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 Post subject: EICO Model 324 RF Output Waveform
PostPosted: Apr Sat 04, 2020 12:03 pm 
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Other topics here include Model 324 output waveforms. Members reply that the nasty looking output waveforms are normal for the 324. The input to the cathode follower is a nearly perfect sine wave, so the distortion is coming after that. In an effort to isolate the distortion source, I replaced the B+ RF blocker cap with an electrolytic. This cleaned up the output some; and now you can see from the photo what distortion is directly caused by the cathode follower. My waveform with the normal RF blocker (1000 pF) is pretty much the same as everyone else's.

Has anyone reworked the cathode follower or maybe the B+ (it's only 54V) to fix the output waveform?

Yes, I know I can't leave the electrolytic there because it pretty much kills any AF modulation - I'm trying to fix one issue at a time,

Here is my 324's cathode follower input (V2, pin 2) and output waveform (V2, pin 3) with a 100 uF electrolytic B+ RF blocker on the final cathode follower (V2, pin 1 to ground):

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sat 04, 2020 12:27 pm 
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You had the answer here:
Quote:
Members reply that the nasty looking output waveforms are normal for the 324.

You don't WANT to fix this---the output is supposed to be distorted---that's how the higher frequencies are generated

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sat 04, 2020 2:04 pm 
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In general, service grade generators use simple designs that are not optimized to do anything more than provide basic signals for troubleshooting and alignment. The waveforms posted by the OP are normal for the 324.

I haven't looked at it in any detail but the distortion is most likely caused by over-driving the buffer stage. Preventing this at all frequencies generated may not be easy.

The 324 only depends on harmonics for the top band. Band F2 (110-435 MHz) is made from the third harmonic of the F1 (37-145 MHz band. All the other bands use fundamentals.

RRM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sat 04, 2020 3:10 pm 
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If you want to clean up one of these signal generators, simply connect it to a tuned circuit at the desired RF or IF frequency, the way it is meant to be loaded when it is connected to the RF or IF stages of a radio. That will remove much of the harmonic content from the output.

The problem with randomly swapping capacitors like this is, above its self resonant frequency that electrolytic is going to turn into an inductor which will cause the circuit to lose gain or generate even stranger waveforms, if it works at all. The 324 was optimized to do what it does; you may be able to "fix one problem at a time" but you'll never get done because everything you "fix" will be at the expense of something else. If you need a low distortion RF signal generator for some reason it would be more satisfactory to sell the Eico and buy an old HP, GR, Boonton, or Measurements Corp. unit which is already closer to what you want, and refurbish it.

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sat 04, 2020 4:45 pm 
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Chris108 wrote:
If you want to clean up one of these signal generators, simply connect it to a tuned circuit at the desired RF or IF frequency, the way it is meant to be loaded when it is connected to the RF or IF stages of a radio. That will remove much of the harmonic content from the output.
.....


This is a little off topic but that's exactly what I did when I needed a 1 MHz sine wave to calibrate a wide-band VTVM. I Used my 324 and with a tuned circuit got the harmonics close to 50 db down.

RRM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sun 05, 2020 9:29 am 
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Thanks for the replies so far, guys.

Chris108: Not sure where you got the idea I was randomly swapping capacitors. As I said, I know I can't leave an electrolytic in there for C5, but are you sure it's because of the reason you gave? C5 is not in the RF signal path. B+ blocking caps are pretty standard and a great idea except the 324 modulates the RF via the (seems crazy to me) RF section's B+. Shouldn't they have gone with a heptode there and left the B+ a nice DC? I guess keeping the manufacturing cost down won over standard design. Nevertheless, I am thinking C5 could be replaced with a better RF filter since all you really want getting to the plate, besides DC, is 400 Hz. You may be right that I should drop the 324 and get a different signal generator. I'm not there yet, and hope I don't get there. I can be pretty obsessive about design. 8^O

RRMan: That is what I was thinking. The input to the cathode follower is only 3.6 Vp-p, which shouldn't be an issue at all except for that (too low IMO) 54 V B+ and the 268 Ohm cathode to ground resistance (seems really low). Maybe once I'm done redesigning the RF buffer stage we can keep that nice oscillator sine wave all the way to the output jack (maybe lose band F2, but I think it would be worth it). Do you have experience building RF gear, maybe ham stuff? : )


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sun 05, 2020 3:03 pm 
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If Eico wanted an electrolytic off the plate of the cathode follower stage, they could have put one there. What they used was a 1,000-pF ceramic disc capacitor which is an RF bypass--intended to function as a bypass to VHF frequencies. It is critical to prevent degeneration in the stage, and yes it is absolutely part of the RF circuitry. Read up on Miller capacitance if you don't believe this--that tube is coupling plenty of RF from grid to plate. Clearly removing audio frequency modulation in that part of the circuit was not part of the plan. Getting maximum usable output over a 150-kHz to 430-MHz frequency range was. "Usable" in the context of a kit signal generator aimed at hobbyists, hams, and TV repair shops, did not require, or include perfect, near-zero distortion sine wave output.

I certainly don't mean to discourage the experimenter spirit here. I think you should try different things and see what happens. That's what the electronics hobby is all about. But just bear in mind before you declare victory that about 60-65 years ago a group of pretty sharp guys in the Eico plant in Brooklyn, NY, tried to do exactly the same thing you are trying to do now, and a lot of clever people have been trying to improve on it since then without unqualified success. You might be able to clean up the waveform at one frequency, but that will likely come at the expense of performance at other frequencies.

