Date   

Re: Grid Bias Adjustment on 475A

 

Jeff,
You'll find my comments below your texts

On Thu, Jan 7, 2021 at 03:15 AM, Jeff Dutky wrote:


I checked C1302 and it doesn't seem to have significant leakage, at least not
according to my cheap component tester. The component tester reports that it
is almost exactly 1 uF, with an ESR of 3.3 ohms, and a "Vloss" of 0.2%. This
is comparable to several 1 uF new aluminum electrolytics that I pulled from
stock.
Your measurements say very little about C1302's leakage, except maybe "Vloss", which isn't clear to me. Capacity isn't critical in this case, ESR even less. Remember, the current through R1303A is only about 100 uA, so a few dozen "parallel"-uA's (of C1302 leakage) will thoroughly disturb intended balance.
Do you have any good cap (electrolytic or not) of between 0.5 uF and 10 uF to temporarily replace C1302 with? I wouldn't leave a cap out because that may result in instability.

Is the voltage across C1304 near 50 V? If less, - cathode voltage will be lower proportionally. C1304 being leaky could cause that.


I'm fairly confident in the reading I got off the cathode voltage, only
because it agrees with the divided voltage I read at the node between R1303A
and R1303B,
It's not the voltage here that counts, it's the current.

and because it agrees with the observed lower voltage of the mesh
bias.
What value do you measure for the mesh voltage?

The high voltage probe is a standard probe I bought new from Amazon. The
specs indicate that it's input resistance is 1000 M ohms.
That should be OK


I'm using it on a Tek DMM916. I have other multimeters I could use, but I feel as if
the Tek is the best of the lot (the others I would use include an EXTECH 22-816, a
Kaiweets H118A, and the CTM in a Tek 2236. I've "calibrated" each against the
others and they all read within 1-2% on my makeshift standards). I tried to
follow the discussion of high voltage probes that went across the group a
couple weeks ago, and I tried to make a reliable measurement, but I'm sure
it's possible that I missed something.
Raymond


Re: 485 as a business dependent daily driver?

Roy Thistle
 

On Mon, Dec 28, 2020 at 04:14 PM, Tom Gardner wrote:


I have 2445B as a main scope I rely on for my everyday work as a vintage audio
repair business. I did bump on a few occasions into 200MHz bandwidth limit of
the 2445B and 485 is really tempting with its smaller size and higher
bandwidth.
Hi:
Just curious... where did you bump into the 200 MHz limit, working on vintage audio?


Re: Grid Bias Adjustment on 475A

Michael W. Lynch
 

Jeff,

Have you ruled out the HV multiplier? A partially shorted or otherwise defective HV multiplier can cause this. On some of the 465 and 465B boards, there is a Jumper wire that will isolate the HV multiplier from ground. It is an easy test on those models. The 475A schematic does not show a jumper, and I do not see one on the picture of the board. Therefore you want to locate and unsolder the ground wire that joins the HV multiplier to the A9 Board. This is a black insulated wire that is at the very edge of the board at about 1 o'clock to the outer Nylon nut that attaches the HV Multiplier to the board. The Multiplier attaches to the top side of the A9 and the wire comes in from the top. If the multiplier is the issue, then your -2400V should come back to near normal.

--
Michael Lynch
Dardanelle, AR


Re: Grid Bias Adjustment on 475A

Harvey White
 

One thing to note on high voltage probes is that they're effectively voltage dividers, but voltage dividers that expect a constant impedance for the meter.  Some higher class meters (and especially ones with lots of digits) have different input impedances depending on the range.  It ought to be in the manual.

The probes I have are designed to work into a 10 meg input impedance.  Often, the lowest ranges have very high input impedances, and you won't know without looking.  Higher voltage ranges are less likely (but maybe barely so) to have very high input impedances.

Harvey

On 1/6/2021 9:15 PM, Jeff Dutky wrote:
Raymond,

Thank you for the advice and analysis. Everything you said makes sense. I didn't really think it was likely that the metal film resistor would have drifted, but I was hoping that I might have found the issue, and that the fix would be relatively easy.

I checked C1302 and it doesn't seem to have significant leakage, at least not according to my cheap component tester. The component tester reports that it is almost exactly 1 uF, with an ESR of 3.3 ohms, and a "Vloss" of 0.2%. This is comparable to several 1 uF new aluminum electrolytics that I pulled from stock.

I'm fairly confident in the reading I got off the cathode voltage, only because it agrees with the divided voltage I read at the node between R1303A and R1303B, and because it agrees with the observed lower voltage of the mesh bias. The high voltage probe is a standard probe I bought new from Amazon. The specs indicate that it's input resistance is 1000 M ohms. I'm using it on a Tek DMM916. I have other multimeters I could use, but I feel as if the Tek is the best of the lot (the others I would use include an EXTECH 22-816, a Kaiweets H118A, and the CTM in a Tek 2236. I've "calibrated" each against the others and they all read within 1-2% on my makeshift standards). I tried to follow the discussion of high voltage probes that went across the group a couple weeks ago, and I tried to make a reliable measurement, but I'm sure it's possible that I missed something.

