Help repairing HP 3314A +5V supply - Page 1
Forum section: electronics / repair

author: reiser4
author_rank: Newbie
timestamp: March 02, 2021, 08:56:41 pm
content: Hello! I have this HP 3314A function generator that doesn't turn on.I've checked the power supplies and I have +15 and -15v. 5V is dead. I did measure some voltages but probably what I miss here is the correct startup sequence. I unplugged W100 and still see 0v.So I'm trying to understand how this PNP configuration works, because it looks fairly strange to me.U101 is an NE555 that should operate around 33khz, but I see always 0v in OUT (and +15V in the trigger). I think the opamp should create some high-to-low pulses and those pulses should move the trigger, but that doesn't seem to happen.Any idea on how to diagnose this fault?Thanks!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 02, 2021, 11:01:45 pm
content: Looks like HP engineers were at it again. Their circuits are complex and they use ICs in most creative ways.I would look at the "usual suspects" before trying to figure out the circuit. Look at the transistors starting with the chassis mounted Q 108.Check the volts on it.And,of course, check the dratted big capacitors. Many an HP unit has failed because of failed cap in power supply.Also, if something is drawing too much current, like a short in another board 5 volt line, I think the 5 V PS will shut down. I think this is what the voltage sense does.Maybe this is in the service manual.

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 03, 2021, 11:55:42 am
content: With my 3314As, there was a problem with the pass transistor sockets. HP used, in my opinion, very cheap sockets, and the transistor(s) were not making good contact. I finally got fed up, removed the sockets, and permanently soldered the transistors to the board.In another case, as the previous poster described, it was one of the electrolytic caps on the output of the supply. It had a high ESR and was suitably replaced, fixing the supply.

author: reiser4
author_rank: Newbie
timestamp: March 03, 2021, 01:08:42 pm
content: Thanks for the replies.Yes, I see some bad contact with the +15V transistor.What do you think about replacing the entire +5V part with a brand new DC DC converter?

author: fzabkar
author_rank: Super Contributor
timestamp: March 03, 2021, 05:09:24 pm
content: ISTM that U102 is a comparator with open collector outputs. I would think that the pulses would be coming in on pin #7.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 03, 2021, 10:39:27 pm
content: The purpose of this circuit is to shut down the 5 V PS when there is a problem elsewhere. If you replace the supply, you defeat that purpose.That could cause problems.That is the purpose of the 5 volt sense. Try disconnecting the boards using 5 volts and see if the 5 V comes up., if you think this could be the problem.

author: reiser4
author_rank: Newbie
timestamp: March 05, 2021, 02:59:19 pm
content: I found some dead transistors in the 5v circuit. I can not find the same part from my usual supplier, so I'm going to replace the whole 5v part with a dc-dc converter. Powering it up from an external supply I see an E30 error that I will investigate when I receive the DC-DC converter that I ordered today. Thanks everyone for the hints!