks to both you and Jules for the fast response From what i see, the tranny (Q908)thats blown has completely blown its top off and i can understand why... The fet blew, shorting gate to drain... hence all the current went through this poor little guy ( Q908, to-92 package).. frying it, the diode CR908 and R909.
the Snippet sent to me by Julian, has no R909, which is whats written on my PCB and therefore i think there are one or two version differences, as mine is the 220V version. The remaining band on the resistor is an orange ( 3xxxx) which makes me think it is a 3k.
any more clues as i'm going to get the parts now and replace and then fire up ;)
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john wrote in news: snipped-for-privacy@funkmeister.agent-j.com:
Well,I'm working from my 2213/15 manual,but the PS circuitry is almost identical. There mare 47 ohm Resistors in the bases of the power osc xstrs,those can open when one of the xstrs fails.The 1 ohm ww resistor I mentioned is in the emitter of both the power osc xstrs.It is very common to fail. Perhaps your crowbar circuit is what failed. This is an overvoltage circuit that is supposed to put the pre-reg into 'burst' mode(current limit) by drawing a large current thru the SCR.
There's a 3 ohm,3w,5% ww R in that circuit along with a SCR (Q935,151-0506-
00,C106B2X283 mfg p/n) and a 51V zener,0.4w,5%.
Your scope should regulate from 90-250 VAC without any internal changes.
-- Jim Yanik jyanik-at-kua.net
snipp..
Well, do as Jim proposes. Followind steps are helpful if you have replaced all other parts the other posters have mentioned.
You can check the circuitry following the hv smps simply by powering the scope from a regulated laboratory DC supply: Locate the 1000 mikro F (uF) cap on the board, apply approx. 45 v Dc from the lab supply /
1.5 A . current limit is very useful (CC/CV-mode lab grade supply).Scope should power up now and be usable partly. HINT: No mains connection for this trobuleshoouting, so unpluq the mains lead from the scope!! Otherwise, you'll fry yourself or other pars ;-).
Be careful not to get more than 49 v DC in this step (or the crowbar circuit will trigger).
If you have excessive current, check also the flyback diode in the primary SMPS circuit. It's likely to blow, and causes a nasty short circuit for the 1000 uF cap. A 200V 2 A fast switch ( < 80 nanosec) diode will be a good replacement.
If the TL594 SMPS IC in the primary SMPS is blown, it can be replaced with a TL 494 ( the 594 is a little bit harder to get, at least here on the european continent).
hth, andreas
snipped-for-privacy@bigfoot.com (TekMan) wrote in news: snipped-for-privacy@posting.google.com:
The crowbar circuit is what was fried on his unit,I believe. When the FET shorted,the full line V was applied to the crowbar.
I surmise that Q908 is the SCR,CR908 is the zener,and R909 is the 3 ohm fusible ww reistor. And yes,the TL594/494 could have blown along with the FET.
-- Jim Yanik jyanik-at-kua.net
Hi Jim,
well, you're right: In most of the 2235 I repaired with a fried smps the fast switch diode is OK. I only had it once in a case of 15 broken 2235's I had to repair that the fast switch diode was blown ( a dead short diode). This was a sour job: It took a while to diagnose this, but finally turned out.
Sympthoms were: blown fuse, dead IC, dead FET and dead fast switch. Crowbar was intact and OK.
Wel, but after rethinking this: The problem can be more clever diagnosed when a variac is handy: With a good fuse (and diagnosed GOOD crowbar) and the dead short FET in the scope, scope should power up partly when one raises AC inlet voltage from 0 to 30V AC. If it powers up, the fast switch diode is ok with confidence 99.9%. If not: I'd recommend you check the diode closely.
Yes, one could use a curve tracer too for powering the scope - if you're lucky to have one ;-)
hth, Andreas
Hi Jim,
well, you're right: In most of the 2235 I repaired with a fried smps the fast switch diode is OK. I only had it once in a case of 15 broken 2235's I had to repair that the fast switch diode was blown ( a dead short diode). This was a sour job: It took a while to diagnose this, but finally turned out.
Sympthoms were: blown fuse, dead IC, dead FET and dead fast switch. Crowbar was intact and OK.
Wel, but after rethinking this: The problem can be more clever diagnosed when a variac is handy: With a good fuse (and diagnosed GOOD crowbar) and the dead short FET in the scope, scope should power up partly when one raises AC inlet voltage from 0 to 30V AC. If it powers up, the fast switch diode is ok with confidence 99.9%. If not: I'd recommend you check the diode closely.
Yes, one could use a curve tracer too for powering the scope - if you're lucky to have one ;-)
hth, Andreas
Hi John,
to answer your question:
Pls. do a google search for this topic, it is covered very often. The component values are already mentioned in the other postings. If you are in europe, consider also a BUZ80 (now something like BUK..-800 , AFAIK) from Siemens as a good replacement for the FET.
hth, Andreas
snipped-for-privacy@bigfoot.com (TekMan) wrote in news: snipped-for-privacy@posting.google.com:
I don't think the pre-reg will fire up at such a low line V. There's a couple of xstrs that supply power to the TL594 pin 12 that act as a delay to allow the input line filter cap to charge up before the PWM IC starts.It will also act as an undervoltage lockout.(normal op range is 90-250 VAC) IIRC,line VAC has to get up to 60 VAC or greater for the IC to start. One can hook up a LV ps across the "housekeeping cap" for the TL594 and slowly raise the V to ~16 VDC and the IC will start,and a pulse train can be scopes at pin10 or a ramp at pin5.
With the scope NOT connected to mains!!!
-- Jim Yanik jyanik-at-kua.net
hi Jim,
the pre-reg has not (and should not) to fire up in the test I described: When the FET is shorted, the the pre-req is not working. The shorted pre-reg FET is a necessity for my test.
What I wanted to do is to isolate the fault: a. Is ist the fast switch diode? or b: is it the crowbar.
hope this make more clear what the proposed test is for.
hth, Andreas
snipped-for-privacy@bigfoot.com (TekMan) wrote in news: snipped-for-privacy@posting.google.com:
You saying "scope should power up" had me thinking otherwise. Now I'm NOT clear as to what's supposed to "power up" in your test.
Do you mean the rectified line V will climb if the diode is not shorted,and the fuse will blow if the diode IS shorted?
You can just disconnect one end of the diode and eliminate it as a fault. The TL594 can be tested without a FET in place to see if it runs,easiest with a separate LV PS as I said.
-- Jim Yanik jyanik-at-kua.net
..snippety for convenience..
hi Jim,
sorry for my poor english: I try to say with "power up": apply power and make the scope show some reaction.
Exactly as you mentioned: Let's assume a shorted FET in the front-end pre-req, and a shorted fast switch diode. If you raise the voltage with the variac, the current will raise quickly and the fuse blows.
If you have a shorted FET, and an good fast switch diode, the voltage will raise. At a certain point, (approx. 35 to 40 V rectified DC accross the 1000 uf Cap). the inverter ( located after the pre-req) will start.
Of course you can test the diode by lifting one end, but the idea i had (doing the test as described): You get a more easily diagnose of the fault without opening the case before you have to open the case.
Of course everybody has it's own ways to start fault search. So don't mind, I just wanted to give my idea. To share with the newsgroup..
hth, Andreas
hi i have a 2235 (mil. version) have the same problem with p/s q908 was removed, does anyone have a part# for this item . and possibly a scan of service manual for p/s thanks doug
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