Signal Generator Died...

[Phrazemaster]

HI all, I had this signal gen incorporated into an oscilloscope and I was using it to callibrate a microphone preamp.

I noticed when I connected the inputs there was a spark, and then the sig gen died! Couldn't find any internal obvious burned parts or anything, so, I'm pretty sad...it was old school; you actually had to CALCULATE the frequency based on the number of cycles per division, etc!

For the life of me I can't imagine how this occurred; I was sending signal INTO the preamp! Sigh. Anyway, I'm hoping the rest of the scope is ok; I'm not going to spend the x hundreds to have it repaired and I'm still a newbie at electronics. It was a Tenma 72-6805 by the way.

Mike

 

[gyraf]

Phantom power and gen's output capoacitor? And if it has a balanced output driver chip, that could be blown as well.

The near-classical problem of putting your TC wizard-series unit into a phantom-powered mic input

Jakob E.

 

[CJ]

scope inputs are usually chassis grounded, so if the sig gen output is chassis grounded,

see where this discussion is headed?

most of the expensive gear that i blew up was due to  ground issues.

take an alka seltzer, then change the fuse.

use a ground lift plug or iso tranny for the scope.

 

[Phrazemaster]

Thanks guys for the hints...yes phantom power switch was on! Looking back...DUH!

I need to see if the mic pre board's phantom power is still working then! I wonder if I could have damaged that in that moment...do you think that could have fried too? I know it's only about 10mw of power though...

As far as the scope's function generator...man those innards are all "greek" to me...couldn't begin to find the grounding for the signal generator...would probably be easy for some of you!

Anyone in the Southern Cal area feel like poking their head into this scope and checking it out with me? Just for fun? I'll buy you lunch for your trouble...

Sheesh, sooo much to learn...this...IS the hard way, isn't it? Lol.

Thanks very much.

Mike

 

[Phrazemaster]

PS for some odd reason I stil get 48v output even with phantom switch disengaged...that's not normal, right? The switch works properly; I've soldered pretty carefully...not sure about this.

 

[Phrazemaster]

With nothing better to do I opened the scope and looked for any obvious problems where the function generator wires connect.

I am attaching a pic of a fried resistor - no CLUE what value it is, but it's a big mama!! I hope I can find the value for this; it's a Tenma 72-6805. Maybe I'll replace the electrolytic caps on this board as well...

Where do you think I could find out the value of this mysterious R515?

And by the way, WHAT is that huge heat-sinked component above the R515?

Thanks!

 

[Phrazemaster]

Ok hard to believe but google really did yield what I believe is schematics!

I am attaching the relevant portion - I'm not clear what the meaning is for the resistor:

68RJ - I get 68R obviously, but what's the J? And 2W seems clear enough...

Thoughts please?

Thanks,

Mike

 

[CJ]

J could mean military, or some spec not related to the actual value of the resistor,

so just get a 2 w 68 ohm,

replacing the resistor might not solve the problem,

if there is a short in a transistor or something then the resistor might fry again,

so after replacing the resistor, plug the scope in for a few seconds and then unplug it, feel the resistor,

if it is hot, then you might have a problem,

if not, then pwr on for 10 seconds, then a minute, get me?

 

[Phrazemaster]

Roger that CJ!!!

Thanks, will-do...I'll report back in this thread tomorrow after doing that...

Thanks!

Mike

 

[gyraf]

"J" is probably tolerance (5%)

If you have found schematic, please post link - that makes it much easier to guess at failure points

Jakob E.

 

[Phrazemaster]

Thank-you. I am attaching the schematic file. The signal generator is what died....I believe that is on page 2 of the pdf. I had to break it into 2 pdf's due to file size contraints here - hence the next post.

**Note that I have replaced ALL electrolytic caps on the board from page 2, as well as the smoked resistor R515. The scope still works, but the function generator does not.

I do appreciate any eyes of intelligence on the schematic to point me in the right direction.

Thanks so much,

Mike

 

[Phrazemaster]

Here are pages 10-12 of the schematics. Got this from google...seems to match the values on the scope, so I do believe this is the correct schematic...

Thank-you!

Mike

 

[s2udio]

R515 is on sht 9, 10r  ....in the X plate deflection circuit....the scope still works ?!
The sig gen is a seperate circuit.........hmm.
Curious ?
AHH as i see.............from the other thread....?
Phantom power........that x deflection circuit would have thrown a wobbly.
A large DC offset ,or i am looking at the wrong diagram  ???

 

[PRR]

> signal generator ....I believe that is on page 2 of the pdf.

No. They hide it well, on page 1. The core is an XR2206 wave-chip. You can look this up, but it probably isn't at fault because there is an AMP pot and a hefty buffer between this and the panel jack where the abuse was applied.

The AMP pot is easy to find. Does it have large signal on the CW pin? If that's a right shape and FREQ-adjustable, you are OK to here.

Buffer Q908-Q915 boosts 3V peak from the XR and AMP pot to ~~7V peaks. It is moderately beefy with 300+mW output devices and 0.15mA current-limit (D923-D924, bias, emitter resistors). It is further protected by 47 ohms in each output collector, and 50 ohm series resistor to the output.

Is it fool-proof into a dead short? If we assume 10V RMS output all dropped in the 50 ohm R969 we have 2 Watts in a half-Watt part. However if the 150mA current limit holds it will clip at 7.5V 150mA peak giving 1.125 Watts peak, slightly less on sine. Peep R969, and adjacent parts while you are in there.

I have some thoughts about possibly stuffing 48V INto an OUTput designed for 7V signal and 24V rails. While this opamp ought to swallow Phantom current, if some transient slews both output devices to an off condition there are an awful lot lof junctions which could be reverse-biased.

 

[Phrazemaster]

Thank-you s2udio and PRR for your eyes and expertise!

s2udio, apparently there is more than one R515...the one I was referrring to, and that was smoked, was on sheet 2...I replaced it, as well as recapped that board just to be on the safe side. That R515 was a 68RJ 2W part.

PRR, you are a genius and miles above my head. You would probably say I shouldn't be messing with any of this, since I clearly don't understand it...you would have a point. I'm still learning; I'll try to decipher what you said and thank-you very much. And yes I'm being very careful; I know CRT's can kill!

I'm not seeing those parts you referred to on any of the boards; I'll keep looking however.

There are 2 huge power transistors on what I believe is the function generator board that have gigantic heat sinks on them, and they also look dark on the top - can't tell if it's glue or if they got fried (you can see one of them in my original post with the smoked resistor). They are Q506 and Q507; I'm going to replace them anyway just to make sure. Doesn't take more than 10 minutes.

Thanks again.

Mike