peterson strobe tuner repair

[mbot]

hi everybody. i'm trying to get a broken peterson strobe tuner model 400 working again and have hit a bit of a wall. here's whats going on and what i know.

everything seems to work except the motor. the power supply seems to be giving the right voltages. the strobe lights are on and respond to an input. the motor lookls like its trying to turn over but just moves about a quarter inch but does not spin.

naturally i began looking at the motor control board. the MJE711 was burnt out and the 3.3ohm resistors were testing high. tried replacing them but they just toasted again. everything else on that board seems to be just fine (although i'm not too sure about how to test the flipflops properly but all three share the same resistance readings). i can't really make any voltage reading on the motor board because it seems like it would start a small fire. with the motor board removed everything is stable and seems good.

what i'm considering now is either that the motor is dead. however bot coils are not open and have the same inductance so i'm not certain how to confirm that its dead.

the other thing i'm not sure about is the signal coming in from the tuning circuit. on the scope the signal looks like a spike up and then down. i don't really know what it should look like or if it could be frying transistors.

in any case, if anyone could point me in a good direction that would be great. i'd like to be able to confirm if the motor is alive or dead if anyone has any tips for that, or any other reccomendations.

thanks!

 

[Resqueline]

Check motor for shorts to ground. Check motor capacitors for shorts (the case of the dual cap should also not be grounded).
Check small transistors (if both output transistors are made to conduct simultaneously they'll fry).
Are the f-f's running off 20 or 25V? (18V is the max for the ordinary 4000 series.) What's their make & type number?
Tuner output should be a 20V monopolar square, duty cycle unknown (& unimportant, f-f's sets that straight again - to 50%).
Replacing the 3.3 Ohm resistors with two (big) 110V bulbs should make the motor circuit safe for testing.

 

[mbot]

the capacitors were in need of replacement and i did that. i could only get a 3.3 microfarad cap to replace the 3.

the smaller transistors seem good. i tried to find replacements just to try with new ones but they are pretty obsolete now (2N3405 and 2n3414) and the best replacement i could find was BC337 which i threw in to try but still no change so i put the old ones back in. i never really trust when websites say something is a replacement.

the f-f's are MFC4040 and run on 25v

as for checking the motor for shorts to ground i'm not too certain what you mean. the one coil is connected to ground. both coils measure about 32-33 ohms.

i will try the lightbulb thing soon (no incandescents in the house right now).

thanks again.

 

[Resqueline]

Disconnect the the coil connection going to ground and then measure both coils relative to the motor frame (ground).
The motor relies on the phase shift introduced by the capacitors in order to run. I figured the diagram said one 2x10uF cap (= 5uF) and one 5uF cap?
I found datasheets on those strange old IC's and it says they have an absolute max voltage rating of 19V.. Maybe there's a 16V zener not drawn in the diagram?
The small transistors can be replaced by just about any switching transistors available, but beware of different pinouts
Yeah, better stock up with different incandescents for testing before they become completely unobtainable, hehe..

 

[mbot]

so there are no shorts to ground in the motor. is that a confirmation that it is good?

when the cap is labelled 2x10MFD does that mean that it is actually 2 5MFD caps? i replaced the dual cap with 2 seperate 10MFD caps. will that be a problem? i just assumed that was what it was but just packaged together for convenience or something.

there are no zeners in the path of those f-f's that i can find but it doesn't look like it's been messed with before and i assume it worked at some point. there is a chance the MFC in hte part number could be something slightly different (its not the best quality schematic) but the 4040 is for sure.

if that cap thing isn't a problem, and the motor is confirmed good maybe i'll try a whole swap of the transistors and see if i can't toast some more? i won't be able to go get bulbs till tomorrow.....

 

[Resqueline]

If you disconnected the ground wire and measured infinite resistances to the frame then that (together with the other equal resistances) is a good sign that the motor is ok.

In the diagram the wires are only connected to the upper two terminals, nothing to the common terminal. That makes it a series connection of the two 10uF caps.
The resulting total capacitance is thus half of each section (5 uF) and it also becomes a bipolar capacitor (meaning it can stand Alternating Current).
If you connected those caps to ground or in parallell it could explain the overload.

If you Google the IC and take a look at a pdf datasheet you'll see an image of the IC. I doubt any other version or type would stand 25V (for very long).
I've seen poor design before, and I would've put a zener across their power supply if this thing turns out to work at all..
Edit: the 470 ohm resistor will drop sufficient voltage at only 9mA total, and each IC draws an average of at least 10mA, so that's the explanation why it lasts..

I'd hold the test until I got some protection in place if I were you, but maybe you like toast better than me..

 

[mbot]

i connected the caps with their negative ends together going into the same hole as the negative was before (which is just for support and doesn't go anywhere) and the positives into the 2 remaining places. sound right?

the image of the IC looks like them but i just assumed the same package would have many types and operating voltages based on the manufacture. you think it would be a good ides to add a zener to ground then?

 

[Resqueline]

Caps sounds like they're hooked up right.

Adding a zener would not be neccessary and might not be useful but it certainly wouldn't hurt.

 

[mbot]

um, about using a bulb in place of the reisitors. i've heard of this before but never actually done it. would i be using just regular light bulbs for this like when prople use them for powering up tube amps and stuff or do i need to get a 2 watt bulb as the resistors are 2 watt? just want to make sure i'm not putting something in that will be worse than before....

 

[mbot]

hooray! so i tried just changing out the whole board (except the smaller resistors and the f-f's) and the thing works. i believe it may have been a simple case of getting the leads wrong on the smaller transistors. i checked them like 10 times this time around. anyways, it works but i would still like some information about using lightbulbs in place of the resistors for troubleshooting. if you have any information to share....

thanks for your help..

 

[Resqueline]

Congrat's!
Sorry I seem to have missed replying to your question the 15th.. Didn't mean to ignore you.

Bulbs pass a more or less constant current; they have a low resistance when cold and a high resistance when illuminated.

In this case the 3.3 ohm 2W resistors can stand 0.78A (ohms law) and the power supply is 50V. The ideal protective bulb would then be 50V & 40W.
50V bulbs are hard to get but a 115V 100W bulb can just as well be used.

It's still no guarantee that the transistor in series with it won't cook, but at least it won't blow up, and you'll most likely get time to turn it off before it cooks.

Same technique can be used in cars. If a fuse blows now & then, replace it with a (H4) 65W bulb. If anything tries to draw more than 3-4A the bulb will light up.
If you need 6-8A just hook up both the low & high beam part of it. It'll work as a short indicator while you work with the wiring harness to find the culprit.