6611C questions - Page 1
Forum section: electronics / repair

author: hp3310a
author_rank: Regular Contributor
timestamp: May 09, 2025, 09:27:41 am
content: I would like to completely disassemble my new 6611C DC source so it can be cleaned properly and for exchanging the VFD.Barely started I'm already hitting a purely mechanical roadblock. I can't get the front panel off. The service manual saysQuoteI'm at point G but I don't dare rotate the panel as far as would be required. I'm 100% certain something would break first, the power switch, the board behind the cover, the left hand side of the panel cover or all of the above. No amount of squeezing and pushing moves the studs far enough out of the holes so the panel can be pulled forward.Clearly I'm missing something. Maybe someone who disassembled (without breakage) a 6611C, 6612C, 6613, 6614C or other equally constructed device's front panel could let me know the trick.Forgot to add the pictures

author: hp3310a
author_rank: Regular Contributor
timestamp: May 09, 2025, 04:19:54 pm
content: After another hour of wrestling this thing I finally got it off undamaged. It requires a certain amount of brutality, the plastic parts are tougher than usual. However, what they didn't say in the service manual is that you really have to pull the power switch extension rod off the switch, so the rod can turn with the front panel when you rotate and twist it to free the left hand side.Another nice trap is the front panel PCB. The service manual just says:QuoteWhat it doesn't say is there's a nut and a washer securing the encoder to the front panel, so you really can't lift it at all. You have to remove the front acrylic window and remove the nut first, then first move it left. Evidently someone was in there before and broke one of the taps of the window.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 11, 2025, 11:01:16 am
content: I'm gonna continue with silly questions here, making this a bit more general about the 6611C (I'll change the thread subject accordingly).Having disassembled the unit I found that one capacitor near the output connector lost some of its outer coating. Maybe this happened when the connector broke (which I will replace). The capacitor apparently is a ceramic disc version with the "220K X5E" printed on it. Not sure what that means, is it 200pF? X5E apparently is a kind of series. What should I replace it with?On the power supply end of the unit there are 5 big electrolytic capacitors which I'm not sure whether to replace them. 4 are Nippon Chemicon, 1 is Nichicon, I can see no trace of leakage. The unit has a MY40xxxx serial number which puts the year of construction to 2000 (if the HP rules apply here too). However a stamp on a metal part of the chassis says Dec 2007, so it should be 2008 or younger. I found matching capacitors on Digikey, though the SME (2x), SMH (1x) and LQ(M) series markings apparently no longer exist.Without desoldering I can't 100% make out cap 4 (see last image), this is what I could read:Code:1. Nippon Chemicon; 25V; 22000µF(M); 85°C; SMH2. Nippon Chemicon; 35V; 2200µF; ?; SME3. Nippon Chemicon; 16V; 12000µF; 85°C; SMH4. ????; 50V; 1000µF ???; 85°C; ???5. Nichicon; 16WV; 8200µF; 85°C; LQ(M)So, is it necessary to replace the caps? If replacement is advisable, what do I go for?

author: hp3310a
author_rank: Regular Contributor
timestamp: May 13, 2025, 06:01:06 am
content: I noticed on my unit, the encoder doesn't have steps, so when you turn it, it's like an endless potentiometer. Is that normal or does this indicate the my encoder is broken? Should it be stepped?

author: Qw3rtzuiop
author_rank: Frequent Contributor
timestamp: May 13, 2025, 08:35:27 am
content: The encoder is without detents. You can replace it with an encoder with detents. For the caps i would recommend replacing the the Y2 caps in the first picture. Those are RIFA timebombs. I wouldnt consider replacing the other caps if there arent any issues visible/measurable.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 13, 2025, 09:01:38 am
content: Quote from: Qw3rtzuiop on May 13, 2025, 08:35:27 amOK, thanks for the advice! There are more RIFA caps (2x 4.7nF on the output and 2x 1.0nF on the line input) but I'm not sure what to replace them with, it seems the format is rather special (judging from what I found at Digikey). I read (here) about replacing RIFAs and some replace old RIFAs with new ones and hope they last longer. Doesn't seem to be the best idea.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 13, 2025, 04:18:45 pm
content: There's another one I didn't realize it was a RIFA. The big blue one, do those usually need replacement, too?

