General test equipment discussion

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General test equipment discussion

Post by Wildcat445 on Mon Sep 01, 2014 9:52 pm

Would anyone, besides me, be interested in a discussion about the repair and maintenance of test equipment that we use on radios? Like VTVM's, scopes, signal generators, that kind of thing. IIRC, we have a member who is knowledgeable about test equipment. Perhaps we could twist his arm into helping out.

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Re: General test equipment discussion

Post by jukeboxman on Mon Sep 01, 2014 10:00 pm

yes I would

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Re: General test equipment discussion

Post by JasonAMFM on Mon Sep 01, 2014 10:21 pm

That sounds like a good idea.
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Re: General test equipment discussion

Post by Dale H. Cook on Mon Sep 01, 2014 10:55 pm

I would be interested. I started with service grade equipment, mainly EICO and Hickok, when I became interested in electronics about 1961. By the time I became an electronics professional in the 1970s I was buying and using older lab grade instrumentation, mainly GR and HP. My shop today is dominated by GR and HP plus a bit of Tek, but I still own some EICO and Hickok that I use in my business in my shop, so I am interested in discussing just about any brand or level of test equipment. Much of what I have is overkill for antique radios, but the general principles are the same, and I have been repairing my own instruments for decades, so I would be interested in discussing anything that others use in their shops or want to work on.
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Re: General test equipment discussion

Post by Wildcat445 on Mon Sep 01, 2014 11:24 pm

Thank you, Dale. How would you like to proceed?

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Re: General test equipment discussion

Post by Dale H. Cook on Tue Sep 02, 2014 8:31 am

We could begin by seeing if anyone has any equipment that they are having trouble diagnosing, repairing, or finding parts for, where another member may have a suggestion. I have gotten pretty good at finding, substituting, or (for mechanical parts) improvising, especially for repair shop equipment from the '40s, '50s, and '60s.

Lab grade instruments can be nastier, as they often are more sophisticated and complicated, can be more difficult to troubleshoot, and have less wiggle room for replacing parts with other than original parts. Case in point - my HP 651A test oscillator recently developed a problem - it covers 10 Hz to 10 MHz in six bands. It is my most used oscillator for AF, IF, and RF signals. It works only above 4 MHz on the top band, and the lower five bands are dead. Troubleshooting this beast is complex, as it incorporates three feedback loops, one positive and two negative. After a couple of stabs (and the HP manual is surprisingly unhelpful with this problem) I decided that a) I need a backup unit anyway, and b) having another unit to compare to this one would help. Saturday at Shelby I bought a another 651A and a very similar 651B, getting both for a fraction of what I paid for my 651A ~20 years ago. I may get to dig into my 651A later this week and will describe the progress here. First, though, I have to finish a couple of non-instrument (plus one Sencore tube tester) repairs for clients using Shelby purchases.

If you have difficult or interesting test equipment troubleshooting or repair stories share them here - no matter how long any of us have repaired instrumentation there are always things that we can learn from each other.
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Re: General test equipment discussion

Post by Ragwire on Tue Sep 02, 2014 8:55 am

I'm not sure I have any interesting stories or major issues at the moment, but I love collecting and fixing up test equipment (and using it on the bench)--both modern and vintage--as much as the old radios. I like building homebrew test equipment, too.

So I would be interested in a thread like this--very much so.

I do have one minor issue/question that is slightly annoying, but experience tells me it is probably the design rather than a tweak or repair--and that is with my old Heathkit SG-8 signal generator. On band A, 160kHZ to 500kHz, the waveform is awful. Looks like third harmonics. I would like it to be able to produce a cleaner sine wave at below band B, if possible. Using its 455kHz setting, though usable, produces a lot of image frequencies. I can post a picture of the waveform is anyone wants to see it.
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Re: General test equipment discussion

Post by Dale H. Cook on Tue Sep 02, 2014 9:40 am

Ragwire wrote:I like building homebrew test equipment, too.

I enjoy that as well, but my projects tend to be rather rococo. For example, I am currently building a box that simulates connecting two pieces of telephone line equipment through a central office. In radio broadcasting we still use a variety of specialized equipment that operates over dial-up telepnone lines.

