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 Post subject: HP 200AB audio oscillator
PostPosted: Dec Mon 03, 2012 6:26 pm 
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Location: Bellingham WA
I have an HP 200AB serial number 606-20937 that I picked up for a good price. Comparing it to the service manual and schematic I found on line for units with serial numbers 7725 and above I see that mine must be a later model since the tube line up is different. Mine uses a 6AU6 rather than 6SJ7 and a 6AQ5 rather than 6K6 in the audio oscillator circuit. The rest of thte tubes are the same. I think there are also differences in the filtering caps from that in thte manual.

My unit is in terrific cosmetic shape, clean and untouched. Does anyone by chance have a correct schematic for my later unit? I've been unable to locate one that matches my unit. Also, how long did HP manufacture these units? I see a QC inspection sticker inside dated 9-'97. That can't be the date of original manufacture could it? Also there was a test equipment inspection tag dated Feb 2003 indicating a problem with power supply filter caps. I plan on re-capping the electrolytics and paper caps as a start. I have not as yet powered it up but previous owner indicated it did work but had a ripple on the output.

Here are a few pictures for those interested. I'm impressed with the build quality and the feel of the switches and the movement of the dial. It's a beautiful peice.

Image

Image

Image


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Mon 03, 2012 6:49 pm 
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The HP manual archive site has two manuals but both are for the 6K6 - 6SJ7. However, the one manual has some user-added markings on the schematic indicating the 6AQ5 and 6AU6 with a couple of notations for other differences. Electrically, there's not a whole lot of difference between the octal tubes and the 7 pin miniature replacements in the 200AB circuit. The 200AB is a real keeper. and can be used as an audio amp of obviously very wide frequency range with that serious inverse feedback output transformer.
That manual with the added markings on the schematic is at
http://www.hparchive.com/Manuals/HP-200AB-008-Manual.PDF

By the way. Those 6K6 tubes in your set look tougher than the normal 6K6 tubes. Are they mil equivalents or special?

-- Rich


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Mon 03, 2012 8:10 pm 
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Joined: Feb Thu 18, 2010 2:45 pm
Posts: 507
Location: Bellingham WA
Thanks for the link to the updated schematic, that will help! I took a look at the 6K6 tubes. They are marked Sylvania 5881 also shows BC AAF. I guess the 5881 is actually a 6L6 tube. I wonder if that is going to be a problem, or in this unit it's a safe swap?


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Mon 03, 2012 9:11 pm 
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Those 6L6 tubes draw over twice the filament current as the 6K6s. The other ratings are quite different as well. I would find some 6K6s. You shouldn't have a problem finding someone that would swap a pair of 6K6s for your 6L6s. Post your needs in the Classified section.

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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Tue 04, 2012 5:09 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 288
Location: Staten Island,NY
I have a bunch of 6K6's I'd be willing to trade a pair for those 5881's.But I can tell you they (6K6's) are readily available on ebay.PM me if interested.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Tue 04, 2012 6:35 pm 
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It would be best to plug in a real 6K6.

Also the tubular caps under there are probably leaky as heck.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 12:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 288
Location: Staten Island,NY
why assume the caps are leaky?I have the same unit,all caps are fine,none leaky at all.The electros were change,but that is what I do on all tube gear.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 12:46 am 
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Location: Ohio 45177
I dunno. I think I see a bumblebee cap in there, those are pretty much always leaky? But keep the leads long when removing them for the ebay buyers. I don't know how long they made those, but I worked on a 200CD that had a printed circuit board in the bottom and I think the date code on some parts were from the 80's. I think it was milsurp. Maybe the mil. kept them going a long time with their requirements. The HP oscillators had a 600 Ohm output impedance and a very high level of audio output, which is something not seen in most of the solid state ones. Oh, and you can lubricate the gears in front to make for smooth tuning. Some of them even had holes in the castings to lube the shafts.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 1:23 am 
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HP always used the highest-quality parts available at the time the equipment was built, but the parts were made by other companies.
Very few were made by HP themselves. Those were specialized parts not available on the open market, or parts with critical specs.

The molded plastic capacitors have a known high failure rate. Two are visible in the photos. I would suggest replacing them.

- Leigh

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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 2:55 am 
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Location: Elmira, NY
I'm curious, how can you use the HP 200 as an amplifier;what applications?

Eddie


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 4:24 am 
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Mil-spec 5881 tubes are tough critters. If you were in the neighborhood, I would happily swap those for a pair of 6K6. The 5881 is a very desirable tube for audiophiles, more valuable than the 6K6. They also take more power, not only in the filament department but, depending upon the bias, a lot more plate current. That may explain why the unit has a hum. It's possible that the power supply is stressed to the point where the electrolytics are not capable of an adequate job of smoothing the DC. Do a current draw check on the B+ for those tubes as compared to a pair pf 6K6.