author: ZGoode
author_rank: Frequent Contributor
timestamp: May 13, 2025, 04:42:43 pm
content: From my understanding that big blue one is probably fine since it isn't made out of that translucent resin which is prone to cracking like the infamous ones. Although it's not like it would hurt if you replaced it with a newer film cap. My general rule of thumb is just to replace anything I see with the name RIFA on it.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 13, 2025, 04:51:24 pm
content: Quote from: ZGoode on May 13, 2025, 04:42:43 pmOK, thanks. What should I use instead? I'm not really sure what I'm looking for, is it a "suppression capacitor"? Sometimes they're listed along side "radio frequency suppression capacitors", are those one and the same thing?

author: ZGoode
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:09:49 pm
content: Since it looks like its part of the AC input sections, any film capacitor with a 250VAC+ rating, 470nF (or 0.47uF as Digikey likes to use), and X2 rated should work. You'll just need to make sure you find the right physical size so that it fits on the board without interfering with other components. Something like this could work, but definitely take calipers to it to get the XxY dimensions (Z is less important as long as you're in the right ballpark)https://www.digikey.com/en/products/detail/kemet/R463I347050N0M/5731179https://www.digikey.com/en/products/detail/kemet/R46KN347000N0M/1930819

author: hp3310a
author_rank: Regular Contributor
timestamp: May 13, 2025, 07:44:11 pm
content: Quote from: ZGoode on May 13, 2025, 05:09:49 pmThanks a lot, that really helps!

author: hp3310a
author_rank: Regular Contributor
timestamp: May 20, 2025, 11:07:18 am
content: So I didn't pay enough attention and ordered a Y2 class capacitor instead of X2. If I understand correctly a Y class will fail open instead of short (X-class), which would basically render its function void but otherwise cause no issue. Any strong objections to using the Y2 or should I solder the existing X2 back in (it is working)?Edit: I used the old one.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 21, 2025, 07:21:38 am
content: I reviewed the photos of my soldering (if you can call it that). Most of the joints (especially those on pads with high thermal mass) are bulb-shaped and have a white residue around them (this is after thorough cleaning with IPA).Some pads look OK (the rightmost 2 of the 5-pin connector in the first picture), but the rest don't look so great.The capacitor joints look especially bad.I marked the joints that I did with red dots, so basically what looks OK is not my doing. I used an Aixun T3A soldering station with two different tips (Chinese knife style and a JBC C245-905 1.5mm bevel tip) at a temperature between 290°C and 320°C using lead-free solder.What am I doing wrong?

author: ZGoode
author_rank: Frequent Contributor
timestamp: May 21, 2025, 05:36:00 pm
content: For those kinda bulbous solder joints, you're probably just not dumping enough heat quickly enough into the board. It looks like there is a pretty substantial copper pour in that area, which loves to suck up and spread heat (good for some scenarios like spreading heat while under use, but not when you're trying to add/remove components). This is a scenario where I really like using my FR301, since even though it for desoldering, it can also dump out a lot of heat really quickly compared to my JBC or T12 station.

author: Kean
author_rank: Unknown
timestamp: May 21, 2025, 05:41:53 pm
content: Yikes. You will need a higher temp for manual Pb free soldering.Try 360°C at least, but only hold it there as long as needed for the solder to flow. That should be quick for the small pads, but could take a while on that big copper pour. Depending on your iron, you might need to go to 380°C or something on that. And you'll need to add extra flux now to get those blobs to flow nicely.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 23, 2025, 06:46:20 am
content: Thanks guys, I followed the advice and redid the joints at up to 370°C and it worked better. I'm certainly not winning any prices with those either, though. I will have to invest into more suitable tips, Steve Gardner likes the chisel ones. Now it's waiting for the new VFD which shall be interesting to solder, too.I also exchanged the extremely loud fan. I'm not surprised the Noctua fans are a lot quieter as they also produce a lot less air flow. At 12V the stock fan that was in the unit produces much more air flow than the new Noctua does (I'd estimate 3-4 times as much).

