HP-8640 Service: My Story|
For additional information regarding repairs to the HP8640 series generator see "Hints and Kinks" page
1. Locating The Problem
On the back of the generator is a large heat sink and extending from the heat sink is a BNC connector marked Aux. Output. Connect a scope or a RF power meter to the BNC connector and turn the generator on. If there is no output at this point, the problem is either the hybrid pre-amp., the supply voltage for the pre-amp. or any of the circuits preceding the pre-amp. If there is no RF at the BNC outlet, turn the generator up side down and remove the bottom cover. Turn the generator so the back is facing you and remove the cover from the AM/AGC AMPLIFIER ASSY which is located in the lower right hand corner. This assembly is divided into two sections, the A26A3 Modulator Assy. is on the left side (which contains the pre-amp. hybrid) and the A26A1 Power Amplifier and AGC Detector Assy. on the right side.
In each section you will see what looks like a large power transistor in a case style similar to a TO-3 or TO-8 transistor. The hybrid amplifiers are contained under the caps of these units. On the PC board, in the left section, locate the end of the 50 ohm coax line with the center connector terminating in a circuit pad. (see Fig. 1)
Turn the frequency range switch of the generator to its lowest range and turn the generator on. With a scope determine if there is any RF at the end of the 50 ohm coax. If there is, and it is a square wave measuring 550 mVp-p, then you have confirmed that everything before the pre-amp is working and the problem is either the pre-amp or the power supply. The only way to really find out if it is one or the other is to remove the pre-amp. hybrid by removing the two screws holding it in place. Once the pre-amp. is removed check the supply voltage that it is +44.6 volts. The +44.6 volt input is clearly marked on the PC. If there is no voltage present or it is the wrong value then proceed to fix the power supply problem. The reason that you have to remove the pre-amp. to check the voltage is that the pre-amp. may have an internal short, which was the case with my pre-amp. The power supply has what is called a crow-bar shut down feature, where if the output is shorted, the voltage goes to zero preventing damage to the power supply.
2. Changing Supply Voltage For The MAV-11
The MAV-11 requires 5.5 Volts while the hybrid amplifiers both run on 44.6 Volts. The 44.6 V line enters the AM/AMPLIFIER ASSY box that contains both amplifiers via a multiple pin connector and then along a PC board trace in the center of the box. There is no spare pin on the connector so a feedthrough capacitor has to be added to bring in a voltage line for the MAV-11.
With the back of the generator towards you, take off the top cover. The AM/AGC AMPLIFIER ASSY is now located in the bottom left corner. Remove the AM/AMPLIFIER ASSY from the generator by following the instructions on the amplifier lid. Once removed, turn it so that the side opposite the hint sink is facing up. There you will find a removable rectangular aluminum plate. After you remove the screws and the plate you will see a row of feedthrough capacitors in the cavity below. From one end there are two wires that go from feedthrough capacitors to the section that contains the hybrid pre-amp. The second wire (white with purple and brown bands) from the end is connected between the feedthrough capacitor +44.6 volt input line and a pad on the pre-amp. PC board that is marked 44.6 V. Unsolder this wire from the feedthrough capacitor. Make a mark on the aluminum plate in the middle between the two end feedthrough capacitors. Drill a hole to fit a feedthrough capacitor. I used a good quality 0.001 uF feedthrough capacitor that I had purchased from Down East Microwave. One end is threaded so that when it is screwed into the aluminum plate the result is a very secure RF connection. Connect the wire that was soldered to the +44.6 V feedthrough insulator to the new feedthrough insulator using heat shrink tubing to cover the solder connection. Connect a 12" length of insulated wire to the outside of the new feedthrough insulator and again pass short length of shrink tubing over the connection. See Fig 2. Now reassemble the AM/AMPLIFIER ASSY and insert it back into the generator.
The other end of the wire just attached to the outside of the new feedthrough insulator is now soldered to a hole at the end of a trace that leads to C4 that is on the power supply rectifier assembly circuit board. I choose this point as the voltage is approximately +15 volts leaving sufficient room to drop the voltage to 5.5 volts for MAV-11 while limiting the current to 60 ma. Also this particular winding has the largest current capability of all the transformer windings. C4 is 8200 uF and large enough that I was not able to detect any ripple on my scope.