Ragwire wrote:Heathkit SG-8 signal generator ... the waveform is awful. Looks like third harmonics.
I have seen similar comments elsewhere about high harmonic content of Heathkit, EICO, and other RF oscillators intended for the service, amateur, and hobbyist markets. It is probably a consequence of the simple Colpitts oscillator design. Let me have a look in a couple of my oscillator reference works (I already have a scan of the SG-8 assembly/ops manual) and see what comes to mind. The only suggestion that springs to mind from looking at the SG-8 schematic is to add a couple of hefty RF chokes to the plate supply lines for the two sections of the 12AU7. There is nothing to decouple the oscillator and buffer sections other than the power supply filter network - decoupling is always used in lab-grade RF oscillators of the period.

Ragwire wrote:I can post a picture of the waveform is anyone wants to see it.
Please do so - it could be quite instructive.
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Re: General test equipment discussion

Post by Wildcat445 on Tue Sep 02, 2014 9:57 am

I suppose the first two pieces of equipment I am having issues with are an old RCA Voltohmyst that I bought in 1966. It got 220 into it when it was set on "ohms" and several resistors got fried. It would be nice to see could it be repaired. Next is a Heathkit VTVM, IIRC model IM-28 (?) that just does not work. The tubes light and the pilot light comes on, but it won't measure anything. I have the instruction and assembly manuals, if those would help.

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Re: General test equipment discussion

Post by Dale H. Cook on Tue Sep 02, 2014 10:06 am

What is the Voltohmyst model number? I likely have an original or scan of the manual.

Check the VTVM model number. I have a scan of the IM-28U schematic (which came from a British gentleman!) but can almost certainly find the full manual online since Heath is ubiquitous.
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Re: General test equipment discussion

Post by Ragwire on Tue Sep 02, 2014 10:00 pm

Dale H. Cook wrote:I enjoy that as well, but my projects tend to be rather rococo. For example, I am currently building a box that simulates connecting two pieces of telephone line equipment through a central office. In radio broadcasting we still use a variety of specialized equipment that operates over dial-up telepnone lines.
Like an SxS switch? That's cool. After working a while in C.O.s back in the '80s, I didn't think I would ever get that clicking and clacking out of my head.  Laughing
Dale H. Cook wrote:I have seen similar comments elsewhere about high harmonic content of Heathkit, EICO, and other RF oscillators intended for the service, amateur, and hobbyist markets. It is probably a consequence of the simple Colpitts oscillator design. Let me have a look in a couple of my oscillator reference works (I already have a scan of the SG-8 assembly/ops manual) and see what comes to mind. The only suggestion that springs to mind from looking at the SG-8 schematic is to add a couple of hefty RF chokes to the plate supply lines for the two sections of the 12AU7. There is nothing to decouple the oscillator and buffer sections other than the power supply filter network  - decoupling is always used in lab-grade RF oscillators of the period.
Good info...thanks Dale!
Dale H. Cook wrote:Please do so - it could be quite instructive.
The first picture is a good sine wave at 1.0MHz--range B.
The second is simply switched to range A and the scope sweep halved. This is what my I.F. frequency signals look like.

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Re: General test equipment discussion

Post by Dale H. Cook on Wed Sep 03, 2014 8:26 am

Ragwire wrote:Like an SxS switch?
Not really - mine does not do any CO switching per se. This was designed first and foremost for testing a class of broadcast codecs that send remote broadcast audio with up to 14 kHz frequency response over copper phone lines by digitizing and bit-compressing the audio and transmitting the data with a 56k modem chip set. The transmitting chip set needs battery, and has to be fed call progress tones including dial tone, audible ring, busy, and reorder. The receiving chip set needs battery and ringing voltage. Both RJ-11 jacks have off-hook detectors.

When the transmitting device goes off-hook it gets dial tone. The chip set detects dial tone and dials. DTMF and pulse detectors detect the dialing and five seconds after that stops audible ring is fed to the transmitter and ringing voltage to the receiver. When the receiver goes off-hook the two RJ-11 jacks are connected to each other, the modem chip sets negotiate, and the audio connection is established.