author: Kean
author_rank: Unknown
timestamp: May 24, 2025, 10:18:03 am
content: I've got a 6612B which I bought around 2012. The VFD is still quite readable, so I've not replaced it. The fan is quite noisy though, so eventually I'll swap that and recap the unit.When soldering the new VFD, you should keep the higher soldering temp if you use lead free solder. Even with leaded solder you probably want to use 340-350°C.You should then be able to solder the joints reasonably quickly as I don't think there will be large copper pours on the VFD traces.

author: wraper
author_rank: Unknown
timestamp: May 24, 2025, 10:22:25 am
content: Quote from: ZGoode on May 13, 2025, 04:42:43 pmThere is nothing wrong with Rifa film capacitors. However metallized film X safety capacitors losing their capacitance due to self-healing is very common.

author: wraper
author_rank: Unknown
timestamp: May 24, 2025, 10:24:37 am
content: Quote from: hp3310a on May 13, 2025, 09:01:38 amJust use regular Y ceramic capacitors.Quote from: hp3310a on May 20, 2025, 11:07:18 amNeither of them should fail short. X capacitors generally self-heal until lose all their capacitance if something goes wrong (except paper timebombs). Y capacitors have stronger insulation and are rated for higher voltage.

author: hp3310a
author_rank: Regular Contributor
timestamp: May 26, 2025, 05:18:34 pm
content: Quote from: Kean on May 24, 2025, 10:18:03 amI did some digging and the stock fan is more powerful on paper than the current replacement Notua. Air-volume is about 50% greater and static pressure about 3 times (not sure how those can be different ratios).So I took the fan apart to see if it could be fixed. It has 2 tiny 8x3x3mm roller bearings (outer x inner x width). Those are available from SKF at about €10 a piece. It certainly will be better but I doubt it will fix all the vibrations, most of those likely come from the wheel wobble and unbalance. If the unbalance is just due to unequal blade lengths this could be improved by trimming them to equal lengths (on a lathe). Not sure if it's worth it.

author: Kean
author_rank: Unknown
timestamp: May 26, 2025, 05:58:58 pm
content: I bought a fan ages ago for it, and looking through my notes it was the Sunon HA40201V4-1000U-A99 - sourced from Digikey.Not sure if that is ideal, but I suspect someone else must have commented on using one on this series of supplies.I know it won't be as quiet as a Noctua, but certainly it can't be worse that the original one after several decades of accumulating dust.

author: Runco990
author_rank: Frequent Contributor
timestamp: May 27, 2025, 04:39:49 pm
content: I replace my fans in all my HP gear with Noctua's or BeQuiet's because I can't stand the noise. I work on audio and need a quiet lab. Remember that this equipment was built for worst case service, so it's generally fine for home use. I have not seen any adverse temperature problems in the last 15 years, so I'll stick with it. My HP 3325A for example could be used to vacuum the house. Now I can barely hear it. It barely even gets warm.When you have have near 30 pieces of test gear, that noise adds up quick!There ARE exceptions, though, like the Tek 2465 scopes. I would not decrease the cooling due to the U800 IC. Wish I could...

author: hp3310a
author_rank: Regular Contributor
timestamp: July 17, 2025, 11:22:43 am
content: After having exchanged the dim VFD for a brand new one, I had the unit running for a couple of hours. After switch-off I noticed the typical sound metal components under stress make when cooling down when tension eases off. So something had heated the whole unit up (the cover was cold).Before putting on the cover I had it running for a couple of minutes (no more than 5) and noticed that the heat sink of U305 is extremely hot to the touch.This is the voltage regulator that provides 5V for the logic board. It is located surrounded by all the big electrolytics of the unit and so heats them up constantly. I think it is this regulator and its very bad placement that requires the extremely noisy stock fan.Lacking real schematics (the service guide has limited info) I was wondering if some knew the type of regulator used?Maybe there is a better solution that could be used instead?If there's not a drop in replacement with better efficiency, I was thinking maybe a little daughter-board that takes the input (11V AC) and produces the required 5V without generating that much heat. Or maybe mount the regulator on the chassis for cooling away from the caps. A third option might be a better custom heat sink, that contains the heat better and connects directly to the main heat sink for the FETs that has the fan mounted to it.

