hp1725a - Page 1
Forum section: electronics / repair

author: jklasdf
author_rank: Regular Contributor
timestamp: February 18, 2016, 10:10:32 am
content: I'm trying to troubleshoot the high-to-low impedance converter on the A1 attenuator board. The problem is the error amplifier U1 is always saturated near the negative rail (about -3.8V), and the output of the high-to-low impedance converter at the bottom of CR1 is too high (about 3.7V with the input set to GND, instead of tracking near 0V).The first problem I found was JFET Q1 seemed to be bad. Vgs was forward biased, and also the gate was at a higher voltage than the other side of R10. In normal operation, very little current flows across R13 into the base of Q3, so both sides of R13 are at about the same voltage. Then there's about a 1.5V drop across R10 (I think because an input bias current of ~75nA flowing into the gate of Q1) which sets the gate 1.5V below the source. The only way Vgs can possible be positive is if current flows from the drain to the gate, or C5 is bad.Luckily Q1 is socketed. When I checked it with a diode tester, it conducted both from gate to drain and also the other way around instead of acting like a PN junction. When I replaced Q1 with a J111 (the only n-channel thru-hole JFET I had on hand), the gate does indeed sit lower than the source as expected. I still need to retake the measurement with a meter with a higher input impedance...a regular 10Mohm input-impedance multimeter forms a voltage divider with R10. But the gate is definitely at a lower voltage than the source. Does anyone have any info on the original Q1? It has a siliconix logo with part number J2367 8329 and the HP part number in the service manual is 5080-9691.Moving on, Vdd for Q1 was being dragged down to about 8V from its nominal 10V, so I'm fed it with an external supply temporarily (draws about 50mA). And also -15VF, because it was being dragged down to about -14.76 (probably less important). The reason seems to be Q3 is saturated. I'm measuring 6.47V at its base, 5.6V at the collector, and 5.56V at the emitter. The source of Q1 measures 7.82V, so it seems like ~30mA flows across R13 into the base of Q3 which seems high, and Q3 really shouldn't be saturated in normal operation when the input is set to GND.Q2 measures -4.013V at the collector, -10.08V at the base, and -10.82V at the emitter. Based on the 11.8V drop across R12, Q2 sinks 20mA of current. This is about double what you'd expect during normal operation based on the nominal values given on the schematic, but this isn't surprising since error amplifier U1 is saturated to the negative rail.Here's my understanding of the low-frequency feedback path: assume the input is grounded, but the output at the bottom of CR1 is higher than 0V. The input at the non-inverting input of U1 is about 0V since R8 and R59 form a voltage divider to the input, and the input at the inverting terminal (from voltage divider R15, R55-56) is greater than 0, so the output of U1 saturates to the negative rail. Q5 conducts causing current to flow through R65, which raises the voltage at the base of current source Q2. Current source Q2 sinks more current, increasing Id for Q1 which increases Vds given a constant Vgs bias. As Vds increases, the source of Q1 and the base of Q3 (at the same voltage since very little current flows through R13, since Ib for Q3 should be small) decreases in voltage and Q3 begins to turn off....decreasing the voltage at the bottom of CR1, and the loop is complete.The annoying thing is the gate of Q1 is completely AC-coupled from the input, and seems to have its operating point set by the voltage at the source of Q1, and it's really difficult to reason about what the voltage there should be.Right now, my thinking is Vds for Q1 is not high enough given the current draw, which forces Q3 into saturation. Does anyone have any thoughts on possible replacements for Q1 or any other thoughts on my reasoning? I ordered a matched pair on eBay for $19 which is hopefully genuine, but it won't get here until next week.Interestingly I'm seeing the same issue on both channels, but the only thing that seems to be common is the +/-15V supply. I'm pretty much supplying both of these externally though. Maybe somebody blew out one channel, and then connected it to the other one?

