Can a TO-39 diode be replaced with a DO-41? HP428B Repair - Page 1
Forum section: electronics / beginners

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 27, 2022, 07:01:11 pm
content: My HP428B seems to have a faulty SZ1521-7, a 6.8v breakdown (Zener?) in TO-39 can. These are pretty much unobtanium.A 1N5921-BG appears to be a functional equivalent but a DO-41 package.Any problems with this substitute? There is also a 12.1v diode in the unit also a TO-39, but that one has a heatsink. Same question, any problem replacing it with a 1N5927BG?Thanks,Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2022, 08:12:41 pm
content: A different zener diode with suffient power rating should be OK. If in doubt I would prefer a slightly higher power rating.As this is an analog meter there should be no true reference voltage, just some supply regulation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2022, 12:01:28 am
content: Probably. Are there any references fed by it? Can you trim them to compensate? Will the tempco/drift/aging be suitable?I'm guessing those are the motivations behind such a special-sounding item, but I don't know anything about the instrument, you'll have to determine this yourself. And more to the point, whether it will meet your own specifications, if not OEM specifications.Note that axial zeners can be heatsinked by soldering a tab onto the lead(s) near the body.Tim

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 28, 2022, 01:02:59 am
content: Thanks Tim and K.This beast has several zeners between 6.8 and 82.5v including a few spec’ed at 5%!I am going with higher power rating. Thanks for the heatsink tip.Jeff

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 28, 2022, 01:45:17 am
content: A much Higher power rating zener might have a higher knee current, and may not work

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 28, 2022, 03:09:50 am
content: Quote from: gbaddeley on July 28, 2022, 01:45:17 amHmmm. That could be challenging as datasheets for some of these seem unavailable. Thanks for the warning. I won't go for an indiscriminately larger power rating if available.Jeff

author: floobydust
author_rank: Super Contributor
timestamp: July 28, 2022, 03:41:34 am
content: My dig gave me: HP 1902-0013 6.8V@55mA 5% 1.5W zener 1N3785B, TO-7 packageedit:1966 Motorola Semiconductor Handbook

author: Kleinstein
author_rank: Super Contributor
timestamp: July 28, 2022, 07:02:57 am
content: From the circuit one get a reasonable estimate ont he Zener current. For the 6.8 V type it like like some 10 V/168 Ohms = 60 mA minus what the 7 V supply takes. So I would not expect more than 420 mW of actual power dissipation in this case. So even a 1.3 W zener could work , though at the nominal power the zeners do run rather hot and less temperature can help.I would not expect the meter to be used for extended times (e.g. 24/7) as it is tube based - so no need to worry about long time drift / aging of the zener.Zener diodes at 6.8 V are usually quite good, with little leakage before the actual zener voltage. The soft knee and leakage is more a problem with zener voltages below some 5 V or more than some 15 V and when used to limit a signal voltage. As a supply regulator the actual zener type is usually not very critical.The TC of 6.8 V zener diodes is usually quite similar, unless it is a special temperature compensated type (actual 6.2 V zener + 1 forward diode). The original zener looks like a normal type.The 7 V seem to be used for a rather non critical supply, so really nothing to worry about with TC, absolute value and noise.

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 29, 2022, 03:44:31 am
content: Quote from: floobydust on July 28, 2022, 03:41:34 amThanks, Flooby.Need to get me that book for sure. But could you tell me how you cross-referenced the HP part number? You are correct, 1902-0013. The SZ10939-446, HP part 1902-3404 is also bad. The 1N3785B appears obsolete as well but available from a few sources, though NJSemi lists it as 400mw max dissipation?!?!?Jeff