author: Kean
author_rank: Unknown
timestamp: July 17, 2025, 07:54:26 pm
content: Service guide for 6612B lists U305 as HP P/N 1826-1597 which is cross referenced as LM2940CT-5.0. There are definitely 3-terminal switchmode drop-in replacements available, but you'd need to ensure any extra switching noise isn't going to be an issue. Mounting the regulator on the metal chassis is certainly a workable solution that helps keep the caps cooler.An alternative is some pre-regulation of the rectified 11VAC, say to 8V with an LM2940CT-8.0 mounted on the chassis. I'd first suggest measuring the temperature and checking with other owners if this is normal. I'm guessing it is, otherwise they wouldn't have included such a big heatsink.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 18, 2025, 08:37:49 am
content: Thanks a lot for looking up the component, the service manual I have doesn't list components at that level.The heat sink temperature is "normal" so it's not my unit that is defective, it's the whole design. From what I can see many 66xxzN units use this design.If my calculations are correct the LM2940C in the unit wastes around 5W. No wonder it is going hot. I'm leaning towards a better replacement, maybe using a step-down regulator like the ADP2301. That should do a much better job in terms of waste and heat.

author: hp3310a
author_rank: Regular Contributor
timestamp: August 01, 2025, 05:40:17 pm
content: So there was some progress I can report on with respect to replacing the hot-running voltage regulator.I kind of accidentally stumbled upon the TI TPS562242, which is a "3-V to 17-V Input, 2-A, Synchronous Buck Converter in SOT-563 Package", says TI. Apart from SOT-563 being really really tiny - you could easily inhale if you're not careful - this wonder of modern electronics does exactly what I want. All you need to do is give it a little infrastructure and it voltage-regulates with little power loss and heat dissipation.So after a initial test on the breadboard I soldered a more durable version to a permaboard. I had no DIP-to-SOT-563 adapters so I botched my own from a TSSOP-8 adapter. This was fiddly but works as intended.The voltage regulator works great, time to test it in the real world. Out goes the old regulator - which is challenging because it is very very well connected to the board through its heat sink and that dissipates all the energy you try to desolder it with (and gets hot in the process).I put the new regulator board on long leads so it can be interrogated easily on the bench.Then power up and it works! That's great but it's even better that it only warms up to 57°C without any cooling and with a very sub-optimal PCB layout (the datasheet suggests how to design whole regulator PCB section so it can dissipate as much energy via the board as possible).The final version of the regulator board could be as small as the DIP adapter PCB, it doesn't have to be though, the heat sink used a whole lot of space.The only downside of this experiment is that there are other regulators that run hot too (albeit with much smaller heat sinks). U304 gets very warm, the heat sink goes over 80°C. That is without any cooling, though, so might be OK when the lid is closed even with low-noise Noctua fan (which is why I wanted to change the voltage regulator in the first place).Since I did remove the biggest heat generator on the board, it might be OK. Should I ignore it?

author: hp3310a
author_rank: Regular Contributor
timestamp: August 22, 2025, 05:30:32 pm
content: So after the initial test I went and designed a PCB for the regulator (JCLPCB made it for me). I basically just followed the design advice TI gives in the datasheet and used their suggested values for capacitors, resistors and the inductor (except for the voltage adjustment set point resistors), so this is in no way "from scratch". For a rookie like me, it's still really really cool to be able to do such a thing.Yesterday the PCBs arrived and today I completed a board:After a dry run with the lab PSU which worked great I connected it to the 6611CAnd turned it on:TA-TA! Works! The voltage is spot on under load...... and the temperature of the chip after a couple of minutes of operation is merely luke warmTI didn't lie when they said that one can use the PCB copper as a heat sink. The difference to the old regulator (85+°C) is nothing short of astonishing (for me anyway).Mission accomplished, I'm really happy!

