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HP-8640 Service: Hints & Kinks

Note: None of the contributors or myself assume any liablility for any damage caused by acting on information contained herein.

Most recent HINTS and KINKS Update is #23: 30 June 2016

If you have a hint or a solution for fixing the HP8640, especially if it involves repairing or finding substitutes for HP parts that are no longer in production please let us know and we will post it on this web page.

Service Hints
  1. HP8640B Frequency display reads VFO directly (250-520MHz), 000.0->no osc.!
  2. If <250 MHz, VFO cuts out (esp. top of band) or O/P noisy, RF Divider U11/U12 area bad?
  3. Normal VFO O/P 250-520MHz, about 0dBm with 0 to +2dB variation over band?
  4. VFO cavity repair is extremely difficult (impossible?) without HP experience and tools.
  5. Aglient Technologies (purchased HP) has online manual, see below.
  6. Sourcing? See HP Part# to industry (JEDEC) #s PDF list by (WA1GFZ) Frank Carcia.

Quick Links:
1. Output dies toward high end
2. Cracked Wheels
3. More Cracked Wheel
4. Caution storing
5. Surface mount MAV-11
6. Replacement type MAV-11
7. Oscillator Failure
8. Osc. Frequency Adjustment
9. Gear Information
10. More Storing Info.
11. Audio irregularities
12. Manuals Online
13. Combo Gear Replacement
14. Final Hybrid Amp. Replair
15. Display, Lock, Gears
16. -5.2V Supply, No Counter
17. Tuning grears crack
18. High Speed Divider
19. Learning hard way
20. (opt 002) dead output amp
21. Jitter Problem.
22. More Cracked Wheel Solutions.
23.
Frequency Lock Problems.




1. Output dies on tuning to high-end on all bands,   November 2004: Markus VE7CA

This may be due to a problem with the 128-256 MHz band ECL divider in RF Divider A10A2, the board located just under the large die-cast cover on top-side of generator. If there is input at the divider over the full freq. range but the output fails above a certain freq. then this fix may apply.

A factory modification was made: 100 ohm variable resister R6 was added from pin 13 of this divider to ground. This mod. increases the sensitivity of the divider. To adjust the resister, tune gen. to high frequency end of band and set resistor half-way between the high and low cutout. If the bias is too high or low, the divider will not clock.


2. Cracked Wheels,   November 2004: Hadley K7MLR

I just finished the first read of your article, quality work, thank you very much. There are two other, very common, problems with the HP8640:

  1. The nylon gears, in the gear trains, of the audio osc. and range switch crack and will eventually fall apart. If caught in time, epoxy will extend their life.

  2. The small finger contacts, mounted to the clear Styrene wheels, located on the range switch assembly and the attenuator will become intermittent. Again with some work and epoxy this is pretty easy to fix. The Styrene wheels are reversible and give new purchase points to re-mount the fingers when they fall off. Every HP8640 I work on gets the epoxy treatment on the wheels because the fault will occur sometime down the road. Just a warning, don't lose any of the fingers during disassembly or you will be up the creek unless you have a hanger queen.
Ed. Note: Picture of a Cracked Styrene Wheel on EB5ABV's Web site is
here. It shows a cracked wheel before repair and the small finger contacts, mounted to a clear styrene wheel.


3. More Cracked Wheel Help,   November 2004: Ken VE3FIT ve3fit @ rac.ca

I read your article in QEX about repairing an HP8640 and it led me to your web site. I greatly enjoyed both. Last winter, I restored an 8640B which I had bought "as-is" on e-Bay. Fortunately, it was a long winter: I did a repair on several cracked Delrin gears and in a nutshell, here it is:
  • Having removed the gear, I put it in a refrigerator's freezer compartment for a few hours. This will cause the delrin to shrink more than the brass hub. Drift out the brass hub. Mine came out with just a gentle tap on a hammer and drift. (Credit the next part to Jeff Liebermann). Use 24 hr epoxy to glue the Delrin back together. Use a round file to open up the delrin centre hole so that the brass hub will just fit back in. Epoxy the hub back in.
I had to replace the final power amp in my unit. Fortunately, I managed to get a 'pull' from Fair Radio, but it cost about US $125. I was just looking on the WJ Communications web site and it looks like they now have several MMICs that could work in the 8640B. Check out their ECG008.

