GW Instek GFG-8216A Frequency Control

[xrunner]

Got one of these off Ebay, listed as "For parts or not working"

It actually works in all aspects except one - the front panel freq. control will not change the freq. The decade range buttons do change the freq. correctly, but the baseline freq. before multiplication still has the same numbers. I have done the laying on of hands, re-seating all that I can, and inspecting the entire board, but I can't see anything wrong. As I said all else works - amplitude, duty cycle, offset, ... I cannot locate the schematic on the internet.

The top board on the left in the pic actually comes off and the thing still generates a frequency, it seems to just be a freq. counter because if removed the front freq. display goes away.

Does anyone here have the schematic or if you have experience with this model know what could be the issue?

[lowimpedance]

In the absence of any circuits, maybe some more detailed pictures of the main board to determine whether it uses discrete or an IC to generate the waveforms.
Most likely it will be some form of VCF, ie voltage controlled frequency via the front panel Pot  . Generating a main triangle waveform by constant current charging suitable capacitors. Then using a sine shaper and squaring cct for the square wave.

[xrunner]

In the absence of any circuits, maybe some more detailed pictures of the main board to determine whether it uses discrete or an IC to generate the waveforms.
Most likely it will be some form of VCF, ie voltage controlled frequency via the front panel Pot  . Generating a main triangle waveform by constant current charging suitable capacitors. Then using a sine shaper and squaring cct for the square wave.

Thanks yea I will tomorrow. Having a brew right now because I'm beat down by GW Instek. I called them in CA and I tried like hell to get them to send me the schematic but they refused.

The tech support guy who I could barely understand kept insisting the pot for the freq. control was bad. I told him, I don't know how many times, I tested that first with my test gear and it has no problems at all. But he wouldn't even consider that it could possibly be any of the hundreds of other components on the board. Bastards. 

[xrunner]

OK, I'll have to eat my words in public.  It was a defective pot - or actually pots - they are two ganged 5k pots. Why my testing showed good I'm not sure, could have been the pressure I put on the pot with my test leads made them make contact inside.

In any case, I gave up for a couple of hours and then I took the board into my living room and looked at it carefully between TV commercials. I then noticed that the pot had a crack in it. I couldn't see it very well when the front panel was on. I went back to the workbench and took to unsoldering the thing. When I pulled it out, it fell apart. The carbon traces were cracked.

I then did a quick and dirty test using four 2.2 k resistors (two per pot). I simply stuck them in the holes where the pot(s) went (I didn't even use solder), simulating the pots in the middle of their rotation. When I kludged the mess back together the frequency did indeed change. The other thing that didn't work before was the VCF input on the back - it wouldn't change the freq. as it should when applying a 0 - 10V input, and now it did. So I was very confident that the problem was solved. Someone smashed the freq. knob against something and broke the pot.

But I then had to find a part to replace it with, and that proved very difficult, as GW Instek will not sell you parts or give you the schematic or parts list. I must have searched for two hours before locating the right size ganged pot in China and Thailand. Since they were only 0.99 cents each, I ordered one each from two different suppliers hoping to get one quicker than the other.

So, again this Ebay find, "for parts or not working", is proving to be a good deal. Here's the pics in case it helps anyone else to fix one of these.

The pot -



This is where the pot was installed -



Main board -



This is the frequency counter board -



This is the front panel -

[Fraser]

Hiya,

Good result to your investigation. Sounds like GW Instek Techs know this is a common failure !

I have one of these with a different (easily repaired) issue. I need to remove the frequency counter PCB but it looks like the rear through hole header has some additional soldered pins that will need to be desoldered first. Grateful for comment on how you removed the frequency counter PCB.

Many thanks

Fraser

[xrunner]

I have one of these with a different (easily repaired) issue.

What issue do you have?

I need to remove the frequency counter PCB but it looks like the rear through hole header has some additional soldered pins that will need to be desoldered first. Grateful for comment on how you removed the frequency counter PCB.

Indeed, they made it difficult to remove, I don't know why. But yes you have to unsolder two pins, and I chose to do it on the bottom of the main PCB. You can see which ones in the pic. 

I'm tempted to just retrofit those two pins to make them like the others. 

[Fraser]

I bought my unit, knowing it was faulty, for £25 from TopLoser. It looked a fun little fun project 

The unit had been given a hard ride by a courier and the case burst open. In the process an axial inductor on the frequency counter PCB was sheared off. I also noted that one pin from the display pcb pins is bent and not inserted into its socket. I suspect that replacing the inductor and correcting the inter PCB pin issue will solve the units problems.

Aurora

[TimFox]

.....
The tech support guy who I could barely understand kept insisting the pot for the freq. control was bad. I told him, I don't know how many times, I tested that first with my test gear and it has no problems at all. But he wouldn't even consider that it could possibly be any of the hundreds of other components on the board. Bastards. 

Whenever I've talked to a factory hardware support guy, he usually knows which parts fail most often, although his boss may not want him to admit it.

[xrunner]

Just in case anyone ever needs the part for the freq. control - this is what you want -

http://www.ebay.com/itm/180915960255?_trksid=p2059210.m2749.l2648&ssPageName=STRK%3AMEBIDX%3AIT

[xrunner]

Still waiting on the slow boat from China. However, I ordered a 10 pin header today so I can convert the way they decided to connect the counter board to the motherboard by having two longer pins soldered to the main board. I think that's just dumb.  I want to be able to unplug the thing without heating up a soldering iron. 

If anyone wants one of these function generators the same group has two more up for bidding here -

http://www.ebay.com/sch/Electrical-Test-Equipment-/92074/m.html?ssPageName=STRK%3AMEFSXS%3AMESOI&_ssn=goodwill-bb

[xrunner]

Finally got the parts I needed - a long wait. Got the pot installed and it now works as it should. I then decided to go ahead and retrofit the strange way they did the header pins from the freq. counter board so that there would be no desoldering required to take out the counter board. I removed the original female header and installed one with one extra pair of holes. I removed the plastic spacer from the original pins and stole one from another header assembly so it would have the proper number of holes. I also cut two pins to the proper length and installed the plastic spacer. I touched up the ends with a file, and it was good to go. Why they didn't do it that way to begin with I have no idea. 

[PJ Bain]

Awesome idea with the stand-off modification!

Just wondering if you have any thoughts on a problem I have with one here. The unit seems to function perfectly fine except on the 100k and 1M ranges. If either of these ranges are used, the display shows the frequency going the opposite way to the adjustment knob (turn knob clockwise and freq goes down). It also doesn't have the same range  as the others which is normally min of about 0 to max of around 3 (at whatever range you pick).

Cheers
Pete

[xrunner]

Just wondering if you have any thoughts on a problem I have with one here. The unit seems to function perfectly fine except on the 100k and 1M ranges. If either of these ranges are used, the display shows the frequency going the opposite way to the adjustment knob (turn knob clockwise and freq goes down). It also doesn't have the same range  as the others which is normally min of about 0 to max of around 3 (at whatever range you pick).

Hard to say. I was never able to obtain the schematics for it - GW Instek will not give them out! 

[rysiontko]

Hi guys,
Do you experience problems like unstable frequency - let's say you want to have 32768Hz, but on the display it varies a lot up and down? Also, when I want to measure the frequency on my subject board, the counter is causing my board to malfunction.