I'm not liking where this is probably leading, and in the hope that I've made a bad measurement or three, I will redo my measurements of the voltages at each (low voltage) node in the circuit. I'm really hoping that I won't have to remove the main interface board.

-- Jeff Dutky





Re: Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

Roy Thistle
 

On Tue, Jan 5, 2021 at 01:25 PM, <rgpierce1@gmail.com> wrote:


I'm currently running the supply into a bank of resistive loads
I'm interested in that. Why the load bank? Is the scope a relative "basket case?"
Did you follow the "hi eff ps" guide posted on Tekwiki?


Re: Grid Bias Adjustment on 475A

 

Raymond,

Thank you for the advice and analysis. Everything you said makes sense. I didn't really think it was likely that the metal film resistor would have drifted, but I was hoping that I might have found the issue, and that the fix would be relatively easy.

I checked C1302 and it doesn't seem to have significant leakage, at least not according to my cheap component tester. The component tester reports that it is almost exactly 1 uF, with an ESR of 3.3 ohms, and a "Vloss" of 0.2%. This is comparable to several 1 uF new aluminum electrolytics that I pulled from stock.

I'm fairly confident in the reading I got off the cathode voltage, only because it agrees with the divided voltage I read at the node between R1303A and R1303B, and because it agrees with the observed lower voltage of the mesh bias. The high voltage probe is a standard probe I bought new from Amazon. The specs indicate that it's input resistance is 1000 M ohms. I'm using it on a Tek DMM916. I have other multimeters I could use, but I feel as if the Tek is the best of the lot (the others I would use include an EXTECH 22-816, a Kaiweets H118A, and the CTM in a Tek 2236. I've "calibrated" each against the others and they all read within 1-2% on my makeshift standards). I tried to follow the discussion of high voltage probes that went across the group a couple weeks ago, and I tried to make a reliable measurement, but I'm sure it's possible that I missed something.

I'm not liking where this is probably leading, and in the hope that I've made a bad measurement or three, I will redo my measurements of the voltages at each (low voltage) node in the circuit. I'm really hoping that I won't have to remove the main interface board.

-- Jeff Dutky


Re: Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

Roy Thistle
 

On Wed, Jan 6, 2021 at 04:58 PM, David Campbell wrote:


Replace ALL the caps
Didn't we have that conversation?


Re: [dead tek 485] Can i fix it ? (newbie here)

Harvey White
 

In most of the manuals of that era (and they got less detailed and less "nice" as time went on....

You'll find that Tektronix published a lot of theory of how their scopes worked.  They also had troubleshooting information, check A, then B, then C, and so on.  They had performance verification, and also calibration procedures.

It's going to be worth it to get a good manual (I can recommend ARTEK, for one) for the appropriate serial number range for your scope, they cost (typically) under 20 USD, and the ones from ARTEK are particularly nice.  A problem with some other scans is that they are low resolution, which means that when you zoom in to get the value or designation of a part, those numbers are just blocks.  Check around, and if there's a good high resolution scan, it's preferable, perhaps even if you need to spend money. However, you can start with the free stuff and spend money if you need to.

With any piece of electronic equipment, it naturally floats or sinks depending on the power supply voltages.  TTL chips, for example want 5.0 volts + .25, or -.25, but won't be guaranteed to function properly at all conditions if the supply voltage is outside those limits.  Tektronix was very good at telling you the voltage ranges, and (almost equally important) the ripple of those voltages.  Once you start looking at the power supplies, you may well need another oscilloscope for measurements that a multimeter just won't do.

First check the supplies, then go from there.

Harvey

On 1/6/2021 4:02 PM, marcosjl31@gmail.com wrote:
Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos





Re: Details on 015-0670-00 Feed-Through Adapter for AM503B?

Jared Cabot
 

Oh nice, if you manage to find one that would be great!

Any info you are able to share on these current adapters (or any other tek gear) would be greatly appreciated! :)


Re: Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

David Campbell <k_8_b_y_p@...>
 

This scope is 40 ( +) years old.* It is not a repair. It is a restoration.*

Replace ALL the caps in the PS. Try again.


