The best-kept test equipment secret of them all...

[N8YX]

Looking to put a decent bench together on a budget?

Be on the hamfest lookout for Tektronix 7000 series mainframes and plugins. Until late last year I had absolutely no idea what this series of gear was all about and it's too good not to share.

Tek made a number of different oscilloscope chassis and measurement plugins in the series from the early 70's until the mid 90s. It was expensive as hell at the time, fairly affordable at present as surplus - and the best part is that a piece from the 70s will work with a piece from the 90s.

Mainframes: Bench and rackmount models were offered. Most came with an on-screen alphanumeric display (which I'll get to in a moment). Of the benchtop series, the two I would look for are the 7603 (a 100MHz unit with a wide CRT) and a 7104 (1GHz; widest bandwidth of any analog scope ever built).

WEBTEKTR7603.jpg tek7104front.jpg

The key to these instruments' flexibility are the many plugins which are offered. The 7603 (at top) has a spectrum analyzer installed while the 7104 underneath it has a standard vertical amplifier/timebase complement.

In subsequent posts I'll discuss each plugin type and any associated quirks, hints and needed accessories.

[N8YX]

First, a further word about the mainframes and what they do. There are a number of them from which to choose; with the exception of the 7104, the 7600 series and above are probably the most desirable as they offer bandwidths greater than 100MHz. Both storage and non-storage variants were offered. If one is planning to use the chassis for housing a curve tracer, multimeter or similar readout-based plugin then any of them (including sub-100MHz B/W models) can be pressed into service.

Tek incorporated an on-screen alphanumeric display which was facilitated through a custom character generator/display board. Depending on age of the scope there are two versions of the board. They're interchangeable can can be retrofitted to a scope which came from the factory without one. The readout enabled display of operating parameters, and - by way of the various plugins discussed in subsequent postings - turned the chassis into much more than a simple oscilloscope.

As with all vintage electronics, the main failure point of these instruments are the power supply electrolytic capacitors and the numerous tantalums which are used for de-spiking purposes on each board.

Do yourself a favor and shop the hamfests before turning to eBay for a 7000 series mainframe. Most offerings these days are from commercial gear dealers whose overpriced wares sit for months if not years...and the estate liquidators following suit.

You may catch a deal on a complete setup from time to time but shipping charges will often equal the price of your purchase.

While a number of custom ICs were used in the equipment, they rarely fail - and there are so many surplus units available that donor parts are fairly easy to acquire.

[X-Rated]

Yes. Those are oldies but goodies. I used one of the bench models back in the early 80's with the spectrum analyzer plug-in. 10kHz? to 1500MHz? I don't remember. It had a beautiful display.

Anyway, looking at pictures on the web, it was a 182T and an 8558B plug-in.

[X-Rated]

Now I have an HP ESA-L1500A that is incredible. (at work) It has a built-in frequency counter that allows you to count the frequency of the signal under the marker to 1Hz resolution. 1Hz may not sound that good, but some of the 900MHz carriers seem to be pretty accurate and 1Hz at 900MHz is about a part in ten to the ninth resolution. (1ppb) Also, counting a signal in a crowd of signals is a pretty nice capability.

[N8YX]

Now I have an HP ESA-L1500A that is incredible. (at work) It has a built-in frequency counter that allows you to count the frequency of the signal under the marker to 1Hz resolution.

I had to do a bit of searching for the model you mentioned and upon checking it out will segue into an observation about Tek plugins:

No matter how nice they are, they're NOT worth more than a stand-alone, single-purpose instrument of the same basic capabilities.

Thus...we begin the discussion with timebases. If a given seller asks more for a timebase than an entire scope with an equivalent vertical amplifier response is fetching, simply walk away.

For any of the 100MHz platforms, a 7B50 (non-delaying), 7B51 (delaying) 7B53A/AN or 7B70/7B71 works just fine. 7B80/85/87s work to 400MHz, 7B90/7B90P, 7B92 and 7B92A to 500MHz. For the 7104 you'll want a 7B10 or 7B15, the latter being a delayed sweep. Pay no more than $100 or so for the fastest units and at most hamfests they'll go for considerably less.

