HP 428B Older Version - Page 1
Forum section: electronics / repair

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 29, 2019, 09:22:01 pm
content: This has me stumped, but still, it's not operating properly at all.Here is the schematic scanned.So I've testing the probe and it is within tolerance.As I've worked my way through the procedure I ended up attest points 3 & 4.They are NOT 180 out of phase as they should be.These measurements were taken with scope on theP1 side of the probe connection on the rear of the front panel.As they were in phase, I worked my way back.to the V8 side of T6, measuring on pin 1 and pin6.Again in phase with 199Vpp & 197Vpp.Working back to V7, pin 2 and pin7, should beout of phase here also, but they are not.Again they are in phase. V22.4Vpp & 22.7Vpp.Test point 1, is close at 8Vpp 2.4vpp hotter than specat 5.6Vpp.The slightly higher voltages isn't the issue.I'm surprised that the V7 6DJ8 I thought wassupposed to flip the sine wave between pin 2 and pin 7.Then V8 the are just amplified grid pin 2 to pin 1 andgrid pin7 to pin 6.I've substituted V7 and V8 with known good tubes nothing changes.I've worked through the steps trouble shooting steps following:5-265-28a.-d. I adjusted B+ to 272.1 on HP 34401a.+12 is 12.433, spec.-7 is -6.43 spec.5-29 a., b, is also where I came up the no phase problem.That and on 5-7 (A) (B) (C). I have 8Vpp, not 30Vpp.But no 180 degree phase shifts.I don't have 30Vpp on A&C tests.Opps I didn't remove V6, my bad.Testing will continue shortly.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 29, 2019, 09:23:01 pm
content: Here is the manual, it wouldn't let me post it in the initial post.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 29, 2019, 11:03:30 pm
content: Here are the Trouble Localization steps thatI couldn't fit into the first post.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 29, 2019, 11:22:03 pm
content: The following are the scope pics.Test point 1.Test point 3.test point 4.Interesting that the phase of the TP3, and TP4 are in phase,the ripple is out of phase.Is this just so far out of balance that we can't see whereit starts and ends? That would be at T5, and it looks asthough there is and adjustment to that.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 30, 2019, 12:38:20 am
content: For Test Points 01.03 and 01.04 that appear out of phase, how is the oscilloscope being triggered? If it triggers on the probe, then they will appear to be in phase. Do you have two probes so both can be viewed at the same time?Notice how the ~1.5 MHz oscillations match up well when the trace from 01.04 is shifted right by 2 1/2 divisions.Edit: Frequency

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 30, 2019, 01:14:41 am
content: Tomorokoshi,No, I didn't notice that.And yes, I do have two probes.So if I trigger the same, that is /up slopethen they will both appear in phase?So if trigger /up slopeand then \down slopeUsing one probe, then they should look 180out of phase. HmmmI think you are better at scope reading then I am.BRB with updated pics.I'll run them at the same time and seehow it looks.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 30, 2019, 01:51:55 am
content: Here are the test points 3 and 4 in sequence.Single, combined, ALT, then X-Y.So, the good news is that the test pointsare out of phase and I learned something newwith my scope that I should have known.Looking at the oscillations...Can these be shunted to eliminate them?Or, is it ringing?I'm looking at the ALT, selected pic,this is how much out of balanceeach side is.I wasn't sure if this was due to aged electrolyticthat should be replaced. It appears this unit leftthe factory in 1969. 50 Year old capacitors.And finally X-Y. Also shows oscillations?

