HP 8165A function generator: no sine/square output - Page 1
Forum section: electronics / repair

author: Devenda
author_rank: Newbie
timestamp: August 22, 2014, 02:22:30 pm
content: HiI just acquired a HP 8165A function generator. It worked fine up until now.The only output I get is a triangle, no sine and only in 20% duty cycle mode I get some kind of signal in square mode (looks like the top of a malformed sine).When in triangle mode I can get a 20% and 80% duty cycle, but selecting 50% sometimes gives me 20%.Just until now I also got a sine when pressing one of the sweep buttons (when not in sweep mode). But this also stopped working...I do have the service manual but I have no idea where to start looking.Maybe someone can give me some pointers?EDIT: I also noticed that, when pressing some of the input mode buttons or sweep time buttons, I get (a part of) a sine function for a very short time.EDIT2: I get a perfect square when below 1kHz (999Hz is fine, 1kHz not). But when changing the freq with the VERNIER buttons, the freq changes but not like expected, often outputting the wrong freq.I can do the same with the sine but I don't get a stable sine, I only see it when toggling for a brief moment, than its gone. Same applies here, no linear freq change when using VERNIER buttons. I do get freq higher than 1kHz but it is malformed ('pushed to the left').EDIT3: This is a summary of the things I found out today:1. Frequency is wrong: displayed and measured is always different.-The output doubles at the figure 1270 to 1280 (ex: 1.270kHz to 1.280kHz or 12.70MHz to 12.80MHz, ...).-performing a sweep works without skip, but the freq is way off-Max freq is ~24MHz (measured, 50MHz on machine)2. I can't get a sine or square wave:-I'm able to get a triangle (only 20% or 80%, 50% only under 1kHz)-The only 'square' I get is a malformed (positive) sine in 20% mode-Sine is very briefly visible when adjusting the freq using the VERNIER buttons3. I tried following the adjustment instructions found in the manual (Section 5-18: Adjusting VCO).-A8TP3 is never 2.56V.-On freq 2.56kHz I get a very low measured voltage-TP3 is 1.9V when freq is 2.55V (which is to low and I can't change that with R318)Hope this helps.

author: mij59
author_rank: Frequent Contributor
timestamp: August 22, 2014, 05:04:49 pm
content: Hi,I can only give some general tips.Start by inspecting the generator.Look for detective components like burned resistors, blown fuses, lose connectors etc.Look for mechanical faults like dodgy switches, relays, connectors, solder joints. ( tap components or boards with a isolated handle of a screwdriver)Check power supply voltages.

author: Devenda
author_rank: Newbie
timestamp: August 23, 2014, 09:38:39 am
content: Thanks for the reply, but sadly there isn't anything visualy wrong with the board/components.It seems to be more complex than expected...Also, see my resent edit

author: mij59
author_rank: Frequent Contributor
timestamp: August 23, 2014, 09:57:17 am
content: Quote from: Devenda on August 23, 2014, 09:38:39 amDid you check the supply voltages, and ripple on the supply voltages ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2014, 10:34:11 am
content: Hi,You can the Service Manual from Keysight:http://www.keysight.com/main/facet.jspx?&cc=CA&lc=eng&k=8165a&sm=gGood Luck!!Jay_Diddy_B

author: Devenda
author_rank: Newbie
timestamp: August 23, 2014, 11:54:00 am
content: Quote from: mij59 on August 23, 2014, 09:57:17 amThere is one signal (4V3) which has a .25V ripple, the rest seems normal.It might have something to do with the VCO, the LFG seems to function (more or less) but the problems get worse with freq above 1kHz...

author: mij59
author_rank: Frequent Contributor
timestamp: August 23, 2014, 01:17:54 pm
content: Quote from: Devenda on August 23, 2014, 11:54:00 amOn page 8-22 of the service manual is the troubleshooting Tree, based on the strange behavior the fault could be in block 5 rather than block 3.

author: Devenda
author_rank: Newbie
timestamp: August 23, 2014, 02:29:45 pm
content: Quote from: mij59 on August 23, 2014, 01:17:54 pmI could be, I just measured the frequency and found that when changing from 1.270kHz to 1.280kHz the frequency doubles. Strangely this is the same for 1.270MHz to 1.280MHz. (And the others to, it always seems to be at 1.270)Now, I don't see the correlation with the fact that there is no sine output (or square with higher than 1kHz)

author: z1zmail
author_rank: Newbie
timestamp: September 27, 2020, 05:01:19 pm
content: hi I have read with great interest the problem of the hp8165a as mine also has a very similar problem and I was wondering if that instrument was finally repaired and if so what was the fault, thanks! Flavio

author: z1zmail
author_rank: Newbie
timestamp: November 18, 2020, 05:39:01 pm
content: I finally fixed it. The problem was in the decoding circuit of the vco controller.Flavio