Oscillator not working right (HP 204C) - Page 1
Forum section: electronics / testgear

author: DanielSpokane
author_rank: Regular Contributor
timestamp: May 22, 2017, 03:24:58 am
content: Help!I have an older HP Model 204C oscillator that isn't putting out a solid sine wave. See the attached pictures. The setting on the oscillator is at 1khz. You will see three things: 1) the bottom of the sine waves is flat, 2) the geometry of the wave seems a little wacked, and 3) it looks like it is actually putting out around 1.4khz not 1.0 khz.I'm thinking adjusting the oscillator to the correct frequency won't be difficult, but seems like the problem with the sine wave (ie flat part and geometry) might be a bigger deal. I also attached a picture of the set up (IE scope/osc/probes/etc,) FYI.What do you guys think? Fixable/calibrate-able? ... or just thrown in the junk pile? I like the look of this thing and would like to keep it for use with audio tube amp trouble shooting.Thoughts? What would cause the problems with the sine wave? Anyone have any experience with this?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 22, 2017, 05:02:18 am
content: Time to start replacing capacitors.

author: edavid
author_rank: Super Contributor
timestamp: May 22, 2017, 05:30:00 am
content: Probably just a bad power supply capacitor. Don't do any tweaking until you've checked the power supplies.

author: oldway
author_rank: Super Contributor
timestamp: May 22, 2017, 08:30:23 am
content: It is a simple and well manufactured test gear and it is easy to fix it.Here the service manual:

author: Tomorokoshi
author_rank: Super Contributor
timestamp: May 22, 2017, 03:36:23 pm
content: Also try this manual:http://bama.edebris.com/download/hp/204c/HP%20204C.pdfIt has the full schematics and parts lists. It lists Sprague electrolytic capacitors in the power supply and other places. I've had those short out in other HP equipment of that vintage.Look at:A1C11A2C1, A2C2, A2C3, A2C4 (Standard)A2C21, A2C22 (Option 02)

author: oldway
author_rank: Super Contributor
timestamp: May 23, 2017, 05:57:20 am
content: If power supply is allright, desolder one side of each electrolytic capacitor and measure it....it is not necessary to check them for ESR.If to low value, replace them.

author: cnqhdszq
author_rank: Regular Contributor
timestamp: May 23, 2017, 10:20:11 am
content: It looks like the output stage does not work well , please check the biggest trasisters on the output stage board .please read the link page .http://bbs.38hot.net/forum.php?mod=viewthread&tid=217746when I repair my WK3245 LCR datebridge , I meet the similar thing .

author: DanielSpokane
author_rank: Regular Contributor
timestamp: June 05, 2017, 05:23:27 pm
content: Quote from: Tomorokoshi on May 22, 2017, 03:36:23 pmOk, so I finally got around to working on this. Note... I have Option 2 (with the batteries). I'm planning to replace capacitor's A1C11 and A2C21 and A2C22.I took the Oscillator apart and found that the batters are severely corroded. See attached pictures. Think that could have anything to do with the oscillator's performance? I never use the batteries... I would like to just remove them. Will that affect the unit?Any other parts you would suggest removing/replacing given the corrosion?... or just clean it up.What do you think?

author: edavid
author_rank: Super Contributor
timestamp: June 05, 2017, 05:41:52 pm
content: Quote from: DanielSpokane on June 05, 2017, 05:23:27 pmYes, the batteries operate as voltage regulators. If they are open circuit or removed, the unit won't work right.I have had success replacing the positive and negative battery strings with 14V zener diodes, or with TL431s. The TL431 works a little better, because you can get the voltage just right (14.5V is good).I also modified the charger circuit to set the zener current to about 16mA. You could probably just use the high charge setting though.I'll have to see if I can find the sketch I made of the changes.P.S. With the batteries missing, there is a 16V capacitor in the power supply that has 40V applied (C22). It must be fairly robust, since I haven't seen one blow up yet.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 05, 2017, 06:16:51 pm
content: Your last picture shows corroded traces and components. You have to remove the components and clean them and the traces, otherwise it will eat through the traces and components eventually.

