----- Original Message -----

From: Mover

To: TekScopes@...

Sent: Friday, January 04, 2013 12:20 PM

Subject: [TekScopes] Re: 2246 Mod A main board overloading the SMPS

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Tom & Francis,

I discovered what it is - the ultra cautious in me had left a 40W light bulb in series with the isolation transformer secondary providing AC line power to the scope. It was there originally to prevent further collateral damage to the scope through my repair and testing. With any load exceeding the load provided by the fan, the SMPS pulls enough current through the light bulb that the AC line power to the scope drops below the 80V required to keep the SMPS running. This shuts down the SMPS and the 40W bulb comes on as the SMPS restarts - absolutely normal behavior. I had completely forgotten about the light bulb till this afternoon when I chanced to glance at it cycling with the SMPS and then I had an AHA moment!

Regardless, I did measure resistances (with a Fluke 116 - don't own a meter recommended by TEK) and here they are:
> +5:100 ?? - mine is 99 ohm
> -5:50 ?? - mine is 41 ohm
> -15:500 ?? - mine is 750 ohm
> +15:400 ?? - mine is 583 ohm
> +7.5:150 ?? - mine is 134 ohm
> -7.5:180 ?? - mine is 143 ohm
> +58: 7500 ?? - mine is almost open ( 8M ohm)
> +130: 9000 ?? - mine is 10,000

Other than the 58V resistance probably becuase my meter can't drive the diodes, my measurements looks fine.

I ran it without the series light bulb and it happily drives a 10 ohm resistor from the 5V and regulation is excellent at 5.05 VDC. With the light bulb in series, it won't run anything more than the fan. Indeed, the fan is not sufficient load to test the SMPS.

Believe I am ready to put the SMPS back into the scope tonight. I apologize to everyone who I misled with my earlier post on this and thanks as always for quick and thoughtful responses. You guys are great!

Priya.

--- In TekScopes@..., "Francis" wrote:
>
> Hi,
>
> several things:
>
> as Tom Jobe suggested, you can do the following:
> with the scope disconnected from the mains, check the resistances
> between ground and each supply. As Tek says, use an ohmeter that is able to forward bias diodes, usually a low range.
>
> Values are
> +5:100 ??
> -5:50 ??
> -15:500 ??
> +15:400 ??
> +7.5:150 ??
> -7.5:180 ??
> +58: 7500 ??
> +130: 9000 ??
>
> these values are taken from a 2245A manual, but should not be
> very different on your scope. If one is significantly lower,
> check the related circuitry.
>
> The fan is not a sufficient load for the SMPS. When I make
> my trials, I use 2 4.7 ?? paralleled on the +5v.
> If the supply still ticks, try to disconnect the crowbar protection,
> or tie together the gate and the cathode of the SCR. Maybe some
> overvoltage due to a dying cap can fire it. Same recommendation
> about Q2208 (base tied to ground).
>
> --- In TekScopes@..., "Mover" wrote:
> >
> > Alas - if only it were that simple ...
> >
> > I finally fixed my SMPS and put it back into the chassis with only the main board. It runs in chirp mode - about every 1 sec there is a clicking sound - the secondary voltages rise and then something gets overloaded and the SMPS shuts down and then cycles again.
> >
> > I quickly disconnected power, isolated the SMPS and again verified that the SMPS is fine on its own with just the cooling fan load with a steady internal 44 VDC. No new or collateral damage thankfully.
> >
> > There are no obvious shorts on any of the supply lines on the main board. The issue may still very well be with the SMPS being unable to handle load even though the fan runs fine and the other voltages are fine.
> >
> > I assume that chirp mode is a common issue with these SMPS supplies. What have others done to (1) find whether the issue is with the SMPS or with the main board and if the latter, (2) which supply line or lines is drawing too much current and how to isolate the problem further?
> >
> > Thanks in advance.
> >
> > Priya.
> >
>

---

From: Don Black
To: TekScopes@...
Sent: Friday, January 4, 2013 4:58 PM
Subject: Re: [TekScopes] Need help choosing a scope and some other RF capable equipment.

You mean it really isn't flat???

Don Black.

On 05-Jan-13 9:24 AM, Jim wrote:

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History teaches us the unfortunate lessons and fates of people who believed that a flat Earth was the center of the Universe.

73

Jim N6OTQ

?

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From: Matt Jodz
Subject: [TekScopes] what is the differences between 465 and 465B?
To: "TekScopes@..."
Date: Friday, January 4, 2013, 3:48 PM

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Another newbie question and let me say thanks for the groups patience beforehand!!? I have been trying to research the differences between the Tek 465 and 465B scopes.? Can I use the 465 Service Manual on the 465B or must I get the 465B service manual?? I have only been able to find an internet comment that the B was an improved 465 and that it is preferred.? Is that because it is a newer design/model or is there some better capability in the B over the plain jane 465?? Can anybody enlighten me?? Thanks.