3. Substituting A MAV-11 For The Hybrid Pre-Amp
I had some Mini-Circuits MAV-11 amps. on hand so I decided to try one as a substitute for the pre-amp hybrid. You may want to choose another model such as the MAV-11SM or ERA-1SM that has a typical gain flatness up to 2 GHz of +/-0.5 dB., 0.02 dB better than the MAV-11, though I don't think it will make much difference because the generator has an AGC circuit to keep the output power at a constant output.
Begin by removing the cap from the existing pre-amp. module. I inserted the module in a vise and using a metal punch I was able to get under the seal of the cap and pry it up and off. I removed the hybrid from the base by tapping the edge with a small screwdriver. I decided to use the original base of the hybrid because it has the connecting posts that connect through to the PC board below and it has room on top for a MAV-11 and associated parts. I cut a small piece of PC board to fit between the connecting posts and extending over the mounting holes of the base. If you use a MAV-11 then drill a hole in the center of the PC board large enough so that the MAV-11 drops into place and the four leads lie on top of the PC board. I used a Dremel tool to cut isolation pads for the input and output pins of the MAV-11.
Facing the back of the generator and looking at the base that holds the hybrid pre-amp., the connecting posts farthest away from the back of the generator from left to right are: output, ground and input. The connecting posts closest to the back are the left post (not used) and the right post, (voltage supply input). From the base input post connect a 4 dB pad in series with a 0.1uF monolithic or surface mount capacitor. The other end of the capacitor goes to the input pin of the MAV-11. Between the voltage supply input post on the base and the output pin of the MAV-11 connect three 470 ohm 1/4 w resistors in parallel. The three parallel resistors should measure about 157 ohms and serve to drop the supply voltage from 15 volts to approx. 5.5 volts and limit the current to 60 ma. From the output pin of the MAV-11 connect another 0.1 uF monolithic or surface mount capacitor to the output post of the base. See Fig 3. for the schematic.
Before putting the AM/AGC AMPLIFIER ASSY cover back on, make sure that the new components do not touch the lid of the cover. I cut a small single sided PC board to cover the MAV-11 and new components and soldered the copper side of the PC board to the screen that is sandwiched between the cover and the assembly box. It took me several hours to perform the above changes. I strongly recommend that you do not hurry when making the suggested changes. You don't want to make a mistake that may cause other problems that are not fixable because of the scarcity of some of the HP original parts.
When I completed the substitution and turned on the generator, with the attenuator set to 0 dBm, the generator output meter read 0 dBm and my RF power meter connected to the output connector also registered 0 dBm. This was with the frequency set to 875 kHz. I was one very happy person. According to my RF power meter the output is within +/- 0.2 dB across the generators full operating frequency range. (I had previously calibrated the RF Power Meter using the signal generator as the source and confirmed the calibration using another HP8640 that a friend owns.) The repaired generator also produces full power across the full frequency range of the generator, which for the HP8640 Avionic Version is +13dBm. The AM and FM modulation features of the generator were not affected by the substitution.
Obviously, the problem outlined in this article is only one of many reasons why a HP8640 series signal generator may not be working. However, it became clear after discussions with a number of fellows hams that had years of experience working with this particular series of generators that the hybrid pre-amp is prone to failure. Hopefully this article will help those who experience a similar problem. It is interesting to note that there is a notation on the circuit diagram for the Avionic Version of the HP8640 indicating that the output amplifier is a 16 dBm amplifier. The MAV-11 may therefore also work as a substitute for the final amplifier hybrid. This would not be the case for other versions of the HP8640 as they typically have a higher output power than the Avionic Version that has a max. output power of 13 dBm. The MAV-11 is listed as having a max. output power of 18 dBm (1 db Comp. Typ). If it was used to replace the final power amplifier in the higher power generators, then the AGC circuit will not be effective unless it is adjusted to recognize that the new amplifier is only capable of putting out 18 dBm max. There may be three terminal amplifiers available that provide more than 18 dBm that would be a better substitute for the final amplifiers in the higher output generators.
I have not been able to confirm this, but I was told that the last year that the HP hybrids were being produced they sold for over $200 US. I paid $2.65 US for the MAV-11's when I purchased them a couple of years ago. Now that is progress!