Re: Grid Bias Adjustment on 475A

 

Jeff,
I haven't followed this thread for a while, so
Just a few points:
- 0.7 V at the base of Q1306 is close enough to 0.6 V as mentioned in the SM, I don't see your point ("only"). If anything, a slightly high base voltage possibly indicates a harder drive of Q1318's base, resulting in a more negative cathode voltage.
- What are you using as a HV probe and what measuring instrument (DMM?), i.e. what load are you putting on the cathode voltage when measuring? The cathode voltage cannot provide a lot of current into your probe while measuring cathode voltage.
- It is unlikely for R1303A to drift low. Also, I can see several paths spoiling measurement of R1303A in situ, to a degree depending on the voltage that your ohmmeter applies.
- I guess you know/realized that R1303B and R1303A together form a voltage divider; the voltage across R1303A should be about - 24M5/500k times the 50 V across R1303B. If R1303A were about 13M7, as you measured, I would expect the cathode voltage to be about - 1400 V, not -1900 V as you measured. Again, it's quite possible that you measure -1900 V because you're loading the cathode supply with your HV probe, and/or the voltage divider that results from the combination of the probe and your DMM (?) isn't correct at the range you're using; many DMM's have different input resistances at different ranges. 11 MOhm is a common value, as opposed to the 10 MOhm you'd expect on all ranges.
- Did you check C1302 for low leakage? If you study the schematic, you'll see that a leaking C1302 will result in a less negative cathode voltage with the regulation (R1303A/R1303B) in equilibrium. You could just (temporarily) replace it with another cap in the same capacity range.
- The less negative voltage (-2.1 V) at the collector of Q1308 than the SM mentions suggests that the base of Q1318 is driven harder than with -3.2 V, which would result in a more negative cathode voltage than intended, not less negative, as you indicate.
- The collector of Q1308 being so far away from the power supplies surrounding it (+ 5 V and - 8 V), suggests regulation is happening

Raymond


Re: [dead tek 485] Can i fix it ? (newbie here)

 

Hi Jose,

I was in almost exactly your circumstance a few months ago: very little experience with electronics, but I can read a schematic and a multimeter (and I know what Kirchhoff's and Ohm's Laws are, even if have trouble spelling "Kirchhoff"), and I have been successfully fixing a small pile of similar vintage scopes (from the 70s and 80s, several 475s, a 475A, a 2215A, and a 2236) since then. It's really not that hard if you are patient and careful.

You've gotten off to a good start by getting the manual, and the troubleshooting tips document. First thing to do (as you've already been told) is to check the power supply rails, and fix each fault in turn.

A working 485 is a wonderful scope, and certainly worth your time and effort to fix up. It sounds like you got a really good deal on it, even if it requires repairs. I can tell you from experience that there is nothing like working on one of these old scopes to teach you about electronics. You will learn a lot, and probably enjoy the process.

-- Jeff Dutky


Re: [dead tek 485] Can i fix it ? (newbie here)

Tom Gardner
 

If it is just capacitors, you can probably fix it.

I've had startup fail due to a marginal C1841.

Use google to search for C911. That, and two others, are 15V tants on a 13V PSU rail, and they aren't shown on the "PSU" schematic.

A working 485 is a delight, particularly since it has a proper 50ohm input, not 50ohm//15pF.

On 06/01/21 21:02, marcosjl31@gmail.com wrote:
Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356


Re: Grid Bias Adjustment on 475A

 

I've been hunting the cause of a low mesh bias voltage in the HV regulation circuit for the past week. I just got my HV probe in and was able to verify that the cathode voltage is low (high?) by about 560 V (I measured a cathode voltage of only -1.89 KV, but it should be -2.45 KV. I calibrated my HV probe by checking the +110 V rail with it and comparing to what I read with normal probes, both readings were the same to within 1%).

Now I'm hunting (carefully) around the HV regulator and it looks like something is wrong around Q1306 or Q1308. The collector on Q1308 is about a volt off (-2.1 V instead of -3.2 V), and the voltage at the base of Q1306 is only +0.7 V.

I also tried to measure the values of R1303 A & B. It looks to me as if there are no alternate paths through the circuit, so I should be able to measure both resistances in situ (unless some other part is failed, which we should actually assume to be the case, since the circuit is not working right). I measured R1303B as 503 K ohms, almost exactly as specified, but R1303A measures only 13.7 M ohms, rather than the specified 24.5 M ohms.

My question is this: how likely is it that the metal film resistor has failed?

It seems like this would be an easy fix, compared to replacing either the HV transformer, or the transistor that gates the primary winding, both of which require significant disassembly of the scope.

-- Jeff Dutky


Re: 485 as a business dependent daily driver?

Tom Gardner
 

At least the 485 doesn't store any calibration constants in battery backed RAM.

When the battery "suddenly" fails, repair is less than trivial.

On 06/01/21 21:53, Ondrej Pavelka wrote:
That sums it pretty much up, I will pass the 485 to a friend of mine who
doesn't have a scope and keep my eye out on another 2445B or 2465B as a
backup. With rifa smoke bombs replaced and the leaky SMD caps out I will
hopefully not have any of the hybrid chips failing on me if i keep them
cool enough.