author: USMC_Spike
author_rank: Regular Contributor
timestamp: September 30, 2019, 03:05:33 am
content: Continued check out of the unit.The probe/meter doesn't read.Not sure how to assume the probe is alright,it passes the preliminary tests; that it,approx 5 ohm reading from probe pin to pinand doesn't fluctuate when cable is bent, moved etc.Doing the simple check out procedure,Figure 5-2 Electrical Zero-Set, OK.Figure 5-3, DC Current Indication, failsMeter fails to move adjusting current up to 100mA.Step 4. Even switching probe arrow around to checkfor proper current flow, nothing.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 01, 2019, 12:10:30 am
content: Don't worry about the ringing. It could be normal, or it could be caused by the main problem.From before: "Test point 1, is close at 8Vpp 2.4vpp hotter than spec at 5.6Vpp." Maybe it's worth while to track down why this voltage is high. It could be due to high feedback drive to V7 for the oscillator to run, or it could be due to reduced loading at Test Point 1 to the other circuits.Looking at Test Point 1 from the 20 KC Oscillator, follow note "40 KC Output To C28 & T4".T4 is in the 40 KC Phase Shifter circuit, and C28 is in V5 Detector Gate Amplifier circuit. Are the components in this path okay? How do the signals look? What about Test Point 6?See "4-45. 40 KC Phase Shifter" on page 4-5.Also, are the contacts on the two selector switches tarnished? Test with the unit unplugged and see what the resistance is between the wiper and the selected switches at the soldered wire terminals. If possible check all settings on both wafers.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: October 01, 2019, 05:38:18 am
content: Quote from: Tomorokoshi on October 01, 2019, 12:10:30 amGood points Tomorokoshi. I assumed high from going 115 to 120V. But then the unit passed the103V to 127V test for regulation. It didn't deviate at all. Also figured that might be a goodsign to not mess with changing the capacitors for a while.Quote from: Tomorokoshi on October 01, 2019, 12:10:30 amI Checked the components in the path. The only hard partis trying to read c28 as it's in series with the transformer.was reading something on the order of 587uF...which madeno sense. I think one other of these ceramic caps tested thathigh, the rest were close around the .01uF value they were supposedto be.Haven't powered up and looked yet.Quote from: Tomorokoshi on October 01, 2019, 12:10:30 amThis took a while as it's a PITA to get to all the switch positions,however, I checked them all and were generally fine. It tooka bit of finagling to reach all and proper measuring.I'll either continue tomorrow evening and post pics then.My LCR meter is out of commission until new probes comein and I can solder them into the fixture.It's a DER EE LCR DE 5000 and the short alligator clips came off tryingto get them around something a bit too large.Also looking around for some of the tubes and I'll replace some of themarginal tubes:All 3 6ah6 tubes are marginal.I found some 6AU6s that are better then what was in there,they are right on boarder of usable for Tektronix screened tubes.I wish I had a source for the HP 6DJ8s, could use some of these.The only one I couldn't test was the 5651 92V zener tube.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 02, 2019, 03:25:28 am
content: "I Checked the components in the path. The only hard partis trying to read c28 as it's in series with the transformer.was reading something on the order of 587uF...which madeno sense."What frequency was C28 measured at? If it was a low frequency like 1 KHz, then the conduction through R50 might make it look like a larger value than it is. Try 100 KHz.For these tests, pay special attention to the electrolytic capacitors C12, C21, and C25. From the schematic, DC ohms for the three should be:C12 = More than R30 of 5600.C21 = R36 + R37 = 1380.C25 = Whatever R26 is set to.At the 40 KC Amplifier V1, check the voltages at R28. Check resistors R28 and R30. Check capacitor C12.At the 40 KC Detector Driver V2, check the voltages at R34 and R37. Check resistor R37. Check capacitor C21.At the Detector Gate Amplifier V5, check capacitors C26, C25, C24.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: October 02, 2019, 08:03:37 am
