Author Topic: HP 105B frequency standard (00105-6013) oscillator teardown (Read 8211 times)

ArthurDent · « **on:** December 23, 2019, 04:07:53 pm »

I thought some of you might be interested in what the inside of the HP 105 frequency standard oscillator looks like. The crystal oven assembly (HP 00105-6013) has been used in a few different HP standards from the mid 1960s onward for several years and it is still a pretty good crystal standard. I picked one up cheap and it seemed to work just fine but when I tied to fine tune it using the EFC there was no change in frequency. After looking at the schematic and double checking everything and even substituting it into a good HP 105B, there was no doubt but that the varicap had failed open.

It took me a while to decide to open the oscillator because I thought I might break something and make matters far worse, but I finally got the tools out. It turned out that it is fairly easy to open it up and get to the innards. I took a number of photos that I’m posting here that may help others who wish to delve inside this oscillator. Having taken it apart once I found there are a few things I would do differently that would make disassembly a lot easier. First, take photos before you disconnect any wires or remove any parts so you won’t have to guess where they go when you reassemble the unit.

On the end with the connectors, remove the 4 screws on the sides and the 3 screws on the end and remove the end plate. After taking photos, remove the wires on the connectors that go directly from connectors to the circuit board and prevent you from removing the circuit board. Loosen the 2 small torx screws on the knob and remove it. There are 2 small purple wires that come up through holes in the board and are soldered to terminal posts- unsolder them or you won’t be able to remove the board.

Carefully lift the board up and move it to one side. The 2 purple wires go into a hard foam cover for the inner oven and must carefully be pried up to remove. It is a tight fit and may be hard to remove so be careful. This cover is also the holder for the sensor that the 2 purple wires go to. Take another photo of the wiring going to the inner oven. Unsolder the 2 10K resistors and the 2 inductors from the top of the inner oven. Unscrew but do not remove the 2 hex head bolts from inner oven. Carefully pull on the 2 bolts to lift the oscillator out of the inner oven. The 2 bolts are held in place by 2 clips on the bottom of the assembly.

If you check the photos you will see how the oscillator is assembled in ‘levels’ that each has different parts of the circuits with the crystal on the bottom level. The level separators/circuit boards have yellow tape around them but in the photos I have removed one piece of tape to allow access to the parts. Replacing any parts could be a big problem because there is almost no space between levels. I managed to get the varicap out but it wasn’t easy. Now I have to find a suitable replacement and reassemble the unit. Although the parts list showed an HP part number, the varicap had Motorola markings on it and I haven’t found any info on it yet. Here are the photos.

doktor pyta · « **Reply #1 on:** December 23, 2019, 04:12:51 pm »

Thanks.
P.S. what a coincidence: http://bbs.38hot.net/thread-838238-1-1.html

5065AGuru · « **Reply #2 on:** December 23, 2019, 04:17:31 pm »

Arthur,

I have several dead disassembled oscillators. If you give me your address I'll remove and send you a varactor for free.

Merry Christmas

Corby Dawson

ArthurDent · « **Reply #3 on:** December 23, 2019, 05:33:53 pm »

Quote from: doktor pyta on December 23, 2019, 04:12:51 pm

Thanks.
P.S. what a coincidence: http://bbs.38hot.net/thread-838238-1-1.html

Very interesting! Great minds think along the same lines, right?

The 2 oscillators I have are serial numbers 1231 and 1271 so are pretty close together in when they were made. The one I opened for repair seems to be different than the one you have in parts layout and the crystal can. It does look like the one you have is in really great shape. One thing is no matter how they look the batteries in your unit are probably questionable (at best) after all these years. The HP 105B I got was cheap because the original batteries in the unit had shorted out and burned some resistors in the charging circuit. I found that the original charging for the 2500mah batteries I used for replacement was too high so I reworked the charging circuit so the battery pack wouldn't overheat or overcharge. The specs say 8 hours for battery power and the new batteries and charging circuit give me 10.5 hours and the unit remains cool.