author: jklasdf
author_rank: Regular Contributor
timestamp: November 04, 2016, 06:12:36 am
content: I got the oscilloscope repaired a while ago. The main problem ended up being blown input transistors, which are luckily socketed (A1A1Q1 for channels A and B on the attenuator boards). The hp replacement part number is 5080-9691, and includes two transistors as a matched pair. I bought a set from ebay, and they're still available, although there's only one set left as of right now:http://www.ebay.com/itm/300867847783?_trksid=p2060353.m1438.l2649The repair: Before buying the replacement transistors I tried substituting JFETs I had on hand to see if it would fix the problem. Unfortunately the only ones I had on hand were the J111, which have completely different characteristics, and don't bias correctly in the circuit. I added a couple of zener diodes to get the bias point correct enough to kind of work, albeit with limited voltage swing:Finally an image on the screen! (although not triggering properly):I ordered a replacement set of JFETs, part number 5080-9691. Unfortunately, even cross-referencing the part number, I don't think these JFETs are produced anymore. On top are the original (burnt out) matched pair, and on the bottom are their replacements. It looks like hp changed suppliers sometime during production:I took some measurements of the new JFETs for Vgs vs Id. The blue squares and orange diamonds show the measured replacement matched pair, and the other four colors/shapes show four samples of a J310 which I ordered later:After doing these tests, I think a J309 would have actually been a closer match to the hp parts. And luckily BG Micro still has both the J309 and J310 in stock as through-hole parts! The pinout is *not* the same as the hp parts though. These measurements are probably not that exact. I made a really quick test setup (used a uCurrent, go eevblog!), and I'm not sure if I actually kept Vds the same between when measuring the hp parts and measuring the J310s. The hp parts do seem to be matched for Vgs vs Id, although not all that closely. You probably wouldn't want to just select two parts at random, but it'd be fairly easy to find a pair that matched close enough from an order of 10 of them.Since A1A1Q1 just acts as a voltage follower for the high frequency components, I don't think using exactly the correct hp part is that important, although it might affect calibration slightly at higher frequencies. The lower frequency components are servo'd by op-amp U1 on A3, and should be unaffected. The higher frequency components should follow the input almost exactly since it's a voltage follower. The bias point is slightly different though, and that probably has the biggest effect on the circuit. I found it interesting that unlike my Tek 2230 (a 100MHz scope), there's no input protection diodes. I wonder if this is an oversight, or if it was too hard to get the performance they needed with protection diodes in circuit. The gate of Q1 seems to be biased by it's (extremely small) leakage current, which flows through R10.Continuing with the repair...inputting a square wave on channels A and B gave the following waveforms:Channel B apeared to only be passing the high frequencies, and channel A didn't look quite right either (both channels set as terminated in 50ohms, and the resistors measured good). Here's a slightly faster square wave on channels A and B, and also on my Tek 2230 to show what it's supposed to look like:For me, the problem with channel B ended up being one of the pins on the connector between the attenuator board and A3 was bent. After taking out the attenuator board, I bent the pin back into place from the other side. I also took out the channel B attenuator board and cleaned the contacts and reseated all of the socketed parts, and that fixed channel A not passing hihger frequencies intermittently. Similarly, most of the boards along the horizontal timebase shaft tend to get unseated with use from the force of turning the knob. I took all of these out, cleaned the contacts, and reseated them, and that helped with intermittent trigger issues. After a lot of electrical contact cleaner, I got my hp 1725a repaired.This oscilloscope has a pretty unique feature for the delayed time base which I have never seen before. You can sweep part of the screen at a slower speed on the main time base, and then the rest of the screen at a faster sweep speed on the delayed time base. I thought this was pretty cool, because I've never seen anything like it before, but it doesn't actually work that well in practice unless the main and delayed time base are at a similar setting. Otherwise the delayed part is too dim to see.Main intensified:Main and delayed on same screen:Other notes:Going through the calibration procedure in the newer hp manual, I can get everything into adjustment, except the horizontal sweep speed and linearity adjustment is extremeley finicky trying to get it right throughout all ranges, especially for higher sweep speeds (sections 5-77 through 5-84). It's really hard to get the left/right side of the screen to match for all sweep speeds.A high resolution copy of the service manual is available from qservice, and I'd highly recommend the qservice copy over the scan you can download for free from hp/Agilent/Keysight if you're working on the scope. The qservice schematics are much much clearer. The hp manual is newer though, if you have a newer serial number unit. Specifically, schematic sheet 11 is pretty much completely redrawn for the time interval marker control circuit. (The hp 1725a has pretty good specs for time interval measurements, and apparently those specs were really hard to meet. The parts placement and adjustment procedure is also slightly different for newer models for that board.

author: bitseeker
author_rank: Super Contributor
timestamp: November 05, 2016, 01:33:54 am
content: Bravo on the details, photos, and especially following up on the solution.

author: David Hess
author_rank: Super Contributor
timestamp: November 05, 2016, 04:02:38 am
content: Quote from: jklasdf on November 04, 2016, 06:12:36 amTektronix called this "mixed sweep" and included it on their dual timebase oscilloscopes in the 1970s and 1980s. I have never found a use for it but have been told it had applications with video signals.

author: Zucca
author_rank: Unknown
timestamp: November 07, 2016, 10:23:15 am
content: jklasdfplease do more repair!

author: jklasdf
author_rank: Regular Contributor
timestamp: November 08, 2016, 03:16:15 am
content: thanks!