Please understand I am not an EE so I apologize for the rudimentary questions.
If you have no electronic experiance, you are in trouble. In that case I would try to find some help in your area.
In Germany self-repair support shops are becoming more and more popular. There you will find tools, knowledge and help (I am supporter of such a group here in my area).
If you have just never worked with such "old style" electronics, I have tried to guide you through the start-up below...
Hope it helps.
Test points 4 shows an incorrect signature (display's A446 when it should be HH46) and test points 7, 8 and 9 are incorrect as well in Normal and Service mode. All the others show correct. The 5v test point, I have 4.82v.
The 5V (4.82V) is ok in my view 3.6% deviation is will in the by TTL tolerance.
But I am not sure, which troubleshooting diagram you have followed. Would help, if you note the page, add some pictures, schematics or similar. Please keep in mind, this is no face-to-face communication, the other side is blind
- I have looked to the path of troubleshooting-diagram on p61 (SM I have supplied for upload) - out of my understanding the start point.
- If you have wrong signature at TP1 - TP5 -> diagram recommends to look to the selftest circuit, located on the main board A1, build by the ICs U25/26/27.
This seems to fit to your description, thus I have attached the part of the board location (p73) and the schematic (p74).
Am I to assume it means check...
U25, U26 and U27 are 8 bit counter (I assume binary types).
1. I would recommend to download first the data sheets of that parts, will help to understand there function.
Look to the part list table, which parts are U25/26/27. Normally that are HP part numbers. There are cross tables on the web (just google for "HP part numbers"...)
2. Next you may want to begin with my below listed check-flow / U26(2,3) means: measure at U26 pin2 and 3 ...
Check U26(2,3) - are the levels at that logik input ok?
--> if not correct == check the board for issues on wiring or bad soldering (repair / than continue)
--> if correct --> check U26 (14) - is there clock signal?
--> no clock == check U28, may have a problem (repair / than continue)
--> clock exist --> has U26(12) clock (output)?
--> no clock == U26 has a problem (repair / than continue)
--> clock exist --> continue same testing with U27 and so forth...
Hope these information helps and is not obvious. Good luck...
BR
PeLuLe