HP 3325A output amplifier  - Page 1
Forum section: electronics / repair

author: tonycox01
author_rank: Contributor
timestamp: August 04, 2019, 08:20:42 pm
content: Hi,I'm trouble shooting a problem with my HP 3325A signal generator's output amplifier. As soon as I turn it on a couple of resistors immediately overheat and if left would cook. I've isolated the circuit board so the only connections are the +15/+5/-15 power supplies. Within a second or two of turning on the power the two 47ohm resistors marked with red dots in the attached schematic hit over 90.C (checked with a thermal camera) and would cook if I left the power on. It seems like a a dead short.I've pulled all the transistors and checked them out - replacing one failed one. Also replaced the two 47ohm resistors. But that did not fix the problem. The resistors v rapidly and very specifically overheat. All the other resistors are good.Could anybody give me some advice on where next to tackle this problem?thanks, Tony

author: duak
author_rank: Super Contributor
timestamp: August 04, 2019, 08:43:44 pm
content: I would confrim that Q219, Q203, Q209 & Q216 don't have any shorted junctions, especially collector to emitter.Have you checked C245 & C246 for being shorted? I expect these are tantalums and as such are susceptible to shorting.Are you able to pull A14 and operate it by itself on the bench? If so, I'd power it with a current limited power supply with say +-10V and poke around the circuit to find open resistors and shorted semiconductors.If not, can you parallel both R271 & R228 with 100R resistors? This may reduce current enough in R242 & R243 enough to allow you to keep the unit on while poking around.

author: Arkytype
author_rank: Contributor
timestamp: August 05, 2019, 01:53:43 am
content: duak wrote:Have you checked C245 & C246 for being shorted? I expect these are tantalums and as such are susceptible to shorting.According to my 3325A Service Manual, C245 & C246 are indeed tantalums.Tony, I'll be happy to get the covers off my 3325A (tomorrow) and note some voltages to look for. Can I assume you are able to change the frequency/amplitude and the front display is working? Do you have the high voltage output option?BTW, there is a troubleshooting guide on page 8-K-1 for the Output Amplifier. It's not comprehensive, but a start.Regards,Lee

author: bob91343
author_rank: Super Contributor
timestamp: August 05, 2019, 04:54:52 am
content: It's interesting that they didn't make a more conventional bias circuit. The two transistors with the marked resistors usually are set to use the bias voltage as a source.Not so here, as they are returned to 15 Volt supplies for some reason. That circuit should draw minimal current. I am skeptical of those components being good. In fact, my take is that there is at least one seriously bad part. With so many parts in the circuit it becomes tedious to check each one but I think that may be the best way to find the ttrouble.I wonder if my HP 3310B uses a similar circuit.

author: bob91343
author_rank: Super Contributor
timestamp: August 05, 2019, 04:59:03 am
content: I checked and no, the ampllifier in mine is much more conventional, sort of like many audio amplifiers. It uses diodes for bias.

author: tonycox01
author_rank: Contributor
timestamp: August 05, 2019, 08:07:49 am
content: Hi duak,On my board C245/246 look like axial format electrolytics (15uF) to me. I have not checked these specifically but I will do - they seemed rather remote from the problem area for me to focus on initially.There is no display on the instrument (maybe a very brief display of some zeros but it goes almost immediately). So I have no control over the input at all. I have the board running with only power supplied ATM - but the resistors overheat so quickly I can only apply power for a few seconds.I've pulled all of the transistors in the amplifier circuit and tested them all - all are OK.I'll take a look at the troubleshooting guide - but its tricky when the overheating happens so rapidly!thanksTony

author: tonycox01
author_rank: Contributor
timestamp: August 05, 2019, 08:16:18 am
content: Hi Lee,No, no high voltage output option on my instrument.Tony

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2019, 12:19:53 pm
content: Quote from: bob91343 on August 05, 2019, 04:54:52 amThat configuration is known as a diamond buffer and provides higher current gain than the more common implementation with two diodes.

author: magic
author_rank: Super Contributor
timestamp: August 05, 2019, 04:38:39 pm
content: Short across R270?Check voltage across R273 to measure Q219 emitter current, it should be no more than 1V / 20mA. Ditto for the negative side.Which transistor was blown?

author: tonycox01
author_rank: Contributor
timestamp: August 05, 2019, 10:00:43 pm
content: I'll check for a short across R270 - although I'm pretty sure the resistance measured almost spot on.I'll also measure across R273 (and -ve side) and see what the drop is across it.Q213 (a 2N5160) was bad and I replaced it.thanks for your advice -I'll check it out.Tony

author: magic
author_rank: Super Contributor
timestamp: August 06, 2019, 07:41:31 am
content: In circuit or out? R270 may be good but some junk could be shorting PCB traces at the same time.If not R273 then the other resistors. There are only 3 places where current may be coming into Q213.You did check that there is no continuity from R242 to ground or +15V, through C228 or otherwise, right?

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2019, 10:54:41 pm
content: The common failure I see in these kinds of circuits is the constant current sources:R271 or R228 could open driving one of the current sources to maximum output. Of course anything which can push the bases of Q203 or Q219 around is also suspect. Notice that capacitors C229 and C241 do *not* go to ground; if they shorted, it would just shut off the associated current source.Or the transistors themselves, Q203 and Q219, could short.

author: tonycox01
author_rank: Contributor
timestamp: August 08, 2019, 07:54:35 pm
content: I'll recheck for a short on R271/228. I thin R271 is one of the discrepancies between the board and the schematic. On the board there is a 22 ohm +15V rail equivalent of R229 but its it not shown in the schematic. A couple of the resistor numbers are also swapped.Also no continuity from R242 to GND or 15V (and through C228).Like Magic say, there can only be three sources of current. I've managed to get a +/-17V variable power supply so I'm going to try and power it at a very low +/- voltage to see if I can find the current source before it all gets too hot....