Fixing a HP200AB Audio Oscillator, where to start?

[rwgast_lowlevellogicdesin]

So i picked up this piece of gear a while back sitting a an e-waste place. I just downloaded the user manual/schematics. I would like to get it functioning properly again! I have an AWG, but this HP is nice because it outputs much higher voltages than a digital AWG.



It is in really nice cosmetic condition, but has multiple problems. The frequency is pretty far off from the set frequency on the dial. Sometimes I have to turn it on and back off and on again to get it working. The connections need a good cleaning and the Variable cap needs some DeoxIT. What I am most concerned about is the distortion, using the 600 ohm impendence posts on the front the wave looks pretty bad



If I plug my leads in to the black/red post I get an even worse waveform. 



I have never worked with tubes before and I am pretty unfamiliar with the issues that arise when a tube is bad, I'm not sure if they weaken of just die. The first waveform looks to me like it could possibly be a tube clipping the Sin wave, but I'm not even sure if that is a thing a tube would do when close to failure.

I went to take it apart and start testing components mainly caps, but these caps are nuts and I'm not sure the best way to get them out. I think these two bigger caps are non polarized and there the kind where you can open the tube and there is smaller ones inside, but I'm not even sure about that yet because I don't know a good way to get them out of the unit.

Here is the first cap



It looks like the only way to remove this guy is to drill out those rivets?

The second bigger cap doesn't even have riveting, I'm not sure but it looks like maybe you are suppose to twist the lead blades, and that is all that is holding it to the chassis???



If anyone on here has worked on one of these guys, or some HP gear of the same vintage and knows the best way to remove these caps I would very grateful. I watched some you tube videos but most are for the 200CD not 200AB and they never show how they actually remove the caps. For now I figure I would just go through the schematic cleaning and testing parts (there isn't not to many) and replacing anything bad, but I don't want to go drilling unnecessary holes to get parts out!

[coppercone2]

Well I have alot of Hp test equipment with those caps and I can tell you think

1) most people say if they test fine their fine
2) I cut a whole bunch apart with a band saw, that tested fine. The only thing I see is browning on some caps but others remain toilet paper white. They smell sweet inside like some kind of fruit chemical.


I replace them all. You use flat grip pliers to straiten out those tabs that are twisted in and they pull right out once smashed flat enough, and can be bent back into place.


You can either try to cut that cap apart and reuse its base to mount a capacitor or make some hardware. The bases kinda smell but glue does seem to stick to it after a good cleaning, so you can drill holes in there after cutting the tabs flush and possibly reinforcing them with glue. Top side inside of the can that you might remove, its aluminum thats like crimped to steel or some thing like that, its not very good for solder. I would recommend drilling 2 small holes in there so you can feed capacitor leads through and wrap them around the soldering tabs on the bottom of the bakelite substrate if you are hell bent on reusing stuff. I don't care for the original can, the new caps have the nice vent in them so they can fail gracefully, and you get more room in the chassis if you ever wanna do some mod.




The nice option however, is to drill the rivets, make a diamond shape PCB that fits over the existing holes, bolt that in place with a capacitor in the middle, and rivet two solder posts to the bottom so you can do your point to point wiring (feed capacitor leads through PCB to wrap it on the binding post). You could even get big threaded studs in the PCB, so you can just bolt it to the chassis without a nut. In this case you just need to select the right hardware and get a diamond shape pcb material cut out (i.e. if there is alot of wires going to the leads, get some big ass solder posts. The one that looks like a chirstmas tree made of disks.

Now if you actually get a plated PCB, you can solder the capacitor down and use traces to lead the capacitor to you feed throughs rather then using silicone and wire pulling to get the capacitor connected to your solder posts. What is important to get similar to original 'quality' is that you use riveted hardware for soldering, so you have nice heat resistant easy to solder and reliable connectors. You could also just solder the wire to a glued down capacitor on a PCB that fits over the original chassis hole, but its weak and looks bootleg


another option is to put the capacitor in the diamond hole after its cleaned, and put alot of JB weld around it to fix it in place near the crimped end so the putty confroms around it but leaves the bottom and top of capacitor undisturbed. This is the easiest solution if your cap can fit in that hole, the one ones are alot smaller, but you have to live with soldering wire to the capacitor leads without some kind of  durable interconnect. If you do this, for best attachment, you need to skin the capacitor to get the JB weld to glue to the metal can not the film on it, incase it wants to rotate.


If you have high voltage on those caps, I suggest making it more robust, if you desire to replace them. The browning I found made me a little nervous....


a cheap chinese distributor can make a PCB for you that has all the holes drilled, so you only need to rivet on the hardware and solder everything in place.. then you can just screw it in. Only challenge is that making sure all the hardware is what you imagined in your design, lots of work to do in selecting the right parts, but it will be legit. You could even get tabs cut into your PCB so you can attach the can if you carefully open it and reform it after gutting and cleaning it for cosmetic reasons (retaining the bent tab mounting method for the can).