It's great that you have offered your web site for posting information regarding the HP8640.


4. Caution when storing HP8640,   November 2004: Terry K7TAU

A retired co-worker (George Steen) has located a fellow that worked at HP on the HP-8640-B RF generator and has been corresponding with him regarding the instrument. Here is something George passed along to me and suggested that I copy you on this information.

"Word of caution: don't store the oscillators on-end (with their tuning shaft pointing up), because if you do so, eventually the internal silicone grease migrates down to (and saturates) the back plate where the transistor oscillator resides. The grease sometimes gets between the plate and the case, causing loss of good electrical connection. End result: the unit drops out of oscillation at parts of the band - - or in bad cases, won't oscillate at any part of the band. " This is a direct quote taken from correspondence between George Steen and Greg Burnett, Retiree of HP.

Thought you might be interested in reading this and perhaps posting this on your web page.

Regards, Terry K7TAU


5. Using a surface mount type MAV-11 and PCB,   December 2004: Angiolo Chiti I5SXN

I am I5SXN, an old Italian radio amateur no more active but I am now only a technician. In 1999 I had the same problems on two generators HP 8640B. At that time HP asked me $600 for that modules. I solved the problems in the same way, as you, but I opened the container of the amplifier and I introduced inside a small printed circuit with an MMIC type MAV11, two condenser and a lot of chip resistors series and parallel to obtain the right current for the MMIC and dissipation using directly the 44 volt supply. I closed the container and in this way if you open my HP8640 you do not see any transformation. I send You the schematic diagram and the printed circuit indications.

PC Board Layout & Schematic



Printed circuit and mounting position of the components. The shape is octagonal for problems with my CAD, I rounded the circuit before mounting in the container. Diameter 25 mm. Distance between pins A and B 12.5 mm, and between B and C 16.4 mm. The circuit was of 0.8 mm thickness, with copper on the lower side. I scratched the copper around the active pins. With small holes and wires I connected the ground pins of the MMIC to the lower copper side. I used the original pins of the module. Another time I repaired the burned final amplifier adding a zero bias diode on a different printed circuit. I hope to be useful to you. Good luck and 73.

Angiolo Chiti I5SXN, E-mail: a.chiti @ rossbauer.it


6. Another replacement type for the MAV-11,   May 2005: Tom Bruhns K7ITM

I just photocopied a mixer article from QEX (July/Aug 2004 issue) and happened to get the last page of an article you apparently wrote about HP8640 amplifier issues. In footnote 4, you mentioned a WJ amplifier. For a little better output power and quite a bit better IP3, have a look at Sirenza SBF-5089. It's rated nominally for 500MHz high end, but the rolloff is gradual and it should be no problem using it in the 512MHz HP8640B. It's probably a bit shy on power output to be used directly in the output stage if you need full power output. The SBF-5089 has a particularly high IP3 (which is why I know about it...), and you probably don't need that linearity in the 8640 output. For power output from a monolythic gain block, have a look instead at the Sirenza SGA-7489, which should give you a bit better IP3 than the WJ part and about twice the output power--maybe enough to actually give full 8640 output. Plus, it's rated for a wider frequency range.

Cheers, Tom Bruhns


7. Help for Oscillator Failure in the HP8640B.,   October 2005: John Klingelhoeffer WB4LNM

John wrote an excellant article in the Sep/Oct 2005 issue of QEX describing how he fixed the oscillator in his HP8640B generator. John describes in great detail the process of disassembling the A3 oscillator assembly in order to get at the oscillator circuit. He included many clear pictures of the inside of the generator and oscillator assembly. If you don't have a copy of the QEX article you can order a copy from ARRL.