author: edavid
author_rank: Super Contributor
timestamp: June 05, 2017, 06:51:16 pm
content: Quote from: PA0PBZ on June 05, 2017, 06:16:51 pmI'm sure that's a good idea, but I have some that have looked that bad for years, and the corrosion has not progressed since I removed the batteries. However, I live in a dry place, not the Netherlands

author: DanielSpokane
author_rank: Regular Contributor
timestamp: June 07, 2017, 04:33:38 pm
content: Since I have Option 02 I’ve been trying to find replacements to capacitors A2C11, A2C21, A2C22. I’m trying to find good replacements caps but having trouble finding exact/close matches. Below are the original capacitor specs from the manual followed by a link to the best replacement I can find. What do you think?1.) A1C11 – from the manual: HP Part # 0180-0140 ; C: fxd Al elect 300 uF + 100% - 10% 10 vdcw ; mfr 56289 (Sprague) ; mfr part number D36546 obdhttp://www.mouser.com/ProductDetail/Vishay-Sprague/TVA12085/?qs=sGAEpiMZZMtZ1n0r9vR22cxOkjJhVyTBhb7YRJnDHUw%3dComment: The problem is the tolerance is 20% not 10%. I found one that is 10% but they are out of stock. Think this one will be ok?2.) A2C21 – from the manual: HP Part # 0180-0149 ; C: fxd Al elect 65 uF + 100% - 10% 60 vdcw ; mfr 56289 (Sprague) ; mfr part number D36978-DSM (TYPE 30D)http://www.mouser.com/ProductDetail/Vishay-Sprague/672D686F060DM5J/?qs=sGAEpiMZZMtZ1n0r9vR22fPFtzULxwAM5yTQhsZksso%3d3.) A2C22 – from the manual: HP Part # 0180-0061 ; C: fxd Al elect 100 uF + 75% - 10% 15 vdcw ; mfr 56289 (Sprague) ; mfr part number 30D107G015DC2-DSMhttp://www.mouser.com/ProductDetail/Vishay/30D107G015DC2A/?qs=E45C3CtnMAYoXtBvN3fonA%3d%3dAlso, given the battery condition and corrosion, I’m planning to clean things up. Based on edavid’s suggestion I’m wondering if I should replace with diodes. Yes, edavid if you can find a sketch that would be awesome. If not, I’m thinking it may be easy to just replace the batteries. Or maybe they are still good? What do you think? Any suggestions on battery replacements? The part info from the manual is below:Rechargeable batteries – from the manual: HP Part # 1420-0015 ; Battery: rechargeable nicad cadmium 6V 225 mAh ; mfr 000LP ; mfp part number 5-B225J

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 07, 2017, 05:24:33 pm
content: Note that for the second two capacitors, I've found that HP equipment of that era tends to run right up to the capacitor rating. As in a 40 V cap might be run at 39 volts. I like to give around 50% margin to the capacitor rating, so perhaps 100 V for capacitor #2 and 25 V for capacitor #3. If they are in a bridge circuit off the transformer any line spikes would go right to the capacitor.

author: edavid
author_rank: Super Contributor
timestamp: June 11, 2017, 12:04:11 am
content: Here's the TL431 version of the regulator, and a photo of the installation.The LEDs are just for bling, and can be omitted.

author: DanielSpokane
author_rank: Regular Contributor
timestamp: June 11, 2017, 02:23:46 pm
content: Thanks edavid!! I'm going to do this. I was just on mouser wondering what battery's to order. Now I'm just going to order some resistors, diodes and a cap. The picture was very helpful too. Thanks!Yesterday I took some measurements.A couple more trouble shooting comments/questions.Need some electronics wizards to help with this. It's boggling my mind!I replaced A1C11 (300uf, 10V) and checked A2C22 (100uf, 15V)--good. I didn't have a good replacement for AC21 (65uF, 60V) so haven't done anything with this one... probably need to replace. Should be getting one in the mail soon.Checked voltages and found that the B+ and B- voltages (at TP2 and TP3) which should be in the range of 13+ and 13- were closer to 7 volts--so voltages are off. Also found that the bias and AGC on TP1 and TP4 could not be adjusted to 1.8V and 0V no matter how much I turned the adjustment pots. Seems like this thing is running at the wrong voltage. If it is a power capacitor the only one I haven't checked/changed is A2C21. Think that could be the problem?... I'm also wondering if the batteries are shot and maybe that is having this effect?... if so, edavid's voltage regulator should fix that.Also, one more tid bit... when I play around with the AGC... I can get the sine wave to look right (fixes the flat bottom distortion) but if I adjust it to look right at 1 kHz when I adjust it to a different hrz rate (lower) the amplitude changes (decrease) and as I adjust it more it will eventually flat line. Weird! Still seem like a power issue?

