Tek 465B overheats?

I find it useful when I've forgotten where the trace is centered and I'm interested in DC level. All it does is short the signal to ground. (Actually, I don't know whether it shorts the probe tip or the output, which would be quite different in the case of a x10 probe. Suppose I should find out before I inadvertently ground something that shouldn't be grounded.)

Jay Levitt wrote in news:MPG.1ab09ba16ed2c04698970f@news- central.giganews.com:

Yes,if you have an accurate (digital)signal generator or time-mark generator to calibrate it.(IOW,some known -standard- to compare to)

First thing is to check the power supplies for the right voltages and ripple(need a scope for this).The manual has the specs.Then,to duplicate the failure,take it out of the cabinet,wrap it in plastic sheeting,let it warm up and fail.Then check the CRT pins to see what direction the trace has gone off-screen,vertically or horizontally,or if the sweep ramp is missing and it's just in holdoff or 'ready to sweep' mode.You may need another scope to troubleshoot this one. Read the circuit description in the manual,its a great help in understanding how the scope works,especially the trigger and sweep circuits where your problem may be.

The GND REF button is just to let you see where you set the trace before applying a signal,without lifting the probe tip off the DUT or using the scope's ground switch. A convenience.

I figure good-quality audio D/A converters should be enough for hobbyist use, at least up to 22kHz. I'm not knowledgeable enough to build an oscillator myself, and I presume that any used calibrators I might find on eBay are themselves equally suspect.

Sure, if I understood electronics :) Guess there's no time to learn like the present, and I did just order the Don Cannon book. Right now, a schematic is as useful to me as assembly source code is to my mom... I'll check out the PS and the CRT pins and see what I find.

Make sure that the fan is running. These actually have a small fan pulling air in the back end.

It grounds the input to the scope. It does NOT short the probe tip to ground. (The probe tip is 9 MOhms away from the point that is grounded.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Jay Levitt wrote in news: snipped-for-privacy@news-central.giganews.com:

Yeah,but 22 Khz is not going to cover much of a scope timebase range.

Actually,for a scope timebase calibrator,one of those small clock oscillators and a few 74C90 divider ICs,and you've got a great TB calibrator;Electronic Goldmine used to sell a kit for making one,don't know why they stopped.You can get a nice 1,2,5 divider ratio from the 7490s,and use a 1 or 10 Mhz clock oscillator module. 100 PPM is more than enough accuracy for a 3% scope timebase.

Good call. I had noticed the fan filter, and even observed that it was fairly clean, but did not follow that to its logical conclusion: "Boy, that fan sure is quiet. You can hardly tell it's even... oh." With that evidence of my powers of observation, and the caveat that this is probably my first serious component-level troubleshooting project...

I did some experimenting with my Fluke 867B DMM/pseudo-scope on the fan card. The card is getting a pretty steady 14.91V on the +15V input, but other than that, the readings seemed to bear no relationship to what was supposed to be seen:

The non-grounded end of R8058: was 3.39V and climbing slowly, should be

Pin 3 of the fan: was 4.28V and dropping slowly, should be 1.6V. Pin 4 of the fan: was 3.84V, s/b 1.6V. Pin 7 of the fan: was 4.81V, s/b 1.6V. Q8067 base: was 14.78, s/b 14.5V. Between C8064 and R8054: was 14.78V, s/b 5.4V.

So something is wrong with this board. I'm not really sure what to do now.. should I just start desoldering components at random and seeing what doesn't meet spec? I've tried testing in place but of course the interconnections are throwing off most of the readings. Is there one particular component that could fail and give me these results? The theory-of-operation section on the fan is small and unhelpful on this matter, and there's no troubleshooting flowchart for it.

Of course, my limited knowledge of electronics doesn't help; I'm not even clear why I should see a fixed voltage coming out of the transistor-capacitor network (is that the right term?), since the output of the transistor should be temperature-dependent, but perhaps the capacitor plays some role in this...

Maybe I oughta buy the calibrated, warranted 465B on eBay (listing for only $70 more than I paid for this, of course) and save this one for parts later when I know what I'm doing! I didn't really need the DM44 anyway...

Jay Levitt wrote in news:MPG.1ab44746271fa9d98971f@news- central.giganews.com:

IIRC,The fan assembly for the 465/B is a Hall-Effect motor,and failed often enough.There's a thermistor that controls fan speed,not the xstr.The driver IC fails,and also the motor itself.Motor bearings also wore out,too.

I'd try to replace it with a simple miniature 12V fan and maybe a dropping R (it runs on 15V unreg,IIRC) or even a 3-terminal regulator IC.You would probably have to modify the rear plastic panel for such a fan.

Yep, it is.. but if the motor itself were failing, the voltages to it would still be right, no?

Right, but the thermistor drives that transistor.. sorry, I had said that but somehow edited it out. The transistor gets +15V in, and the base is attached directly to the thermistor IIRC, so I'm not sure why the manual says there's a specific voltage on the output.

Yeah, probably.. not sure I've seen a fan quite like this one, though - it's not a typical exhaust fan. More like a water wheel, and I can't quite figure out the airflow. Hmm.

Jay Levitt wrote in news:MPG.1ab49bcd925b7ed4989720 @news-central.giganews.com:

often

dropping

would

The fan is not an exhaust fan but an intake fan. In other words the fan sucks, not blows. (:>)

;-)

Does the fan blade turn freely?

IIRC, that board, etc, is pretty hard to get to on the 465B. Mine was a bit noisy and I just decided to live with it.

The DM44 really isn't much of a bonus these days. I'm partial to the

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Yep, the blade turns no problem manually.

Nah, more like hard to figure out how to get to. There are two screws on the fan board that are accessible through holes in the board next to it, but I couldn't separate the board from the fan blade. I took off the whole back cover (which required the purchase of an impact screwdriver since the screws were all stuck) and then discovered all I had to do was remove a set screw from the fan-blade shaft.

Actually, this one's a 465B too - I had read enough to know to prefer the 'B' model. I presume I will eventually need a parts scope anyway, so maybe (since I already know this scope's out of calibration and I don't have a calibrator or the skills to build one) I'll just buy a calibrated one and use this as my parts scope. I certainly don't need yet another multimeter, especially DC-only, but I had been told that the DM4 was useful for its frequency counter.

I guess I could probably move the DM44 to the new scope.. anyone know if that's an easy task?

Here's one for $59 ouch!

Jay Levitt wrote in news: snipped-for-privacy@news-central.giganews.com:

Well,both the motor and the drive IC failed often enough.Yours probably is the IC failing.I don't recall if that IC is still available or not.

Jay Levitt wrote in news:MPG.1ab5111a5feb79b0989723@news- central.giganews.com:

DM44's time,1/time functions are limited in accuracy.It's not really a "freq counter".Check the DM44 manual specs.

Well,you have to remove and transfer the DM's floating power supply,that PCB mounted in the center channel of the scope chassis,next to the CRT neck,the ribbon cable to the power xfmr,and make REAL certain you don't mis-connect the DM44 ribbon cables to the timing switch board;I have a DM44 PCB that had that done and it fried the time functions,so I made it into a standalone DMM,using two of the DM plastic covers back to back. Other than that,it's not very hard.