I have have had a very reliable?HP8640B that I bought as a parts unit years and rehabbed into a working state, including replacement of the tricky differential gears with brass ones. It has been a most faithful runner up until the other day when I was using it to align a receiver, when the RF output suddenly stopped, and the counter went to 00000. I powered it off and on, and the RF returned for 10-20 seconds and fell silent again. In further power cycles, the RF will occasionally come back on for a matter of a few seconds before failing again.

My first though was of course that one of the power supplies had finally lost a filter cap. So I checked them all for proper voltage and any ripple None of them had any sign of excessive ripple and all voltages were all spot on EXCEPT the +44.6. As long as the RF is being generated, it's happily at 44.6, but as soon as the RF quits, it rises to 47-49V and the voltage trim pot has no effect. I checked all the discrete components on the 44.6 board including swapping out the U2 723BE regulator. No luck.

I placed a scope directly on the output of the oscillator, both on the counter side and the main output, and it appears there is no RF oscillation happening at this point. In looking at the schematic, I was unable to see any use of the 44.6V supply in this area, so I'm baffled. It appears to use +/-20 and 5.2 but not 44.6. I've heard of heroic repairs of the RF cavity circuitry, but I am praying it's not failing there.

Any tips or thoughts of what to check next are most appreciated.

Scott

?

I have not checked for bad solder joints on the pass transistor. I will give all of them a close inspection tomorrow afternoon.?

One thing I did notice today, is that along with the odd 44.6V behavior, the -20v directly at the connection on the RF oscillator drops significantly at the same time the RF fails. So this is a new clue. Not sure how this would also affect the 44.6V supply, but it seems this would likely cause the oscillator to fail. Perhaps there's a bypass capacitor failing somewhere on the rail. Time to take a closer look at the smaller electrolytics scattered around on the various boards.? Perhaps a marginal edge connector somewhere.?

I guess I have reason to hope the main RF oscillator is not at fault, unless there's a faulty component drawing the rail down within that circuit. I did not notice any excessive heat forming in that area, so I suspect not.?

The search continues.?

Scott

@Jim: I too am a big fan of freeze spray. It can really root out faults that would otherwise be very difficult to diagnose with bench equipment, especially intermittent issues like this. I gave each of the regulator sections a nice dose of freeze spray, without any revealing results, except for the one drawback of such techniques, namely the condensation that can result after cooling the boards, which in this case would throw the regulators off until they'd dried off. Ordinarily, I avoid freezing entire sections of a circuit at once, preferring to freeze each component separately with a nice steady and directed drip from the freeze can. This not only helps eliminate the condensation issue, but saves on the freeze spray, which is not inconsequentially pricy. Unfortunately, without any extender cards, it's nearly impossible to just freeze a specific component at a time, so, firehose freeze spray it is! Anyway, it didn't expose any temperature sensitive issues on the regulator cards.

@George: I definitely plan to re-flow all of the TO3 connections, and clean the sockets and pins. Even if it doesn't fix this issue, it's good hardware hygiene in any case. I'll put all of the power transistors on a curve tracer too, and run them through a wide temperature sweep. None of them generated any notable heat while in circuit, but a flakey P/N junction can decide to fail at relative room temperatures. I will say, the day this failed, the temperature in my shop (uninsulated barn actually) was probably in the mid 40°s, F. Admittedly a silly temperature to be aligning a receiver, but it was intended to be a rough alignment. I was bored, what can I say?

@Dave: As long as I've had this 8640, is has always been positioned horizontally, so I would hope this would preclude the possibility of the lubricant creeping into the electronics. I do agree that the behavior of the 44.6V supply floating high when the oscillation stops, seems like the regulator's reaction to being underloaded, perhaps when the circuits it supplies, take a nap. Thanks for the tips on where these los might reside.?

I really need to make or buy some 15 pin extender cards. Troubleshooting such circuits without, is like trying to fish coins out of a storm drain. Anyone know a good source of such, that aren't priced by concert scalpers?

Say, anybody know any disadvantages of using freeze spray as a duster or a duster as freeze spray?? Turn one upside-down to make it work as the other, of course.? TIA.

Jim Ford

Sent from my T-Mobile 4G LTE Device

-------- Original message --------

From: Flannel Tuba <flanneltuba@...>

Date: 3/1/22 12:12 PM (GMT-08:00)

To: [email protected]

Subject: Re: [HP-Agilent-Keysight-equipment] HP8640B RF Fails Several Seconds After Power Up

@Jim: I too am a big fan of freeze spray. It can really root out faults that would otherwise be very difficult to diagnose with bench equipment, especially intermittent issues like this. I gave each of the regulator sections a nice dose of freeze spray, without any revealing results, except for the one drawback of such techniques, namely the condensation that can result after cooling the boards, which in this case would throw the regulators off until they'd dried off. Ordinarily, I avoid freezing entire sections of a circuit at once, preferring to freeze each component separately with a nice steady and directed drip from the freeze can. This not only helps eliminate the condensation issue, but saves on the freeze spray, which is not inconsequentially pricy. Unfortunately, without any extender cards, it's nearly impossible to just freeze a specific component at a time, so, firehose freeze spray it is! Anyway, it didn't expose any temperature sensitive issues on the regulator cards.

@George: I definitely plan to re-flow all of the TO3 connections, and clean the sockets and pins. Even if it doesn't fix this issue, it's good hardware hygiene in any case. I'll put all of the power transistors on a curve tracer too, and run them through a wide temperature sweep. None of them generated any notable heat while in circuit, but a flakey P/N junction can decide to fail at relative room temperatures. I will say, the day this failed, the temperature in my shop (uninsulated barn actually) was probably in the mid 40°s, F. Admittedly a silly temperature to be aligning a receiver, but it was intended to be a rough alignment. I was bored, what can I say?