8. Oscillator Frequency Adjustment HP8640B.,   January 2005: Markus VE7CA

If you set your HP8640B so that the frequency counter reads exactly 10.000 Mhz but the output frequency is not exactly 10.000 Mhz, you can easily calibrate it yourself.

  1. Remove trim strip that holds front panel window in place. Gently pull window up and out and remove it. Allow generator to warm up for 2 hours.

  2. While listening to WWV at say 10 MHz in a AM receiver,(with a a short length of wire attached to the generator output), if your generator is off frequency even though the frequency counter reads 10.000 Mhz, then you will hear a beat note over-riding the WWV carrier. Adjust the time base adjustment pot (available through the hole in the front of the counter casting) until the oscillator is zero beat with WWV at 10 MHz, i.e. the beat note disappears.

  3. Replace front panel window and trim strip.

9. More Gear Information for the HP8640B,   March 2005: Jim, K8JL

In order to purchase new plastic gears for the HP 8640B one must specify the Diametral pitch (DP), the Pressure Angle, and of course, number of teeth, bore, thickness, etc. The DP is 48. A new 20 deg. pressure gear does NOT mesh correctly with the original gears. Hence, I conclude the pressure angle is 14.5 deg., the other choice. I have not tried a new 14.5 gear with the originals, however. I used Super Glue to mend split gears and it worked well. Clamp the pieces together for a few hours. I could not pull the joint apart with my hands.

NOTE: The latest available manual from 1992 for the 8640-B covers the "new" assembly and it gives a detailed gear specification for the combination gear:
"48-T 48-DP 14.5 DEG-PA".
Regards, and tnx for your help. Jim, K8JL.


10. More Information regarding Storing the HP8640.,   January 2006: Jim Korenz N8PXW

Hi Markus:

I E-mailed you awhile back about HP8640B problems. I stored my 8640 face up. When I read your webpages, I saw the comment bout intermittent frequency operation. Sure enough, I had 7 dead sections in the tuning range, including both ends. Additionally, the generator would not lock, even after an hour warmup. I E-mailed Terry, K7TAU, for help. He recommended operating the unit face down, to let the silicone grease flow back. After three months in this position,the HP8640 had only one dead spot in the middle of the range and would lock. I turned the unit off and let itset flat for another couple of months. I turned it on before Xmas, and lo and behold, the oscillator works perfectly. The bottom line, you don't have o open the unit and dissemble and clean the cavity oscillator to restore proper operation. You can let it sit face down, but it will take some time to come back to full operation.

Thanks for your help, and your webpages.


11. Information regarding variable and fixed audio irregularities in the HP8640.,   September 2006: Markus VE7CA

After finding and reading through the HP repair information on your site, I thought the following may be of some interest to fellow HP user's. My HP8640A developed a fault causing the variable and fixed audio oscillator to stop working. I located the problem to the thermistor module and discovered that the fault seemed to be an open circuit between the common point (A) and the two series connected thermistors to connection point (C). The path between (B) to (A) was ok. No information is forthcoming in the HP manual regarding this component.

After trying several sources for a replacement and in most cases only being offered a complete replacement board for £50 plus vat etc, decided a cheaper cure must be sought. Trying various remedies I recalled that a solution might be to use series connected lamps to replace the whole original thermistor sealed assembly. I happened to have a supply of miniture 16v lamps so connected 3 in series and placed them on the top of the pcb soldering directly to the pads and interconnecting with thin wire.

The idea worked OK apart from a slight bouncing of the o/p amplitude at switch on. The output remains stable otherwise. It may not be the best of "fixes" but it cost nothing to achieve and will certainly suffice for my needs.

Regards, Steve GW4ZDU


12. Information regarding manuals for the HP8640,   July 2007: Markus VE7CA

Aglient Technologies who purchased HP has now posted operating and service manuals for many of the older HP test equipment.

Click here to go direct to the HP8640 service manual site:
HP8640 Manual.


