HP 3312A repair help. - Page 1
Forum section: electronics / repair

author: NPG
author_rank: Contributor
timestamp: February 15, 2025, 11:20:25 pm
content: I have an HP 3312A that is intermittently losing it's mojo.See attached scope traces.Ch3: Output signal.Ch4: Sync signal.First image, expected output.Second image, very broken output.Instrument settings.Frequency: 1 KhzAmplitude: 0.5V PtPModulation: None.Wave Sym.: SymmetricalWaveform: Sine, Triangle, Square.Malfunction:Intermittent, observed on all wave forms.Usually happens when changing frequencies, but sometimes happens on instrument start or just randomly.Can sometimes be cleared by maxing out the frequency dial then bringing it back down to desired setting, or changing frequency range.It's definitely failing on the falling edge of the waveform.Theories:1. Component U201, described in paragraph 4.25 of manual is faulty.2. Possibly some of the capacitors making up the diode switch described in paragraph 4.21 of the manual are faulty.I should be able to confirm one of these once I take the instrument apart and check TP#3.Before I start tearing apart vintage equipment that I barely understand, I would appreciate the input of some people smarter than me who may have run into this before..Thanks. :-)

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 16, 2025, 02:21:31 pm
content: If you haven't already, make sure you use contact cleaner on the pushbutton switches and the pots. HP has a habit of running signals through multiple switches and you can get all kinds of misbehavior if any of them are dirty.

author: NPG
author_rank: Contributor
timestamp: February 16, 2025, 06:20:47 pm
content: Quote from: wn1fju on February 16, 2025, 02:21:31 pmDeoxIT is on the to do list for when I take it apart. :-)Waiting for a set of Posidriv screwdrivers to come in before I start.By the look of some of the screws on the case, some Neanderthal attacked them with a Phillips head screwdriver in the past.I sure don't want to make that worse.

author: NPG
author_rank: Contributor
timestamp: March 07, 2025, 08:45:26 pm
content: While waiting for the Posidrivs to show up, the following dawned on me.1. HP uses a lot of gold contacts in their products.2. The DeoxIT D series product that I had, ( ~ $13 for 5-6 Oz. in 2020 ) probably was not the best choice.3. A DeoxIT G series product would be better.I find some DeoxIT G series product for ( ~ $29 for 5-6 Oz. ). OK, prices have gone up in five years and the G series has always cost more than D series.It came in today. I obviously didn't thoroughly read the item description, and somehow stumbled into a good deal.Do you all think I'll have enough?LOL!

author: NPG
author_rank: Contributor
timestamp: March 07, 2025, 11:12:43 pm
content: Just found this.https://speakyssb.blogspot.com/2021/12/hp-3312a-small-repair.htmlHis broken output looks a lot like mine. Something else to check.

author: NPG
author_rank: Contributor
timestamp: March 08, 2025, 08:16:20 pm
content: Quote from: NPG on March 07, 2025, 11:12:43 pmI visually inspected the caps in that area, they look fine.

author: NPG
author_rank: Contributor
timestamp: March 08, 2025, 08:26:46 pm
content: It looks like someone may have been in here before.1. The two large caps at the back of the top board appear to be newer, and are not placed on the board as neatly as the other components.2. There is a large resistor near test point 4 on the bottom board that does not look like other photo's I have seen of this board online

author: NPG
author_rank: Contributor
timestamp: March 08, 2025, 08:27:56 pm
content: Quote from: wn1fju on February 16, 2025, 02:21:31 pmRan into the issue of not having the right tool to remove the knobs from the front in order to get easier access the the switches / pots.Does anyone know what the right tool for that job is?Also realized that I had a cracked offset knob.Does anyone know where I can get a replacement?I ended up just carefully applying DeoxIT from the back of the switches / pots and then working them to get the product into them.Results:1. The switches and knobs work way easier.2. The intermittent waveform generation failures appear to be gone. I ran the unit for ~ 3-4 hours last night while switching through frequency ranges, without any issues.3. The frequency dial is definitely out of calibration, as I think part of it's previous issues had been calibrated out of the output signal. The output signal frequency is now 10-20% above what is dialed in.I'll definitely have to re calibrate the unit.

