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Re: 2246 Mod A main board overloading the SMPS

Tom Jobe
 

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Hi Priya,
I think all that the 'chirp mode' is telling you, is that the voltage drop across R2201 is too high, which fits right in with the over current damage you have already repaired.
With the scope assembled (and turned off), check the resistance at each test point on J1204 using one of the ground pins?on J1204.
I thought I had a list of power supply resistances from a?working 224x, but I can not?seem to?find it today. Maybe someone will have such a list for you, if not I will take the cover off of a working 224x and make a resistance list when I get home this afternoon.
My first guess is that you have a bad capacitor of two on the secondary side of the main transformer T2204.
I'm assuming that you have checked that you do not have any of those?problem diodes marked ZS or ZM. Those diodes rectify all but the?two 5 volt supplies, and there are about 17 of them on the power supply PCB. The original part number for them is 152-0400-00 and there are lots of nice ultra fast diode replacements available if you need to change them.
tom jobe...
PS If you do check your power supply resistances, please post your results.
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----- Original Message -----
From: Mover
Sent: Friday, January 04, 2013 6:07 AM
Subject: [TekScopes] 2246 Mod A main board overloading the SMPS

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Alas - if only it were that simple ...

I finally fixed my SMPS and put it back into the chassis with only the main board. It runs in chirp mode - about every 1 sec there is a clicking sound - the secondary voltages rise and then something gets overloaded and the SMPS shuts down and then cycles again.

I quickly disconnected power, isolated the SMPS and again verified that the SMPS is fine on its own with just the cooling fan load with a steady internal 44 VDC. No new or collateral damage thankfully.

There are no obvious shorts on any of the supply lines on the main board. The issue may still very well be with the SMPS being unable to handle load even though the fan runs fine and the other voltages are fine.

I assume that chirp mode is a common issue with these SMPS supplies. What have others done to (1) find whether the issue is with the SMPS or with the main board and if the latter, (2) which supply line or lines is drawing too much current and how to isolate the problem further?

Thanks in advance.

Priya.
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