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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Tue 08, 2019 3:42 am 
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Location: Powell River BC Canada
Check for a lead dress problem that would couple the horizontal CRT drive to pins
7 and8 back to V 201B causing it to squedge.

This may be panel wiring proximity back from the intensity modulation jack 501.


Does the CRT have audible resonances you can hear in a quiet room
around 2000 Hz ?

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Tue 08, 2019 9:09 pm 
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"squedge"? "lead dress"? What do these terms mean?

Yes, I can hear high pitched noises coming from the tube, but I can't identify their frequency.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Tue 08, 2019 9:45 pm 
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I noticed that I gain a peak when I attach a probe to pin 1 of V201... https://imgur.com/a/u4koqnl

Maybe that will help with the diagnosis. I also swapped C203 from the original electrolytic to a ceramic, if that matters.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 12:37 am 
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Greetings to ZombieElvis and the Forum:

ZombieElvis wrote:
Maybe that will help with the diagnosis. I also swapped C203 from the original electrolytic to a ceramic, if that matters.


This doesn't make a whole lot of sense to me. According to the schematic with parts list, C203 is a 50uF 25 volt electrolytic. I know of no ceramic capacitors that can even begin to approach that value. You may wish to confirm the actual value of C203 and ensure that it is, in fact, 50 uF.... that is 50 microfarads, or 50,000,000 pF (picofarads).

What is the stated capacity of your scope probe according to the Rigol documentation? Most scope probes are specified to have a certain resistance in ohms shunted by a certain value of capacity, usually in the few pF range. If your scope probe is for example, 5 pF, then try hanging a small mica of that value from V201A pin 1 (grid) to ground. I doubt it will distort the sweep waveform very much and it may kill the oscillation.

I would also be interested in some scope camera photos of the waveform on V201A pin 3 with R203 (presumably horizontal gain) at various settings including especially minimum setting.

The waveform at pin 6 of V202 would also be of interest.

Regards,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 1:16 am 
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Location: St. Louis, MO
Jthorusen wrote:
I know of no ceramic capacitors that can even begin to approach that value.


https://www.mouser.com/ProductDetail/81 ... R1E476MRT6

As far as I'm concerned, MLCCs are magic.

Let me get those other waveform pictures for you.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 3:45 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
ZombieElvis wrote:
"squedge"? "lead dress"? What do these terms mean?

Yes, I can hear high pitched noises coming from the tube, but I can't identify their frequency.


Squedge means a parasitic oscillation at a frequency not related to the intended one.
This was what I saw on your screen save.

Just winging the word origin, I might suggest it means what starts on the square edge
of a fast rise time signal.
http://www.encyclopedia-magnetica.com/d ... /squegging

Hint: When the scope is acting up try and get the Rigol to sync with the parasitic by
kiting the time base to nanosecond range.

Everyone knows engineers do not design things that wont work well. So those
hired to make this true have a sub lingo to describe certain scenarios.

Often lead dress is responsible.
Attachment:
Lead dress lingo.JPG
Lead dress lingo.JPG [ 131.99 KiB | Viewed 388 times ]

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 8:52 am 
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Greetings to ZombieElvis and the Forum:

Gaaalleee.... Shazam! What'l they think of next?!

Seriously, in thinking about this a bit, aluminum electrolytic capacitors of more or less conventional design (especially those of the same vintage as your scope) have a certain amount of self-inductance. This inductance appears as an L in series with an ideal C. Thus, you will often find disc ceramic capacitors in parallel with electrolytics in circuits where it is necessary to bypass RF, or you may find large numbers of electrolytic capacitors distributed around a board, all bypassing the same buss.

Ceramic capacitors, on the other hand, work well into the VHF range. At the time both your scope and I were young'uns, there weren't any high value ceramic caps. So, the Eico engineers used an electrolytic to get the coupling value they needed, and wouldn't have thought to see what the circuit did with any other sort of cap because they didn't exist at that time.

Bottom line: I would go back to an electrolytic cap for this spot in the circuit... at least until we can nail down what is happening. Spurs are hard to track down and kill at the best of times; better not to deviate from the original design until we figure out what is happening.