author: NPG
author_rank: Contributor
timestamp: March 10, 2025, 07:21:11 pm
content: Quote from: NPG on March 08, 2025, 08:27:56 pmI have a quick question here, as I am definitely a newbie at this.HP 3312A service manual. Page 5-11. Section 5-33 "Frequency adjustment"sub-section b"measure the dc voltage across A2R124 (between J1 pin 6 and TP2).Is the TP2 above referring to the test point 2 on the bottom board; or is there a TP2 on the top board that I just haven't found yet?I would really appreciate some clarification before proceeding.If anything blows up during this adventure, I would rather it be due to accident, as opposed to ignorance.Thanks :-)

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 10, 2025, 10:46:33 pm
content: It is TP2 on the A2 board which looks like the attached picture.

author: NPG
author_rank: Contributor
timestamp: March 11, 2025, 01:39:17 am
content: Quote from: wn1fju on March 10, 2025, 10:46:33 pmThank you so much. :-)Can't believe that I completely missed the fact that J1 & J2 are present on both the top board and the bottom (A2) board.

author: NPG
author_rank: Contributor
timestamp: March 12, 2025, 06:11:19 pm
content: Another question RE measurement polarity.HP 3312A service manual. Page 5-11. Section 5-33 "Frequency adjustment"steps b & c.step b."measure the dc voltage across A2R124 (between J1 pin 6 and TP2) Connect the low side of the digital voltmeter to J1 pin 6. Record this voltage to 1mv resolution.Negative probe on J1 p6, positive probe on TP2.Voltage recorded +2.853 to +2.854 (fluctuating).step c."Connect the dc voltmeter across the A2R119, A2R121 combination (between J1 pin 3 and TP1) Connect the low side of the digital voltmeter to J1 pin 3. Adjust A2R121 such that the voltmeter reading differs by < 1.5 mv from the voltage measured in step b.Negative probe on J1 p3, positive probe on TP1.Voltage recorded -2.853 to -2.854 (fluctuating).If polarity does matter, then the measurement from step c is about 5-6 volts out of alignment. I will need to adjust it until it reads +2.583 - +2.584 volts.If polarity does not matter for this, then I am dead on.Does the polarity of these measurements matter?Again clarification would be appreciated.Thanks.

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 12, 2025, 10:20:14 pm
content: I tried that measurement on my 3312A and get what you got. So apparently there is a mistake in the published adjustment procedure. Wouldn't be the first time there is an error in an HP manual.Besides, at least on mine, you couldn't adjust A2R121 to give you more than about 1/4 volt of change anyway.And further, if you look at the schematic, J1 pin 6 is the -14V rail. You put your negative probe on that and the positive probe on TP2. TP2 can't be a higher negative voltage (like -15V), it must be a lower negative voltage (like -12V). So your measurement will be positive.Now you put your negative probe on J1 pin 3 which is the +14V rail and your positive probe on TP1. By similar reasoning, TP1 can't be higher than +14V, so it must be lower and you will get a negative reading.

author: NPG
author_rank: Contributor
timestamp: March 13, 2025, 03:03:46 am
content: Quote from: wn1fju on March 12, 2025, 10:20:14 pmI had a feeling that might be the case, so continued on as if polarity didn't matter.Finished the frequency adjustment section 5.33.Not sure if I should have, but I used some DeoxIT G series on A2C205, A2C216, and A2C217, as they would not turn initially.The output signal frequency is now 1-2Hz to 100-200KHz off from what is dialed in.Definitely an improvement.In the performance testsTest 5.9 Pass with the exception of the 6 and 13 settings for 100KHz & 1MHz they are about 100-200KHz low.Not really worried about it, as I can tweak the dial if I need to get it dead on. May try to calibrate it out in the future.Tests 5.10 & 5.11 Pass.Tests 5.12 & 5.13Haven't been able to figure out if I can set these up with what I haveTek TDS 744A ScopeHP 3478A MultimeterHP 5334B CounterHP 8616A Pulse / Func Gen.Any ideas if I can do those tests with the above equipment.Tests 5.14 & 5.15 Pass.Test 5.16 PassFor AC RMS measurements, the HP 3478A has a bandwidth of 100KHz. Setting the Range HZ on the 3312A to 10Khz allows the test to be run with an HP 3478A.That's were I stopped for today.Quote from: wn1fju on March 12, 2025, 10:20:14 pmThat makes sense.I wasn't sure if what I was seeing was what I was supposed to be seeing.Thank you for taking the time to check that out on your 3312A. :-)