I replaced the 32V zener in the power supply with a 30V zener and replaced the 1.6 and 5 ohm resistors in the charging circuit that had burned up with 2 10 ohm resistors mounted on the rear panel. It might take longer to recharge to a lower level but it should last longer and run cooler.

jfphp · « **Reply #4 on:** December 23, 2019, 06:42:36 pm »

You can find the schematics diagrams in the service chapter of the 5065A first edition, 5245M, 5248M... manuals and the parts list is in the parts chapter but there is no layout drawings of the different stages. Main problem is the aging of the quartz after about 25 years of continuous running (!) : you will exceed the tuning range of the screwdriver adjust capacitor and playing with the other caps of the oscillator will degrade the phase noise.

ArthurDent · « **Reply #5 on:** December 24, 2019, 12:34:16 am »

Quote from: jfphp on December 23, 2019, 06:42:36 pm

You can find the schematics diagrams in the service chapter of the 5065A first edition, 5245M, 5248M... manuals and the parts list is in the parts chapter but there is no layout drawings of the different stages. Main problem is the aging of the quartz after about 25 years of continuous running (!) : you will exceed the tuning range of the screwdriver adjust capacitor and playing with the other caps of the oscillator will degrade the phase noise.

Not necessarily true. The present mechanical adjustment left on my oscillator that was made in 1967 (over 50 years ago) is from +6hz to -9hz so it will take a very very long time for it to age out of range.

Leo Bodnar · « **Reply #6 on:** December 28, 2019, 01:57:24 am »

What does ADEV for 105A/105B look like for 1 to 10,000 seconds?
Leo

5065AGuru · « **Reply #7 on:** December 28, 2019, 06:40:01 am »

Leo,

I just finished plotting 20 of the 0105-6013 oscillators used in the early 105 as well as a few other instruments.

Most were in the low to mid parts in 10-12th out to 1000 Sec, although a few did in the high 10-13ths.

I have found that selecting from 10811 oscillators will usually give better performance at least for the really good ones.

My reference Quartz units are a selected 10811, a selected FS1200, and an HP 106A.

Cheers,

Corby

bob91343 · « **Reply #8 on:** December 28, 2019, 07:43:07 am »

Generally, a varicap is just a diode that has been measured for capacitance. I have tested 1N4004 for instance and it has the same characteristic. I would not hesitate installing a small diode for frequency control.

To be safe, I'd first measure its capacitance with various bias voltages. If the curve seems about right, I'd use it.

Most good bridges include provision for bias, and also use small enough excitation so as not to cause the diode capacitance to shift much, or ever forward bias.

Leo Bodnar · « **Reply #9 on:** December 28, 2019, 10:03:17 am »

Very interesting, I wonder how phase noise looks like for these. It would be a good exercise to redesign the sustaining amplifier using modern transistors and techniques that were not available at the time, while keeping the crystal and environmental design intact.
Do you have a way of measuring crystal parameters directly?
What is the working temperature of the oven? 105A/B were designed in late 1960's, while SC-cut crystals were not produced until early-mid 1970's.
Cheers
Leo

Quote from: 5065AGuru on December 28, 2019, 06:40:01 am

Leo,

I just finished plotting 20 of the 0105-6013 oscillators used in the early 105 as well as a few other instruments.

Most were in the low to mid parts in 10-12th out to 1000 Sec, although a few did in the high 10-13ths.

I have found that selecting from 10811 oscillators will usually give better performance at least for the really good ones.

My reference Quartz units are a selected 10811, a selected FS1200, and an HP 106A.

Cheers,

Corby

German_EE · « **Reply #10 on:** December 28, 2019, 10:37:34 am »

Designed and built when the letters H and P meant something

Thanks for this!

