Fluke 8800a repair

[sycho123321]

Hi everyone,
   Today I scored a fluke 8800a for 5$  ! It was known not working and I thought that it might be something really easy to diagnose and fix. And if non reparable, so what it would be a learning experience for 5 bucks. Straight away I downloaded the manual ( http://bama.edebris.com/manuals/fluke/8800a/ ) And started working through the troubleshooting section. all the voltages were fine except for at section 4-52, f ,where it should be 5v +- 0.2v, but it was at 4v. I also went ahead and tested the adc (section 4-75) since on the schematic I did not see any 5v going into the adc. (I may be totally wrong!)  None of the tests passed. By the way I do have a o-scope I tested with. From what I can tell the adc is dead, right? unless the low 5v has something to do with it. On the display there are multiple decimal points no matter what range I am in, and there is always a mildly loud buzzing coming from the multimeter, sometimes it is louder on a few of the ranges.

Thank you very much!

[RJFreeman]

all the voltages were fine except for at section 4-52, f ,where it should be 5v +- 0.2v, but it was at 4v.

ok, the 5V rail is going to drive the logic in the multi-meter, and some older logic circuitry could be very sensitive to low supply voltages, and this could very well explain

On the display there are multiple decimal points no matter what range I am in,

so while it is only a volt low, i would try and sort this out first.
Also given it is making a buzzing sound, this could be indicative of excessive ripple, maybe due to dried out Electrolytics? what do these supply rails look like with your oscilloscope ?

[sycho123321]

Thank you for the fast response! The rails look fine on my oscilloscope, and there are no telltale signs of dried out electrolitic caps. Also I forgot to mention in my origninal post that there is no visible damage on any of the boards. I think maybe the buzzing could be comming from  the main transformer.

[Shock]

I've not looked at the schematic but check everything on that rail, pulling current or connected to that component to be certain.

[tautech]

Not sure if you'll find something in this thread:
https://www.eevblog.com/forum/testgear/old-fluke-multimeters/

You could maybe ask the OP drtaylor, for advice.

[sycho123321]

Thanks tautech for that awesome thread! And shock, thanks for the advice I will check around.

[sycho123321]

Quick update, I removed the ohms converter board, and the main buzzing is gone but i've noticed a high pitched buzz which once I removed the ac converter board that buzzing stopped. But the problem of the ranging and reading of the voltage still exists. So i'm dong what shock suggested

[wiss]

I had a 8800 with a blown controller,  it did show those dots on the display if I remember correctly...
Fixed it by getting a controller from a 2100.

[Shock]

Also low volts can be a problem with voltage regulation and filtering, again too lazy to check schematic.

[sycho123321]

I had a 8800 with a blown controller,  it did show those dots on the display if I remember correctly...
Fixed it by getting a controller from a 2100.

You're talking about u11 ,the main Ic, on the 8800a right

Edit: Oh the display controller, that would make sense, but all the ad/c tests have completely failed

[sycho123321]

Yeah I'm guessing either what shock said or  blown u11

[sycho123321]

Doh!! I don't think that what I was measuring is a 5v rail, the manual says to measure between tp9 and tp8 putting the ground lead of the multimeter to tp8 (which is -18v relative to ground) and the other lead to tp9 ( which is -14.3 relative to ground). So what I am effectivly doing is meesuring to difference. Why would the manual want me to do that?

[wiss]

There is a bootstrap supply for the input buffer that can fail...

[sycho123321]

There is a bootstrap supply for the input buffer that can fail...

Just went through the testing procedure, It passed

[dacman]

Doh!! I don't think that what I was measuring is a 5v rail, the manual says to measure between tp9 and tp8 putting the ground lead of the multimeter to tp8 (which is -18v relative to ground) and the other lead to tp9 ( which is -14.3 relative to ground). So what I am effectivly doing is meesuring to difference. Why would the manual want me to do that?

The 7805 regulator is referenced to the -18 Vdc rail.  The OUT and COM of the 7805 (U19) can be measured at TP8 and TP9.  Check the voltage across C22 (it should be at least 7.1 Vdc at the low end of the main line input voltage).  Check the voltage into CR31 (acV).

[sycho123321]

Doh!! I don't think that what I was measuring is a 5v rail, the manual says to measure between tp9 and tp8 putting the ground lead of the multimeter to tp8 (which is -18v relative to ground) and the other lead to tp9 ( which is -14.3 relative to ground). So what I am effectivly doing is meesuring to difference. Why would the manual want me to do that?

The 7805 regulator is referenced to the -18 Vdc rail.  The OUT and COM of the 7805 (U19) can be measured at TP8 and TP9.  Check the voltage across C22 (it should be at least 7.1 Vdc at the low end of the main line input voltage).  Check the voltage into CR31 (acV).

Thanks for the reply dacman, The voltage across c22 is a little low at 6.8vdc and the the voltage into cr31 is at 4.5vac. Should i pop a new 7805? as I have a few laying around

[dacman]

The input to the rectifier seems low.  Can you check the position of S12 (115/230 Vac switch)?  I suppose the 5V output could be loaded.

[sycho123321]

the switch is set to 115v which is correct for where I live

I also swithed out the 7805 for good measure, still at 3.8 so looks like the input is low

[dacman]

Those values seem low (from the transformer to the 5V output).  Try to exercise S12 a few times.  The 5V supply is either loaded or the transformer has low output.  Can the 5V supply be isolated?

[sycho123321]

I fliped s12 a few times, That didn't work. Also I think I may have inadvertantly measured the wrong in of the diode rectifier. But I have to go on an errand so I will be gone for a few hours. I will make sure I measured the right pin when I get back.

[sycho123321]

The input into the 7805 from the rectifier is 6.4vdc that seems low to me. And the input to the rectifier is 8.5vac So i did measure the wrong pins  . I think this means bad rectifier, or am I wrong?

[sycho123321]

Looks like something is loading it down, I powered  the 7805 i sucked out with 6.4v and wit is working fine.

[sycho123321]

WE"VE GOT SOMETHING    I revomved the 7805 and powerd the 5v rail from my bench power supply annd when I turned it on it showes overrange on everything, but the power draw max is only .37 amps, so nothing loading it down. But nomatter what I try none of the ranges will measure any thing but at least I temproarly fixed the 5v rail so I can trobleshoot this problem!! 

[dacman]

I think your rectifier is bad.  The regulator requires at least 7 Vdc at its input (and because the input is unregulated, it should be 7 Vdc at 103.5 Vac line power input, else a line regulation check would fail, and at least 7.8 Vdc with 115 Vac line power input).  What is the Vac across C22?  The ripple usually goes up with failing rectifiers.  (The rectifier is a hard working part, continuously turning on and off with relatively high forward current.)  I'd also check those electrolytic capacitors.

Also, the input to these regulators normally have what may seem to be a lot of ripple, which the regulator must take out.  (They may work with a lower input voltage if the input voltage is smooth with relative little ripple, such as from a lab supply.)  Read the datasheet for your regulator.  Most passive regulators I've seen want 2 or 2.1 Vdc more voltage at the input than the output.  I've replaced several rectifiers due to low voltage at the regulator.  (And I want to know if the unit will work at its lower power line voltage specification, which can be directly tested using a Variac.) 

[dacman]

A generic cross for CR31 is KBP02 or 2KBP02 (I think).