Re: TEK 475: Dead on Start Up.


Mark Vincent
 

Replacing the filter cans is a good idea. I did it in mine using high temp and long life types. If you can get low ESR types for these in addition to the long life and temp., good. Raising the values is fine. The 1.5A bridge rectifiers should be 4A. If you have any that are higher in voltage rating of the original, 200V, fine. It will not hurt to put in a higher voltage type. The higher current is the main thing. Make sure to put jumpers between the negative pads if you use snap-in or radial leaded types as replacements. The output filters on the supplies can be raised in value, e.g. 10mfd to 47mfd. I use Nichicon ULD or UHE for these. The former has the lower ESR. The UHE is also a great choice. C1304 and C1374 I put in a film type. Decoupling types can be left in unless bad.

Check VR1416. If open, use BZX85B9V1.

Check all 151-0367-00 transistors. Any that are leaky C to E, use KSP10BU as replacements. The pinout of these is BEC.

Some resistors I replaced with a higher wattage because the originals were too small and the heat changed the resistance making it out of tolerance or open in some cases. R1378 is likely out tolerance. I used the Vishay VR37 series as the replacement. The symptom of this resistor being too high, about 28meg and higher, is the trace will be bright at turn on then dim down as it warms up over 15 minutes. I have seen this problem enough times.

Mark

Join TekScopes@groups.io to automatically receive all group messages.