HP-3421A Service Manual is required.

[youngwug]

There is one HP-3421A that does not turn on (+15, -15, +5V is dead).
The measured value of C708 where oscillation is occurring inside the capacitor is 495(0.205).
C708(25V, 470uF) was replaced.
And when you turn on the power, +16.9V is measured in TP700, +7V in RT700, and +6.6V on top, regardless of S502(Switch).
However, +15V, -15V, and +5V cannot be measured.
I suspect the U702(LM393N, Dual Differential Comparator), but it is not easy to analyze because there is no circuit diagram.
If the “Wakeup Logic” on the left side of the figure can be related to S502, the problem is complicated, so I need a schematic.
Please tell me where to buy the service manual schematic(pdf or jpg).

[wn1fju]

http://www.hpmuseum.net/collection_document.php

Search on page for 3421A.

[coromonadalix]

some links

3421A with only one ROM   talks

https://groups.io/g/VintHPcom/topic/27149599?p=Created,,,20,2,0,0::recentpostdate%2Fsticky,,,20,2,0,27149599

hp museum service manual attached
do a 3421a search,  will be easier to find what you need

thks @ wn1fju    never saw this website.

[youngwug]

coromonadalix & wn1fju
Thank you so much.
It's best.