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 Post subject: HP 650-A
PostPosted: Oct Mon 19, 2015 1:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Eagan, Minnesota, USA
What type of signal generator is this HP 650-A? How do you set it up? The manual I downloaded doesn't go into using the instrument. I don't have any leads for it either. Any info on this would be appreciated.

(Note: This is a very large generator. I've googled HP 650A and a lot of smaller
things come up that look nothing like this model. This one is serial number 4774).

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 2:20 pm 
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Location: Erie, PA
Aside from being a big boat-anchor this was an excellent performing and stable piece of equipment. This oscillator covers the entire audio range and RF up to 10 MHz. The only output impedance is 600 ohms and its attenuator will be calibrated as such. Set up and use is as straightforward as it gets; probably why the manual doesn’t explain operation as they really didn't need to for its intended customers. It is ranged tuning for the frequency and attenuation for the output power. Think of the attenuation as a volume control.

Being early 1950’s vintage the filter capacitors are more than likely dried out and will likely need replacing before use, along with any paper caps that might be under the chassis. There are no special output leads for this- you can get an adapter that converts the banana jacks to a BNC connector to make it easy.

-Mark-


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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 2:21 pm 
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Joined: Jul Wed 22, 2009 3:07 pm
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Hi Frank,

This is a generator giving output from 10Hz to 10MHz.
Internally it consists of two different generators, a Wien-bridge generator from 10Hz to 100kHz, and phase-shift oscillator from 100KHz to 10MHz. These sources are switched over as you change the frequency range.
Here are some links explaining the oscillators:
https://en.wikipedia.org/wiki/Wien_bridge_oscillator
https://en.wikipedia.org/wiki/Phase-shift_oscillator

The output is 600 Ohm, max 3V RMS, and there is an output attenuator from +10dB to -40dB.
It looks like you could use simple test leads with banana plugs.
There is also a built in voltmeter showing the output voltage before the attenuator.

The section on the operation in the manual is certainly enough.

Here is a web page that has the manuals for the for the generators with the various prefixes:
http://hparchive.com/hp_equipment.htm

Regards, Peter


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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 2:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1766
Location: Eagan, Minnesota, USA
Thanks Mark and Peter for giving me a start with this. It sounds like I just need to hook up some test leads and experiment a little.

I've had this sitting for awhile, and wanted to make a decision on either parting it out or learn how to use it. It sure is a boat anchor, and even reminds me of my Collins 51-J4 when I worked behind the front panel. So over the weekend, I replaced all of the paper capacitors. Also replaced the electrolytic located on the flat board where the three 6AK5 tubes are located near the front. And I replaced a 20 mfd @ 450 VDC single cap near the 6L6 tubes that was easy to get at.

The remaining electrolytics are still in there. I brought it up on a variac and the filters didn't heat up. Had it run for about three hours, then had to knock off for the night.

I just couldn't scrap this since it was in really nice shape.

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 2:37 pm 
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The 650A covers 30Hz to 10MHz. This would be useful for audio applications and lower RF frequencies. Unfortunately it does not have any amplitude modulation capability which would limit its usefulness for testing radios. It does have a nice output attenuator to set the level accurately.
To use, tune to desired frequency with the frequency dial and frequency multiplier switch. Set the amplitude by adjusting the amplitude control and using the meter in conjunction with the attenuator setting.

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 4:56 pm 
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Location: Norfolk, VA
The 650A and it's successor, the 651B, were test oscillators, used to test frequency response of measuring equipment. We used both, but the 651B was a better unit, with less distortion. I con't recall ever using it beyond 5MHZ, and we almost always used it as a good "flat" source for checking VTVMs, Wave analyzers, and some low-end scopes for frequency response.

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 19, 2015 10:51 pm 
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Location: Littleton, MA
Frank wrote:
What type of signal generator is this HP 650-A? How do you set it up? The manual I downloaded doesn't go into using the instrument. I don't have any leads for it either. Any info on this would be appreciated.

(Note: This is a very large generator. I've googled HP 650A and a lot of smaller
things come up that look nothing like this model. This one is serial number 4774).

