motor speed control

[cosmo]

Well, I'm still searching for a good scope. A Leader 1021 just went for 112 plus 30 shipping. I wonder if I should have bid another time on that one. I just bid 80 on a Tektronix 7834 Storage Oscilloscope with Plug-Ins. I get the feeling the desirable models are fluke, HP, Leader, maybe tektronics., maybe in that order? Although I'm sure model and age factor in as well as brand and price.

I think the 555 chip was a great idea, but like you said, maybe a two transistor flip/flop might be the easiest route. Maybe someone could point me to a good simple diagram to make a compatible flip flop replacement for that IC1. I saw a variety of flip flop circuit diagrams online but maybe too much variety,..TMI again.

Besides bidding on a scope, I also bid on a function generator, which I thought would replace the LHS of that circuit. I bid about 20 plus 20 shipping, which is probably massive overkill for what I actually need to fix the thing. I figure it could work remotely, like the pot control, and maybe even better. But the more I think about it the dumber the idea sounds.

I guess I could replace the IC1 with an equivalent two transistor flip flop and see if it works, in lieu of having a scope for the time being.

 

[Mitchekj]

I've never been wronged by a Tektronics... Fluke and others certainly are the big names in quality, but I wonder how much you're paying for the brand name sometimes.

(Now multimeters, yep, Fluke 100%.)

 

[Resqueline]

How about trying the simple diode peak detector first?

 

[55pilot]

Tek is and has been #1 for a few decades. HP (now Agilent) is #2. Fluke is a good company, but their portable scopes have been very limited in capabilities. For what you are doing, they may be a good choice. I have no experience with any others.

One other area you may want to consider is a few "USB Scopes" that you find in hobbyist magazines. These are basically ADCs with a buffer, a USB interface and software. The software runs on the PC, captures data from the USB interface and displays it like a scope. These go for a few hundred dollars new. I have personally never used one, but there have been some good reviews. If you can not find something, I am sure I can dig something up.

---55p

 

[cosmo]

You could make a crude peak-meter add-on for your multimeter from a diode (1N4148) and a small capacitor (10-100nF).
The time constant of C69 dumping into R59 is just 1.55 microseconds, with an amplitude of up to 14V and a repetition rate of up to 120Hz.
1.55u * 120 *14 = 3mV ballpark average DC on R59 at top speed, matching what you measured. But this only tells us there are pulses, not their amplitude.
If Q24 has an increased internal series resistance then this would widen and flatten the pulses, making for a similar average DC voltage.
If you measure a good peak amplitude on R59 then it means IC1 is bad, but I'm sure we'll be able to adapt a simple two-transistor circuit like this to replace it.

OK, I'm supposing the diode in series with the cap will let the cap charge up to the max voltage, and then I should measure the voltage across the cap with the multimeter after being charged up through the diode. I will try to get one of the larger caps so the multimeter doesn't drain the charge too fast. One place was sold out of those diodes but I think another place has them in stock.

I tried to short out Q24 between A and C to see if it would change the state of IC1 from hi to low, but it didn't seem to work.

 

[cosmo]

There might be an opportunity to buy a B&K Precision 40Mhz Oscilloscope Model 1541B. Would that be a worthwhile opportunity?

 

[55pilot]

I have personally not had experience with B&K or this particular model.

I hope you are not looking at eBay item number 400036889575. That one has been listed at least since June of last year, which sends up HUGE red flags for me. The estimated retail price is also outrageous. The kicker is that what is pictured is not the model in the description. There are a number of places on the internet that are selling it for about $350.

If you are even remotely considering being in the $300 range, I would not bother looking at anything other than TDS210. It is a professional scope and the best value. Its big brother, TDS220 used to be by home scope for many years. I bought mine right when it came out and it was the best $2K I ever spent on a tool.

Do a search on eBay for "TDS 210" as well as "TDS210" You should also look for TDS220. It is the higher performance version but not worth the extra money for you.

---55p

Edited to add: You may also want to consider the following: Prop Scope. It is a USB scope. I have not personally used it, but Paralax has a good reputation.