BTW, Steve's comments on lead dress are right on; simply re-routing a piece of wire in a critical circuit can have profound effects on what that circuit does.

Regards,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 9:37 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
Yes, R203 is Horizontal Gain.

Here's those pictures. I noticed that V201 pin 3 is different from my earlier posting of that pin. Also, touching the probe to that pin will now fix the picture on screen as well.

As for V203, it seems that R203 will increase the amplitude of that sawtooth. I also noticed that bump on top of the wave. Increasing R203 will move that bump to the right, toward the peak. You can see it in the pictures.

I just did a voltage test on V201 where you had me add that little .01 ceramic cap. I need one a LOT bigger to cope with voltage that high!

Let me try swapping C203. I got an electrolytic of the same values.


Attachments:
File comment: different from previous post of V201 pin 3
V201 pin 3.png
V201 pin 3.png [ 40.34 KiB | Viewed 369 times ]
File comment: Low setting for R203
V202 pin 6 (1).png
V202 pin 6 (1).png [ 38.51 KiB | Viewed 369 times ]
File comment: Higher setting for R203
V202 pin 6 (2).png
V202 pin 6 (2).png [ 40.29 KiB | Viewed 369 times ]
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 9:57 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 209
Location: St. Louis, MO
I swapped in that electrolytic cap. I got the same result.

I now noticed that the probe must be grounded on pin 1 to give a correct picture now. I also noticed that the bump on top of V202's pin 6 waveform is different. It can't be moved with R203 any longer. I just gains amplitude along with the rest of the wave.


Attachments:
V202 with electrolytic cap for C203.png
V202 with electrolytic cap for C203.png [ 41.85 KiB | Viewed 368 times ]
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 09, 2019 10:10 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 209
Location: St. Louis, MO
radiotechnician wrote:
ZombieElvis wrote:
"squedge"? "lead dress"? What do these terms mean?

Yes, I can hear high pitched noises coming from the tube, but I can't identify their frequency.


Squedge means a parasitic oscillation at a frequency not related to the intended one.
This was what I saw on your screen save.

Just winging the word origin, I might suggest it means what starts on the square edge
of a fast rise time signal.
http://www.encyclopedia-magnetica.com/d ... /squegging

Hint: When the scope is acting up try and get the Rigol to sync with the parasitic by
kiting the time base to nanosecond range.

Everyone knows engineers do not design things that wont work well. So those
hired to make this true have a sub lingo to describe certain scenarios.

Often lead dress is responsible.
Attachment:
Lead dress lingo.JPG



Basically you're saying it's picking up interference from how the wires are positioned? You're talking about this picture, right? I have an updated picture of that pin's waveform and the ugly stuff on top of the peaks is mostly gone. I may have pushed around some wires right there when I fixed my hack job of replacing C401 with 3 individual caps. I did that after the original picture was taken.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 10, 2019 12:55 am 
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Greetings to ZombieElvis and the Forum:

ZombieElvis wrote:
Basically you're saying it's picking up interference from how the wires are positioned? You're talking about this picture, right? I have an updated picture of that pin's waveform and the ugly stuff on top of the peaks is mostly gone. I may have pushed around some wires right there when I fixed my hack job of replacing C401 with 3 individual caps. I did that after the original picture was taken.


Yes. that's essentially correct. The interference isn't external in this case though, there is stray coupling allowing part of the signal from further downstream to feed back into the earlier stage with the correct phase relationship and sufficient amplitude to sustain self-oscillation. Improper lead dress can allow both internal feedback and external noise to get into the circuit. It depends on the design of the circuit and how close to critical the existing design is for feedback to occur.

At this point, it looks as though you have an acceptable sweep waveform, although there is still a bit of "fuzz" present. Why don't you take a look at the waveform as presented on the Eico and see if it is satisfactory. If it looks OK to you, then you can quit... you're done. If you want the last little bit of sharpness to the trace, then you will have to start experimenting with ferrite beads, low value bypass caps or other tricks to clean up the waveform still further.

