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On Tue, Mar 22, 2022 at 12:04 AM, Jeff Dutky wrote:

thanks for the suggestions, I will try making some measurements with the cover
off. Is it safe to operate this scope with the cover off, or do I need to
provide extra cooling on the hybrids?

I looked at 2445 SM and there is no warning about constant air flow unlike the 2440 which needs constant air flow for the 500MS/s CCD samplers (I have a 2440 and CCDs get hot). I wouldn't be worried about operating 2445A without the cover, especially if it has good space around it for natural air flow. Even on 2467B with a lot more heat working on the scope without forced air cooling is fine.

...

I"m a little confused by the construction of the attenuator, however: there
are only three stages to the attenuator, the first switches between 1 MΩ and
50 Ω input, and the other two are both marked "÷10", but I'm clearly able to
select attenuation that is not a simple multiple of ten. How can this
attenuator give me all the 1-2-5 ranges? Is there switchable attenuation
happening inside the pre amp hybrid?

Yes, this is described on page 3-15 bottom right paragraph.

Ozan

On 3/21/2022 10:43 PM, James55 wrote:

Great reply Harvey.

Made me smile. Thank you.

Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards

I take it you mean via the connectors?

and the chassis.

Do you mean disconnect any ground wires?

That's because you keep removing parts. Stop doing that unless you can justify removing them

Fair enough. The two diodes had a leg lifted in order to help find the last 'good point' with the voltage readings. All components are back in now other than the +15v rail, which still has a dead short lurking somewhere.

So, voltages...

The CR1721 +65v lines are reading 173.5v and 86 volts

In the hope that it may illuminate the situation, I just did a series of quick measurements 'across' the components of the +65v section, which are as follows;

R1722 - 128.3v - good since the input of U1724A should be high impedance
R1723 - 18.7v -
VR1722 - 20.2v - GOOD
VR1724 - 23.3v - normaly 56 volts,
C1724 - 23.3v - way low since the input of U1724A should be high impedance
CR1724 - 0.25v - GOOD
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v - way too low, possibly because +65 is low
VR1726 - 9.2v - GOOD
R1727 - 26v
R1725 - 5.8v - bad, since the input of U1724A should be high impedance
C1725 - 3.39v - bad, since the input of U1724A should be high impedance
R1726 - 16v - understandable since +65 is low
CR 1723 - 0.573v - GOOD
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v - way bad
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

As has been mentioned, 26 volts across 3 ohms would be a current drain of 8 amps.? It's not.? I think that (as has been mentioned), R1734 is open.

That leaves the only way to get any current to the +65 a path through VR1722 (allows 4 ma at worst), and then a possible path through CR1723 through Q1736.

I'd also suspect the op amp U1224A.

Harvey

+65v test point currently 25.32 volts (obviously)

There is lots I wish to add in response to the earlier post however it is very late here.

I'll get back on it tomorrow evening.


James

James,

There is a serious anomaly right at the beginning of your voltage tracing. Pin 8 of U1724 should be at +22V, regulated by the Zener VR1725. It looks very much like either VR1725 or U1724 is bad. The voltage on pin 2 of U1724 should be around 9V even with the low value of the +65 supply, you could confirm that the voltage on VR1726 is 9V. It could be that the op-amp is drawing a large input current due to the input voltage being more positive than the supply voltage so this isn't an immediate indication that U1724 is bad (somebody please correct me if I have this wrong!).

Keep at it and don't get too tired, you will probably save time in the long run.

Best wishes,

Roger

On 2022-03-22 10:37 a.m., James55 wrote:

Hi Mark,
... for sure I was tired last night, but hey!
If you have a 466 handy then that could well be a game-changer.

I have a 466 here but it died while I was testing it out. I haven't opened it up to investigate yet, but it needs to happen.

Don't mind helping out other 466 owners to the extent I can... Have never repaired one before, though. I follow threads like this one for hints. :)

--Toby

Thanks for your supportive words.
There has been some excellent advice throughout this thread, which has transformed my understanding of general fault-finding as well as certain things that I had read about but never really worked on in practice, such as op-amp feedback circuitry.
If I'm honest, there were times when I thought of giving up on this scope, however finding the crossed transformer wires was the missing part of the picture, and now I can see no real reason why this can't be repaired.
I'm very comfortable with electricity and electrical properties even if I don't yet have the necessary understanding of how certain components interact when placed in placed within particular circuitry. But that is coming.
I'm not looking to be a design engineer, however the experience and capability to recognise circuit weak-points is where I would like to get to, so projects such as this scope are actually a Godsend.
This scope will be repaired.
All the best.
James

Hi Mark,

... for sure I was tired last night, but hey!

