HP 8657B - No RF Output - Page 1
Forum section: electronics / repair

author: ZeroTolerance
author_rank: Contributor
timestamp: December 21, 2015, 09:28:43 pm
content: I am repairing a HP 8657B RF Signal Generator. I noticed all the functions on the front panel work, but not getting any RF out of the type N connector on the front. I have removed the PSU from the device and noticed that the +5.2V rail was only outputting +4.2V. I tried adjusting the trim pot, but only got +4.6V max after turning it all the way up. I have since replaced the LM337T VReg. I have not yet powered it back up, as I am still testing the power supply. All other outputs though seem to be within tolerance on the Power Supply (A14).This device is from 1990, should I go ahead and replace all caps in PSU as well? Now I need to investigate why I am not getting any RF Output.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 21, 2015, 09:37:20 pm
content: Look for short tantalum caps first. I did repair one some time ago (currently for sale on ebay) and it only had a few shorted tantalum caps (those yellowish ones, the metallic ones are much more reliable, generally) on the low frequency PLL / VCO board. Fault isolation works best if you unplug board by board and look at the 5V rail

author: ZeroTolerance
author_rank: Contributor
timestamp: December 21, 2015, 09:39:35 pm
content: Thanks for the info madI will definitely do that. If any others have some insight as well, please post.I will update this post once parts arrive, have been installed, and power supply tested.

author: cncjerry
author_rank: Unknown
timestamp: December 23, 2015, 06:28:12 am
content: One of those generators is prone to output amp failure. I saw a few repaired with mmic type amps instead of the HP part. I don't think a mmic would drag the +5 down that much though.

author: ZeroTolerance
author_rank: Contributor
timestamp: January 02, 2016, 08:31:33 pm
content: The unregulated rail (+5V, +15V, -15V) are all not within tolerance, but on the regulated portion of the circuit I am reading everything within spec.I am reading +11.00V on the +5V pin (J9)+29.47 on the +15V pin (J9)-29.37 on the -15V pin (J9)I find it hard to believe that CR2, CR6, CR4, and CR5 are all faulty???

author: SeanB
author_rank: Super Contributor
timestamp: January 02, 2016, 08:39:39 pm
content: Fine as unregulated inputs so the diodes are fine. Faulty diodes almost always reduce voltage, that they are high says there is low load on the rails. If outputs are fine then power supply is working.

author: ZeroTolerance
author_rank: Contributor
timestamp: January 02, 2016, 08:45:39 pm
content: I kind of figured, but was not sure. Yes, the power supply and transformer were removed from the device to correct the +5V output, which was reading +4.6V with the trim pot max'ed out. It is now outputting at +5.2V as spec'd. I corrected this by replacing LM337T and adjusting trim pot accordingly. Now I need to see what other problems this unit may have.

author: ZeroTolerance
author_rank: Contributor
timestamp: January 03, 2016, 03:09:59 am
content: Now, the A12 Frequency Doubler is bringing down my -5V supply to -2.6V AHHHHH!!!!

author: ZeroTolerance
author_rank: Contributor
timestamp: January 22, 2016, 02:11:04 pm
content: Well I found out why I wasn't getting any RF Output. One of the attenuators is broke; It's missing a piece of brass.The Frequency Doubler is still bringing down the -5V rail though to -3.2V. The voltage seems to slowly increase over time to -5V. Unfortunately, there is no schematic from the manufacturer on the Frequency Doubler.

author: bob91343
author_rank: Super Contributor
timestamp: April 15, 2019, 05:38:25 am
content: I realize this is a thread over three years old. But these problems plague many owners of these otherwise fine signal generators.I recently repaired my doubler. It's not so sophisticated that you can't find certain problems.My doubler failed when the SPDT solid state switches that cut it in and out, failed. I replaced them and all is well.Others have had the problem with the +5V being dragged down. You can troubleshoot it by removing the regulaor and supplying +5 V from an external PS that is current limited. If the load is heavy or the regulator is bad you will discover quickly.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: April 17, 2019, 11:30:48 am
content: Mine does that also. The -5.2v gets dragged down and the doubler stops. When it recovers, it works fine again.I haven't been able to spend much time with it lately, but I will try the external supply trick.I would find it odd if those SPDT switches are the culprit in dragging the supply low.I also don't know why HP decided to send a -5.2v supply right to the doubler, as that seems to be the only place it goes when I traced it.There is already -15 in there.But with no service info, we're on our own.I know these issues have been discussed before, but no one has really nailed down what the culprit is.Slightly shoddy design from HP, with all of the similar issues.

author: bob91343
author_rank: Super Contributor
timestamp: April 17, 2019, 04:02:37 pm
content: I don't think it's a shoddy design as much as shoddy documentation. I would be proud to create a generator that works as well as this one. But supplying no diagram of a major component is inexcusable. Forcing a sincere technician to reverse engineer an expensive unit isn't good documentation.Having said that, I have many other major complaints about the documentation. On the surface it appears that they made a good effort but when you try to follow the flow of signals and commands it falls short. Locating major parts is another deficiency.It's not just this unit; I am going nuts with a 3455A as well. My 3456A was obtained at a good price because the HPIB function was intermittent, causing the unit to stop working. Luckily, that has fixed itself and it seems to work reliably now. My 5328A had to be cobbled up from three units to make one good one, and I am happy. There is still one issue but it's not causing problems for me.Since the latter unit only works up to about 600 MHz I bought a cheapie Chinese 2.4 GHz counter and am mostly satisfied with it. For just a few bucks it has given me good value and service. While it has a TCXO, it's no match for the oven oscillators in the generator and counter. But if I need a good calibration and can work reasonably quickly, I can compare it at a lower frequency and then measure, making appropriate correction.The hobby has to include repair. But it's a learning experience.I am eager to see the results of your testing.