author: floobydust
author_rank: Super Contributor
timestamp: July 30, 2022, 01:56:57 am
content: For fine test equipment such as this, used by the US government, aviation, military - the components are listed in thedatabasewhich is not cached by search engines. For parts in the 1960-1970's it can help for finding a manufacturer, alternate part numbers, tidbits of specs etc. Click on part number only to get meaningful search results.Motorola SZ zener diodes or SZG I've encountered but with no datasheets to be found. HP 1902-3404 Diode Zener, 82.5V 5% DO-7 PD 0.4W TC=+0.082%, from HP8671B. I think it's a 1N3811 TC=+0.090% and no data on Motorola SZ 10939-446 or SZ 30016-446.HP started with the5651gas tube reference rated 86V@1.5-3.5mA for around 0.3W and later changed over to an 82V zener.428B schematicit looks like under 2mA through R106, R107 so a 1N4762A 82V 1W would be ok? You can trim the +272V regulator, it's no big deal as far as exact Vz. Zener tempCo/drift is better and this instrument might be insensitive to rails drifting because it's using AC excitation? But the extra heatsink is suspect then, is it for a cooler running zener or is there higher dissipation. The 12V rail would have nothing for current availability with 2mA in the 82V zener, so I must be missing something.For a given small zener current, I'm not sure what the penalty is for using a larger1N5375B82V@15mA 5W part.I don't see a short or failure that could overload the two zeners, maybe shorted C68 or C63 could do it. Or R105 would be cooked. Did you find out what killed the zeners?

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 30, 2022, 03:01:42 am
content: You are all helping beyond the call of duty.I haven’t gotten very far along into troubleshooting beyond some obvious.Several tubes were bad- ReplacedR117, 118, 119 and 120 were all cracked, some open. All replaced with a bit higher wattage rating.Power supply checks failed, namely -7v (measures at -3v) so CR9 suspect, 12v measures 11.7 (close enough?), 82v measures 68 hence CR10 suspect. Have new Q1 and Q3 not yet installed. A few other resistors, not 105 though, were out of spec and replaced. Also a few caps were way out of spec and replaced. 272v is spot on as adjusted by the pot.Current complaint is can’t zero the meter. It pegs below 0 on all ranges. Meter appears to test OK going full scale at the appropriate directly supplied current (not home at the moment so don’t remember the specific numbers.)So replacing CR9 and 10 seemed to be the appropriate next step, as well as maybe Q1 and 3.Thoughts?Thanks again,Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2022, 06:27:27 am
content: The 11.7 V for the 12 V supply are well good enough. The manual allows for +-1 V. The -7 V should also be within +-1 V, to not super critical, but -3 V is bad. It is still rather unlikely that a 6.8 V zener has drifted to 3 V - usually they fail short.I point I would check is also the difference from the -17 to -7 V, as this should give an idea about the current consumption of the main tube circuit and the zener current.If there is a basic function (though with a wrong zero), chances are high that Q1 and Q3 are working OK.The service manual has quite some information on the alignment. After changing tubes and quite some resistors capacitors this part would very likely be needed.The fluxgate circuit needs some trims to get the correct phase and symmetric drive. A magnetized proble could be a problem too - in the non working state exess current could magnetize the probe.A poorly adjusted drive and phase could cause some offset.