ArthurDent · « **Reply #11 on:** December 28, 2019, 03:15:10 pm »

Quote from: Leo Bodnar on December 28, 2019, 10:03:17 am

... What is the working temperature of the oven? 105A/B were designed in late 1960's, while SC-cut crystals were not produced until early-mid 1970's. ...

The temperature marked on the crystal is also marked on the outside of the assembly. Here's the one I'm working on.

Leo Bodnar · « **Reply #12 on:** February 19, 2020, 03:30:51 pm »

I got HP 105A that has a full scale reading (100+) on the meter in "OVEN" position even after a good hour warm up.

The output frequency is 4.9999981MHz

Does this sound like a faulty thermal sensor and overheating oven/crystal?

Leo

5065AGuru · « **Reply #13 on:** February 20, 2020, 06:29:52 pm »

Leo,

I'd give it a couple hours as the coarse oven might be out of whack.

The coarse oven is just used during warmup to shorten the time.

The unit will work fine without it but just takes a while to warm up.

If after extended warmup you can adjust to 5Mhz exactly I'd say it's working.

Then if your oven reading is still 100+ I think there is a resistor to select to put it on scale.

Cheers,

Corby

Leo Bodnar · « **Reply #14 on:** February 20, 2020, 07:31:37 pm »

Thanks, Corby,
I am replacing tantalum caps on the heater controller PCB with ceramic X7S ones (they are not capaciatnce-critical anyway.)
The thermal fuse for ~110V fast heater was blown as well.
I'll give it another go.

I don't know the oven temperature, it does not seem to be an SC-cut since it's dated early 1970's. If it works, I will sweep the oven temperature to find the turning point.

Leo

5065AGuru · « **Reply #15 on:** February 20, 2020, 11:42:05 pm »

Leo,

If the coarse heater fuse was blown I'd just leave it out and live with an extended warmup. Since most of these stay on 24/7 that usually is not a problem.

You are right, it in not an SC cut. Most probably AT which you can verify by plotting the warmup curve.

Cheers,

Corby

Leo Bodnar · « **Reply #16 on:** February 24, 2020, 04:55:53 pm »

I stuck a thermocouple inside the foam near the Aluminium can containing XO - under the temperature controller.
I am getting 60-61C which is a bit too low I think. There is no turnover temperature recorded on th outside of the enclosure and I don't want to rip it all apart just yet.

Temperature controller has some weird bridge oscillator design that I have not seen for ages. I'd like to model it before I do any more modifications.
One of my suspicions is that the heater is possibly damaged. Its resistance is 100R - I am not sure what its nominal resistance is.

Leo

5065AGuru · « **Reply #17 on:** February 24, 2020, 06:05:17 pm »

Temp is marked on the crystal. Two that I have opened show 68.6C and 67.5C.
I'll see if I can measure the two heater windings resistance.

Cheers,

Corby

5065AGuru · « **Reply #18 on:** February 24, 2020, 06:55:24 pm »

Both oven windings (assuming both fuses are good) can be measured without opening the unit.
The fast warmup heater (two pins on the side) reads around 240 Ohms.
The normal heater (read between the M and +24V pins) reads around 102 Ohms.
So if your normal heater was the one that read 100 Ohms it OK.
There is a small hole on the two pin end the allows you to reach the pot that adjust the temperature.

Cheers,

Corby

Leo Bodnar · « **Reply #19 on:** February 24, 2020, 07:47:49 pm »

Thanks, Corby, for taking the time to measure your units.

Looks like the heater is not faulty then. I suspected it could have been two 100R in parallel and one winding open-circuited.

The NTC seems fine - it drops its resistance with increasing temperature.

The problem then might be either in the faulty or misaligned 200R pot (it was set for just 10R) or components drift.
I am still curious to understand why HP used Wien bridge oscillator as a temperature controller. The only reason I can come up with is safety - if it stops oscillating then heater cuts off (final stage is AC coupled.)