content: TomorokoshiQuote from: Tomorokoshi on October 01, 2019, 12:10:30 amSo, moving right along....From Test Point 1 to C28 & T4. Signal is same 8Vpp instead of 5.6VppTest Point 6 Voltage high 14.8VAC vs 10VAC.Quote from: Tomorokoshi on October 02, 2019, 03:25:28 amModified probe for LCR meter and meausred:@10kHz = 8.35nF (.449ohmESR) IN SPEC@100kHz=7.97nF (7.57ohmESR) Border, but close enough for Gov't work.Spec is 10KpF =- 20%10kHz in spec,Quote from: Tomorokoshi on October 02, 2019, 03:25:28 amDifferent test Meters used are Fluke 112 : HP34401AC12 = More than R30 of 5600; 1.7 MOhm : 40KOhmC21 = R36 + R37 = 1380. ; 1.5 KOhm : 1.5 KOhm (R36 981 Ohm + R37 483 Ohm = 1464 Ohm total)C25 = Whatever R26 is set to ; 1.3KOhm : 1.4 KOhmC12 = 44.5uF (.445 ESR)C21 = 11nF (2.12 ESR) 10uF -10% +100%C25 = 56.7 uF (.77ESR) 50uF -10% + 100%At the 40 KC Amplifier V1, check the voltages at R28.R28 = 786 OhmR30 = 5.3 KOhmCheck capacitor C12, see up there.Quote from: Tomorokoshi on October 02, 2019, 03:25:28 amR34 = 49 OhmR37 = 483 OhmQuote from: Tomorokoshi on October 02, 2019, 03:25:28 amC26 = 10.2 nF (7.24K ESR) 10KpF =-20%C25 = 467.0 nF (.80ESR) 50uF -10% +100%C24 = .471 nF (.82ESR) .47uF +-%Pics are attached.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: October 03, 2019, 11:34:31 pm
content: UPDATEI managed to get some replacement tubes to substitute tubes that were weak.It didn't seem to make any difference to the higher than normal voltage parts thatI've listed prior to this post.The manual specifically warns not to make any adjustments to the meters functionuntil the trouble is identified and repaired. So I haven't delved into that at all.Two things I know, most voltages are close, some are high, in the V6, V5 Detector area.While the probe passes the ohms check etc, it doesn't seem to read miliamps at all.In fact when I go through the process of checking the meter and it's reading,the probe has no effect at all. at 1ma, or 100mA, etc. When I adjust the zero pointor zero to the 1 for the 100mA reading it makes no difference whether the probehas the wire in its jaws, arrow left or arrow right, or not connected at all.So I'll go through the manual a few more times.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: October 05, 2019, 07:17:32 am
content: UPDATE ***** UPDATE ***** UPDATE ***** UPDATE ***** UPDATEAs I was thinking about this HP428B...I kept asking myself, why HP Designed the zero meter knobsdifferent than other HP units I have? Typically, when you turn a knob tied to a meter to the right,the meter needle moves to the right and vice versa.So, I was going to check into reversing the leads to the knobs.But wait, there is more.I also kept thinking the probe and checks are way to stable for something supposedto be able to pick up changes in the earths magnetic field, Ja? Stimmet?So I started looking again at the meter plug and how it screw into the jack whichis screwed to the face. I wondered if someone had gone in here and screwed withit and maybe soldered the leads to the jack incorrectly based on pin number.I mean HP didn't make troubleshooting this thing very easy. Along with that,they certainly didn't place pin number designations on any of the drawing forthe probes. They didn't ID the tube pin numbers either...that wasn't and issuefor me. But the probe pins it was and going to Amphenol's web site trying tofigure it out took some time. I didn't know HP used the jack from mic cable. Go figure.Trying to figure out if the leads to the pins on the inside of the front panel weresoldered in incorrectly. Tracing it back etc.Wait there is more!Then I found it. Ta Da.The ground lead wire was broken but the insulation was hung on the pin!Then when I moved it with my orange stick...it fell away.I cleaned it off trimmed the end and stripped it, resoldered it viola.Powered it up and it worked. When through some checks...like turning down the variac voltage to 103, the meter reading sags to be below 0 scaleand just hangs there even going back to 120V. It won't go back to the zero adjust point.that it until about 127V. Or I think I had to turn the meter off and on again.So meter sagging like that: Capacitors drying out? Regs weak? Diode Tubes weak?When I did the meter calibration check at 120 V, just a few tweaks of the calibration potto dial it in and it remained spot on.It failed the last of the quick check though...This is 5-37 Final Check.5-405-41 100 mA passa. 100mA cal to passb. It didn't like moving down to 103 a few times but eventually made it back.c. 300 mA pass, lower ranges pass5-42 Meter Tracking5-43, 5-44 I think it passed.5-45 Bandwidth (loop gain)5-46 this manual is incorrect, from Table 5-2: it should be step 13 DC LOOP GAINnot step 18 EARTH FIELD EFFECT.it failed the DC loop gain.spec is +.5 to -.5Actual was +.024 to -.023A really small amount.So that is where I am for now. Will get back to it as time permits.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 05, 2019, 01:30:32 pm
content: Wow, good to know!

author: USMC_Spike
author_rank: Regular Contributor
timestamp: October 08, 2019, 10:23:59 pm
content: I'll post some proof of cal pics when I get the chance,the only problem with those is they are three pics each.This means I'll have to crop then paste them into onepicture each.OrMaybe I'll have the larger meter pic and I can past the readoutsfrom the power supply and HP 34401A along the side of the HP428b Meter.I'll figure it out.