[NQ6U]

Good info, Fred, thanks. I'm headed out to the Yuma Hamfest this weekend, I'll keep an eye out for some of this. A spectrum analyzer would be a nice thing to have.

[N8YX]

Carl, an area friend scored an entire setup (7844 dual-beam unit w/ all plugins) for the princely sum of $25.

No, I'm not kidding...and it all works.

Vertical amp time:

There are a bunch to choose from. Be on the lookout for the 7A11 (single-channel, 250MHz), 7A12 (two-channel, 120MHz), 7A18 (two-channel, 75MHz...great for pairing with a curve tracer or other low-bandwidth module), 7A24 (two-channel, 400MHz), 7A26 (two-channel, 200MHz), 7A19 (single-channel, 500MHz) and 7A29 (single-channel, 1GHz).

A number of specialty vertical plugins exist. Most of these sit on dealers' shelves, never to move because they're far overpriced.

I just scored a nice late model 7A13 (100MHz differential amp) for $40. Of course it's "as is" and uncalibrated but one of the very first things I did when the 7000-series gear came rolling in was to buy any needed calibration fixtures. Thus, I can service and calibrate any of the stuff (including 500/5000 series plugins) to Tek standards.

You use this particular plugin for comparing two voltages and displaying the result. A typical example is tracing an AGC voltage through a receiver: Look at the AGC Amp output with one channel and the pathway through each sub-module or connector header with the other.

[N8YX]

Since Carl touched on the need for a spectrum analyzer, I'll delve into those.

Tek offered several in the 7000 family: 7L5 (20Hz-5MHz), 7L12 (100KHz-1.8GHz), 7L13 (1KHz-2.5GHz), 7L14 (10KHz-1.8GHz; w/ storage) and 7L18 (1.5-18GHz; 60GHz with an external mixer. Also has a storage function.)

The most useful to the amateur (in order) would be the 7L14/7L13/7L18/7L5, though if you can snag a 7L12 for a song by all means do so. It doesn't have a digital center frequency readout but is a great addition to the bench nonetheless.

It's easy to blow the mixers in the units if you transmit full power directly into them. If at all possible, examine the prospective purchase when it's powered up in a mainframe and looking at its internal calibration generator signal. Of course, these can be fixed so don't let the "as is" part put you off if you spot one going for less than a C-note.

Pricing...the HP unit Jerry mentions is currently fetching ~$3K and is in many ways far advanced beyond the Tek equipment. That doesn't matter to the average hobby user or low-volume repair lab except where cost is concerned. Kindly remind clueless sellers that there's no way a 30-year-old analyzer should fetch the same amount as its new, state-of-the-art equivalent. You should realistically spend no more than $400 for a 7L13/14 or $500 for a 7L18.

[X-Rated]

I mentioned 2 HP units. One from the 1990's that you quoted the price, and the other is from the 1980s and would be much less expensive. The 182T, even relatively new, I tried everything to keep it operational well, but I remember now having to pull the plugin and re-seat the connector in the slot. I didn't like that, but the calibration was fair and the trace was excellent. I just had the spectrum analyzer plugin.

I would agree that Tek is better than HP at least until the 90's. HP had issues. For the most part, HP/Agilent had cleared up and became more competitive.

[N8YX]

I mentioned 2 HP units. One from the 1990's that you quoted the price, and the other is from the 1980s and would be much less expensive. The 182T, even relatively new, I tried everything to keep it operational well, but I remember now having to pull the plugin and re-seat the connector in the slot. I didn't like that, but the calibration was fair and the trace was excellent. I just had the spectrum analyzer plugin.

I would agree that Tek is better than HP at least until the 90's. HP had issues. For the most part, HP/Agilent had cleared up and became more competitive.

What was the bandwidth of your 182T analyzer plugin, Jerry?

FWIW...my area friend (who got the deal on the 7844) has a couple 182Ts and swears by them. IIRC, one's going to be coming up for sale - it's a 500MHz B/W unit. I'll check with him this weekend if anyone is interested.

90s era HP - would love to lay my mitts on a 4396A 1.8GHz network/spectrum analyzer. Besides a few remaining Tek 5000 pieces plus a 7934/7104 mainframe combo, that's really all the bench still needs in the way of equipment. With an average retail used price of $6500 it'll be a year or so before fundage permits.