Leo

tkamiya · « **Reply #20 on:** March 19, 2020, 09:56:29 pm »

My recently acquired HP105B also stays at full scale in OVEN setting. I was told by someone the spec for zener diode changed between earlier and later oscillator modules, so that it needs to be checked. I have not done that. It's working fine as is. Sure enough, there are signs someone has replaced components in power supply module. It's very possible component values are off.

ArthurDent · « **Reply #21 on:** March 20, 2020, 01:39:34 pm »

As I mentioned in post #3 I made changes in the charging circuit in my 105B. I replaced the original shorted batteries with smaller sub-C cells. The new batteries powered the 105B nicely but improvements in battery chemistry in the last 50 years created charging problems for me and the new batteries were also overheating because of a too high charge rate. To correct this I replaced the zener diode with a lower voltage one and replaced the two resistors that determine the charging current. It has been working fine since then. Reread my previous post to see what I did.

tkamiya · « **Reply #22 on:** March 20, 2020, 01:46:19 pm »

Oh, that was YOU! I knew I saw it somewhere..... and I didn't realize it was in this thread.

I am thinking of doing exactly the same.

uncle_bob · « **Reply #23 on:** February 03, 2021, 02:48:45 pm »

Quote from: 5065AGuru on February 20, 2020, 11:42:05 pm

Leo,

If the coarse heater fuse was blown I'd just leave it out and live with an extended warmup. Since most of these stay on 24/7 that usually is not a problem.

You are right, it in not an SC cut. Most probably AT which you can verify by plotting the warmup curve.

Cheers,

Corby

Hi

Just to keep things straight ... The crystal is most likely a BT cut. That was HP's "secret weapon" back before the SC.

Bob

Xtremexp · « **Reply #24 on:** March 14, 2021, 05:47:17 am »

Hello forum members,

Here I am with HP 105 unit. I picked up as I love to collect such engineering marvels. it was last calibrated in 1998 by hp, the calibration seal was still intact. I powered up, as people reported, the meter goes above on OVEN, 18.7V and 5MHz switch position, the meter shows ~30 on both 1MHz and 100KHz.

Only 5 MHz output is working. the frequency is 5MHz (I do not have precision device to measure it to full accuracy of the 105 device) but 1MHz and 100KHz output is not there.

I opened up the unit, it has ovenized oscillator 10811A (which is newer than the pictures above posted, i think). I am wondering what would be first step to look into to find out the issue on 1MHz and 100KHz output. Any suggestion at this point is helpful.

Regards,
Xtentec

tkamiya · « **Reply #25 on:** March 14, 2021, 05:57:41 am »

These days, an only output frequency that will be useful to you will be 10MHz. For that, you'll have to tap off of the output of 11801 and bring it outside through buffer amplifier of some kind.

As to other frequencies, first thing I do is to unplug cards one at a time and closely inspect, then plug it back in. That it self fixes a lot of problems.... Even though they are all gold plated, contacts can be iffy after a decade or two.

Mine had a failure in voltage regulator transistor and filter capacitors. So it makes sense to check the power supply voltages as well.

BE CAREFUL! HP105B has mains voltage on front panel near battery charging switches! I learned it the painful way.

uncle_bob · « **Reply #26 on:** March 14, 2021, 01:50:29 pm »

Quote from: Xtremexp on March 14, 2021, 05:47:17 am

Hello forum members,

Here I am with HP 105 unit. I picked up as I love to collect such engineering marvels. it was last calibrated in 1998 by hp, the calibration seal was still intact. I powered up, as people reported, the meter goes above on OVEN, 18.7V and 5MHz switch position, the meter shows ~30 on both 1MHz and 100KHz.

Only 5 MHz output is working. the frequency is 5MHz (I do not have precision device to measure it to full accuracy of the 105 device) but 1MHz and 100KHz output is not there.

I opened up the unit, it has ovenized oscillator 10811A (which is newer than the pictures above posted, i think). I am wondering what would be first step to look into to find out the issue on 1MHz and 100KHz output. Any suggestion at this point is helpful.

