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-   -   Modernizing classic analog oscilloscopes (http://www.diyaudio.com/forums/equipment-tools/212543-modernizing-classic-analog-oscilloscopes.html)

hollowman 12th May 2012 01:48 PM

Modernizing classic analog oscilloscopes
 
This topic got almost no attn a while back ...
"http://www.diyaudio.com/forums/parts...illoscope.html"
... but since I just got an eBay-purchased vintage (1976) Tektronix 465 to work (and work well), I thought it would be worth a redux.
Now, I'm not talking ancient (= tube) 'scopes here (nothing wring with them but they are not very practical).
Remaining in analog, but moving into the sold-state world ... the Tek 465 may be the most popular o'scope of them all: so solid-state workhorses from the early '70s onward are IMO worthy of some forum attn ;) (The Tek 465 was actually recommended to me, off-list, by several DIYAudio long-time members!)

There are a few web sites and articles on repairing the Tek 465, but not much on really IMPROVING it.

I'm guessing the same tweaks that we use to revamp and improve classic or under-performing audio gear applies to test equipment as well.
E.g., the 465 uses a few classic dual-ch op-amps. This site ...
Repairing a Tektronix 465 (power supply, z axis amplifier, vertical pre amplifier issues)
... claims that subbing those OPAs for more-modern NE5532 may help a little.
Dunno :rolleyes:
But replacing vintage and/or worn-out electro caps ... and perhaps adding more bypassing and improving rectification ... may tighten things up.
Which brings me to the query ... what exactly is the $ (perhaps overrated) procedure of re-calibrating and "certification"?
Thx for any input you can provide!

Loudthud 13th May 2012 02:30 AM

First of all, do you have a 465{nothing} or a 465B? (Maybe a 456M?) You better know that when it comes time to find a service manual. The wrong manual will be useless. There may be some slight differences depending on serial number. Not much point subing opamps unless one dies and you can't find an exact replacement. There are some Tek made IC's, you'll need an exact replacement. If you can find a dead scope like yours for spare parts, buy it. I killed a circuit breaker in my 576. Replacements are going for $25 on ebay. Keep the spare in a low humidity environment. Humidity is the enemy of the high voltage parts that are unobtainium.

Calibration requires a little bit of specalized equipment and some time. At the beginning or each section of the calibration procedure, there is a performance check. If it passes, most techs will skip to the next section. They won't take the time to get dead flat square wave response or tweek the input attenuators dead flat with one "comp box" so your probe will look the same on both channels. They'll just get it close enough to pass the performance test. If you want the best, you'll have to do it yourself.

hollowman 13th May 2012 03:00 AM

loudthud:
I have 465{nothing}. It was made in 1976, S/N B313496.
I got it from eBay a few years ago. It was orig. owned by the FAA and was/is in immaculate cosmetic cond. inside and out. FYI, I bought it DEAD for (total incl. S/H ) $96. The non-function was due to a shorted tantalum (this is the most common failure on these units -- great YouTube video on it). Replacing it brought it back to life.
I found the "orig" Service Man. online.

Your suggestion to get a 2nd unit for spare parts wise. I live in Southern Calif. where humidity is not really an issue. IAC, I keep it in a tight HVAC-controlled, dust-free environ.

You noted:
"If you want the best, you'll have to do it yourself."
How does one do this? Where to begin?

BTW, I think the upgrade I hear most freq. about is replacing electro caps. Ergo ...
What about those orig. large metal electro caps in the PSU? How long do electros in that style (form that era) last (time-wise and/or chemistry-wise)?

Loudthud 13th May 2012 05:21 AM

Aluminum Electrolytic life depends on how often they are used and how they are stored. They can last 20 to 30 years under the best conditions. Tek power supplys are conservatively designed and don't put too much stress on the parts. Failure of the Tantalum caps is rare.