author: jeffjmr
author_rank: Regular Contributor
timestamp: July 30, 2022, 09:12:39 pm
content: Thanks Kleinstein.I can’t confirm basic function because if power is on, the meter is pegged below 0. I demagnetized the head carefully a few times. Meter pegs below 0 even with the probe disconnected.Alignment is certainly indicated but not sure how I can proceed since first step for ALL tests is “zero the meter”. Since CR9 is directly associated with the meter circuit, I thought I’d start there.You did not mention the 82v circuit being so far off (68), CR17.Could you elaborate on what you mean by looking at the difference between the -7 and -17v?Thanks,Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2022, 08:41:39 am
content: The 82 V zener by itself is not important. The question is if the main 272 V supply is within reasonable limits. The trimmer R109 can adjust quite a bit.So if the 272 V are OK the lower zener voltage (82 V) should not matter. So no need to replace the 82 V zener.The 12 V also were OK, so not need to change CR10Much too low a voltage for the -7 V and a wrong value at the -17 V would however be a problem.So changing CR9 may be a good idea.The -17 V voltage is generated from the -7 V with a series resistor. So measuring the voltage over that resistor (R102) gives an idea on what current is flowing.A somewhat off zero is not good, but also not that serious a problem.With basic function I meant getting an about linear change in the output from a change in test current.There are quite a few trimmers for adjustment and with so many parts changed there is a chance one would have to redo the adjustment.The adjustment can effect the zero point somewhat.I would still be reluctant to change much before the -7 and -17 V are OK.It is still not clear if this about a pure adjustment or still a repair. As far as I see it the procedure in the manual assumes a working circuit. So It may be better to first to a few checks before, to find possible defects before changing the adjustment too much.1) Check the amplitude at TP6/TP7.2) Symmetriy of the drive waveform, if needed trim R98.3) The 40 kHz gate/detector drive waveform (e.g. trim L7 for maximum amplitude)The 40 kHz waveform should be resonable sine and about the right amplitude.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 01, 2022, 01:12:24 am
content: 272v is spot on.-7 is at -3.47 and -17 is at -11.3. Measured drop across R102 confirms those numbers with a 10.8v drop or 60ma. R102 is new and 170 ohms, spec is 169 ohms 1%.The diode I have on hand is a 1N5921BG. That should suffice for CR9, correct?Will get to the other checks you recommended soon.Thank you!Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2022, 08:05:14 am
content: The replacement zener should be OK. Before soldering in the new one, one could check the old zener, if it is really bad.With about the right current through R102 it is unlikely that most of the 60 mA flow though Q3. So chances are high that CR9 is in deed bad.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 02, 2022, 04:17:08 am
content: Some progress. Sort of……A new problem is that the manual has errors, even the manual corrections have errors! Anyway,I replaced CR9 as well as Q1, and C24 which appears to have been a wrong part (removed cap was .39uf 10% and the manual calls for .47uf 5%). I now have appropriate voltages on all range settings 1A and above. The manual is unclear about the range setting for checking power supply voltages.I also have some basic functionality, but only on range settings 300ma and below, the polarity is reversed, and expectedly since the alignment is likely way off, readings are off by about 60-70%. Lastly, the zero adjustment pots have no affect.TP7 has the called for sine wave, but J1 pins 2 and 1 have vastly different waveforms and amplitudes. 6v p-p is called for with a ragged triangle wave; one pin has a very ragged triangle waveform of about 5v but the other has a cleaner triangle wave of over 10v p-p.The CR17 82.5v zener which was at 68v is now at 92v. Otherwise the power supply voltages are correct as are the plate and grid voltages on V9 and V10.More testing to follow but still wondering how far I can get while still unable to zero the meter.Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2022, 05:30:35 am
content: The supply voltage should not change much with different ranges. If in doubt the votlage should be in the valid range with all settings.It is off that the CR17 voltage would change so much with changes to the 7 V zener. Getting a voltage well above the 82 V nominal is a bad sign. So replacing CR17 would be a good thing. If 82 V are hard to get it would be possible to use 2x39 V or 3x 27V in series.Getting an inverted polarity indicates serious problem: so the basic function is not there and it looks like something is really bad. So one would have to do an basic adjustment.Nomally there should be relatively little signal at pins 3/4 of the the J1 connector and thus good symmetry at pins 1 and 2. As the feedback loop seem to be not working, it may be better to first start with the feedback diconnected (the jumper in sereis to L6 open). This would avoid interference from this side and make the signals easier to interprete.