Regards,
Xtentec

Hi

The first thing you will need is a manual for the later version of the 105. Most of what's out there is for the earlier version. With the manual, working out what's what is do-able. Without it, you will spend a lot of time tracing out circuits and guessing what they do.

Bob

Xtremexp · « **Reply #27 on:** March 14, 2021, 05:45:45 pm »

Thank you for valuable feedback and alert on mains voltage.

For sure I will check the power supplies first. I will keep updating as I find about more about this unit.

Thanks!!

5065AGuru · « **Reply #28 on:** March 15, 2021, 12:43:41 am »

Xtentec,

Hi, You have a nice unit.

The 1Mhz and 100Khz outputs are derived from self starting regenerative dividers.

They are in series so if the 1Mhz divider is inoperative the 100Khz will not get an input.

Best to look in the manual and troubleshoot the 1Hhz divider.

Cheers,

Corby

Xtremexp · « **Reply #29 on:** March 15, 2021, 01:29:19 am »

Quote from: 5065AGuru on March 15, 2021, 12:43:41 am

Xtentec,

Hi, You have a nice unit.

The 1Mhz and 100Khz outputs are derived from self starting regenerative dividers.

They are in series so if the 1Mhz divider is inoperative the 100Khz will not get an input.

Best to look in the manual and troubleshoot the 1Hhz divider.

Cheers,

Corby

Thank you for the clue. 1MHz divider is also not working. I probed the power supplies, the 24V rail is 29V and same for 18.7V rail. I suspect that 29V on 18.7V rail brought down all modules? It is scary. I found possible suspect (Q8 and zenor diode) on the A5 board. lets see how the fix goes. Technically I am still wondering even if the 18.7V rail is 29V, the 1MHz module should work as I see the 1MHz module has it's own internal voltage generation.

For guys who is interested on the unit. I attached picures of my unit. It's really nice unit. I love it.

5065AGuru · « **Reply #30 on:** March 15, 2021, 03:23:10 am »

Take a look at the correct manual.

The version you want is at:

http://ftb.ko4bb.com/manuals/132.1.207.129/HP_105B_Late_Model_Quartz_Oscillator_Manual.pdf

The power supply should be +18.7V.

Cheers,

Corby

5065AGuru · « **Reply #31 on:** March 15, 2021, 03:31:32 am »

Bob,

Per your guess:

"Just to keep things straight ... The crystal is most likely a BT cut. That was HP's "secret weapon" back before the SC."

The crystal cut in the older version oscillator is AT!

The BT cut was used in the HP 10544A

Cheers,

Corby

tkamiya · « **Reply #32 on:** March 15, 2021, 09:42:59 pm »

I just received a second HP-105B. Transformer inside is dated 5/31/89, so it's probably close to the last version. If you need me to measure or take picture of something, let me know. I'll be glad to help.

This thread is somewhat confusing to me as the ORIGINAL poster had a unit with older oven at 5MHz and latest discussion is for HP-10811. I think they key will be if the oscillator is working or not. Everything else, you can repair....

http://hparchive.com/Manuals/HP-10811AB-Manual.pdf

As you can see on page 8, it takes two input. 24V for pin 14, 12V for pin 3. With just that, you should get a signal at 10MHz on pin 1. Please see respective ground connection on that chart. You CAN buy 10811 variant fairly cheaply but I *think* ones used on HP105B was hand selected version. Bob can probably attest to it.

**update**
I just re-read the second OP's post. He has 5MHz out, so at least OCXO is working and divide by two circuit is working. Phew.
\**update**

Likely, the rough control on the front panel won't work. On mine, fiberglass shaft burned out and metal blade fell out. I had to remake mine. I have a dimension for that somewhere on this forum.

I'll be working on fixing few thing on my new one. Good luck with yours.