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sun 05, 2020 8:27 pm 
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If you really want to clean up the output I think you'll have better luck by doing a redesign rather than trying to modify the existing one. I'd start by ripping out all the RF stuff except the oscillator and go from there. Once you get a buffer stage that will pass the signal without distortion you can worry about modulation, an output stage and an attenuator . My gut feel is that you can probably get a decent waveform up to a couple of megahertz and with a lot of luck maybe even to 10. The chances of going all the way up to 145 is zilch.

This can be a fun exercise or one of frustration, depends on what you want out of it.

The 1958 Allied catalog lists the kit at $26.95, not dirt cheap then.

I built mine some time in the late 60's. I think it was $29.95 or so then. Allied listed the wired version for $44.95 in the '69 catalog.

Have fun.

RRM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Sun 05, 2020 9:02 pm 
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The cathode follower would have been biased into conduction if they wanted a linear circuit. But a linear circuit won't MODULATE - feed two signals in and you'll get the same two signals out. To make it modulate, it has to run near cutoff so that one signal is switching the other one on and off. To make it multiply accurately, a much more complex circuit is required.

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 1:32 am 
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RRM: You BUILT one in the 60's? Then you're the best guy so far to understand where I'm going. As you know, there are only two wires coming up from the bottom of the chassis to the RF section (filament and B+) and there are no terminal strips up there. Every component is soldered to the tube socket except the tank coils. What a mess! This is one of the things that makes me think manufacturing cost played a big part. There's other things that make me think that, like using chassis ground to carry filament current. Heck, using chassis ground everywhere instead of single point ground - practically begging for ground loops.

That's great that you were building back then. My first tube project was a keyer for my Eico 720. I think the project plans came from QST. I was only 13 then, but I had my novice license (WN9KQA). So you can see I'm relatively new to the Antique Radio site, but I've been into electronics over 40 years. Have a good one!


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 1:44 am 
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Of course manufacturing costs mattered, they always do. If you can't sell it you don't make money. The design EICO came up is simple and works well for the intended use. Take a look at the schematic for an HP-606 sometime. That's a wideband generator that puts out clean waveforms but it wasn't aimed at hobbyists, hams, and service shops and it cost a lot more than $26.95.

As for mounting things on the tube socket, that keeps the leads short and that's what you want unless you like stray inductance and capacitance.

RRM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 2:42 am 
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I agree with both your points. Electronics were expensive back then and they needed some way to keep costs down to stay in business. Plus all the 324 really needed to do was provide a good enough signal to do alignments, and a lot of parts weren't critical. I remember that if he didn't have the exact tube with him in the caddy on a house call, the TV repairman I worked for back then would stick in the closest one he did have.

I thought about stray inductances. But I don't see that kind of worry in the FM section of tube radios. Well, the 324 goes up to 145 MHz while FM stops at 108 MHz. Would it really be a factor if they ran the components to a terminal strip next to the socket? Heck, look at that band switch, it carries all those frequencies... Just thinking.

Do you think if I reworked the output cathode follower components to a terminal strip I wouldn't be able to keep a clean signal?


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 3:33 am 
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Look, there's no way to tell what will happen with any given configuration until you try it. In general, you want to keep leads as short as possible and dress things so as to avoid coupling. All sorts of weird things can happen otherwise, like extra oscillations, working at some frequencies and not others, etc.

RRM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 12:40 pm 
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You may be in the middle of a common situation, to wit:
In attempting to solve problems that do not need to be solved, we sometime create problems that do......
Ask me how I know this.......;)

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Mon 06, 2020 3:24 pm 
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As a 89 year old retired tech I call it looking for problems in all the wrong places.


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Tue 07, 2020 10:08 pm 
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Thanks guys. I needed it back together so I could use it to align a Zenith. So the cathode follower, for now at least, is safe from my 'fixing' it. I did do important updates before putting it back together:

- Conversion to BNC
- Finished recapping
- Added a pre-attenuator BNC output for my frequency counter
- New power supply with a silicon rectifier and bigger electrolytics
- Replaced old cord with a new 3 prong cord and removed the two caps from the inlet power to ground


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Wed 08, 2020 12:04 am 
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FuzzyDriver wrote:
removed the two caps from the inlet power to ground

Those are there to keep the RF off the incoming power lines which then feeds into whatever you are trying to align.

DM


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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Wed 08, 2020 4:40 am 
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Replace them with type Y2 capacitors. They're important.

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Wed 08, 2020 7:30 am 
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FuzzyDriver wrote:
I thought about stray inductances. But I don't see that kind of worry in the FM section of tube radios. Well, the 324 goes up to 145 MHz while FM stops at 108 MHz. Would it really be a factor if they ran the components to a terminal strip next to the socket? Heck, look at that band switch, it carries all those frequencies... Just thinking.......

"undocumented C and L" is a HUGE issue in FM radios, and packaging and wire routing is extremely critical. Go into a typical circuit in either the RF or LO sections and move a few wires by just 1/8 inch, and you'll be up all night trying to get the thing aligned.

On the main topic: Now that you have "fixed" the distortion in the output waveform, how is the signal strength at 145MHz?

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 Post subject: Re: EICO Model 324 RF Output Waveform
PostPosted: Apr Wed 08, 2020 5:21 pm 
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If you want clean RF output forget about the EICO and other service-grade signal generators, which weren't designed for clean RF, and buy a used lab-grade signal generator. Bear in mind, though, that an older lab-grade generator may need some repairs and calibration, and that likely would require instrumentation and experience that you may not have. My GR Type 805-C was the king of AM standard signal generators ca. 1960. It cost nearly $2,000.00 then, and after renovation, replacing some bad mods made by a previous owner, and recalibration, it pretty much meets the standards for AM broadcast transmitters. It now surpasses the factory specs (by about a factor of 10 for most specs) as it did when it was new.

Image

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