author: floobydust
author_rank: Super Contributor
timestamp: August 03, 2022, 06:24:33 pm
content: There's no high current path that could push a new CR17 past 82.5V so I suspect your measurements. For this age of test equipment, I expect all electrolytics to need replacing and using a multimeter to test them is a flawed idea. They can have high leakage current or break down at higher voltages. Or you have lots of hum from C66, C69 (12V), C62B (272V), C12, C65 etc. being low value. The AC+DC can fool your multimeter into giving bad readings, you have to know how it reads DCV with say 200VDC and 50VAC on top of that.You have to be careful the power supply is not simple.The 12V rail zener is shunting current sourced by the head drive amplifier V8. You can confirm V8 cathode current across R99 15k is around 5mA.For the -17V and -7V rails, current is pulled out of the zeners. Either way, a low measured Vz can be due to low load current/failed capacitor/bad tube and not the zener's fault.Low -17V, see what V6 is doing. That would upset Q3 and cause the -7V to be low as well. The rails interact, it was the way to save money in the era.I would look at just getting the drive oscillator working and calibrated. Don't run it with the probe head disconnected, there would be no load for V8. I would use a pair of dummy load resistors at P1-1,2 then center-tap grounded.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 04, 2022, 03:05:34 am
content: Thanks Flooby and Kleinstein. Your help with this is deeply appreciated. Wish I were in a position to respond in kind.I am using a Keithley 2015 and two HP3478A meters for all voltage measurements, a DE-5000 for cap testing pulling a lead and retesting if the number in situ look bad. Also using my Rigol MSO5104 for waveform analysis and rough backup AC voltage, as well as frequency, and the Keithley for frequency. All the electrolytics test good with the DE-5000 (and are Sprague by the way.)I will get to your recommendations in a day or two, Flooby as I finally have a few days off from work.The rails are now good, and the first three oscillator alignment steps are spot on. Also all waveforms shown on the schematic look good but some p-p voltages seem high. My manual is a reprint, and I’m having trouble reading the voltages on the waveform icons. I have an original manual on order.The meter now can be moved off the negative stop with the zeroing pots, but it is very unstable. Also I have basic functionality, meter moving in concert with changes in current. Still reversed polarity, and I have checked the probe assembly carefully, as well as degaussed it.The immediate issue is the next step in the oscillator alignment calls for adjusting L7 for peak reading. But the coils in this unit have no screw or hex slot. The holes are perfectly round. See picture. How does one adjust this coil?Still reading 92vdc on the cathode of CR17. I will put the scope on more points when I get back to it, to check for hum. Also will check the current as you suggested.More to come in a day or two. Thanks again.Jeff