Uky · « **Reply #33 on:** March 16, 2021, 01:44:35 pm »

BTW:

The last version of the HP105B (that have a yellow LED for AC interruption) featured a redesigned PSU board (A5) using "modern" IC's such as 3-terminal regulators. I uploaded an addendum to the manual describing the Oscillator (10811-version) as well as the A5-board. It should have ended up in KO4BB's "HP105"-directory but the addendum ( a .ZIP-file) with two .PDF's inside instead is in the root (?) folder on his server. This .ZIP-file seems to be the only one that also has the schematic drawing for the A5 board in it.

Xtremexp · « **Reply #34 on:** March 23, 2021, 04:31:16 am »

An update on my unit. I fixed the 18.7V rail by replacing the transistor Q8 on the power supply board. Now I have outputs in 5MHz, 1MHz and 100KHz.

My next goal is to add 10MHz output on the system as the oscillator has 10MHz output. Any idea of buffering the output to get 10MHz signal to drive 50Ohm load is welcome.

Also if anyone needs help on probing signals then please feel free to message me.

Thanks all.

tkamiya · « **Reply #35 on:** March 23, 2021, 04:58:41 am »

I just did that to my second HP105B.

I did it by tapping off 10MHz directly from a socket where 10811 (OCXO itself) socket. It then goes into a buffer amplifier with relatively high input impedance so it won't affect the original signal level to other circuit match. An output connector (SMA) was placed 1 1/4 down from center of the coarse tuning hole.

The amplifier is THIS circuit from John Miles:
http://www.ke5fx.com/norton.htm

Please read second from the last paragraph under "Source Material and..." He has words on circuit loading and how to deal with connection to HP10811.

I have a PCB for it if you'd like.

Xtremexp · « **Reply #36 on:** March 23, 2021, 05:01:13 am »

Quote from: tkamiya on March 23, 2021, 04:58:41 am

I just did that to my second HP105B.

I did it by tapping off 10MHz directly from a socket where 10811 (OCXO itself) socket. It then goes into a buffer amplifier with relatively high input impedance so it won't affect the original signal level to other circuit match. An output connector (SMA) was placed 1 1/4 down from center of the coarse tuning hole.

The amplifier is THIS circuit from John Miles:
http://www.ke5fx.com/norton.htm

I have a PCB for it if you'd like.

Sure I am interested in the PCB and curious to see how your unit looks like. I sent PM to you.

tkamiya · « **Reply #37 on:** March 23, 2021, 05:30:15 am »

I don't have inside picture. I don't want to open it as it is going through an aging cycle. But I can give you an outside image. The modification for output port is right below the coarse adjust hole. I mounted a bulkhead SMA 1 3/16" away. Center to center. Looks pretty stock doesn't it? Location of this is kind a tricky because there is a "lip" inside.

What I have done is, on a PC board directly behind the OCXO, tap off 18.7V and feed it into 7812 with 0.1uF in the usual way. Mount the 7812 onto an aluminum bracket (right next to the existing one). Tap off 10MHz from pin 1 of the OCXO through 100pF capacitor. Ground goes to the nearest ground. That goes into a thin coax. That coax connects to the input side of the buffer amp. (J1) J2, the output goes to a short coax harness to a connector on the front panel.

I removed the Nicad pack completely. I have everything connected to a big UPS. I don't need per-unit backup. I mounted the buffer amp in that space using studs.

Xtremexp · « **Reply #38 on:** March 23, 2021, 05:43:44 am »

Quote from: tkamiya on March 23, 2021, 05:30:15 am

I don't have inside picture. I don't want to open it as it is going through an aging cycle. But I can give you an outside image. The modification is right below the coarse adjust hole. Looks pretty stock doesn't it? Location of this is kind a tricky because there is a "lip" inside.

What I have done is, on a PC board directly behind the OCXO, tap off 18.7V and feed it into 7812 with 0.1uF in the usual way. Mount the 7812 onto an aluminum bracket (right next to the existing one). Tap off 10MHz from pin 1 of the OCXO through 100pF capacitor. Ground goes to the nearest ground. That goes into a thin coax. That coax connects to the input side of the buffer amp. (J1) J2, the output goes to a short coax harness to a connector on the front panel.