A panel switch allows other conditions to be simulated after dialing, including busy, reorder, and no response from the CO.

Ragwire wrote:The first picture is a good sine wave at 1.0MHz--range B. The second is simply switched to range A and the scope sweep halved. This is what my I.F. frequency signals look like.
Yup, that looks like third harmonic. I'm stacked up with work for the next couple of days but when I find odd moments I will look at what some more expensive signal generators have used for decoupling. Do you have any unused RF chokes kicking around? I have a bunch, all measured and sorted by value (stored in the main parts cabinet in my Explorer for work), from purchasing many of the old Archer-Pak choke and coil assortments at Radio Shack back in the day. If not I think Radio Shack still carries 100 uH RF chokes so we could maybe try those.
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Re: General test equipment discussion

Post by Ragwire on Wed Sep 03, 2014 10:23 am

Your switching box is way over my head. Laughing

I have a few various scrap coils and a new (or two) 100uH Rack Shack R.F. choke(s) in my parts boxes for general purposes. I'll give it a try and see what happens.
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Re: General test equipment discussion

Post by Dale H. Cook on Wed Sep 03, 2014 12:15 pm

Ragwire wrote:Your switching box is way over my head.
The logic in it was getting beyond what I wanted to cope with. The problem with hardware-based logic for a complex project, whether CMOS or TTL, is that it can get so complex that it becomes a major problem to troubleshoot. Early in the development of this project I bit the bullet and switched to an Arduino Uno microcontroller board to implement the logic, as it needed only a handful of glue chips such as latches and buffers. The Arduino learning curve was not as steep as I expected because the programming language is essentially a subset of C, and the power and flexibility of using a microcontroller board has allowed me to incorporate features that I would not have added with hardware-based logic. I have a couple of other projects in mind that I will also implement using Arduino - I built myself an Arduino Uno prototyping station to simplify the development process, and already have a couple of Unos on the shelf for future projects.

Ragwire wrote:I have a few various scrap coils and a new (or two) 100uH Rack Shack R.F. choke(s) in my parts boxes for general purposes. I'll give it a try and see what happens.
Let us know here if that helps. If it does not we'll drop back and punt - there should be some relatively painless way to improve that A band waveform.
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Re: General test equipment discussion

Post by Ragwire on Wed Sep 03, 2014 11:52 pm

I would like to learn Arduinos...but not yet...still having fun with tubes.

So, on the SG-8: Apparently it's not an SG-8 model after all, or maybe it is an early version of same. I've had the cover off plenty of times but never paid any attention to the tubes. When I went to find the pinout for the tubes it turned out that mine has only a pair of 6CU4) I think they are) triodes. Anyway, I put a 100uH RF choke in the plate supply but it didn't make any difference. However, I also stumbled onto the "good-enough-for-me" solution, I guess: Some time ago I installed a homebrew broadband transformer coupled, solid state amplifier inside of it to boost the RF output by a factor of ten or so when I throw a switch. I never had it on a scope with the booster amp inline before, but it cleans up the wave almost perfectly, and I can still turn the attenuator down to basically zero for low output levels. I suppose the inductance of the hand-wound coupling transformers is filtering out the extra harmonic content?
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Re: General test equipment discussion

Post by Dale H. Cook on Thu Sep 04, 2014 9:19 am

Ragwire wrote:Apparently it's not an SG-8 model after all ... mine has only a pair of 6CU4) I think they are) triodes.
They are probably 6C4 triodes, making it probably an SG-6 or SG-7. Have a look at the manuals - my copy of the SG-7 manual is partial but does have the schematic and parts list:

[url]http://plymouthcolony.net/starcity/radios/files/Heathkit SG-6.pdf[/url]
[url]http://plymouthcolony.net/starcity/radios/files/Heathkit SG-7.pdf[/url]

Note that on the cover of the SG-6 manual the "6" is crossed out and an "8" written in, but it is an SG-6 manual, not an SG-8 manual. The SG-8 uses a 6C4 triode audio oscillator and a 12AU7 dual triode RF oscillator-modulator:

[url]http://plymouthcolony.net/starcity/radios/files/Heathkit SG-8.pdf[/url]

It sounds like your transformer has cleaned it up, though, which is what counts. What kind of transformers did you wind - perhaps either toroid or air core??
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Re: General test equipment discussion

Post by Ragwire on Thu Sep 04, 2014 11:43 pm

That's it! 6C4 tubes.
You're a life saver...I couldn't find a schematic. Thank you thank you!