On Tue, Dec 29, 2020 at 2:41 PM Chuck Harris <cfharris@erols.com> wrote:

Nothing can be made 100% reliable as a daily driver, so no,
your 485 might break, and it might need an expensive repair.

If that is your only consideration, go with something new.

The 485 is a nice scope, but with its tiny screen, and lack
of readout and cursor, most would find it tedious to use as
a daily driver. When the 2465 came out, the 465-485 scopes
disappeared from lab benches very quickly. We were getting
them in government scrap lots by the ton back then.

-Chuck Harris

Ondrej Pavelka wrote:
Hi folks,

I scored 485 with few missing knobs and very blurry trace. It was for
about $30 so I didn't expect much from it.
I have 2445B as a main scope I rely on for my everyday work as a vintage
audio repair business. I did bump on a few occasions into 200MHz bandwidth
limit of the 2445B and 485 is really tempting with its smaller size and
higher bandwidth.
Would you say if I invest the time and money into the 485 I can bring it
to state where it can be 100% reliable dependable instrument?
Otherwise I will pass it onto a friend who will repair it to have it as
his only scope but if this has the potential to take the place of the 2445B
(as much as I will miss the cursors) ?









Re: [dead tek 485] Can i fix it ? (newbie here)

J. L. Trantham
 

Jose,

Happy New Year to you as well.

I've resurrected several of these over the years and they all had shorted tantalum capacitors on various boards that bring down the power supply.

There are some 'combs' that you can remove to isolate the various boards. If you find the pin for the various voltages and check resistance to ground you can likely identify the board with the shorted tantalum. Once fixed, don't be surprised to discover that it failed again. That leads to further checks for shorted tantalums. The ones that survive the 'resurrection' not infrequently fail on the reapplication of correct voltage and fail, thus starting the process all over again.

It's doable and, compared to some of the issues, relatively simple.

Start with getting a manual.

Good luck.

Joe

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of marcosjl31@gmail.com
Sent: Wednesday, January 06, 2021 3:02 PM
To: TekScopes@groups.io
Subject: [TekScopes] [dead tek 485] Can i fix it ? (newbie here)

Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos


Re: [dead tek 485] Can i fix it ? (newbie here)

Roy Thistle
 

On Wed, Jan 6, 2021 at 02:12 PM, <marcosjl31@gmail.com> wrote:


Do you think fixing this 485 scope is within my reach, with my basic knowledge
?
Well... I won't give my opinion on that... but I observe... many of us posting here will spend many posts/hours directing you by remote control. In that regard much depends on your ability to follow directions and make good observations.
Someone already suggested to start doing stuff to the power supply... but you haven't yet told us whether you've checked to make sure there is mains power, the proper fuse is in the scope, and the correct value, the scope is set to the proper voltage, and whether the line cord is good, or if the power switch is good or not.
Maybe you already did that.


Re: [dead tek 485] Can i fix it ? (newbie here)

Jim Ford
 

Happy New Year to you as well, Jose'!  Yes, Reed is the resident 485 expert.  Lucky you, finding one for dirt cheap!   Jim FordSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: marcosjl31@gmail.com Date: 1/6/21 1:02 PM (GMT-08:00) To: TekScopes@groups.io Subject: [TekScopes] [dead tek 485] Can i fix it ? (newbie here) Hi guysHappy New Year to all.I'm new in the group. I've bought a working (supposed to) Tek 485 for a few euros here in France :  received it and tried it for ~10mn. I powered it off and cannot power it on again...I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.=> I'm not an electronics expert but know how to read schematics, use a multimeter and  a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....     Do you think  fixing this 485 scope is within my reach, with my basic knowledge ?I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356I'll be glad to ear from you, any advice will be really appreciated !Thanx in advance.RegardsJose L. Marcos


Re: [dead tek 485] Can i fix it ? (newbie here)

Roy Thistle
 

On Wed, Jan 6, 2021 at 02:12 PM, <marcosjl31@gmail.com> wrote:


I powered it off and cannot power it on again.
Can you say more about what they told you when you bought it. (They said it was/was not working? They thought it was a T.V.?)
Can you say more about what happened when you powered it on the first time? (What did you see, smell, hear? Did a fuse blow?)
Do you have another oscilloscope, to use for troubleshooting?
Do you know how to properly use a 400 series scope?
Did you read the user manual? (or at least look through it?)


Re: [dead tek 485] Can i fix it ? (newbie here)

Bob Albert
 

Seems like a power supply issue.  Measure the voltages and go from there.
Bob

On Wednesday, January 6, 2021, 02:12:13 PM PST, <marcosjl31@gmail.com> wrote:

Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France :  received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and  a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
    Do you think  fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos

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