author: DanielSpokane
author_rank: Regular Contributor
timestamp: June 13, 2017, 11:58:51 pm
content: Quote from: edavid on June 11, 2017, 12:04:11 amIs this the right TL431? I've never worked with voltage regulators.http://www.mouser.com/ProductDetail/Texas-Instruments/TL431CLP/?qs=sGAEpiMZZMvAX9OfPh%252b2NR%2fmZDP9ZixDD7ptP4Lq%2faE%3d

author: DanielSpokane
author_rank: Regular Contributor
timestamp: April 06, 2018, 08:50:39 pm
content: edavid recommended the voltage regulator below to replace the corroded batteries in my HP 204C oscillator which is not behaving correctly. (Thanks edavid!) I have all the parts and am putting this together.Also included is a picture in which I drew on the original schematic where I believe the regulator needs to be placed. Look correct?I'm having a little trouble understanding where/why R5. Does it replace R23 in the original schematic?... or remove S4 and R24 and replace with new 680 ohm parallel to R23?Any help would be much appreciated.

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2018, 09:15:41 pm
content: Quote from: DanielSpokane on April 06, 2018, 08:50:39 pmR5 goes in parallel with R23, with S4 left open.Or, as I mentioned, you could probably omit R5 and leave S4 closed. I didn't test it that way.

author: DanielSpokane
author_rank: Regular Contributor
timestamp: April 06, 2018, 09:29:04 pm
content: Awesome, thank you!I'll wired up and test tonight. I'll post pictures. one thing I was noticing is it looks like the board layout of my 204c and your layout are a little different. Is that a fuse in the upper center of yours above the Transformer with the wires connecting to it? Mine does not have that . If so is that something you added? Thanks again

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2018, 09:33:48 pm
content: Quote from: DanielSpokane on April 06, 2018, 09:29:04 pmThere are a few different versions.Yes, it's a fuse, and it is original.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 07, 2018, 02:16:28 pm
content: A simpler fix would be to just replace the batteries with two 13 volt zener diodes like the 1N4743. The HP 204 will operate from +/-11 to +/-14 VDC so these voltages aren't too critical as long as they are close to the same values but because the power supply circuit is using the batteries to get the identical + and - voltages you have to use shunt regulation like the TL431 previously mentioned or zener diodes to replace them. If you decide on the TL431 regulators and eliminate the LEDs shown in the circuit, check the output voltage to see if that change lowers the output requiring adjusting the resistors.On cleaning the board, I have used vinegar to clean a similar situation in an HP 3586 and it does work well. Make sure you get all the crud off, especially if there's any in the green connector, because when powered, galvanic reaction can cause any remaining crud to continue corrosion. Rinse well but avoid getting the transformer wet and make sure the board is completely dry before you power it up.

author: edavid
author_rank: Super Contributor
timestamp: April 07, 2018, 03:54:02 pm
content: Quote from: ArthurDent on April 07, 2018, 02:16:28 pmI tried that first, but I found the supply voltage was too low to give the best distortion performance. You really need selected 14V zeners (1N5244). Much easier to use TL431s.

author: DanielSpokane
author_rank: Regular Contributor
timestamp: April 07, 2018, 06:25:59 pm
content: Finished it this morning and works perfect!! Thanks again for all your help and schematic and picture. Got it calibrated and runs great. Below are a couple pictures. You can see the new voltage regulator board in one of the pictures. The other is the test setup and the oscilloscope output. Also cleaned up the corrosion, etc. Excited to have this functioning.Thanks again!

author: pishta
author_rank: Contributor
timestamp: November 24, 2020, 03:50:57 am
content: in 3.24 of the service manual it describes clipping of waveform as a sign of low battery voltage. It also states that if 1 peak is clipped check bias voltageTo all that find the batteries are shot, they can be rebuilt using the same (almost) NiCad button cells at 5 stacked for a 6V pack. I found 170mah rated ones for $0.20 a piece (in 10 or more) at:https://www.surplusshed.com/pages/item/R3181.htmlthe OEMs were rated at 225mah IIRC and all mine were shot, leaking and smelled like bad milk! Much cheaper than 2.50/pc on Ebay considering you need 20 of them.

author: edavid
author_rank: Super Contributor
timestamp: November 28, 2020, 04:10:56 pm
content: Quote from: pishta on November 24, 2020, 03:50:57 amI don't think it's a good idea to do this unless you really need battery operation. The primitive charging will cook the new batteries quickly and then you have more cadmium waste to dispose of.