Re: tektronix 2235 , blown switchmode

From: john (john_at_removeagent-j.thiscom)
Date: 07/16/04 

Date: Fri, 16 Jul 2004 21:35:48 +0200

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 ;)

On Fri, 16 Jul 2004 00:16:25 +0000 (UTC), Jim Yanik <jyanik@abuse.gov>
wrote:

> john <john@agent-j.com> wrote in
> news:opsa7aefh90tb1i5@funkmeister.agent-j.com:
>
>>
>> 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
>>
>
> here's some data I compiled while at TEK;
>
> TO-220 FET SPECS
>
> +
> 151-1136-00 MTP12N10E 100V 0.16ohm 14.0A
> 151-1141-00 IRF730,STP300H 400V 1.0 ohm 5.5A
> 151-1151-00 IRF710/MTP3N40 400V 3.6 ohm 1.5A
> 151-1152-00 IRF820/MTP475 500V 3.0 ohm 2.5A
> 151-1171-00 BUZ71A/MTP15N05E 50V 0.12ohm 14 A
> 151-1214-01 IRF830 500V 1.5 ohm 4.5A
> 151-1245-00 MTP6N60E 600V 1.2 ohm 6.0A
> 151-1286-00 MTP4N80E/BUK456-800 800V 3.0ohm 4.0A
>
>
>
> 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. 

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