Good Luck,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 10, 2019 4:16 am 
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Location: St. Louis, MO
No, it's still doing the blurry trace thing just off the middle of the trace, like here: https://i.imgur.com/Gq8jjLP.jpg

It only goes back to normal IF I try to measure it with a probe. I may have to try your 5pF "fix", which sounds like a juryrig if I ever heard one. I'll be basically faking a probe hooked up all the time, won't I? How to I know my probe's capacitance anyway? My DMM doesn't go down past 100 pF or so. I tracked down their data: https://www.tequipment.net/Rigol/PVP215 ... pe-Probes/


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 10, 2019 8:34 am 
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Greetings to ZombieElvis and the Forum:

The probe capacitance is right there in the specs: Assuming you are using the X10 position on the probe, the shunt capacitance represented by the probe is 10 pF +/- 5 pF. If you are using the probe in the X1 position, then the shunt capacitance represented by the probe is 50 pF +/- 20 pF.

So, a 10 pF cap would be in order to try. You also might want to look into placing some ferrite beads on the wires leading to sensitive points. A ferrite bead on the lead to pin 6 of V202 as close to the tube socket as possible would be the first place I would try. You could also try one on the lead of C201 that connects to the grid of V201A. The 6C4 is good to at least 150 MHz and probably higher as an amplifier and oscillator. This would not be my first choice for a tube that is unlikely to see more than 1 MHz or so in normal service.... and no more than 100 KHz when being driven by the internal sweep oscillator. I suspect the circuit was always marginal and subject to individual tube characteristics and wiring layout (lead dress).

Generally speaking, you can add shunt capacitance wherever you need to as long as you don't distort the linearity of the sawtooth ramp.

Good Luck,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 10, 2019 4:45 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11808
Location: Powell River BC Canada
If the Eico 470 is producing a horizontal line, another approach to see how linear the
sweep is involves the intensity modulation jack, J501 on the front panel. With your function
generator, feed a square wave into this circuit. If done correctly, this will produce series
of bright/dim spaces along the line that should be equally spaced.

If you have a keen ear, while listening to the CRT sing to you, this technique may even
add harmonics to the sound.

So remember, Thompson , using a cathode ray tube, learned a great deal in 1897.

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Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sat 12, 2019 12:34 am 
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I got out my plastic screwdriver and fiddled with the wires as best I could to see if I could find which wire was causing that interference. I didn't find any. So I cut out that .01 cap you had me put in, then soldered in a 47 pF mica cap to V201 pin 1 and ground. I plugged in my cheap function generator and it works! No more blurry trace and it's a beautiful wave!...almost. The peaks are a little off. I almost complained about it, then just to be sure I tried that FG in my other oscilloscope. It's got ugly peaks too! It wasn't my old scope, it was my $10 function generator!

So I plugged in my B&K E-200D signal generator instead. MUCH better sine waves! Then I plugged in my Accurate 153 signal generator. WOW it makes ugly waves! Guess I'm opening him back up in the future.

So am I done?


Attachments:
FG output.png
FG output.png [ 39.96 KiB | Viewed 325 times ]
FG output on Eico.jpg
FG output on Eico.jpg [ 228.76 KiB | Viewed 325 times ]
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Tue 15, 2019 3:12 am 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
Well am I?


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Tue 15, 2019 4:59 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Insofar as adding 47 uuFd to V201 all 'appears' OK.

You have gotten past one gatekeeper, will the next be more powerful ?

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Steve Dow
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 16, 2019 12:30 am 
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Greetings to ZombieElvis and the Forum:

It looks to me as though you are. Congratulations on your perseverance! Enjoy your scopes... both of them.

Regards,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 16, 2019 6:03 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The Heath and Eico products of the past have no great value in this century.

However the potential for learning electronics from the restoration theory issues
cannot be easily be found by googling.

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Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 16, 2019 10:00 pm 
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ZombieElvis wrote:
Then I plugged in my Accurate 153 signal generator. WOW it makes ugly waves! Guess I'm opening him back up in the future.

The signal generator is working as expected. Service-grade signal generators do not generate a clean sine wave. It's usually a pretty ugly looking waveform. It's not a problem as the tuned circuits in the receiver under alignment or test will filter out the harmonics and give a nice sine wave to the rest of the circuit.

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