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Re: Grid Bias Adjustment on 475A

 

Michael,

You are probably thinking of the mesh bias value that led me down this path. The mesh bias, which is supposed to be about -150 V, is only -110 V, which appears to be consistent with the decrease in the cathode voltage.

I'm not sure how to disconnect the ground lead to the HV multiplier. It appears to be mounted on the reverse side of the main interface board by two nylon nuts, but I don't see any electrical connections. I haven't found any discussion in the service manual of the HV multiplier.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

 

So this is my current thinking about this problem:

1. I think that the issue has been pretty well isolated to the HV transformer, HV multiplier, and regulator circuit.
2. It sounds like the regulator is trying to do its job, which means that most of the components are good.
3. It also sounds like the regulator is not working especially hard, which suggests that the HV transformer is probably okay.
4. That leaves a couple of HV parts, and the transistor that drives the primary winding (Q1318) as suspects.
5. All the remaining suspects are inaccessible without removing the main interface board.

I would be overjoyed to be proven wrong on that last statement, but I'm perfectly willing to undertake the task of disassembly. This was meant to be a parts scope from the get-go, and everything I'm doing on it is in the interest of learning and practice.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

Mlynch001
 

On Wed, Jan 6, 2021 at 10:14 PM, Jeff Dutky wrote:


The cathode voltage is still -1890 V. I made sure to select a specific range
on the 916 when making this measurement, to ensure a known impedance.
Jeff,

So is the -2450 TP actually at -1890? I thought that you had quoted a quoted a much different reading in an earlier post. If the HV multiplier was bad, I would not expect that -1890V reading. To be sure, I would still disconnect the Ground lead to entirely eliminate the possibility of a bad HV Multiplier.

--
Michael Lynch
Dardanelle, AR


Re: 485 as a business dependent daily driver?

Chuck Harris <cfharris@...>
 

And yet, there are folks willing to recalibrate your 2465s
for around $150. Is that really any worse than other repairs?

The NVRAMS seem to last more than 30 years if they are not
cooked to death, or otherwise abused. FRAMS are expected to
last about 140 years.

Given that you will certainly be willing to pay $150 for a
dinner and a movie with your sweetie, ...

I wouldn't calibrate a 485 for less than $350. It is a real
complicated job... when compared to a 2465. Takes easily
twice as long, and when you are done is not nearly as stable,
nor as accurate.

-Chuck Harris

Tom Gardner wrote:

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: Grid Bias Adjustment on 475A

Mlynch001
 

On Wed, Jan 6, 2021 at 09:21 PM, Raymond Domp Frank wrote:


ISTR that many HV probes have their own resistor to ground, in parallel to the
meter's. At least Fluke's 80K-6 and 80K-40 have one. Maybe something wrong
with your 80K-40?
80K-40 does use a separate resistor ground. If you fail to connect it, the readings will be wrong.

--
Michael Lynch
Dardanelle, AR


Re: Grid Bias Adjustment on 475A

 


Replacing C1304 made no difference, by the way.
Not surprising since the voltage across it is about 50 V as you report. Thanks for checking.

Bed time for me (5:15 AM, CET).

Raymond


Re: Grid Bias Adjustment on 475A

 

Raymon,

I'm just reporting what I saw on the other meter. The DMM916 manual is sadly terse on this particular detail.

I might imagine that it switches in a higher impedance mode momentarily during auto-ranging, but I guess I could just have a peek at what it's doing with a scope to be sure (I already had some fun doing that with the capacitance mode).

Replacing C1304 made no difference, by the way. The cathode voltage is still -1890 V. I made sure to select a specific range on the 916 when making this measurement, to ensure a known impedance.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

 

On Thu, Jan 7, 2021 at 04:28 AM, Jeff Dutky wrote:


I am under the (entirely uninformed) impression that "Vloss" is supposed to be
leakage. I really need to get a proper LCR meter (or fix up my grandfather's
capacitance bridge). The device I'm using was a $25 kit from Amazon, and came
with essentially no documentation. I'd say that it's better than nothing, but
I really can't be sure of that.
AFAIK, it is uncommon for all but the most professional LCR meters to indicate leakage in the range of a few uA's. It's dependent on applied voltage as well.


The voltage across C1304 is a little over 50.5 V (I don't see the exact value in my notes, only the phrase "diode drop?"
That's fine (50 V supply - one diode voltage drop (CR1304)).
Vloss indeed seems to stand for voltage loss of a capacitor. It could be related to dispersion. Difficult to directly convert to leakage current between two voltage levels.

I see there seems to be some straight relation between mesh voltage and cathode voltage in your measurements. That's to be expected (with limit at Vmesh about -180 V) and supports the idea that the circuit regulates to a wrong cathode voltage.
If you're courageous, you may temporarily solder a resistor of say 2M2 in parallel with R1303B to see if the cathode voltages goes about 20% more negative, or 4M7 for a 10% (roughly) change. It's not risky.

You said you measured R1303B did you? It may have drifted up (that's their usual direction).