I removed the Nicad pack completely. I have everything connected to a big UPS. I don't need per-unit backup. I mounted the buffer amp in that space using studs.

No need to open it now. It looks amazing!! such a beauty

uncle_bob · « **Reply #39 on:** March 23, 2021, 01:02:58 pm »

Hi

Corby has done a few 105's, 5065's, and others with the Clifton Labs buffer. I have one example and it seems to work fine.

https://www.dxengineering.com/parts/dxe-z10000u-kit

Power it off the 18.7V rail, attach the input to the 10811's output. Bring the output to a BNC somewhere ( either put in a new one or re-purpose one that's there).

Bob

tkamiya · « **Reply #40 on:** March 23, 2021, 03:56:42 pm »

I have a few Clifton Lab buffer amp here, too. They work well. They are especially nice on low level signal.

An only limitation I thought undesirable for my need was its output level. At around 5 to 6 dBm it starts to saturate. If it could do just a couple db more, it would be a perfect ready made solution.

Uky · « **Reply #41 on:** March 24, 2021, 12:38:49 pm »

I once made (or rather: copied) a low noise Frequency Doubler, 5 -> 10 MHz design referenced by NIST. I think the original design was found on Bruce Griffiths home page.
See attached picture. I have schematic, layout (Cadence OrCAD/Allegro ".BRD") and gerbers should anyone be interested.

Xtremexp · « **Reply #42 on:** April 12, 2021, 03:44:46 am »

Thanks tkamiya for the amplifier buffer. I modified same way that tkamiya added BNC on the front panel. I added the PCB between the power supply and the 1 MHz divider space with existing PCB support. Now I have 10MHz source.

Thanks all for the support!!

tkamiya · « **Reply #43 on:** April 15, 2021, 07:55:36 pm »

Looks almost stock doesn't it?

As you already know, I have two. First one drifted quite a bit when I bought it and it took good 6 months to get to the ultimate stability. Second one has been running for few months but it is still drifting. (frequency goes lower and lower....)

I hope you don't have the old battery elements themselves in there. Those are NiCd. They can leak and can cause severe corrosion.

trobbins · « **Reply #44 on:** September 09, 2022, 12:49:29 am »

I'm part way through repairing a 00105-6013 crystal oven assembly used in a HP5065A.

It's first identifiable fault was leakage to ground on the 115Vac fast warm-up heater which is detected by IR measurement of mains input to earth, and also tripping a safety ELCB. But that heater is not pedantically needed, so the next fault was magic smoke from the connector end, which led to a few other faults being identified. The main problem appears to have been decades of long term high temp operation, perhaps including overheating, such that the internal foam insulation has hardened and charred in places, and insulation of wires in the ribbon-cable that passes between ends had degraded. Repairing the poor interconnect wires, as well as two shorted diodes used in the voltage doubler that provides the heater temp feedback, and a charred resistor that may have seen a bad interconnect, showed that the circuitry was still working, but ad-hoc conduction from the normal heater to ground meant there was still a fault situation - and the internal can had to be excavated from the hardened foam coffin to explore further. Extracting the can showed the heaters were resistance wire wound on the outer surface of the can, and that insulation tape had degraded and the charred foam was provided a leakage path to the can. Removing charred leakage paths has allowed the 100 ohm normal heater to return to a high resistance to earth (still some degraded insulation but no overt conduction paths) even when powered - enough to warrant a return to service, so I'm just about to reconstruct the ribbon wiring and somehow centre the internal can back in the outer assembly and retest to see if all functions have recovered. There is still a risk that the heater could form a low resistance path to ground again, in which case the heater could plausibly be stripped off and an array of fixed resistors affixed to the can as a psuedo heater.


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Author Topic: HP 105B frequency standard (00105-6013) oscillator teardown (Read 8211 times)

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