Last update 10/8/07

HJFP2 JFET Micpre Kit Tech Support Page: 5/5/5

Tech Forum for the HJFP2 kit: This is a link started by one of the people who bought the kit, to help take some of the tech support burden off me.  Thank you Jason!  Please try these resources first before emailing.  If you can't find the answer you are looking for, then we are here to help.  Please don't take offense if my reply is short, or a link back to this page.  If I think your question would benefit others, I will post it here for everyone.

Updates / FAQs:

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5/5/5:  RECOMMENDED MOD:

Change R13 and R19 from 68Kohm 1/4w to 127Kohm 1/4w.  (Actually anything in the 125-175k range will work). 

Reason: this resistor sets the output driver bias current.  Increasing this resistor value cuts power dissipation down by about 40% in the ZTX603 and 653 transistors, with no degradation in output drive into 600 ohms, full bandwidth.  This is the value in the current design.  If you are not sure which value you have, just look at the parts list.

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5/24/04:  We discovered a drilling error on the last run of HJFP2 pcb's.  The holes for C1 and C2 of the audio cards are 0.03, not 0.04 like they are supposed to be.  These need to be enlarged to get the parts in.  If you have no means of doing this, please email and we can figure out the easiest solution.  Any drill size between 0.04 and 0.063" is fine.

C3 and C5 of the audio channel parts list should read "330uF or 470uF".  The kits includes 330uF caps for C3 and C5, but 470 can work if the other parts are back ordered.

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4/27/04:  NEW HJFP2 PILOT LIGHT:  Some people had noise issues with the pilot light, which were solved by using a simple metal tube over the lamp (explained below).  A new LED pilot light is available which we have switched to (shown below, on top, in comparison to the original).  The LAMP has an integrated resistor, so it connects directly to 25.5vdc (which is the audio supply rail in the micpre).  If you choose to switch, make certain you connect the side with the "+" sign (shown below, new lamp is on top), to the +25.5vdc, and the other side to ground.

Mouser part number: 607-1091QM1-24V $2.44

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4/8/04:  TESTING THE POWER TRANSFORMER:  The power transformer for the HJFP2 kit has dual primaries (for 120/220vac input) and dual secondaries, each 22VAC.  The AC input are the wires with the extra yellow jacket.  For 120VAC, connect the red and yellow wires together (with yellow jackets) and connect this with one side of ac.  Connect the violet and black wires  (with yellow jackets) and connect this to the other side of AC.  Remove the wires going to the pcb.  With your DMV in AC volts, measure between the red and green wire, it should read 26.2VAC RMS or close to it.  Next, connect to the brown and blue wires and take the same reading, it should be the same, 26.2VAC.  These are "no load" measurements, but should indicate if there is a problem with the transformer.  If you powered the unit up with either a cap or rectifier in backwards, you could have damaged it.  A faulty transformer cannot cause the circuit to read a negative voltage, this is more than likely the result of something in backwards, like the rectifier.  If you did put the rectifier in backwards, you should replace all the components in that regulator circuit.

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4/5/04:  The pictures below show the orientation of the ZTX603 and  ZTX653.  The notches on the back represent the side of a TO92 (this package style) that would normally be round.  The silver side is the back of the part.  If your preamp seems to have no gain, or very low, it is likely these are in backwards.  Though I haven't tried it, a couple people have and said the parts were not damaged after they put then in correct.  This is not always true, tubes tend to be pretty forgiving, solid state devices are generally not.

HJFP2 PSU Assembly Pictures

HJFP2 Audio channel / Chassis Assembly Pictures

ALL UPDATES BELOW APPLY ONLY TO KITS SHIPPED BEFORE 4/20/04

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2/17/04:  We discovered a bad audio pcb today, it had a hairline short between two traces.  This is not in the board file, it is an imaging flaw.  To our knowledge this is the only board that was had this problem.   The short was right where the +24 and +48 wire to the audio channel.  As the two traces wrap around  one of the pads on C6, there was a minute short between the two.  This can happen if anything gets onto the board when it is imaged (like a hair).  We checked all the boards we have here, and they are all ok, but just to be safe, examine the pcbs in your kit held up to a light to ensure there is nothing suspicious.  More than likely it was a fluke.

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2/15/04:  NOISE INDUCED IN CHANNEL 2 FROM PILOT LIGHT.

Someone brought to my attention a few days ago that they noticed an increased noise floor in channel 2.  This person bought two HJFP2 kits, and told me one unit was fine, the other had noise in channel two.  Though I had not encountered this (I tested running the pilot from AC vs DC and found no induced noise), I pulled all the pilot lights we had here (47), and tested all of them in circuit.  What I found was 5-7 of them did in fact induce noise to a varying degree ranging from barely audible to quite noticeable with the gain cranked.  I selected the worst two, and found that the induced noise was also a function of the DI jack and pilot light orientations, and by rotating the two you could hear the noise get louder/softer.

