Tektronix SG 503 repair

[Charles]

Just finished working on this old beast. The display was flashing,
indicating an open amplitude control loop. The output vernier had no
effect. There is an op-amp error amp and it is socketed (741) so that was
easy to try first. No joy. Looking at the schematic it seemed the peak
detector diodes would be easy to check. Turns out that they are under two
covers but still not too hard. One cover is glued down but was easy to pry
off. One diode was open. I tacked in a 1N914/1N4148 substitute and
amplitude control was regained and no more flashing display. The output at
250 MHz is within 1 dB. Not too bad.

To fix it right, I am looking at a MMBD452LT1G.

 

[Look165]

A 1N4148 will never be able to work at 250MHz.

Rtaher try 1N3595 or equivalent.

Charles a Êcrit :

Just finished working on this old beast.  The display was flashing,
indicating an open amplitude control loop.  The output vernier had no
effect.  There is an op-amp error amp and it is socketed (741) so that
was easy to try first.  No joy.  Looking at the schematic it seemed the
peak detector diodes would be easy to check.  Turns out that they are
under two covers but still not too hard.  One cover is glued down but
was easy to pry off.  One diode was open.  I tacked in a 1N914/1N4148
substitute and amplitude control was regained and no more flashing
display.  The output at 250 MHz is within 1 dB.  Not too bad.

To fix it right, I am looking at a MMBD452LT1G.

 

[Phil Allison]

Look165 wrote:

A 1N4148 will never be able to work at 250MHz.

**

Rtaher try 1N3595 or equivalent.

** Err - that is a very slow, low leakage diode.


.... Phil

 

[Phil Allison]

Charles wrote:

To fix it right, I am looking at a MMBD452LT1G.

** That is a dual Schottky.

http://www.onsemi.com/pub/Collateral/MMBD452LT1-D.PDF


Readily available and cheap enough:

https://www.digikey.com.au/product-detail/en/on-semiconductor/MMBD452LT1G/MMBD452LT1GOSCT-ND/3462360


.... Phil