B&K 3011B signal generator, no output - Page 1
Forum section: electronics / repair

author: Johnnieboy
author_rank: Contributor
timestamp: April 17, 2023, 02:05:13 am
content: I was checking a tube amp and intending to inject a signal into the grid of one tube, but attached to an incorrect pin. Afterwards I was not getting any signal output. I opened the generator and found no obvious issues. Anyone familiar with this model that can offer a suggestion. Later units of this model have a fuse installed immediately before the output. Mine has no such protection. Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: April 17, 2023, 03:25:44 am
content: What position was the output switch in and what voltage/current do you think it was subjected to by being connected to that pin? I suppose you'd just have to start poking at stuff starting at the output and working your way back. Here's a schematic of the output stage.

author: Johnnieboy
author_rank: Contributor
timestamp: April 17, 2023, 12:11:11 pm
content: I was injecting a 1k sine wave. The voltages could've been pretty high. I was expecting to see something blown up, but really there was no physical evidence of anything abnormal. I realized I was not outputting anything on all 3 waveform outputs. One thing for sure, I'm adding a fuse in that output circuit when fixed!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 18, 2023, 06:05:17 pm
content: I would check the output transistors in place first with a diode checker to see if the PN junctions have about .6 volt across them.It is likely you blew themAlso you could start by checking the output resistors for proper values. They may be blown without visible damage.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2023, 06:26:48 pm
content: Let's say it was subject to +100V at the output. That means that about 1 amp of current would flow through R518 + R519, Q507 and R515. That means 0.5A through each of R518 and R519, which means 50V across them and 25W dissipation, 50X their rating. They are small metal-film resistors and very well might blow quickly and quietly. R515 is a 47R 2W carbon composite, that seems much less likely to blow quickly. Q507 has a rated collector current of 0.6A, so a brief 1A pulse likely won't kill it. Also, if Q506 or Q507 were blown, whether short or open, I don't think I'd expect "no output" but either noisy, clipped output or large DC offset. So, I'm thinking R518 and R519 are blown and that's the first thing to check.

author: Johnnieboy
author_rank: Contributor
timestamp: April 19, 2023, 04:09:22 pm
content: Checked all the resistors in your statement and all are spot on value. Thanks for your input bdunham7

author: wasedadoc
author_rank: Super Contributor
timestamp: April 19, 2023, 04:22:27 pm
content: If you put +100V on the output I would expect that Q503 would cease to be a happy bunny. Or a parrot.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 19, 2023, 05:06:09 pm
content: Have you measured the voltage at the junction of the two 10 ohm resistors? It should be close to 0V. Have you verified the supply voltages?

author: bdunham7
author_rank: Super Contributor
timestamp: April 19, 2023, 05:09:56 pm
content: Quote from: Johnnieboy on April 19, 2023, 04:09:22 pmOK, so what do you have for other test equipment? I would check the +/-15V and =/-22V power supplies and then start poking my way through the output amp with an oscilloscope starting with the base of Q501 all the way through to the output. Also, you might just check with an ohmmeter from the output pin of the BNC (probing from the center of the BNC on the outside of the front panel) all the way to the junction of R516 and R517, then to the base of Q502. You should get ~50R then ~18K.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 05:19:21 pm
content: Most of the time when parts fail there is no physical evidence, you need a lot of power before something craters. Test the transistors with your multimeter, and if those are all good (unlikely) then start checking resistors in the output stage.Next time you need to inject a signal, do so through a film capacitor of sufficient voltage rating.

author: Johnnieboy
author_rank: Contributor
timestamp: April 19, 2023, 05:30:35 pm
content: Q503 seems ok, thanks

author: Johnnieboy
author_rank: Contributor
timestamp: April 19, 2023, 05:58:09 pm
content: +/-15 volts good. +/-22 volts showing +/-28 volts dc?

author: Johnnieboy
author_rank: Contributor
timestamp: April 19, 2023, 06:11:56 pm
content: Junction of r16/r17 is -25vdc.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 19, 2023, 06:47:31 pm
content: Q507 might be shorted.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 07:28:13 pm
content: Just check all of those transistors, you can use the diode test function on a multimeter, each transistor should appear as two diode junctions, one B-E and one B-C. Polarity is swapped for PNP vs NPN.

author: Johnnieboy
author_rank: Contributor
timestamp: April 20, 2023, 09:12:27 pm
content: Replaced Q507 and all is well. What a great site, thanks to all, every suggestion was noteworthy. This site is fantastic!

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2023, 10:40:28 pm
content: Glad to hear you got it working, thanks for following up with the outcome. Next time you kill it you'll know right where to look.

author: Johnnieboy
author_rank: Contributor
timestamp: April 21, 2023, 01:52:33 am
content: I will! Thanks again.

author: floobydust
author_rank: Super Contributor
timestamp: April 21, 2023, 05:47:52 pm
content: You don't want to go directly to the "grid" anyway, as that scuttles any bias and +ve excursions draw grid current.Just add a blocking capacitor, a 10k series resistor etc, anything to protect the signal generator and not upset the DC bias.

author: Johnnieboy
author_rank: Contributor
timestamp: April 21, 2023, 09:43:20 pm
content: Thanks, good advice.

author: Johnnieboy
author_rank: Contributor
timestamp: April 22, 2023, 01:16:30 am
content: One last question, before I add a 200ma fuse to the output. The manual I found online said it should be a slow blow fuse...does that make any sense? I would have said a fast blow type.

author: bdunham7
author_rank: Super Contributor
timestamp: April 22, 2023, 02:34:40 am
content: Quote from: Johnnieboy on April 22, 2023, 01:16:30 amI agree that a fast-blow is probably appropriate, but don't count on it to save your transistors if you have a similar event. It will prevent a fire though.

author: Johnnieboy
author_rank: Contributor
timestamp: April 22, 2023, 02:49:03 am
content: I plan on using both the fuse and a blocking capacitor on the output.

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2023, 05:48:22 pm
content: Even a fast blow fuse isn't going to prevent damage from excessive voltage. It takes almost no current for a high voltage to fry a semiconductor. If the fuse blows, it will be because the transistor shorted.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 22, 2023, 08:01:37 pm
content: Glad it is fixed! One possibility is to place an MOV from the junction of the two ten ohm resistors to ground.https://www.digikey.com/en/products/detail/panasonic-electronic-components/ERZ-V07D270/227552I do not know the maximum peak to peak output swing of this signal generator, so the MOV linked might not be the best choice.

author: james_s
author_rank: Super Contributor
timestamp: April 23, 2023, 01:09:05 am
content: I'd be concerned about the capacitance of a MOV affecting the performance of the signal generator. It can go up to 2MHz.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 23, 2023, 10:38:29 am
content: Did not know that (I was thinking audio frequency).OK, how about two zener diodes in series connected cathode to cathode. The point is, if high voltage transients are expected in the future, then some form of limiting is a good idea.