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 22, 2025, 08:07:38 pm
content: this is a low-noise linear supply. I am afraid the mod can add some noise.

author: hp3310a
author_rank: Regular Contributor
timestamp: August 22, 2025, 08:22:39 pm
content: Care to elaborate? The regulator in question supplies the logic, not the output.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 22, 2025, 09:28:42 pm
content: You might be right that it can only affect the digital side of things. There is still a chance the noise can leak to DACs and such. Perhaps indirectly since nobody cared about shielding and distancing from the linear regulator, and now the replacement might emit all sorts of EMI.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 07, 2025, 06:58:42 am
content: I finally mounted the new regulator, exchanged all the electrolytics that have been fried by the old one (they where still good, though) and fitted a new fan (this time a Sunon MF60251V2-1000U-A99, the Noctua did die for unknown reasons. The fan operates at 10.5V maximum and that is just too little to provide any meaningful cooling with the slow turning Noctua. With the Noctua the exhaust temperature at full load after 20 minutes was at 50°C, the Sunon gets it down to 43°C.Anyway, my 6611C does produce some strange spikes in the otherwise very low noise output.Before anyone says it's the voltage regulator, the spikes are there with the old regulator as well (see photo below). It's not the new regulator.The spikes appear at a frequency of about 160kHz and are about 5mVpp. The load is purely resistive, so it's not coming from outside the unit. Any idea what might be causing this?

author: feedback.loop
author_rank: Frequent Contributor
timestamp: September 07, 2025, 08:33:20 am
content: This setup is not shielded, and, probably, the scope is picking up all sorts of stuff from the environment. Try with a coax cable.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 07, 2025, 10:17:54 am
content: Don't think that it's induced externally. No load and no cables, still there.

author: gamalot
author_rank: Super Contributor
timestamp: September 07, 2025, 10:40:05 am
content: It comes from the power supply of the VFD which is close to the output terminals.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 07, 2025, 11:08:09 am
content: Quote from: gamalot on September 07, 2025, 10:40:05 amThanks, I had a suspicion that it might have to do with the VFD. So how do I get rid of it?

author: gamalot
author_rank: Super Contributor
timestamp: September 07, 2025, 11:14:03 am
content: Quote from: hp3310a on September 07, 2025, 11:08:09 amFirst, don't worry about it. This spike can easily be filtered out at the load. Just try using a CBB capacitor, say, a 1µF one, and see for yourself.If you absolutely must do something, try shielding the VFD from the output terminals with some grounded metal sheet or something.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 07, 2025, 01:18:50 pm
content: Quote from: gamalot on September 07, 2025, 11:14:03 amI'll try that, thanks for the suggestion!

author: feedback.loop
author_rank: Frequent Contributor
timestamp: September 07, 2025, 04:28:38 pm
content: Here is what I measure. And exactly the same thing with the output turned off.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 07, 2025, 04:29:35 pm
content: I tried it the cheap way by better twisting the front terminal wires from thisto how it was supposed to beUnfortunately that didn't change the spikes, not even a little.However, I then measured the noise at the rear terminals and this is how it looks therePretty. Damn. Excellent.That should also prove that my logic supply regulator doesn't impact performance. Even if it did it is still well within specs.

author: d-el
author_rank: Regular Contributor
timestamp: September 08, 2025, 11:42:06 am
content: I think the problem was not in the twisted pair. Either the oscilloscope power supply was noisy, or there was extraneous interference in the AC line, or electromagnetic radiation.I still don't understand what the advantage of replacing the linear regulator with a switched dc/dc. After recap use at home, this power supply will last for decades.But I would replace the tantalum capacitors in the backlight module (and I did). A typical problem with old tantalum capacitors for high voltages (20-60 V) is leakage and breakdown. I replaced them with ceramic capacitors of similar capacity.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 08, 2025, 04:53:40 pm
content: If the spikes originated in the scope, they would have shown up in the measurement on the rear terminals as well. I will report back when/if I make progress shielding the VFD. I think that's better than shielding the wires which span the entire case (if the VFD is the source of the noise). I'll change the tantalums as well, thanks for the suggestion.The motivation for this is that I want to use a fan which much less airflow than the original. That isn't easily doable because the regulator gets extremely hot if not cool by a rather stiff breeze (>90°C uncooled). And since it is surrounded by all the big electrolytics I wanted to get rid of the oven. Sure, the device will be rarely used compared to industrial use and it likely would have survived me anyway.I'm a rookie with a lot to learn, I wanted to see if I can do it without ruining the unit. I did and I leaned something about the regulator and the fan control circuit. Mission accomplished.