The input and outputs are both solenoid wound on straight powdered iron cores. I don't remember the turns now. Probably like 100-200 turns--in the several millihenry range.
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Re: General test equipment discussion

Post by Dale H. Cook on Fri Sep 05, 2014 7:49 am

Ragwire wrote:The input and outputs are both solenoid wound on straight powdered iron cores.
Do you have a winder? Winding that many turns by hand would appear to be a bear of a job. I've looked at some on eBay but the more versatile one are more than I want to spend. Most of what I wind these days is on toroids, anyway.
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Re: General test equipment discussion

Post by Ragwire on Fri Sep 05, 2014 9:51 am

No, I just put the core (or form for an air-core) in a 90 degree drill chuck adapter in a vice and put the wire spool on a rod made so there is a little friction to turn the spool--to keep the wire tight--then turn the chuck's input gear shaft with one hand and guide the wire laydown back and forth with the other. So kind of a slap-together-as-needed winder, I guess.
What I really would like to build is a universal winder that I can just crank and it will do the rest automatically. I have plans somewhere but never took the time.

As an aside, regarding test equipment...ever seen one of these signal tracers?

It is the same style as my SICO TW-11 tube tester with its wooden case. This CA-11 tracer has its grid leak detector/amplifier (a 1T4, 7-pin miniature pentode) inside the wooden probe handle. Battery operation only. I just bought this and would sure like to find a user's manual for it.

I thought it was interesting as I have never seen one before.
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Re: General test equipment discussion

Post by Dale H. Cook on Sat Sep 06, 2014 12:41 am

I've never seen a CA-11 before, and don't know where you could find a manual. I presume that you've checked all the usual suspects such as BAMA. It does look really neat.
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Re: General test equipment discussion

Post by Ragwire on Sat Sep 06, 2014 8:18 am

Yeah, looked everywhere I can think of. I put a WTB ad up on the "other" forum too.
The seller said he'd keep an eye out for the manual when they continue cleaning up for the estate sale from which it came. Not necessary--but it would be nice to have that manual.
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Re: General test equipment discussion

Post by mikeinkcmo on Mon Dec 08, 2014 12:50 am

I enjoy working on TE and that is how I accumulated the gear I now have.   I spent a lot of time on the auction site buying good lookers that didn't work, or didn't work well.   Fixed them, and cal'd them as best I could at the time, used most for a while and sold those I didn't think much of or didn't really need, for a profit.   This allowed me to buy more and better gear, and ham radio stuff as well.

Service grade boxes use harmonic rich outputs purposely so they can be used on higher bands.   There are a few that didn't do that but most did.


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Re: General test equipment discussion

Post by Ragwire on Mon Dec 08, 2014 10:57 am

The sine looks good on my SG except for that one range, but you raise a good point about the harmonics. I just ordered a new signal generator (as in really new in box and solid state). I thought it would be nice, though I really have what I need to do what I do. I'll post pics when it comes.

Rob
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Re: General test equipment discussion

Post by mikeinkcmo on Tue Jan 13, 2015 10:41 am

Have you received your new generator yet? Some pics would be nice.

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Re: General test equipment discussion

Post by Ragwire on Tue Jan 13, 2015 9:03 pm

I did, and I am quite impressed for what it is. I expected a flimsy little thing, but it is a heavy, metal cased unit with a sturdy cable, and it works very well.
Max. is 450 MHz on harmonics, 150 MHz on fundamental.

Rob







You can see, if you look closely, a bit of harmonic on this 25MHz, 1 VPP sinewave:
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