If you have a 466 handy then that could well be a game-changer.

Thanks for your supportive words.
There has been some excellent advice throughout this thread, which has transformed my understanding of general fault-finding as well as certain things that I had read about but never really worked on in practice, such as op-amp feedback circuitry.
If I'm honest, there were times when I thought of giving up on this scope, however finding the crossed transformer wires was the missing part of the picture, and now I can see no real reason why this can't be repaired.
I'm very comfortable with electricity and electrical properties even if I don't yet have the necessary understanding of how certain components interact when placed in placed within particular circuitry. But that is coming.

I'm not looking to be a design engineer, however the experience and capability to recognise circuit weak-points is where I would like to get to, so projects such as this scope are actually a Godsend.

This scope will be repaired.


All the best.

James

James,

Your typo could have been when you were tired. The corrected voltage across the 3 ohm resistor is much closer to what it should be. It voltage across it seems low now. If you want, I can get my 466 open and help you if I can. All the great posts from others has been great to read. I know all of us would like to know what you find is wrong. Thanks for confirming about the other three parts I asked about yesterday.

Mark

Jerome,

The procedure for adjusting the variable osc. is:
"Set the COARSE ZERO control about 10 degrees above right horizontal,
and the FINE ZERO control at full capacitance, index horizontal to the
right. Set the internal screwdriver control, C2 (see Figure 5-3), at
mid-range, slot vertical. C2 is mounted on the FINE ZERO capacitor. Set
the variable oscillator also to 140 kc by adjusting the tuning slug in
T1 (Figure 5-3). The variable-oscillator signal appears at the GUARD
VOLTAGE terminal."
All that said, I have found several inconsistencies between Tek manuals
for the same device, but of later printing before.

The Factory Calibration calls for 20 degrees. But remember that this is merely a preliminary setting before further tuning of T1 is done with the S-30. If the latter works (meter can be zeroed in the iterative process) then the initial setting (degrees) was simply OK.

As for the "bounce", it is definitely not normal, as at times it can
become quite savage, pegging the needle at either end of the scale.

If you perform the the procedure in message 126515 to tune T1 then you don't need the display meter at all. After that you can pay attention to the meter problem.

Albert

Hi chaps.

Ozan,

Having a quick look, R1724 is 324mV

Q1732 did fail before but was replaced. It currently tests ok using diode mode on the DMM and when replaced just now with a 2N5551 the Base reads 25.90v with the Emitter at 25.48v.
Q1734 is a new TIP31C following my carelessness and Q1736 also tests fine in diode mode.

Mark,

R1734 is difficult to reach but it reads 4Ω in circuit and measuring again, reads 0.062v, NOT 25.25v (not sure what happened there?)
CR and VR 1718 don't show any obvious failings, whilst C1737 isn't shorted and measures correctly at 3.6?F


On a different tangent, and I know this is slightly off the page of study and not sure if this will help, however, the 140v TP reads around 98v, yet when lifting the cathode of VR1718 the Test Point goes up to 160v. The +65v remains the same at about 25 volts. VR1712 is not faulty as it's cathode reads 179.9v. Anyway, just thought I would add that.

I did grab a selection of diodes yesterday whilst in town, just in case...

bonjour cher Monsieur

My 130 LC was from LLNL salvage yard 1980s, cost $0.30/ lb.

Have the test fixtures UHF...binding post.

Still in cal and working fine

I can only mention a few generally tips...

Never recap a vintage instrument without a good reason as,the original caps (except lytics) are probably OK and may be selected or special type.

Find the exact serial numbers manual, revisions are misleading

Checking tubes on a high,quality tube tester, Gm not just émissions

Clean switches contacts and tube sockets, old tin or silver plated are oxidized

Beating near zero is normal

Especially useful to check scope and problems inputs capacity

Classic TEK design!