--- In TekScopes@..., "Mover" wrote:
Indeed, the fan is not sufficient load to test the SMPS.

In my post in the original thread I was not implying that the fan alone was sufficient to "test" the power supply, merely that in order to fault find on a non working/tripping supply it was a convenient load/indicator of the power supply ability to function. Some switching supplies will not run without a certain load, this one will run with just the fan attached. Most faults can be rectified just with the fan as an indication. Unfortunately Tektronix omitted power supply expected load information, unlike Philips/Fluke for which scopes I have built full load test boards. JC

Looking at getting either a 454 or a 475 and would appreciate any opinions as to reliability and ease of maintenance and repair for these two scopes.? The 454 is working fine and the 475 needs some (hopefully) minor repair.? I will be using them for hobby repair of older radio and test equipment.? Which would you prefer?? Thanks.

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Thanks Don,

Yes it is 200MHZ. I will see if I can fix the listing. Where are you?

Jerry Massengale

-----Original Message-----
From: Don Black
To: TekScopes
Sent: Fri, Jan 4, 2013 4:08 pm
Subject: Re: [TekScopes] Need help choosing a scope and some other RF capable equipment.

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Hi Jerry,
??? ??? ??? ??? Nice looking scope, too far away for me. However, you've listed it as a 200 MHz scope in the title but only 100 MHz in the description. I guess I could look it up but which is correct? I suspect 200 MHz.

Don Black.

On 05-Jan-13 5:03 AM, jerry massengale wrote:

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Hi,

Sounds like Hybrid battery work. There are several nice 475s on ebay. Just search for tektronix 475. If you like, I have a 3 channel 7704A on ebay. See item 290839243077. It should serve well and other options can be added.

Jerry Massengale

-----Original Message-----
From: sknaugler
To: TekScopes
Sent: Fri, Jan 4, 2013 10:34 am
Subject: [TekScopes] Need help choosing a scope and some other RF capable equipment.

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Sorry, this is long. Maybe you folks can help with our project, point us to sources of RF help, or at least help me find a scope. But because this is a scope forum I'll start with the scope part. I should add that a DSO would be nice, but an old fashioned CRT analog scope may be perfectly fine.

We need a scope to examine a waveform that we want to apply to a pair of electro-chemical cells wired in series. Currently we power the cells with 1.0 VDC, and they normally they draw between 3 and 5 A. It is thought that cell performance would improve if we can cause the chemicals in the cells to resonate, hence the desire to superimpose an AC signal on top of the DC. We have been able to explore frequencies up to 100 kHz for square waves and pulses and 300 kHz for sine waves, but not surprisingly those lowish frequencies did not help.

We now want to explore frequencies from about 1 MHz to as much as we can afford on a shoestring budget, but no less than 30 MHz for square waves and pulses and 50 MHz for sine waves. 100 MHz would be nice, but there's that budget issue.

The waveforms, at least initially, would be limited to 1 V maximum, and never going negative. We are thinking that the maximum AC amplitude would be 0.5 V peak, or 1.0 V peak to peak, and that would be superimposed on a 0.5 V DC bias voltage. A minimum AC amplitude would be 0.05 V peak, or 0.10 V peak to peak, and that would be superimposed on a 0.95 DC bias voltage.

We'd also like to monitor current if we can find a shunt that does not distort the waveform.

An issue is that I can't guarantee that the final wiring will give us a common ground point so we may need a scope and lead set up where can can measure two differential voltages. (Our Tek 222 with it's isolated inputs is nice in that regard, but at 10 MHz bandwidth won't work for these higher frequencies.)

So if you guys can point us in the right direction for what models of new or used scopes might work for us. Or even sell us one of your surplus working scopes.

Local Craiglist scopes available that appear to work from pics and description: (I know, buyer beware.)

Tek 2465 (not A or B) $350 firm. (Seems a little high)
Tek 2215A, $100 firm (Not quite enough bandwidth)
Tek TDS 3052B (No price given, and I don't know much about this other than labeled 500 MHz)
Anritsu MS2711D (I know nothing about this brand or model)

We're also going to need an arbitrary waveform generator, and the "superimposed RF on top of DC" power supply. These could be the subject of another posting if there's any interest, or we could keep those non scope subjects off forum.

If it makes any difference, our 3 person start up is in Seaford, Delaware.

?

History teaches us the unfortunate lessons and fates of people who believed that a flat Earth was the center of the Universe.

73

Jim N6OTQ