I think the other items that turned up on your Google search are likely to be 651A or 651B test oscillators, which are the transistorized version of the 650A.

As others have posted, the 650A is an R-C oscillator useful for video amplifier frequency response testing, as a general purpose audio frequency source, for driving impedance bridges, and other laboratory uses. It doesn't have as much attenuation range as a good RF signal generator, lacks modulation, and may not have sufficient frequency stability for use in aligning radios.

I found it useful to make up the model 65A-16D output cable. It's described in the manual as the "Output Divider Cable".

Image
(link to hi-res image)
The manual provides a complete description of the 65A-16D cable, so I constructed one for my 650A.

Image
(link to hi-res image)
I used two small Hammond die-cast aluminum boxes. Rather than hard-wire the cable, I added BNC connectors at each end, so I can use whatever length of BNC patch cord is convenient for the current bench setup.

Image
(link to hi-res image)
The top side features a dual five-way binding post for the 300Ω output.

Image
(link to hi-res image)
I used fiber insulated shoulder washers to isolate the banana plugs from the case.

Image
(link to hi-res image)
Here's the inside of the source side, with the 594 ohm series resistor.

Image
(link to hi-res image)
And here's the 6Ω termination end.

Image
(link to hi-res image)
I also made up a 12:1 divider to provide about a 50 ohm output impedance.

Image
(link to hi-res image)
The 50 ohm output is the BNC connector.

Image
(link to hi-res image)
The divider consists of a 549Ω resistor and a 49.9Ω. This gives about a 48Ω output impedance, factoring in the 600 ohm output impedance of the oscillator. I should have used a 52.3Ω instead.

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 Post subject: Re: HP 650-A
PostPosted: Oct Tue 20, 2015 1:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1766
Location: Eagan, Minnesota, USA
Thanks for the additional information guys! I'm going to build an output divider, and appreciate the detailed information on building one. Seeing the process to build it is going to be a big help for me.

I had it on for another two hours or so last night, and the filters stayed cool. For the heck of it, I hooked up a speaker, and I can hear a signal come through. So it's doing something.

Will have some more time this weekend to explore.

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 26, 2015 1:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1766
Location: Eagan, Minnesota, USA
I spent the weekend working on this generator. I have all of the paper capacitors changed, the 10 mfd electrolytic on the small upper generator board changed, and one 20 mfd on top of the chassis that was very easy to do. Also replaced the two original 6L6GB tubes that were just about dead, and one 6SJ7 that was weak.

From what I can tell, all of the ranges work, and the voltage meter looks stable between ranges for a given frequency and attenuation.

My question is about the remaining original Mallory FP filter capacitors. I've been running this for several days for as long as ten hours a day, and they seem to be ok. None are getting hot, and the power supply voltage matches the manual.

Is it possible that HP got a better grade of capacitors that could continue to be ok?

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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 26, 2015 1:57 pm 
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Location: Erie, PA
Frank wrote:
Is it possible that HP got a better grade of capacitors that could continue to be ok?


Absolutely, many of the electrolytics used in HP equipment were the best available at the time. Last year I worked on a HP/Dymec 2401A from the early 1960's, and only around 50% of the electrolytics could be justified in needing replacement after thorough tests of capacitance and leakage with a Sencore LC53. With HP and other high end test gear, it's best to only fix what is actually broken.

-Mark-


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 Post subject: Re: HP 650-A
PostPosted: Oct Mon 26, 2015 2:24 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1766
Location: Eagan, Minnesota, USA
Thanks Mark for your reply. I'm thinking that I'm going to run it some more, and if it remains ok, then leave things as they are. I'll be using this just occasionally anyway, and not every day for hours on end.

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 Post subject: Re: HP 650-A
PostPosted: Nov Mon 02, 2015 3:37 pm 
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Mark Said;
Quote:
With HP and other high end test gear, it's best to only fix what is actually broken.
+1, most, if not all first tier test equipment manufacturers use(d) mil spec or equivalent quality parts.

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