80's HP 6114A schematics? - Page 1
Forum section: electronics / repair

author: schopi68
author_rank: Regular Contributor
timestamp: April 23, 2015, 08:37:06 am
content: I am currently repairing/adjusting a HP 6114A that i bought on ebay. This device has been built after 1980, so the schematics from the original documentation (1972) may be somewhat outdated.Mainly i only had to clean the device (the cables are managed in small PVC tubes... these tubes are terrible, after 30 years they are sticky from the softeners leaking out of the material. I had the same in my HP 6827A). See picture... it looks like oil but these are the softeners.The 10-turn current potentiometer had to be changed, no big issue (only one minor aspect - the actual Bourns device from this line has less sealing than the original one in the device).Every other aspekt in the device seems to work perfectly - but when testing the crowbar circuit i saw something i currently do not understand: After fireing the crowbar the Output Voltage changed to approx. -0.5V. How can this happen? The crowbar thyristor in the original schematics should just make a short between the output connectors, there is no negative power source at the thyristor.Any hint before i have to reverse-engineer the schematics (without disassembling the device...)?

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2015, 09:50:41 am
content: http://exodus.poly.edu/~kurt/manuals/manuals/HP%20Agilent/HP%206104A,%206114A,%206105A,%206115A%20%20Operation%20&%20Service.pdfThere is a detailed explanation of the crowbar principles in section 4 P846 Mb Service manual:http://elektrotanya.com/hp_6104a,6114a,6105a,6115a.pdf/download.html

author: schopi68
author_rank: Regular Contributor
timestamp: April 23, 2015, 10:02:39 am
content: Quote from: tautech on April 23, 2015, 09:50:41 amI already knew this section. This is out of the documentation from 1972."...placing a short across the output...driving output voltage and current towards 0". It does not explain why fireing the crowbar gives -0,5V. So there must be a design change or the crowbar circuit in my device is defective and shows a behaviour that i still can not explain even with the documentation in my hand.The second link you gave does not work here (shows no document). Is this documentation newer?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 23, 2015, 12:24:23 pm
content: Hi .It's working normally I would say. When the thyristor fires it has two actions 1/ it places a short across the output AND simultanously pulls down the 7.5V rail via cr16 to the -Vout node, this rail provided the drive for Q2,Q3 pass transitors and also Q1 drive stage so all these will now be switched off. The return path for current from the 7.5 V rail will now be via CR15 (forward through the reverse polarity protection diode) from the -Vout rail to the 7.5V gnd ref which is at the +vout rail.This probably is what you are measuring at the output, it's the Vf of Cr15 returning bias current's to aux gnd(+vout). The current from the 7.5V rail is enough to hold the thyristor on (~ 10mA or so) even when theres no current through the pass transistors.

author: schopi68
author_rank: Regular Contributor
timestamp: April 23, 2015, 02:21:24 pm
content: I had tested this with an 10 Ohms resistor on the output. So 50 mA must have been on the output.I am still missing the source for the negative voltage on +vout. The only assumption i can made is that it comes from VR5 and R40 after pulling down the 7.5V rail. Hmmm... Vr5 should be capable of withstanding up to 200 mA, current should be limited by R41 and R3 to a value below the 50 mA that i had on my output resistor.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 23, 2015, 03:56:38 pm
content: I though I just explained it,The negative V source your measuring is from the +7.5V supply, when thyristor is conducting then the +7.5 V aux becomes connected to -Vout rail and as is usual your aux gnd point is your +Vout rail ,hence flow is from -Vout to +Vout (aux earth) Via cr15 or any connected load .Why do you think this is not the source of your neg voltage measured at the output ?Vr5 and r40 are there to derive a neg -4.3V point relative to the +Vout node which here is defined as 0V. They are part of the return path for the 7.5 rail of course, but you dont need for those components to be there (just a wire return path would have done ) in order to get negative potential between +Vout and -Vout.Regards

author: schopi68
author_rank: Regular Contributor
timestamp: April 24, 2015, 07:24:33 am
content: Quote from: Kevin.D on April 23, 2015, 03:56:38 pmYou forgot to explain that the CR16/CR20 path will rise -Vout to a level above +Vout.Maybe i struggled about the term of "pulling down" 7.5V to -Vout. It depends on the point of view... After thinking about the schematics during driving to work today it became clear to me. So everything is fine now.