Here's my experience with this piece. A fairly hefty signal is needed if trying it as an amp since the high inverse feedback limits the gain. Fun stuff to play with.
http://www.ohio.edu/people/postr/bapix/HP200AB.htm

-- Rich


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Wed 05, 2012 4:22 pm 
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Joined: Feb Thu 18, 2010 2:45 pm
Posts: 507
Location: Bellingham WA
I've ordered a pair of NOS 6k6 and will swap out the pair of 5881. I can use those for other projects. Learning that the output tubes were incorrect also made me think that perhaps that was the reason for the hum or ripple on the output. I plan on replacing the black individual molded plastic caps, then slowly powering it up on variac once the new tubes arrive and assessing the unit at that point. Who knows, maybe I can leave the e-caps alone for now at least.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 12:27 am 
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Location: Ohio 45177
If you decide to cal it, I think that model may have the plus or minus 2 percent dial accuracy, also. And I have not seen one that tracked perfectly even if you tried to align per the manual. That is why they give you the 2 percent. So if you need a very critical freq, attach a counter. I think you would only have issues trying to find parts if they are major parts or assemblies. And those can be salvaged from cheapo parts units. Caps, resistors, tubes, no problems. I would use fresh NOS tubes for the outputs though. Probably same brand and vintage, too.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 2:13 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 288
Location: Staten Island,NY
I've had mine about 8 years.When I got it I recapped the electros,usually necessary on old tube gear.All the non-electros,including those big black ones,were and still are fine.I went thru it a couple months ago,still good.If you're going to change anything "on the fly",I would suggest changing the electros.Just put your dc meter on those caps,if they aint leaking,dont worry about them.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 3:04 am 
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Joined: Jun Fri 01, 2007 9:46 pm
Posts: 177
Location: Chattanooga, TN
You might want to try finding a spare lamp, the one in the osc circuit. I have no idea if they are still made or not, but the exact lamp is critical for proper operation. It will never burn out (doesn't even light up) but the filament could be broken from physical shock.

Vince


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 3:56 am 
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Location: Maryland 20709, USA
Elrick wrote:
Who knows, maybe I can leave the e-caps alone for now at least.
While I don't recommend using the original caps, if you choose to do so, please reform them as follows:

Power the unit up for ONE minute, then off until the same time the next day.
Repeat at the same starting time on subsequent days for 2, 4, 8, 16, 32, and 64 minutes.

At that point the caps will be reformed properly if such is possible.
It's really a question of the integrity of the seals, which you can't evaluate. If the seals are intact, the caps should reform.

Be sure to monitor the current drain and general operation of the unit and shut down immediately at any sign of a problem.

I've used a lot of HP equipment from that era containing the original caps without problems, so it's not an unreasonable expectation.

- Leigh

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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 3:48 pm 
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When I replaced the caps in my VTVM, after trying to reform them failed, I had them in my hand and noticed that one felt heavier than the other.

Truth is, I only tried to reform one of them. I figured if I had to replace one, I might as well replace both, so didn't even try to reform the other one. I tried reforming them by connection to a power supply and slowly raising it by watching a milliammeter. I actually got to over 400 volts before suddenly the trend reversed itself and leakage became excessive.

But as Leigh said the seals go bad and then the electrolyte dries out. Out of curiosity I weighed them on an electronic scale I have. The one I thought was heavier weighed 11.2 grams. The lighter one was 10.4. Just out of curiosity I just might try and reform the heavier one... ;)


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Thu 06, 2012 9:15 pm 
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On the last two HP 200s I worked on the electrolytics were still OK, so I didn't change them out. I think if the oscillator has been sitting unused for years the caps may dry out, but if it sees any kind of regular operation the filter caps may last a lot longer.


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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Fri 07, 2012 1:54 am 
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.....


Last edited by Mikeinkcmo on Jan Fri 25, 2013 5:13 am, edited 1 time in total.

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 Post subject: Re: HP 200AB audio oscillator
PostPosted: Dec Fri 07, 2012 2:17 am 
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Location: Maryland 20709, USA
Sol wrote:
Out of curiosity I weighed them on an electronic scale I have. The one I thought was heavier weighed 11.2 grams. The lighter one was 10.4. Just out of curiosity I just might try and reform the heavier one... ;)
That's very interesting. I've never seen an actual weight comparison before.

I'd be interested in the results of your reforming test on the heavier one. :D

Thanks.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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