13. Combination Gear Replacements. ,   October 2007: Markus VE7CA

One of the problems that many owners of the HP8640 series signal generators face is finding a replacement for the combination gears (( "Fig. 8-97, A9 Peak Deviation and Range Switch Assembly" it is called: Item Number 13 and 19 (they are identical!), Reference Designator A9MP12 and MP13, Description: Combination gear. ))

Recently I recieved an email from Mr. Wieschhoff in France regarding the Split Gear Combination Assembly:

"After having followed the conversation of some interested circles in early 2007 about the re-manufacture of the combination gears in the HP generator 8640B I tried to get some information about the follow-up and result. The early generators manufactured containing what I would call the "old" dev./freq. switch assembly. The difference to the "new" one is that it uses standard type switch segments where as the "new" ones uses a printed circuit type rotary whiper discs to switch. Apparently both assemblies were manufactuerd in parallel for some time, since the manual from 1976 covering the generator version HP8640-A shows still the "old" assy. The latest available manual from 1992 for the 8640-B surely covers the "new" assembly and for the fist time I see it gives a detailed gear specification for the combination gear: "48-T 48-DP 14.5 DEG-PA". It may also have been given in manuals between 1976 and 1992 but I do not have any. In ALL manuals quoted (including the one covering the "old" version) the part numbers for these gears are identical: 1430-0773 and 1430-0774. In fact both different numbers describe identical parts. Why HP did this I do no know but it may have been for internal procedural reasons. I used a 0734 replacement gear bought from HP some years ago (unfortunately at the time I only bought one) in place shown for the 0774: no problems."

Best regards, Reinhart


14. Final Hybrid Amplifier Replair,   November 2008: Markus VE7CA

I recently made a small SMD board to plug in in place of a blown output hybrid in a 8640B. I used the Philips BGA6589 and could reach about +6dBm. Whilst this got the machine back in working order it was not as linear when modulated really only got to 0dBm.

I started to wonder why the hybrid had died since it showed output on the meter but none at the output pin. I concluded the either the output C had become detached or itís bond wire to the output pin broken. So I mounted the hybrid on a hardwood mandrel and centred it in the lathe and gently turned off the top.

Fig 6. Final Hybrid Amp. with lid removed.





As the picture shows the bond wire to the output was vaporised! The output C had a slight coating of gold sputter but was otherwise OK. A small semiconductor pad capacitor which had been bonded to earth had also melted and was undoubtedly the path to earth for the blast of volts which took out the hybrid.

I soldered on to the output pin an 0805 50V 0.1 and connected it to the original output C with a single strand of 0.2mm. The added capacitor is because the old output C has now 10K of leakage. I re-inserted it in the 8640B put a piece of cling film over it and all is back to normal.

Click Here to view a copy of the circuit for the final Hybrid Amplifier.

"The schematic attached is an educated guess at the hybrid by looking closely at the blown up photo. The resistor values are scaled by measuring the square area of a couple of known resistor values and applying this to the other resistor tracks. Capacitors are estimated. The inductors are calculated. This excellent job was done by Richard, G3SHK who made this sort of stuff in a past-life at Philips labs".

I am pretty certain a good number of old output hybrid may have been zapped by working on valve kit as the DC input voltage of the spec is very low. If anyone has hung onto the hybrid I recommend peering inside! Mine had a sapphire substrate but since you might encounter Beryllium Oxide turn at low speed with care and wear a face mask!

73 Robin G3LBA


15. Various: Display - Lock Range - Split Gears,   January 2009: Mark Kolber WB2WHC

Mark has kindly provided details of several remedies that he performed to his ailing HP8640B in (.doc) format. #1 describes how he fixed his display, +5 Volt supply and added a coupling capacitor to the EXT input of the the counter to it is AC coupled. (Mark used one 0.001 uF coupling cap. I would suggest using two 0.001 uF capacitors in parallel). #2 details how Mark increased the Lock Range. #3 talks about the split gear situation common to the HP8640B.

