Ok, so this schematic is most likely the correct one then? I understand the difference in the start up resistors and how they modified it. But what about the different readings and values present on the previous resistors I was testing?

Or do you think the problem is elsewhere like at the transformer to UC2804 circuit? I am just wondering what I am looking for when I find the circuit you mentioned.

Ya I was amazed at the resolution it had. I read somewhere to use a scanner bead and cover it with something black to keep out ambient light out. So I scammed it I'm at the highest setting it would go and it was clearer then using a magnifine glass. So did you come to the same conclusion that resistor in question had "100" printed on it?

 

Quote:
Originally Posted by diebog View Post
So I went ahead and desoldered Q1 and here are my findings.
R28....121 ohms
R29......7.2 ohms
R24......1k Ohms
R27......7.2 ohms
125K ohms between pin 5 and pin 6.

SO does this mean anything?

Some of the part numbers on the schematic are different from those on the board : R28 and R29 on the schematic are R27 and R29 on the pcb, 6.8Ω.

Let's use the part numbers on the schematic to avoid confusion:

These resistors sense the current flowing through the transistor.
When the transistor is switched on, current flows from DC+ through R11 to pin5 of the transformer, through the transformer then to the drain of the transistor which is connected to pin2 of the transformer. The current goes through the transistor and R28 and R29 in parallel to DC-.

R28 and R29 are in parallel so the combined resistance is 3.4Ω and the voltage drop is Id x 3.4Ω where Id is the drain current.

This voltage is fed to the current sense input (pin3) of the ic through R25 and is used by the ic to control the time the transistor is switched on and off. That is why the ic is called a current mode controller: when the current rises to a certain value, the transistor is switched off.

The transistor current is also switched off by the feedback network which I will describe in the next post

Timescope

 

R24 is 10 ohms and it drives the gate of the transistor from pin6 of the ic.
Zoom in to Q1 and R29, R27, R26 and R24.Check for continuity between the source terminal of the mosfet on the left and the right hand side of R27 and R29. Also check for continuity between the right hand side of R27 and the left hand side of R25 ( located above the ic U3).
Timescope

 

Ok, checked and yes there is continuity between source and each side of R27 and R29. And there is continuity between Right of R27 and left of R25. What does this mean? Are there supposed to be continuity there? Just remembered that you were talking about R27 and R28 were switched from schematic to board? I am testing on the board going off the silk screened numbers. Is that correct?

 

Baldor got back to me and says this is what the engineering dept. says is for that motor. It is the same schematic I believe, but has a few more schematics for another board separate from this one that is part of the speed control.
https://workspaces.acrobat.com/?d=-CkKH5wfm00xRh9NSHmzFA

 

Unsolder D13 and test it as it may be faulty. It should read the same as the others.

I am checking these on the circuit board. Should I remove them? I replaced r29 when I placed a digikey order and when I had the old one off they did ohm at 6.8 but in the circuit they measure 7.2 When I decoded 6R8 the r is basically a decimal point which gives the value 6.8 so that's what I ordered.

The resistors should read 3.4 ohms in circuit because they are in parallel that's why I asked you to check continuity.

Timescope

 

I erased the post cause I decided to check it to make sure it was bad with another tester and it tested fine. So please ignore my incompetence

 

One of my posts above I had compared 3 different testers and they ranged from 6.9-7.2 So I am confused what that means. Does that mean that when they are in parallel that the resistance of the 6.8 is cut in half which should read 3.4? When I use continuity and put leads on each end of the r27 and r29 (on the board) they both show continuity.

 

I tried 3 different testers and the both 6R8 on the board measure:

Fluke ohm/voltage multimeter: 6.9
Radio shack mini multimeter: 7.2
actron cp7665 multimeter: 7.0

So is it possible these multimeters are not accurate? The fluke I have is the most expensive so I would guess it would be a closer to the actual reading

Both 6.8Ω resistors are in parallel. You may have a break in the circuit bard trace.

Check the resistance reading between the source of the transistor and DC-.

Timescope

 

So what is a good way to go about finding the break? Desolder R27 and R29 and follow the circuit with continuity tester?

 

Yes, and use a magnifying glass. The resistance reading between the source and DC- should be 3.4 ohms.

Timescope

 

Sign in is required to access the document. Don't worry as we will continue to use the one we have.

Timescope

 

So R27 ohmed at 1.7 M ohm which I think is the problem. The one I replaced was fine so I am going to put that one in place of R27 and see what happens.

 

Sign in for the last pdf I posted? Thats weird. I published it like the others so anyone should be able to see or download it.

 

Never mind, I posted that before I checked the 6r8 I removed. It is bad also. I really dont want to put in another order for one 6r8 smd, can I replace it with something else? Or do I just need to order another one? Dang it! why didn't I order 2! lol

I have a 392 smd which is 3.9, is that close enough to just use one of these instead of 2 6.8?

 

392 should be 3,900 ohms. The value of these resistors is critical for correct operation of the circuit as they set the peak current. I would suggest that you order the correct value and another UCC2804 just in case it is defective.

I promised to explain the feedback network. I will do that later tonight.

Timescope

 

ok thanks, hey there are a bunch listed under UCC2804. Which do I get?

 

There is a "D" after that number on the UCC2804 so when D is added I found this: http://www.digikey.com/product-detail/en/UCC2804D/296-11388-5-ND/382278 is that correct?

 

It looks correct.

Timescope

 

Ok good, cause I already ordered it. Guess ill have to wait a few more days to fix this. When you have time later, just wondering why I don't replace it with one 3.4 resistor then using 2 in parallel? Seems like the manufacture would have one less component to have to install to board. As you can tell, I have allot to learn. I have been reading up starting at vol 1 but its hard to sit in front a computer for hours.