--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:

An isolation transformer allows you to make measurements safely. You don't isolate the line powered instruments, but isolate the load. You don't ever want the case of an instrument to become live.

I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."

While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.

Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.

Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to –225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst.

If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before – search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek – "differential measurements- a primer".

If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative – a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe.

Steve

?

On Mon, 11 Mar 2013 06:35:34 -0000, "Michael" <af7u@...> wrote:

>This post is a follow up from post #7360, and my 7L13 with center frequency LED readout all zeros. I am a bit further along with my investigation. I now understand the operation 'somewhat' more. I have looked over the diagrams and taken some voltage readings, I also understand the basic DVM logic and readout operation which sends data to the LCD and the CRT. I have an article from a Jan, 1975 '73' magazine which talks about the Fairchild 3814 digital voltmeter. I must admit I am not too clear on the 'Analog' section of the meter. On board 'A2100', diagram 12, the input to pin 2 of U2110 comes from plug P2540 pin 4. Looking over the A2500 board shows that pin 4 goes to chassis ground. Why would pin 2 from U2110 go all the way back to A2500 to reach chassis ground.

U2110 (LM308 style low bias current bipolar operational amplifier) is
configured as a differential amplifier. The signal ground that the
tuning voltage uses may not be at quite the same potential as the
signal ground that the readout uses so the differential amplifier
subtracts the difference. The differential amplifier will also
subtract low frequency noise between the grounds.

>Anyway, the A2000 board seems to be working, albeit the frequency of the clock oscillator is not 200kHz as the diagram shows, it is running at 153.7kHz, though amplitude is good at 5 volts, I would think that would affect calibration but not operation. Back on A2100, the tuning control voltage at R2104 is good, it swings from + - 9.6 volts. The output of U2110 swings + - 10.7 volts. The DCPL Supply rails 15, -15, and 5 volts are all good. The reference voltage at the cathode of VR2115 (Zener) is 3.1, not the 3.3 volts is shows. Voltage at TP 2100, (output of U2130) is 11.65, a bit higher than the listed 11.3 volts. The voltages around the N-channel FET (Q2120) is Drain = 3.37 volts, Source = -3.1 volts, and Gate = -3.5 volts, are those in the ball park?

Those voltages all sound good to me and they are mostly the ones I
would have asked you to measure. In the worst case they add up to a
calibration problem and not a complete failure.

>Needless to say there is NO signals present, waveform, 1, 2, 3, and 4 are all missing. Lines 'E' and 'F' are both quiet, the voltages on 'E' are 7.08 volts, on 'F' the voltage is 5.12 volts which swings a bit higher when I adjust center frequency, up to 5.44 volts. Of course 'E' has no signal as would be expected without drive from U2145 there is nothing to trigger the multivibrator U2030 on board A2000 sending a signal back up line 'F'. I keep thinking of this in terms of a frequency counter, I keep thinking there should be a driving input signal. I try to keep in mind it's a voltmeter, and keep in mind the circuit description given in the manual. It describes a ramp signal coming from the output of U2110, I don't understand that statement. The 'Tune' voltage is just a DC voltage, plus to minus. The minus input of U2120 goes to chassis ground??? What 'RAMP' are they speaking about?

E is 7 volts? That is quite a trick since the pull-up resistor
(R2149) is connected to +5V and the output of U2145 (LM311 style
comparator) is wired open collector (pin 1 is the emitter of the
output transistor and wired to ground) so it can only pull down and
not pull up. On the receiver side, U2010D is part of a 74L04 and also
has a 5 volt supply voltage so even if it failed, it can not pull up
to 7 volts

U2145 (LM311) is probably bad but you might want to look for a bad
socket or open trace. Nothing should be able to make its output go to
7 volts except an open on pin 4 or failed part.

The operational amplifiers and comparator are all TO-99 8 pin metal
cans which are difficult to find these days. If you can not find
metal cans, I would try replacing them with 8 pin DIP parts which
conveniently have the same pinout. I have done this before by cutting
the DIP leads short and soldering little wires to them. Cutoff 1/4
watt resistor leads work well.

Texas Instruments still sells TO-99 LM311 comparators but they are $10
each.

U2010 may be damaged as well. I think it can be replaced without
problems with a 74LS04 or even a 74HCT04 which are both more common.

If you have to order the LM311, pick up an LM308 and 74L04 replacement
as well just in case. The LM308 operational amplifiers can be
directly replaced with the Linear Technology LT1008. An LT1012 or
lots of other devices can be used if you pull the compensation
capacitor. JFET or MOSFET input devices should work as well.

>Lastly the manual's circuit description on page 5-10 talks about a positive going ramp from U2110, aren't these just DC levels, what 'Ramp' are they talking about?

I think the ramp they talk about is the output of U2140 (another
LM308) which is configured as an integrator. The frequency (and
slope) of the ramp is proportional to the measured voltage.

>Okay, I see there are a lot of questions here. I had better post this and hope for the best. It's a great 7L13, it works well, but without a 'Center' frequency display, it's pretty much just a toy. Also, I do appreciate the feedback~!~ I am not accustom to working on test equipment, my experience has been mostly all in Hi-Fi, but this has been very interesting.

I had to add a few pages of the 7L13 manual to my book of stuff I do
not have so it took a bit longer to respond to your post. The 7000
series spectrum analyzers look like fun. I will have to keep an eye
out for one.