It's pretty hard to find an exact replacement for the Aluminum can caps. The easiest replacement is a radial lead type. Current production examples are smaller so space isn't a problem. Consider 105C rated parts or long life types if you can find them. Note that you may have to jumper some of the minus terminals to complete connections on the PCB. You'll need a pretty big soldering iron to unsolder the old parts and you'll have to improvise mechanically securing the replacement caps. Use hot glue, RTV or tie-wraps. You could remove the guts of the old caps and install radials inside if you want to go to that much trouble.

The first thing you tweek is the power supply. There is usually one "reference supply" with a tweek, all other supplys use it as a reference so no adjustment is required with the possible exception of the high voltage. The cathode of the CRT runs something in the -2KV to -5KV range, the anode runs off a multiplier to something like 10KV to 20KV, you don't even measure that.

Starting at the input connector there is the AC/GND/DC switch, then an X1/X10/X100attenuator. Those attenuators need to be tweeked to set the input capacitance. You use a "Comp Box", a simple box with a 1meg resistor in parallel with a mica cap of whatever the input capacitance is supposed to be, IIRC 20pF, between the generator and the scope. You can use a low frequency function generator, 1KHz or 10KHz to tweek those caps and resistors. The manual will tell you what to use.

Next inline is the JFET follower and the X1/X2/X5 amp. Any tweeks in that area usually require a good fast pulse generator. The service manual will outline specs needed. Something with a rise time faster than 2nS would be needed for a 100MHz scope. The rest of the Vertical amplifier is tweeked with the pulse generator. After the step response is tweeked, a leveled sinewave generator is used to verify the 100MHz bandwidth. This type generator has a level detector at the end of the cable so that the amplitude is constant.

Horizontal amps are only good for about 1MHz. Then they are tweeked for sweep speed accuracy with ramps from the sweep generator. There may be some tweeks in the trigger circuits and ramp generators. A "Time Mark Generator", a special generator that puts out pulses that make it easy to tweek and verify all the horizontal ranges of the scope is needed. They usually have a 100MHz crystal and divide down for slower pulses, higher ranges are done with sinewave multipliers. You force 500MHz through a 100MHz scope to check the 2nS per div sweep speed.

The value to the audio diyer is getting the squarewave response as flat as possible at all frequencies. There are tweeks in the ranges that will be used in audio. You hate to find that the little rinkle your amp makes in a 10KHz square wave is caused by your scope.

DF96 13th May 2012 01:40 PM

I would hesitate to redesign a Tektronix scope. Those guys did really know what they were doing. Yes, your innocent opamp 'upgrade' is a redesign. If you ruin the performance of an audio amp by an 'upgrade' then half the listeners (and the perpetrator) will think it is an improvement, but a 'scope deals in facts not emotions.

Conrad Hoffman 13th May 2012 05:54 PM

Quote:

Originally Posted by DF96 (Post 3021820)
I would hesitate to redesign a Tektronix scope. Those guys did really know what they were doing. Yes, your innocent opamp 'upgrade' is a redesign. If you ruin the performance of an audio amp by an 'upgrade' then half the listeners (and the perpetrator) will think it is an improvement, but a 'scope deals in facts not emotions.

+1 on that! There's almost nothing you can do with a given vintage scope that will provide an improvement. Those old ones are getting long of tooth and they tend to run warmer than audio gear. I'd be looking at caps with great care. As the regulars know, I promote measurement over wholesale replacement, but you have to measure both value and loss. At least put an esr meter on it.

Scopes tend to sit around for long periods between use, and that's really bad for caps. The advice about low humidity is also right on. Some older Tek scopes tended to have HV transformer failures, but stored and used under low humidity conditions failures will be rare. The lubricant in the controls gets stiff over time, and plastic couplings get brittle. Clean and relube so you're not stressing things every time you turn a knob.

If you're not using it, it's still a good idea to power up your scopes (and other equipment) about once a month for a couple hours. That will keep moisture under control and the caps in better condition.


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