author: floobydust
author_rank: Super Contributor
timestamp: August 04, 2022, 03:51:28 am
content: That's a threaded ferrite slug with a slot. You'd use an old 'TV alignment tool' plastic or I have made them out of scrap plastic and whittle to a screwdriver tip.Be very very careful, the slugs are brittle and easily break. They are usually stuck. Don't use a screwdriver. The metal detunes and the torque is too high and she cracks.I prefer to leave them alone rather than risk breaking one, unless I have the right tool and the slug is not corroded stuck.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 05:55:33 am
content: Measuring just the top side of CR17 measures the 82 and 12 V zener combined. There 92 V is perfectly OK.If the adjustment of L7 is difficult (no suitable tool / stuck - more from some glue than corrosion) one could also just measure the signal and look if it is OK. The important point is not the amplitude, but the phase of the demodulator drive signal to T3 relative to the amplified sensor signal to T2 with a resonable DC current (e.g. 10 mA) at the probe and open loop.The more modern diode bridge should get away with a relatively small amplitude compared to the old tube based demodulator. So the amplitude at T3 should not be that critical.For the adjustment from scratch it may help to do it first without the fedback loop, as the closed loop can cause confusion. So open the jumper for the "Feedback Disconnect". In addtion have the zero adjust trimmers in the center to not get a signal that way.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 04, 2022, 01:02:25 pm
content: Quote from: floobydust on August 04, 2022, 03:51:28 amThere is no slot at the bottom of that hole. There are two very tiny depressions there but no tool will ever be able to get enough purchase there to move it. I’m thinking of desoldering the coil, soaking in some penetrating oil to loosen, alcohol to clean then electronic cleaner/lube and resolder. Overkill?QuoteCould you clarify? Should they be in perfect phase, 180 degrees out, which? What makes for an “OK” signal here?Thanks,Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 02:06:02 pm
content: The depression in the movable core part is small, but with a suitable tool one can usually move it. Even a flat head srewdriver of suitable size is not that bad, though the strength of ferrites can vary. Ideally the change of tubes and resistors around there should not change the resonance. So there is a good chance that one can get away without an adjustment here. If really needed one could alternatively trimm the capacitor part by adding parallel capacitance / choose a slightly smaller capacitor.That would be the method if the trim range is insufficient.The signals T3 and T2 should be more or less in phase or 180 deg. out of phase - which one depends on the polarity of the test-current to the probe. Ideally it needs some DC current at the probe to get a 40 kHz signal at all. The 40 kHz signal strength is about proportional to the current, including a 180 deg. phase change when the current changes the sign.The change in the phase from adjusting L7 should be some +-30 deg. max without the amplitude going down significantly. In addition the proble tuning part has a similar effect: a phase shift and search for maximum amplitude. So for the initial adjustment the maximum amplitude may the better initial target, and adjustment for the phase would be only after the signal path to T2 is checked and tuned.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 04, 2022, 02:23:50 pm
content: Quote from: Kleinstein on August 04, 2022, 02:06:02 pmThis makes me wonder if I don’t have a particularly noisy bench. With the probe as far away from any interference as possible, I still get the 40 khz signal. Perhaps if I get further along in the alignment procedure that will go away.Any ideas on why the needle wavers continuously and the zero pots are so sensitive? I can get the needle off the negative peg, but can’t get it to stay steady on zero, and the slightest movement makes it swing wildly. I have cleaned both pots and checked the smoothness of resistance changes so it is not the pots.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 07:38:25 pm
content: The proble uses a flux gate magnetometer to sense the rather weak magnetic flield of the DC current. From the drawings it looks like parts of the probe are not in the 20 kHz AC flield and thus can hold quite a bit of residual magenitization. Mechanical stress for example can also effect the magnetic properties and this way the residual field.I have once build a flux gate sensor and a nasty source of changing magnetic field was turning the office type chair with some steel in in the back. As the sensor was very sensitive I could even see this some 3 m away. I would not expect the sensor here to be that sensitive, but 1 mA of DC current is still not a strong field.There is a chance that there is still something wrong with the circuit / probe. E.g. dirt on the clamping part of the probe can make the proble more sensitive to external fields and possibly the clamping force.For the adjustment and debugging it would really help to get rid of the feedback loop, as this can introduce quite odd effects and it makes it much more tricky to interprete the signals.The wrong polarity would suggest that there is still some major problem in the closed loop case.As an additional complication the zero adjust adds some 40 kHz signal to the probe. In the open loop case one should be able to use the zero trim pots to counter the residual 40 kHz signal. The scope probe may be as senstivie as the tube base amplifier. So one should be able to directly see the probe signal.If the drive signal is good (symmetric) and there is no residual magnetization, there should be rather little 40 kHz in the zero field case. As the probe can be quite sensitive even little residual magnetization may be visible.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 05, 2022, 06:58:05 pm
content: More progress. Alignment going well. Next step calls for minimum AC voltage by “padding” either of two 6800pF mica caps.I can guess that “padding” means increasing capacitance. Seems an odd, very time consuming method of trying to get to a minimum voltage reading.Can someone confirm or explain?Update on L7. Removed and it was obvious someone else had been into it before. The ferrite was broken and part of it was no longer in the case. I found a place that claims to have them but they were closed by the time I called. We’ll see next week if the have them and the price. So I’m done for this week.Thanks,Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2022, 08:55:49 pm
content: The adjustment with the mica capacitor looks like is for symmetry of the oscillator, mainly to get C53 and C54 with the same capacitance. Unless C53 or C54 have been changed, chances are that the balance is still OK. So this would likely be just a simple check.The 2 equal resistors would check the the voltages are indeed opposite and same amplitude. 50 mV difference with some 18 V of amplitide is quite a tight tolerance.There are kind of 2 resonators with C53 or C54 and the halfs of T5. So they are coupled but should still work best if symmetric.For the L7 trim, there would be the alternative to adjust C27: the missing ferrite part would lower the inductance and would thus need a little extra capacitance in parallel to the existing C27. So one may get away without replacing the adjustable core part.

author: floobydust
author_rank: Super Contributor
timestamp: August 06, 2022, 05:09:14 am
content: Quote from: jeffjmr on August 05, 2022, 06:58:05 pmThe slugs have standard sizes and threads, that can help track one down. I think the powdered iron slugs rust and jam in there.Micrometals threaded coresSurplus Sales of Nebraska