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 13, 2025, 12:06:54 pm
content: I wouldn't spend too much time on the performance tests. If you can successfully go through the adjustment procedure and everything adjusts properly, I would call it a day. Look at the various outputs on your scope with the various 3312A settings and if everything "looks" right, be happy. The 3312A is a medium fidelity function generator, so it's not like you are going to get NIST-calibrated performance! In other words, if the triangle wave segments look like straight lines, as best as you can tell on the scope, I wouldn't argue the linearity performance test of <1%. You would have to start changing components if it wasn't and you (and I) probably don't want to jump down that rabbit hole.As far as your test equipment list, you might be on the lookout for a distortion analyzer. That is useful both for the adjustment and verification for the 3312A's sinewaves. The HP 33X series often comes up on eBay for around $100. Similarly, a spectrum analyzer would be nice, but now we are talking serious money.I am fortunate enough to have every piece of test equipment suggested in the 3312A service manual. But I took one look at the performance tests and decided it was too much of a bother.As always, just my opinion.

author: NPG
author_rank: Contributor
timestamp: March 13, 2025, 06:27:19 pm
content: Quote from: wn1fju on March 13, 2025, 12:06:54 pmExactly why I didn't go through the entirety of the adjustment procedures.After going through 5.33, in which some of the variable caps changed value on contact, and after reading the last two paragraphs.Basically, calibrate it as close as you can get, put the covers back on, check it in 10 minutes to see if it moved. Repeat. :-)I knew this wasn't a high precision device; just precise enough for bench work. :-)Quote from: wn1fju on March 13, 2025, 12:06:54 pmAgreed, not going that crazy. I'm not after performance that I just can't get out of an HP 3312A.As remarked earlier, I am somewhat new to this.At this point, I am doing the performance tests to better understand how this thing works; and for the fun part of figuring out how to do the tests with what I have, if possible.For instance, I have a whole page of notes on how to do test 5.17 with my Tek scope; which I will probably post later.Test 5.18 didn't pass on point d. High by ~ 250 mv; but who cares, it's a sync output.5.19 ^ 5.20 passed.Quote from: wn1fju on March 13, 2025, 12:06:54 pmThanks, I'll keep an eye out for them.All of my HP test gear has come from ebay for ~ $100 a unit or less.In place of those I used the FFT function on my scope to do test 5.14.https://youtu.be/s_cVP5gu4SY?feature=sharedQuote from: wn1fju on March 13, 2025, 12:06:54 pmAgreed.If I had not started out with a partly broken instrument that I did not really understand, but wanted to fix.If I had not decided to turn the performance tests into a learn what I can experiment; I wouldn't be doing this at all.Once I'm done with the performance tests that I can figure out how to do, I sure don't plan to do it again anytime soon. :-)

author: NPG
author_rank: Contributor
timestamp: March 13, 2025, 06:52:30 pm
content: Another couple of questions RE Performance Test 5.21HP 3312A service manual. Page 5-7.Performance Test 5.21 "Internal Modulation Source Output Level Test"Step a. "Connect the MOD INT/EXT output of the 3312A to the input of an oscilloscope using a 10Kload across the output of the 3312A.1. I am guessing that this 10Kload would be the same thing as a 10Kfeedthrough terminator like the 50ones used in some of the other tests.Where can I find one of those 10Kones?2. Does the 10Kload go on the MOD INT/EXT connector or on the main output of the 3312A?I am guessing MOD INT/EXT, as the next test refers to connecting MOD INT/EXT to an instrument (unterminated).Thanks. :-)

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 13, 2025, 10:08:41 pm
content: Yes, it's the output of the modulation. Just take a BNC (female-male-female) tee at the scope and hold a 10K resistor in the tee's open port between the center hole and the outside metal.I suppose you can buy a 10K feedthrough terminator, although 50, 75 and 600 ohms are the standards. Google it. My guess is it will be in the $25 to $50 range. Not really worth it since you are not likely to come across many instances where you need 10K.

author: NPG
author_rank: Contributor
timestamp: March 14, 2025, 01:49:35 am
content: Quote from: wn1fju on March 13, 2025, 10:08:41 pmI've already been looking for that, nothing yet.In the mean time, this ought to do it.BNC to N tee back to BNC to clips to decade resistor box set to 10K. With an N to BNC for the scope.LOL! That's got to be the wildest thing I have ever built in five minutes. :-)I'm sure that somewhere, an engineer is rolling in their grave or screaming incoherently.Note to self, buy some BNC tees. :-)