Bon courage


Jon

Hi Jeff,

My guess is that the in-between attenuations are indeed done fully electronically. It is hard to maintain bandwidth if you vary gain by large factors all-electronically, but factors of a couple are eminently doable. But I haven't looked at the schematics yet, so I can't confirm that's what's done here. But from the info you've given, it's very likely.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

Tom, Ozan,

thanks for the suggestions, I will try making some measurements with the cover off. Is it safe to operate this scope with the cover off, or do I need to provide extra cooling on the hybrids?

Tom, I DO have the schematics, and I think I see the cap and resistor that you are talking about: just after the attenuators there are C100, marked ".001" on the schematic, and R100, marked "470K". I would be very happy to discover that this is just a bad capacitor.

I"m a little confused by the construction of the attenuator, however: there are only three stages to the attenuator, the first switches between 1 MΩ and 50 Ω input, and the other two are both marked "÷10", but I'm clearly able to select attenuation that is not a simple multiple of ten. How can this attenuator give me all the 1-2-5 ranges? Is there switchable attenuation happening inside the pre amp hybrid?

-- Jeff Dutky

On Mon, Mar 21, 2022 at 11:04 PM, Jeff Dutky wrote:

...
Ch 1 (otherwise the scope seems to be in excellent working order). I am
testing the bandwidth using a Tek 067-0532-00 calibration fixture (constant
amplitude signal generator) for the range from 65 MHz and up, and a TinySA for
the range below 65 MHz. Channel 1 rolls off at about 7 MHz (yes, SEVEN, single
digits, not a typo) and then has a gradual decline in sine wave amplitude,
while channel 2 is essentially flat up to about 75 MHz than has a much steeper
decline up to 125 MHz where it then levels out to much more gradual decline.

Hi Jeff,
7MHz is pretty low so the signal degradation should be easily visible. Why not check for the waveforms at test points 6/7/8/9 before and after the preamp as shown in the schematic before swapping hybrids?
Ozan

Hi Jeff,

I don't have the schematics in front of me, but look for a defective cap that bypasses a large-valued resistance between the attenuator and the input of the preamp. The resistance is there to provide some protection against overloads. The bypass is to maintain frequency response. If that cap goes completely open, the bandwidth is usually well below 7MHz (perhaps even audio bandwidth). It's worth a shot, anyway. (Easiest way to test is simply to put another cap in parallel with it -- say, 1nF or so. If your bandwidth goes way up, you've found your culprit.)

Good luck!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

I have a 2445A (w/ DMM, acquired mainly for the rare front cover) that I am now evaluating, and it looks like there is a significant bandwidth problem in Ch 1 (otherwise the scope seems to be in excellent working order). I am testing the bandwidth using a Tek 067-0532-00 calibration fixture (constant amplitude signal generator) for the range from 65 MHz and up, and a TinySA for the range below 65 MHz. Channel 1 rolls off at about 7 MHz (yes, SEVEN, single digits, not a typo) and then has a gradual decline in sine wave amplitude, while channel 2 is essentially flat up to about 75 MHz than has a much steeper decline up to 125 MHz where it then levels out to much more gradual decline.

I have separately verified the output of the '0532' using my 2465, and the output appears to be constant amplitude up to at least 400 MHz (I was surprised to see that my 2465 does not start rolling off till 400 MHz, when the scope is only specced to 300 MHz: that's a lot more headroom than I've seen in either a 475 or a 2236). I did not bother to verify the output flatness of the TinySA, since I was not using it near the top of it's sine wave range.

I have checked the DC response of both Ch 1 and Ch 2 using a voltage reference, and both channels seem to have full amplitude with a DC signal.

I have not made systematic measurements of the attenuator settings, but from my ad hoc testing they all seem to be working as expected, which leads me to believe that the bandwidth limitation in Ch 1 must be after the attenuators.

My plan of attack is two-fold: first, I will remove the pre amp hybrid from channel 1 (U100), clean the contacts with IPA, and reinstall U100 to see if that cures the problem. If that does not cure the problem, then I will swap U100 and U200 to see if the problem follows the hybrid. If the problem remains on Ch 1 even after swapping the hybrids, then I will need to come up with new plans.

My question is about working on the hybrids. I understand that they are delicate and easily damaged. The service manual does not have much to say on this. Specifically I have not found any information on how to safely re-mount the hybrids (how to tighten the screws in a way that won't damage the substrate/destroy the hybrid). If anybody has any advice in how to properly handle the hybrids when reinstalling them, I would really appreciate hearing it.

