Oscope ?

[salvarsan]

The cal point is a square wave. It's used to adjust the probe. There's a probe compensation, and with the cal point you adjust the probe for the best square wave. (flattest top). I'd still recommend a good signal generator.

I'm trying to ease gtrplayr1976 into deep water a small step at a time so as to allow easy assimilation of concepts and minimize trauma.

You louts are throwing him into the shark pool but mentioning only after the fact that sharks can sense electric fields so he'll have to ditch his cell phone.

Dare I suggest that one proceed gently and slowly with a noob?

As for dummy loads, just eBay it.

It's cheaper than grabbing a handfull of 100 watt brown devils and crufting them into a proper resistance (cough!) ...unless you like that kind of thing.

[JoeM]

Dare I suggest that one proceed gently and slowly with a noob?

Oh, yes, by all means salvarsan. But I felt the use of the cal signal for someone just getting started is even more difficult to use being a square wave, and possibly difficult to connect to unit under test. Plus there's no easy way to adjust the cal amplitude (some scopes maybe - my old 561A for example), and no frequency adjustment.

[gtrplayr1976]

Nope that's not it. What I have for loads are 2 rack panels with an array of 20 resistors on each, 1000W per panel & a fan behind that (fan rarely used unless really big power amps are on test). Got the resistors half a buck apiece on surplus.

ALSO a couple of "portable" loads, each one a box with resistors inside, about 3x4x6 inches. One of THOSE singed my workbench. (insert DUH smiley here = me) Without heatsinking or fan those boxes can get hot. Don't find out the hard way.
Here's a nice 50W 1% 8 ohm @ Mouser, $5.08 ea. get 4, set up series & parallel pairs for 100W 16 & 4, and series/parallel for 200W 8 ohm. Affordable, and tough BUT you will have to bolt to metal panels to properly dissipate their heat. Any castoff heatsink or chassis or panel should do. You may be able to find similar R's for sale on surplus somewhere.

71-RH0508R000FE02 Mouser part #

linky:

http://www.mouser.com/ProductDetail/...P0g27agDHvk%3d

Thanks Leo_Gnardo ! That will save me some cash too. I have some heat sinks on an old Peavey powered PA I can use also.

[km6xz]

I am using variable resistors just like the one you showed, except it is 10 ohms. With two slides it can give 4 and 8 ohms. Or set the slide in the middle and use the ends for 10 ohms which is close enough for a 8 ohm amp transformer. If you need 4 ohms, connect the wires to the slide in the center and connect the two resistor ends together with a clip lead and have a 5 ohm load with more power handling capability than if you used just the slide and one end.
When you measure the output across the load, use the AC meter on your meter, and watch on the scope for the highest amplitude that is not clipping. Multiply that voltage times itself then divide by the load resistance. Having 10 ohms makes it a "in your head" calculation. That is the RMS power output. If your meter has True RMS detector in it(it will say on the outside since it is a valuable trait to have), you do not even have to take the reading just below clipping since the waveform does not matter for an accurate RMS reading if the meter has that capability. That is useful because guitar amps are regularly driven well into clipping in actual use.

[km6xz]

For not much money you have the makings of a very competent shop for your amps and experiments. You will learn much faster with this level of gear which is calibrated and reliable than junk. The 2213 scope is the little brother to one of my favorite tech scopes, the 2236.
I bought the one on my bench for $3600 when it was first introduced back in the mid 80s. It has a digital display for time, voltage, resistance, continuity, and temperature. So it can be used as a frequency counter and with the optional TXCO time base, a very accurate and low drift time base(just the TCXO option was $600) but unless you are setting clock frequencies for precision RF circuits the stock time base is good enough. The side panel test lead jacks can be used or it can be switched to be monitoring whatever the probe on Channel 1 is seeing. So a power output measurement is just a scope probe on the load where you see the wave form and read off the true RMS voltage to 6 digits on the display.
The 2213 does not have that digital readout but it is a solid but light weight scope that once it is calibrated, will last you for many years with consistent and repeatable display.

Now, a cheap Chinese variac and you are all set to test, experiment, learn, build and invent.....
I have a 800 watt Chinese metered variac that cost only $100 here so in the US it ought to be only $50. You can get a digital AC current meter to add to it in addition to the built in voltmeter, on ebay for $6 and have a very useful tool. It will tell you precisely how much current your amp under test is pulling from the AC mains. Good to know if there is a problem.

[gtrplayr1976]

I am using variable resistors just like the one you showed, except it is 10 ohms. With two slides it can give 4 and 8 ohms. Or set the slide in the middle and use the ends for 10 ohms which is close enough for a 8 ohm amp transformer. If you need 4 ohms, connect the wires to the slide in the center and connect the two resistor ends together with a clip lead and have a 5 ohm load with more power handling capability than if you used just the slide and one end.
When you measure the output across the load, use the AC meter on your meter, and watch on the scope for the highest amplitude that is not clipping. Multiply that voltage times itself then divide by the load resistance. Having 10 ohms makes it a "in your head" calculation. That is the RMS power output. If your meter has True RMS detector in it(it will say on the outside since it is a valuable trait to have), you do not even have to take the reading just below clipping since the waveform does not matter for an accurate RMS reading if the meter has that capability. That is useful because guitar amps are regularly driven well into clipping in actual use.