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 06, 2022, 02:52:27 pm
content: That’s not how this one appears to be constructed.The coil is fixed and has half a ferrite fixed around it. The upper half, shown on right in pic sits atop the bottom half and adjusts by rotation to align or misalign with gaps in the bottom half. Again, the top half is the only part that moves, and there is no screwdriver slot or hex wrench slot to facilitate rotation. How that small change in alignment of the two halves can vary the voltage by any significant amount is a mystery to me. The whole assembly is only 16mm in diameter. There are six pins on the bottom. Those depressions in the center are part of the bottom half which does not move, not because it is jammed but because the coil wires protrude through the gaps. It is not meant to move.Now that I know how it works I am going to reassemble hoping the small chip missing from the top half is inconsequential. The metal housing was crimped tightly preventing the top half from rotating. Now that I have corrected that I believe I will be able to adjust it with slight pressure from a pencil eraser.If I measure C53 and C54 and find them matched, can I assume that will result in the minimum voltage asked for?Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2022, 03:18:29 pm
content: The adjustment would be with the small central part of the core of L7. Rotating the halfs of the core again each other would also work, but would be a rather unusual way of adjustment, but could work. With the part missing the inductance likely is a little on the low side anyway. So likely keep them aligned to get the near maximum inductance.L7 together with C27 forms a resonant circuit and it may only take a relatively small change (a few percent) to have have quite some effect on the resonant curve. R116 only gives a little extra damping. If L7 is out too far one can just add to C27 if needed.C53 and C54 are only part of the symmetry - though possibly a major source of tolerance. There is also a little effect of the tube. So it would not be about measureing C53 and 54 out of circuit, but testing in the circuit as described with the 2 additional matched resistors to make it a bridge. Ideally do the test twice with swapping the resistors and this way average out errors in the resistor matching. Still no need for overkill, as the following tubes can also have tolerances and there is an additional adjustment with R98.For the drive adjustment it may help to have pin 3 and pin 4 of the sensor shorted or the link for the feedback loop open to avoid an asymmetry in the probe from current flow. Other initial adjustments for the amplifier would anyway work better with the feed-back link removed.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 06, 2022, 06:34:02 pm
content: So something is way off, as the voltage specified for that step is way off.The page attached is from the wrong manual. My serial number starts with 0995A. HP’s serial numbering scheme back then leaves a lot to be desired. The manual from HP’s archive part number 00428-90003 dated 1970, says it is for serial numbers starting with 995 and above. I assumed this was applicable to my unit.A version of the manual of which I have a very poor, mostly unreadable copy of is 00428-90005 dated 1983 and says it is for serial numbers 0995A and later. 0995A vs. 995?!?!? Older units start with 131 or 601! That is the one I suppose is for my unit. Other discrepancies between instructions in the different manuals are some state disconnecting feedback while the other makes no mention of it or specifically says to reconnect it!!!!!!I compared the alignment pages as best I can and they read practically the same, except the newer manual adds that the voltage expected for the test in paragraph 5-42 is 2.5-3.3v RMS. I am getting 102mv. Steps 5-36 to 5-40 are all good.V5 tests fairly good though not super strong on my tube tester. The 12vdc feeding R49 is my measured rail of 11.7vdc. The 16.2 vdc shown for the resonant circuit is 15.8. But the waveform shown right above it which look like 10v p-p sine, is a jittery .5v p-p sine.Again, the bottom half of the L7 core does not move. The coil is firmly attached to it and wires from the coil pass through the gap on the bottom half of the core. The top half is free to rotate when assembled but how they expected you to rotate it is a mystery. Either way, I don’t believe any setting of L7 is going to raise the voltage from 102mv to 2.5v.Your knowledge of the circuitry leads me to believe you are already referencing the schematic, but if a full schematic is needed I’ll post it.Thanks,Jeff

author: floobydust
author_rank: Super Contributor
timestamp: August 06, 2022, 08:07:34 pm
content: edit: with the powered instrument off, you can inject a sine-wave at the C28 0.01uF cap and use a scope to find the resonant frequency of the V5 input phase network at the grid, to see if it's alive.It's a Ferroxcube pot core 1408PA250-3B7 material. I've never seen an adjustable one before. That would be 14mm OD, AL=250mH/1,000T per core pair.source: old 1964Ferroxcube databookthen it became Epcos, TDK,YageoP14 series. Philips did make them too.Could not find 14mm parts here:Surplus Sales of Nebraska Ferrite: Pot Cores - Slugs & Miscellaneous