-- Jeff Dutky

Hi James,
See my comments below the measurements.

On Mon, Mar 21, 2022 at 07:43 PM, James55 wrote:

R1722 - 128.3v
R1723 - 18.7v
VR1722 - 20.2v
VR1724 - 23.3v
C1724 - 23.3v
CR1724 - 0.25v
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v
VR1726 - 9.2v
R1727 - 26v
R1725 - 5.8v
C1725 - 3.39v
R1726 - 16v
CR 1723 - 0.573v
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

+65v test point currently 25.32 volts (obviously)

-------
VR1722 will set bottom of R1722 at ~20V above TP+65. Your measurements of TP+65V=25.32V and bottom of R1722=173.5-128.3V=45.2V match so VR1722 is looking good and voltage at the bottom of R1722 is correct given TP+65V voltage.

Voltage drop across R1723 is 18.7V. This says 18.7V/15k=1.25mA is flowing somewhere. Unfortunately you didn’t measure voltage across R1724 but voltage across C1724 and voltage across VR1724 are same (23.3V) which leaves zero voltage for R1724 (please confirm). All other indications (CR1724 is not forward biased, VR1724 has less than 56V) also say current is not flowing into the opamp output. This doesn’t mean there is anything wrong with that branch, simply something else is pulling the current away. Other path is through diode CR1723.

From the voltages none of the devices Q1732, Q1736, CR1732 have gross failures. Perhaps they became leaky or damaged in a subtle way. I don’t like testing components without having a good guess of which one failed but this may be a case where testing Q1732, Q1736, and CR1732 may identify a bad component. My guess is Q1732 is bad, given its low Vbe but still passing an emitter current almost same as the lost current at the base node. You can start with Q1732 to minimize work.

Ozan

On Mon, Mar 21, 2022 at 08:20 PM, Mark Vincent wrote:

R1734 with 25,25V across it?! That much voltage across a 3 ohm 1/2W resistor
would blow it up.

---
That is what I thought too but C1737 voltage is 25.32V and Q1734E is at 25.39V. Voltage across R1734 is the difference between them, 25.25V must be a typo. James also noted +65V point is at +25.32V.
Ozan

James,

R1734 with 25,25V across it?! That much voltage across a 3 ohm 1/2W resistor would blow it up. That resistor being open would cause the low B+. It is possible C1737 is shorted. That would also open that resistor. You may have already replaced it. If so, then back to that 3 ohm resistor. If you do not have or can get a 3 ohm resistor, a 2,7 ohm will work. See about CR and VR1718.

Mark

DC504A/ FC503/ DM502A/ PS503A
The PS503a dual tracking knob is off by 5 volts
all others work and seem to be in really good shape
waveforms for reference

Medical bills VA wont cover is reason for selling.
Thank you !
Also have 475A in excellent condition if interested.

Asking $375 and shipping from zip code 34450

Great reply Harvey.

Made me smile. Thank you.

Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards

I take it you mean via the connectors?

and the chassis.

Do you mean disconnect any ground wires?

That's because you keep removing parts. Stop doing that unless you can justify removing them

Fair enough. The two diodes had a leg lifted in order to help find the last 'good point' with the voltage readings. All components are back in now other than the +15v rail, which still has a dead short lurking somewhere.

So, voltages...

The CR1721 +65v lines are reading 173.5v and 86 volts

In the hope that it may illuminate the situation, I just did a series of quick measurements 'across' the components of the +65v section, which are as follows;

R1722 - 128.3v
R1723 - 18.7v
VR1722 - 20.2v
VR1724 - 23.3v
C1724 - 23.3v
CR1724 - 0.25v
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v
VR1726 - 9.2v
R1727 - 26v
R1725 - 5.8v
C1725 - 3.39v
R1726 - 16v
CR 1723 - 0.573v
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v

+65v test point currently 25.32 volts (obviously)

There is lots I wish to add in response to the earlier post however it is very late here.

I'll get back on it tomorrow evening.


James

Michael,

We talked about this on the fb group, I'd appreciate it if you could send me the STL file to my address here.

Also, would this be the kind of thing that would be appropriate to have on the TekWiki?

-- Jeff Dutky