  #1 Display+   #2 Lock Range   #3 Split Gears


16. -5.2V Supply - No RF - Counter,   January 2009: Johan, KC7WW

Johan described solutions to several problems he inherited after he purchased a HP8640B.

Click to view KC7WW's article.

17. Frequency Tuning Gears,   January 2009. Markus VE7CA,

Recently I noticed that when turning the main frequency tuning dial I reached a point where I began to feel resistance and if I continued to turn the dial further I heard a loud click. This did not sound good so I decided to investigate. After removing the bottom cover from the HP8640B, the first thing I noticed was that some of the range assembly gears were cracked.

Fig 5. Split Gears in my HP8640B Combination Gear Assembly



Following the directions on Page 7-71 of the manual I removed the Switch Assembly. Don't forget to take lots of pictures so that you can refer to them again when putting everything back together. You need two different sized allen wrenches, a 1/16" (inch) and a 0.050" to remove the knobs. I found that the set screws in the knobs were very tight and my cheap set of allen wrenches stripped so I had to purchase a high quality set. Referring to page 7-70 in the HP8640B manual, gear numbers 13 and 14 were the two effected gears. Do not remove the two small gears (which mesh with gear #13 and #14) from shaft numbered #12. When you take the front gear off, remove the whole shaft (#12) by gently pulling it out. Remember these gears are very old and there are no replacements readily available. You can then remove the rear gear #14.

The gears have cracked apart at the weakest point where the set screws go through to the brass centre hub. The gears have shrunk for some reason so they can not simply be glued back together because the brass centre didn't scrink along with the delrin gears.

Fig 6. Here my delrin gears after dis-assembly



Using a file I carefully filed the centre of the delrin gears so that when replaced over the centre brass hub, the two pieces met. Leave enough room for the glue and the delrin gears centre rough so that the glue has something to adhere too. I strongly advise taking your time so you get it right the first time! It is also necessary to enlarge the hole around the set screw with a small file. This is really important. If you don't make the wholes large enough, when you try to put the set screw back in, the set screw will pry the delrin gear apart again! Check it once and once again before gluing. I was not able to find any 24 hour epoxy however I did find 5 hours expoxy. In order to hold the gears together while the glue sets I used a hose clamp. Be very careful that you don't tighten the camp too much or you may crush the teeth! Make sure there is no glue between the teeth and the gear is flush with the brass hub.

Fig 7. Hose clamp holding gear together while glue sets



I recommend that you leave the gears in the clamp for 24 hours before removing them.

Now all you have to do is re-assemble the gear assembly following the directions in the manual. Hopefully you have taken notes and photographs in case you forgot some important detail!

17.(B) Fixing the snapping sound. On the left side of the main oscillator cavity there are two pots (VT and FM Gain Compensation pots) with gears attached that turn in conjunction with the main tuning dial. Being I had a probem with the gears binding I removed the oscillator cavity to investigate and saw that the lower pot gear was split. Mark WB2WHC, in Hints and Kinks #16, notes that he had a similar problem. The snapping sound I heard occured when the gear driving the two pots tried to jump over the gap in the gear that had split apart. After glueing the split gear together I re-assembled the gears and as Mark suggested, set the two pot gears so they didn't mesh with the centre gear at the set screw positions. You can do this because these pots do not turn a full 360 degrees. I also re-set the Cavity oscillator "end stop postion" as per the instruction on page 5-36 and 5-37, making sure that the VT and FM Gain Compensation pots did not reach their stops first.

Since I had all the knobs off, I did a thorough cleaning of the knobs and front panel. After re-assembling everything and setting the knobs to their correct position I was happy that the Main Cavity Tuning Knob turned without binding and the frequency display corresponded to the Range Switch setting. I let the generator warm for 2 hours and then following the instructions in Hints and Kinks #8 I calibrated the frequency read out. (See page 5-35 in the manual for further details.)

18. HP8640 High Speed Divider,   April 2009: Nevel N2GX

Nevel kindly shared his solution to repairing his HP8640 High Speed Divider Circuits. This is a very creative solution describing how Nevel subsituted presently available IC's for original HP parts which are no longer available.