Thank you. What do you mount the resistors with ,and what do you mount them on ?

[km6xz]

Tubular power resistors usually have mounting hardware available that connects little brackets onto either end of the resistor and lifts the resistor 1-2 inched above whatever flat piece of wood or metal you might have floating around. The metal cases power resistors need to be mounted on a heatsink because they dissipate their heat by conduction. Teh tubular resistors dissipate by convection air currents or radiation, and are not dependent on a metal heatsink. Both work fine.
If you have the tubular resistors and no mounting hardware, go to the hardware store and get a couple metal angle brackets (L brackets) used for carpentry/wood construction. Also get a threaded 1/8 or 1/4 inch rod about two inches longer than your resistor, plus nuts for the thread, some wood screws short than the thickness of your mounting board. Screw down the L bracket to the wood the same distance apart as the resistor. Bore out one of the nail/screw holes on the vertical sections of the L brackets and to fit the rod.
Push the rod through the first bracket, through the center core of the resistor and through the other L bracket and fasten the nuts on both sides. Secure, cheap and strong mount for dummy loads.

[soundmasterg]

For not much money you have the makings of a very competent shop for your amps and experiments. You will learn much faster with this level of gear which is calibrated and reliable than junk. The 2213 scope is the little brother to one of my favorite tech scopes, the 2236.
I bought the one on my bench for $3600 when it was first introduced back in the mid 80s. It has a digital display for time, voltage, resistance, continuity, and temperature. So it can be used as a frequency counter and with the optional TXCO time base, a very accurate and low drift time base(just the TCXO option was $600) but unless you are setting clock frequencies for precision RF circuits the stock time base is good enough. The side panel test lead jacks can be used or it can be switched to be monitoring whatever the probe on Channel 1 is seeing. So a power output measurement is just a scope probe on the load where you see the wave form and read off the true RMS voltage to 6 digits on the display.
The 2213 does not have that digital readout but it is a solid but light weight scope that once it is calibrated, will last you for many years with consistent and repeatable display.

Now, a cheap Chinese variac and you are all set to test, experiment, learn, build and invent.....
I have a 800 watt Chinese metered variac that cost only $100 here so in the US it ought to be only $50. You can get a digital AC current meter to add to it in addition to the built in voltmeter, on ebay for $6 and have a very useful tool. It will tell you precisely how much current your amp under test is pulling from the AC mains. Good to know if there is a problem.

Stan,

What do you think of the Tektronix 2235 scope? Any experience with them? I have two of them that I use and so far they have worked well for what I need. I've heard things about some of the Tek scopes of this era and transistors not being available should something fail, but not sure if it applies to this model or not.

Greg

[km6xz]

Hi Greg, the 2235 is a good 100Mhz scope as you have found out. They were a line that was created specifically for service techs and not as a lab type scope so it is cheaper, lighter in construction and uses more integration. Although it was cheap, about $2400 when new, which was cheap for Tektronix, they proved to be very reliable and most made are still functioning without any maintenance.
I would not be concerned about parts availability, it is not likely to fail any time soon and if it does, getting a parts unit for $50 or less should provide just about any part needed.

The only difference between the 2235 and 2236 is the 2236's digital display and DMM. It is even possible to convert yours to the 2236 since getting a junker 2236 would probably have the few modules needed to install the DMM. There is one pc board, swap the front panel and outer case for the opening for DMM probes and add one wire to the Cha 1 input and a connector to couple the display into the time base for frequency, delay time, time difference etc measurements.
You scopes should outlast a 1/2 dozen of the low cost digital scopes. Its more sensitive than a 465 with 2mv/dv so and has a very quiet front end pair of vertical amps. You can see and characterize low level noise reading better than on even most lab scopes, and probably any digital scope.

[pontiacpete]

I have a 2336. It's been a very reliable scope for the past 5 years.
This past month something happened to it. I don't know if I am describing this right, but one side of the trace went out,
so in order to get a trace across the screen I have to move the horizontal to one side.
If I start a new thread is this something you guys might be interested in helping me repair?
I haven't opened it yet but I expect that august will be slow enough for me to put it on the bench.

[Tom Phillips]

I have a 2336...If I start a new thread is this something you guys might be interested in helping me repair?...

You can get specific help on Tektronix scopes by joining the group at TekScopes : All about classic Tektronix CRT o'scopes . You often get help from people who worked at Tektronix on the scope. Sometimes a member of the original design team even chimes in.

[gtrplayr1976]

I have been looking for a decent price on these. Most of what I've seen on Ebay are kind of high,but if that's the only place then so be it. Thanks !

[g1]

Price of what? Variacs?

[gtrplayr1976]

Price of what? Variacs?

Power resistors

[g1]

Try Mouser, Digi-key, Newark etc.