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2022, 10:02:16 pm
content: The adjustment of L7 would not be about rotating the whole lower or uper half but moving a small (e.g. some 4-5 mm diameter and maybe 8 mm long) screw like part in the center. The outer part is normally fixed and the parts aligned to match.The correct / wrong trimming of L7 can have a quite large effect. From a crude estimate of the resistance R116 relative to the impedance of C27 the Q of the resonator could be as high as 200 though more likely more like 100. So with the right setting the amplitude could go up by up to a factor of 100 and the waveform would naturally get more sine like.One could look for the resonance of L7 by some external excitation (e.g. to V7 cathode) with the meter not powered. With the broken core chances are the resonance would be a bit higher than the target 40 kHz.Is the signal at C51 about correct - it looks like it should be some 0.5 Vpp ?

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 06, 2022, 10:20:32 pm
content: “edit: with the powered instrument off, you can inject a sine-wave at the C28 0.01uF cap and use a scope to find the resonant frequency of the V5 input phase network at the grid, to see if it's alive.”I assume I would sweep and look for max amplitude? Say 20-60mhz or so?And I see now how that tiny center piece might be threaded to adjust L7. I’ll have to remove it and disassemble again to try to free it.Jeff

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2022, 10:32:32 pm
content: The frequency range of interrest would be some 20-60 kHz, not mHz or MHz. The resonance would show a likely rather narrow maximum in the amplitude.

author: floobydust
author_rank: Super Contributor
timestamp: August 06, 2022, 11:56:22 pm
content: If L7 is in bad shape, it will have low inductance or mangled windings as well. If it's still out of circuit you could try measure its inductance- goal is ~16mH end-end which gives around 40kHz with C27 1,000pF, for resonant frequency.It's a bit odd V5 grid voltage is centered around +16.2V yet the cathode TP4 is less, run off +12V. So I would also check the +16.2V is good. With +ve grid voltage (wrt cathode) you could bias V5 on hard and draw grid current (seen as small voltage drop across R48) which would load the phase circuit tuning tank down a lot. That could be happening too.Does that make sense? If V5 was biased wrong (grid current) it would load the phase tuning circuit down heavily and kill signal amplitude there. V5 plate would have low voltage then as well I think.

author: jeffjmr
author_rank: Regular Contributor
timestamp: August 07, 2022, 01:42:25 am
content: Sorry. Hi meant khz but typed mhz.There is a very obvious peak at 43.6khz. Falls off slowly below but sharply above.Manipulating the top half as best I can only raised the peak to 45.2 which is where it is now. I can’t get it lower. Online calculator says I need 1277nf to get 40khz. But really need to pull the C27 to measure to calc the inductance now. Barring the ability to find a new L7 I guess I’ll try to get an appropriate cap.I mostly understand your point about the grid voltage, Flooby. Still have a lot to learn about tubes, but I can check the volts and current after I get this resonant circuit squared away.Kleinstein, I checked the C51 wave some time ago before the alignment attempt and it looked good though I can’t read the specified voltage. You mention .5vpp so I’ll check that and the other waveforms again also.I’m taking up way too much of your attention but it is sincerely appreciated.Jeff

author: Michael Terrell
author_rank: Newbie
timestamp: April 26, 2023, 07:21:51 am
content: I think that is a Stabistor, which is a special grade of reference diode. HP used them in their lab power supplies. They are temperature compensated, and are basically obsolete now that newer IC based references are available. They are built with a Zener and a normal doide in series to reduce most thermal drift