Click to view a PDF file of Nevel's article in PDF.

19. Lesson from Jeff King, ZL4AI, Learning something the hard way. September 16, 2009

With my last HP8640B, I undertook the careful file out and epoxy gluing of one of the Delrin gears. I clamped the parts together with a spring type clamp. In a hurry, wanting to speed up the gluing process, I decided to heat up the gear. I figured about one minute in the microwave would not be too much.

BIG MISTAKE.

Basically the gear melted! The Delrin material melts at very low temperatures.

DO NOT HEAT UP IN A MICROWAVE UNDER ANY CIRCUMSTANCES.
I hope this warning will prevent others from destroying their gears. Jeff ZL4AI


20. My (opt 002) had a dead output amp., 4 May, 2013 John Antone, VK3ZAF

My unit (opt 002) had a dead output amp but luckily I found a replacement part on Ebay (1st May 2013) the vendor has a few of these and they work fine, search under HP Agilent 08640-67025. They are not the specified amp as the handbook calls up 8640-67002 but they have the same specs except they need Vcc of +23V and +44V I chose a 90 resistor so that it would run on 30V. Info attached is what I did and the outcome.

Installed in sub assembly A26A1 is the 08640-67025. Please note that this amplifier has Vcc of +23v to +44v MAX, therefore it cannot be just plugged in or the 44.6V supply will zap it!. At the back of sub assembly A26, is where feed thru capacitors provide power to the components, the output amplifier and the pre-amplifier, the original output amplifier 08640-67002 is powered by 44.6V fed from the rightmost feed thru. Unsolder this connection and insert a 90 ohm 3 Watt resistor into the circuit. I chose two 47 ohm resistors connected in series mounted on a bracket ( all components found in my junk box) Iím sure a much more elegant solution could be effected! This powers the Amplifier (0840-67025) with about 30 volts. When removing the old amp and installing the new one pay attention to anti static installation techniques.

After, I tested all frequencies, Flatness 512Mhz to 1024 Mhz about 0.2 db except at mid band where it rises to about 0.5 db over a range of about 50 Mhhz. Lower bands is about 0.2-0.3 db. Phase noise and FM and AM modulation unaffected. Measurements made with 8553B and 8555A

The 08640-67025 could be used to replace output amplifier on standard version of the 8640A or B.

Cheers VK3ZAF, jantonel @ bigpond.net.au

21. Jitter Problem., 15 Jan, 2014 Bodo, DL2FCN

The signal of my generator jittered a bit, so +/- 30 Hz, sometimes more, to be seen clearly in the waterfall display of my SDR. The jitter was present in the unlocked and locked condition.

The reason was the relay contact of the reed relay K1 "FM enable" on the FM shaper and PLL filter board.

So I took a miniature relay and glued it on the existing relay K1, the contacts of the new relay just soldered in parallel to the contacts of relay K1. It solved the problem.

Dont know what happened to the reed-relay contact, normally the relay is sealed and made for low current/voltage signals.

Anyway, the jitter is gone and I am again happy with my old generator HB8640. vy 73 de Bodo/DL2FCN

22. More Cracked wheel solutions., 8 Oct 2015 Dr. Ramkumar Ramaswamy, Bangalore, India

The transparent rotors on the range/max deviation assembly - the ones with the small fingers - also have two brown buttons on each one that counter balance the force of the contacts. These buttons slide into small holes on the rotor and are not glued into place. The manual makes no mention of them. So when you remove the rotors, chances are that these buttons will simply fall off and you will lose them. This is to be taken care of while taking apart the range/deviation assembly.