@Dave: As long as I've had this 8640, is has always been positioned horizontally, so I would hope this would preclude the possibility of the lubricant creeping into the electronics. I do agree that the behavior of the 44.6V supply floating high when the oscillation stops, seems like the regulator's reaction to being underloaded, perhaps when the circuits it supplies, take a nap. Thanks for the tips on where these los might reside.?

I really need to make or buy some 15 pin extender cards. Troubleshooting such circuits without, is like trying to fish coins out of a storm drain. Anyone know a good source of such, that aren't priced by concert scalpers?

Say, anybody know any disadvantages of using freeze spray as a duster or a duster as freeze spray?? Turn one upside-down to make it work as the other, of course.? TIA.

Jim Ford

Sent from my T-Mobile 4G LTE Device

-------- Original message --------

From: Flannel Tuba <flanneltuba@...>

Date: 3/1/22 12:12 PM (GMT-08:00)

To: [email protected]

Subject: Re: [HP-Agilent-Keysight-equipment] HP8640B RF Fails Several Seconds After Power Up

@Jim: I too am a big fan of freeze spray. It can really root out faults that would otherwise be very difficult to diagnose with bench equipment, especially intermittent issues like this. I gave each of the regulator sections a nice dose of freeze spray, without any revealing results, except for the one drawback of such techniques, namely the condensation that can result after cooling the boards, which in this case would throw the regulators off until they'd dried off. Ordinarily, I avoid freezing entire sections of a circuit at once, preferring to freeze each component separately with a nice steady and directed drip from the freeze can. This not only helps eliminate the condensation issue, but saves on the freeze spray, which is not inconsequentially pricy. Unfortunately, without any extender cards, it's nearly impossible to just freeze a specific component at a time, so, firehose freeze spray it is! Anyway, it didn't expose any temperature sensitive issues on the regulator cards.

@George: I definitely plan to re-flow all of the TO3 connections, and clean the sockets and pins. Even if it doesn't fix this issue, it's good hardware hygiene in any case. I'll put all of the power transistors on a curve tracer too, and run them through a wide temperature sweep. None of them generated any notable heat while in circuit, but a flakey P/N junction can decide to fail at relative room temperatures. I will say, the day this failed, the temperature in my shop (uninsulated barn actually) was probably in the mid 40°s, F. Admittedly a silly temperature to be aligning a receiver, but it was intended to be a rough alignment. I was bored, what can I say?

@Dave: As long as I've had this 8640, is has always been positioned horizontally, so I would hope this would preclude the possibility of the lubricant creeping into the electronics. I do agree that the behavior of the 44.6V supply floating high when the oscillation stops, seems like the regulator's reaction to being underloaded, perhaps when the circuits it supplies, take a nap. Thanks for the tips on where these los might reside.?

I really need to make or buy some 15 pin extender cards. Troubleshooting such circuits without, is like trying to fish coins out of a storm drain. Anyone know a good source of such, that aren't priced by concert scalpers?

Ok.? Just wondering because it seems whenever I need one, all I have is the other!? So tempting to just turn it upside down....

Jim

Sent from my T-Mobile 4G LTE Device

-------- Original message --------

From: 搁别苍é别 <k6fsb.1@...>

Date: 3/1/22 2:30 PM (GMT-08:00)

To: [email protected]

Subject: Re: [HP-Agilent-Keysight-equipment] HP8640B RF Fails Several Seconds After Power Up

works for me, although seems the condensation is worse with canned air, may be my imagination.
搁别苍é别

On 3/1/22 12:42 PM, Jim Ford wrote:

Say, anybody know any disadvantages of using freeze spray as a duster or a duster as freeze spray?? Turn one upside-down to make it work as the other, of course.? TIA.

?

Hello George, from 12,000 miles away.

?

If only I had met you when I lived and worked in the UK.

?

I have a couple of 8640Bs that I’ve managed to keep going – the odd capacitor failing, and those dreaded gears going U/S.

?

A member of the radio club asked me to look at an 8640B that wasn’t working. There was no RF output and gears were so badly cracked you couldn’t turn the knobs. Someone must have tried because one of the wings on the band-change knob is missing. Now that I’ve replaced gears and lubricated bearings, the band change is fine. Unfortunately, even Walter of Sphere Research in Canada can’t help with that knob; can you?

?

I now have reliable RF output, but no frequency readout. I purchased another frequency readout module from a person scrapping an 8640B, who claimed his counter had been working. But, in fact, neither counter works. Can you help?

?

73 de Brian, VK2GCE

?

?

Hi

I have repaired a number of HP8640B signal generators with similar faults, the TO3 sockets for the power supply pass transistors go faulty with time resulting in poor Base and Emitter connections.

G Edmonds G6HIG

Greetings,?

I am finally getting back to this project and have narrowed the problem to a failing transistor in the A3 Main Oscillator. Fortunately this is a relatively easy part to get to once you know its secret hidey-hole under the large hex cap on the top of the oscillator body. (See this OCRed article from the Sept/Oct 2005 QEX magazine:??and search for HP8640 There's a marvelous description of how to disassemble the A3 unit to get to the electronics, and how to spot one of the most common problems -- loss of either +20v or - 20v at the Q1 transistor socket).

Anyway, having found the main oscillator transistor A3 Q1, and demonstrating that it was failing using a Huntron curve tracer (and some freeze spray!), I am now looking for a suitable replacement. The HP part number on the parts list is 5086-7082, whereas on the schematic itself, it's listed as 5086-4282.

So can anyone direct me to an equivalent to either a 5086-7082 or 5086-4282 NPN transistor, or a spec sheet that I can use to try and match the original?

Thanks,

Scott