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
On Behalf Of wshawlee2
Sent: 12 March 2013 00:20
To: TekScopes@...
Subject: [TekScopes] Found 7854 Boards and more in the big Sphere Storage
clear out...

I know somebody out there asked me for 7854 boards, but I have totally

track of who. I found a big pile of them (when did we get THOSE??? no
idea..), all neatly wrapped from a salvaged scope. including the middle

select board, with all the lighted PB switches, one of the first things to

worn out and damaged. this looks very minty. also found some 7094
boards...

Anyway, if interested, email me, $50 for all of them. also a nice 7633R

mounted storage scope for $50.

Tek 465M stuff, have working modules, top and bottom case covers, and a
front cover! if you are a fan of this great model, we can make you very
happy for peanuts.

We still have tek logic analyzer pods and cables, and several Tek

next week Stan and Dennis will be up, so this is the only chance to get

transported back to the tek museum in oregon for just some gas money. it
will be truly heartbreaking if you miss this chance and long for one of

items.

Still have 3 ea. 11402 11K frames, and a DSA602A, clearly these will

benefit from cheap transport, as they are pretty big and heavy. next week

the last chance for them.

Several people asked what happens to the left overs, here it is:
the robotics and electronics students association at our university get
everything else, to savage or use as they wish.

all the best,
walter
sphere research corp.

walter2 -at- sphere.bc.ca




------------------------------------

Yahoo! Groups Links

From: sbirdasn
Subject: [TekScopes] Re: Sony/Tektronix 318 manual with full schematic
To: TekScopes@...
Date: Tuesday, March 12, 2013, 4:41 AM

?

Try www.ebaman.com.

It contains a copy of the army version service manual, it is complete and a good scan.

You have to register to get access to the files, but it's free.

You will find it in the Test Equipment-> Tektronix-> Logic Analyzers section.

It's a direct copy from the original Tektronix service manual, complete with typographical errors, etc. that exist in the Tektronix service manual.

I'm curious as to the full symptoms of your 318 other than being unable to select 20nS clock rate.

I don't see how it could limit the clock sample rate, and not show some other indication that it thinks it's a 338, like incorrect setup menus such as the Setup menu being populated with 4 pods of signals (318 only populates Groups 1 & 2 w/ signal names for pods A & B, respectively). Or, not fail numerous tests of the hardware in respect to ACQ/Glitch capture SRAM. Either in the normal startup diagnostics, or any of the manual diagnostic tests.

Keep us posted on your progress.

Sbirdasn.

?

Of course. I should have done that in the first place. Here is the link to a spread sheet sowning parts to re-cap the OVPS of a 2465B:
http://f1.grp.yahoofs.com/v1/bF4-UQa4lyGEp6utufRdlNhXEK6fip8OC-bMdb1wtFsYserp-dPNaydifJoV59DLIdkwq4bZ_ErE-tFr1oF3i16dPrSCdVuiihw/2465LVPS_ReCap.xls

This is a spread sheet done by Random Walk and is a very complete listing including links to Mouser parts on the web. I verified the list against the service manual for a 2465, identified a few extra I wanted and ordered parts.

--- In TekScopes@..., Dave C wrote:
>
> Can you please give the link?
>
> Thanks,
> Dave
>
>
> The spead sheet posted at Tekarc was very helpful and still relevant.
> ...Mac
>

?

As I have learned so many times the hard way, there are several apps on the web that do this automatically.

?

One I found to be particularly helpful is this one that will calculate to any precision you desire down to 1% with any values you might find in your junk drawer and it tells you the deviation from your desired percentage and which of the values will give the closest match.

?

?

Dennis

?

From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Reed Dickinson
Sent: Monday, March 11, 2013 12:58 PM
To: TekScopes@...
Subject: [TekScopes] Parallel resistance spreadsheet

?

Nothing works, back to the drawing bored!

On 3/11/2013 12:54 PM, Reed Dickinson wrote:

?

Well gang, may be the third time will bear fruit.? I shortened the file name.? If this does not work then save the files PAR.* in a folder and open with your spreadsheet program.? I did not test it in Access but it works in Lotus.

Reed

On 3/11/2013 12:48 PM, Reed Dickinson wrote:

?

I just tried to read the file PARALLEL.WK1 that came back form Tekscopes and it was corrupted.? I will re-attach another copy.? Lets see how this one works.

On 3/11/2013 12:28 PM, Reed Dickinson wrote:

?

There was a thread a few weeks ago about a parallel resistance
calculator. I created a spreadsheet that allows you to instantly select
a pair of 5% resistors to yield just about any resistance value
desired. I used the @VLOOKUP function in Lotus 123 and every value of
5% resistor from 10 Ohms up to 36MOhms is included. I will attach a
copy to this note. Please reply if any improvements can be incorporated.

Reed Dickinson
reed714@...

?

?

I just received a type 134 current probe amplifier with a newer power supply
I hadn't seen before. (see attached picture)

Instead of the classic power supply, Tek used a power cube (model 3310) that
came from Engineering Design and Sales.

Tek had their logo and part number (015-0058-02) engraved on the front.

Tek completely changed the power connector to the 134. They are now using a
2 prong power connector like you would find in automobiles.



It is a very big ugly thing. The power cube is 3 ?" x 2 ?" x 2".



Here is the important part:

Input: 120V, 50 to 400Hz, 10Watts

Output 26.5VDC @ 200mA



This is clearly way over rated for what the 134 requires (>16V @ 40ma) so
that tells me that just about any power cube that outputs >16V will work.

?