Apart from that there are some fairly standard tips:
(i) Put the gears in the sun if you can - makes epoxy set much faster - in 2 hours or so instead of overnight. Certainly microwaving is silly and I am surprised someone actually tried it.
(ii) Wash the gears in isopropyl alcohol or acetone before gluing. Acetone dissolves many plastic but NOT delrin. I used acetone and swished the gear around. Even though one gear just had a crack and had not come apart that is enough for the acetone to get in and clean it.
(iii) Let the overnight epoxy sit for an hour before you apply. It sets just a bit and loses its viscosity so it does not spread around all over after that. So if you have a crack through a tooth, you can and should apply the epoxy to the tooth halves and then stick together, gently wiping away the excess with a toothpick (pun?.
(iv) Place the gear and brass insert face down on a flat surface so that the insert and gear face are level. Tap the insert a few times with your finger to make sure it has made contact with the flat surface.

One more tip which I discovered unexpectedly at the end of a hard day's work trying to open the set screws with a 0.05 allen hex key made by Allen - the original - in vain. All 4 screws were so tight that these ALLEN keys stripped each time. This problem seems to be universal. I tried applying heat with a soldering iron to the screw, tried immersing things in Liquid Wrench for 8 hours...nothing worked. I found a tip somewhere on the web by a guy who recommended taking a 1/16 bit and filing it down to 0.05. I tried that as a last resort - took one of the standard-sized driver "bits" with a 1/16 hex end, laid it flat and carefully filed each side till it started fitting into the 0.05 hole. Then I tapped it into the hole gently with a plier so as to get a snug fit, and presto - it worked and all 4 opened with just a finger twist.

23. Frequency Lock Problems., 30 June 2016 Dr. Jim K N8PXW

My HP 8640 Problems. I noticed in your trouble shooting tips that all zeroes on the display means no cavity oscillator. That is partly true. In my case a low -5.2V power supply, about -1.5V, caused all zeroes. It turns out that both cavity buffer amps use the -5.2V supply, and a low voltage output will cause no oscillator output.

Here is my story. From day one my unit had frequency lock problems. Even after an hour warmup, I would lose lock after a couple of minutes. This was frustrating to say the least, but I thought that it was normal. This year I talked to a person in Dayton who fixes HP8640's and he said they should stay locked for hours after warmup. After returning home I did some checks. The manual says the unit should say in lock for 1/4 turn of the fine tuning control, and drop out by 3/4 turn. OK, mine says in lock for 1/2 turn in each direction. So that's not my problem. I let the unit warmup overnight. Next morning my display read all zeroes. After removing the top cover, I noticed that the -5.2V led was glowing weakly. As I watched, it got dimmer and dimmer till it went out. OK, regulator board problem. The ua723 regulator chip drives a 2N3053 which drives a 2N3055 pass transistor. I replaced the 2N3053, same problem. Replaced what I thought was the 2N3055, same problem. OK it must be the 723, the local shop had an NTE replacement a NTE923 in the ten pin can for $9. I replaced the part, powered up, and everything came back to life. I then let the unit run overnight. Next morning, all zeroes.

Now I'm stumped. The local shop had only one part. I found a parts dealer 50 mi away and made the trip the next day. This time I double checked everything. I assumed the pass transistor was on the rear heat sink cooled by the fan. Wrong! I traced the wires on the motherboard to a transistor mounted on a flange on the bottom on the chasis, accessable after removing the bottom cover. I replaced the 723, the 2N3053, the only thing left was the pass transistor, 2N3055. When I tried to remove the part, it wouldn't release from the chasis. I finally pried it free with a screwdriver. Apparently, someone had replaced it before, but instead of using silicone grease, thermally conductive, he used silicone rubber, a thermal insulator. I replaced that transistor. I then checked the other power transistors on the rear heatsink. One was also silicone rubbered, but the others were dry, and needed new grease. After regreasing the above, I measured the transistor's temps, the coolest was 98 deg, the warmest 108 deg, a little warm, but you could keep your finger on the case. I then ran the HP8640 all night, and in the morning everything was good and the freq still locked.

Bottom line, if the last signal generators were made in 1991, the latest models are 25 years old and the pass transistors should be checked. My problems apparently stemmed from the pass transistor over heating and eventually causing the 723 regulator to fail.

If you have any solutions for problems you have encountered for the HP8640 series signal generators, please let us know at: