BK Precision 4011 Function Gen- needs help

[guswin95]

New to the forum.
I picked up a non working BK Precision 4011 for cheap at eBay.
Unfortunately it is not easy to find a schematic or SM to help.

I found a burned resistor R612 on the output daughter board and a slightly over heated R140 (56 ohms 2W) on the main board.

The equipment works but the output is unstable in frequency and amplitude.

Any help is appreciated. Reached out to BK for help with documentation. So far no luck. The pdf that is on the web is very poor resolution and does not match exactly my unit.

 

[Mooly]

A circuit is pretty much essential, however I would suggest that the 56 ohm is probably OK and the slight discolouration normal.

The other resistor appears to feed a transistor. Is the diode above the transistor a zener ? If so (and its only a slim chance) then you may be looking at a series pass regulator of some description, or could it be part of an output stage?

I would try and measure the resistor out of circuit to see if it has some kind of value. It may well read higher than it should (if at all) but it would be a starting point. You could also measure the voltage on each end to try and gauge a suitable guestimate.

We need a circuit though.

 

[guswin95]

Hello Mooly

I found a poor quality pdf of the circuit. Your suggestions are right on. And I suspect a short somewhere driving too much current over the poor resistor. Will update with findings. I believe it has NPN PNP output driver with symmetrical values on the burnt resistor so hopefully I can derive its value from the other half.
Thanks

 

[Mooly]

That sounds like a plan, particularly if it is a symmetrical stage of some sort.

Good luck

 

[infinia]

New to the forum.
I picked up a non working BK Precision 4011 for cheap at eBay.
Unfortunately it is not easy to find a schematic or SM to help.

I found a burned resistor R612 on the output daughter board and a slightly over heated R140 (56 ohms 2W) on the main board.

The equipment works but the output is unstable in frequency and amplitude.

.

Use the web schematic for guidance it s/b >90% correct, barring all the reference designators.
On the output board, you can find the value of R612 by inspecting the complementary side transistor circuitry, it is symmetrical. 1st Verify all the DC supplies esp. to the Op-amp and output collectors. You could remove the output transistors and verify op-amp and DC is fine. If the frequency is truly unstable the problem is on a different board.
I suspect the original problem is an overloaded output section, E.g without the attenuation selected. It's easy to do although the outputs fuse shown in your image should normally of blown.

 

[Mooly]

I'm just looking at the pdf now. Its hard to see anything around the output stage that could go up in smoke like that. The rails are fed from 47 ohm series feeds.

The output is equally padded out.

Could that burnt resistor be that 49.9 ? ohm directly on the output socket (via the switch). If someone shoved a high voltage up there then it would burn.

 

[guswin95]

Some quick removal of components proves the issues may lay somewhere else.
The burn resistor (47 ohms) still measures the correct value and the transistor appears correct.
Replaced resistor and the +22v and -22V are present.
Will investigate further

 

[Mooly]

Try looking at the signal coming into that sub panel. As infinia suggests, if the actual frequency is altering then there is another issue somewhere.

 

[guswin95]

Will do...

Need to continue with another repair meanwhile. At least this one I have full documentation and am familiar with it.

Will keep posting progress tomorrow.

Thank you to all.

 

[guswin95]

Finally I took another look at this instrument today.

I looked at the power supply lines for ripple and instability. I did not like the noisy +5vdc. A flaky looking power supply capacitor looked suspicious. Took them out and although they measured correctly, I decided to replace them anyway with a set that I had with better specs (50V vs 35V) and a bit bigger than the 2200uF original set.
Additionally, I replaced both small caps around the +5V regulator for better brand (Elna) and bigger values and voltage ratings (100uF instead of 47uf and 22uf vs 10uF).
After the swap, the line looks much better and the instability went away.
I cleaned the pots with Caig Deoxit and made sure to check everything again. Smooth output.

In conclusion it works well and its stable with a good output. I verified with scope and frequency meter for correct performance.

 

[Mooly]

Excellent news, well done