If all this doesn't bring you any further, you may consider temporarily disconnecting the input of the HV multiplier, as Michael L. suggested. The assumption that the regulation circuit isn't pushing hard may be wrong; the cause must lie somewhere...

Raymond


Re: Grid Bias Adjustment on 475A

 

On Thu, Jan 7, 2021 at 04:46 AM, Jeff Dutky wrote:


The Tek DMM916 has an input impedance of 10 M ohms, except in auto-range mode
(the default) where the impedance appears to be 11 M ohms.
I can't imagine the input impedance being different in manual from auto-range mode.

Raymond


Re: Grid Bias Adjustment on 475A

 

Harvey,

The Tek DMM916 has an input impedance of 10 M ohms, except in auto-range mode (the default) where the impedance appears to be 11 M ohms. I checked my brand new Kaiweets and saw exactly what you described, that the impedance varies quite a bit by range. Good things to know.

I splashed out a bit of cash on the 916, both because it has an auto-hold feature that looked (and is) very useful, and because it has a bar graph display (shallow, I know, but I miss my old analog meters). I've been finding, however, that it was money well spent: the Tek meter is head and shoulders better than any other digital multimeter I have ever owned (probably because all the others were cheap garbage). I understand that there are two models that came after the 916, the Tx1 and Tx2, but I'm not willing to pay what people are asking for those.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

 

Raymond,

I am under the (entirely uninformed) impression that "Vloss" is supposed to be leakage. I really need to get a proper LCR meter (or fix up my grandfather's capacitance bridge). The device I'm using was a $25 kit from Amazon, and came with essentially no documentation. I'd say that it's better than nothing, but I really can't be sure of that.

I have a bunch of 1 uF Al electrolytics, brand new, that are rated for 50V that I can sub in to test. Before I removed C1302 I measured the voltages from the base of Q1306 to R1304. The voltage across C1304 is a little over 50.5 V (I don't see the exact value in my notes, only the phrase "diode drop?").

The mesh voltage I measured was -110 V. I measured that with both the DMM916 and with the 2236 when checking the waveforms presented in the schematic.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

Mlynch001
 

On Wed, Jan 6, 2021 at 08:58 PM, Raymond Domp Frank wrote:


Michael,
The regulator circuit voltages Jeff reports don't indicate that the circuit is
pushing hard to achieve the cathode voltage.
Raymond,

Thanks for explaining that.

--
Michael Lynch
Dardanelle, AR


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.
It depends on the person... but, I would think that the workflow that you have developed with the 2445B might be impaired by using a 485. Taking cursored measurements on the 2445B is different than taking measurements on the 485.... where you don't have cursors... and where you have to count gradients, from the 485's smaller screen.
That seems obvious... but one might have learned to count gradients, and refer to knob settings... but, still might find productivity decreased... moving from a scope with cursors.
It's what one is used to using too.... If you use one instrument daily, for a long time, you tend to get to know it very well.
I'm not sure why one needs a 485 ... as a there are many lower bandwidth, lower priced, purely analog scopes (sans cursors) out there, with very nice displays, and larger screens too... if one wants a back up to a 2445/2465


Re: Grid Bias Adjustment on 475A

 

On Thu, Jan 7, 2021 at 04:14 AM, Harvey White wrote:


I've got a Fluke 8840 which has such a high impedance it takes minutes for an open circuit to go away. I'm
rather certain that would not work well with a standard HV probe on that range..
ISTR that many HV probes have their own resistor to ground, in parallel to the meter's. At least Fluke's 80K-6 and 80K-40 have one. Maybe something wrong with your 80K-40?

Raymond


Re: Grid Bias Adjustment on 475A

Harvey White
 

OK, just checking. Not sure what his meters' input impedance is, and 1000 megs works into a 10 meg (the probe resistor ought to be 990 megs, I think).

I've got a Fluke 8840 which has such a high impedance it takes minutes for an open circuit to go away.  I'm rather certain that would not work well with a standard HV probe on that range..

Harvey

On 1/6/2021 9:56 PM, Raymond Domp Frank wrote:
On Thu, Jan 7, 2021 at 03:23 AM, Harvey White wrote:

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,
I brought that up in my earlier post. Jeff reports that his probe is 1000 MOhm (!) and his deviation is far more than the often-found variation in DC input impedance.

Raymond





Re: Grid Bias Adjustment on 475A

 

On Thu, Jan 7, 2021 at 03:41 AM, Michael W. Lynch wrote:


Have you ruled out the HV multiplier? A partially shorted or otherwise
defective HV multiplier can cause this.
Michael,
The regulator circuit voltages Jeff reports don't indicate that the circuit is pushing hard to achieve the cathode voltage.

Raymond


Re: Grid Bias Adjustment on 475A

 

On Thu, Jan 7, 2021 at 03:23 AM, Harvey White wrote:


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,
I brought that up in my earlier post. Jeff reports that his probe is 1000 MOhm (!) and his deviation is far more than the often-found variation in DC input impedance.

Raymond


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

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