SOLUTION:  Basically all you need to do is shield the pilot by sliding a cylindrical piece of metal tube over the pilot, and ground it to the chassis.  With the worst pilots I found, it totally eliminated the noise.  The picture below shows a HVTP2 we built up here with the shield in place.  The shield shown is a 5/8" ID (inner diameter) copper coupler for water pipe, available at True Value hardware store for 29 cents.  Even if you don't have the noise problem, I would suggest doing this since it can't hurt.  Make certain the tube is snug against the front panel, and can't make contact to either side of AC.  I also tried a piece of thin sheet metal, and a 0.005: thick piece of Mu Metal wrapped into a tube, both also removed the noise.

Someone commented on the choice of copper as being bad because in contact with steel, and moisture, it can cause corrosion.  The metal is electro galvanized, so it is not bare steel.  Second, if there is any significant amount of moisture in the unit, you will have a lot more problems than potential corrosion.  Excessive moisture will eventually degrade just about everything electronic, especially the pots, transformers, and switches.  Personally I do not think it is an issue.  If you are concerned, you can just as easily buy a small steel piece of tubing, I chose copper because you could solder to it.

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2/5/04:  The instructions for assembling the psu tell you to install B30, test the 25.5vdc supply, then install B30, and test the +48vdc supply.  The second B30 should be B50.  The idea is you test one circuit, make sure its ok, then test the second.  I explain it verbally so I think most people were able to work around it.   If you have two meters, you can install both bridge rectifiers (B30 and B50) and bring the two supplies up at once. 

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1/27/04:  The psu parts list calls out two 15Kohm resistors that can be either a 1/4 or 1/2W.  The circuit requires a 1/4W.  We stock 1/2W resistors for another project, so I left the option open to use either.  FYI, you can substitute higher wattage resistors, just like you can (typically) substitute higher voltage caps.  You might notice that the caps C3 and C5 are 35V, when in the circuit the never see more than 2vdc.  The reason is the higher voltage cap (for this part) has better specs.  C4 and C6, the two big audio coupling caps are 100V, and chosen for the same reason.  When I choose caps for audio coupling, I am looking for the low dissipation factor, with electrolytic caps, I look for low ESR (effective series resistance).

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1/27/04:  The anti rotation hole on the front panel is for rotary attenuator switches, not the pot (this is mentioned in the instructions).  The pot doesn't need one, and the tab should be snipped off.

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1/24/04:  The updated parts list you received electronically has the date 1/1/00 on it, the date means nothing, it is simply a spreadsheet function that displayed the wrong value (in Excel its called the "NOW()" function).  I don't know why it displayed that date when I printed the sheet, but it simply is not important.  Again the change was this, R34=27ohms, not 33.  The 1N4007 diodes should be described as "rectifier diode", not "bridge rectifier". 

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1/23/04:  The updates below apply only to the first handful of kits shipped.  We have resolved all the minor problems below on all kits in stock.

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1/23/04:  A couple parts in the pictures provided below were sub's for the prototypes, so they may look different than yours (or in the case of some caps, be a higher voltage).  This is mentioned in the instructions, follow the parts list you have; the pictures are a general guide.  Additionally, quite often substitutions are required due to back ordered parts, on the first run we had 4.  So the appearance may be different than the ones shown.

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1/23/04:  The holes for C1and C2 appear to be drilled 0.032, not .04 like they are supposed to be (the two proto runs we did they were fine, and there was no change made to those parts for the production run).  You will need to widen these holes slightly to get the caps to install without force.  I will sort this out with the pcb shop.  All the boards we have in stock will be fixed prior to shipping.

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1/23/04:  The 2x +48 leds were deleted from the audio channel parts list, and as a result did not get kitted.  This only applies to audio parts lists dated 1/2/04.  These will get sent out with the heatsinks mentioned below.

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1/22/04: We just realized that only 4 small TO-92 heatsinks were packed in each kit, not 8.  This is totally my mistake, sorry.  I told our tech to put in 4, I failed to say per channel.  I will either order them next day air, or find them locally, and send them to the people who got kits from the first run (I know exactly what kits got only 4, because we photographed and weighed the contents of each bag in each kit).  They will come priority mail so you will have them quick.  In the meantime install the ones you have on U2/U3 (the NPNs driving the output) of each channel.  In reality they aren't even needed (none of the 3 original prototypes had them).  Last, the ones in your kit stand vertically and are square (they are in the psu bag), not round like in the HJFP2 prototype (#4) shown below.  The square ones you got are spec'd to dissipate more power than the round ones.

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1/22/04: C4 = 10uF, C6 = 3 or 5uF not the other way around.  The audio parts list has them reversed  See the picture below for the proper placement.  C6 can actually range from 1uF to 10uF with no effect on performance.  The pictures below show a 5uF, these were back ordered until March, so I sub'd the 3uF.

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1/15/04:  The PSU parts list calls out two 15k 1/2W resistors, it should read 1/4W or 1/2W.  The correct parts are in the kit.

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1/15/04:  There are 4 x 20kohm (R10/R16 x 2 channels) and 4 x1.82kohm (R12/R18 x2 channels) 1/4W resistors in the audio resistor parts bag that are not needed.  They are replaced with the select resistors in the active device bag.  These were the original design values shown in the Tape Op article.  In the development of the micpres, a 2nd source for the 2n5457 JFETs became available which had about 3dB lower gain, which for this circuit was better.  The new JFETs required different resistors.  Also the 4 JFETs in the kit are all matched for gain, with select resistors to optimize their performance included with them.

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