Electronics > Repair
General Radio 1396B Tone Burst Generator - Understanding power supply
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barbaroja:

--- Quote from: coppercone2 on November 29, 2024, 05:07:37 am ---and in this case, it does look like a RC filter, that is normally not seen so much, because its wasteful. I think with the RC filter design (it looks like a CRC, Pi filter) they are definately thinking about bode plots when they build it that way, aka they want the frequency response to go out further, meaning capacitor parasitics might be being considered

I am kind of thinking if you combine the 600 and the 800, you might still want to put a 50uF or something in place of the 600, to get in on whatever they were trying to do with what looks like a RC filter


And that resistor, if you just have it right after a big cap, it might choke out the circuit? And if you put the resistor first, it might surge the circuit (the capacitor is too big)

I think, espeically if you put new diodes, the first cap can actually be made quite a bit bigger, and the second cap after the resistor.. you might wanna stick to the value they have. Like a 680uF or 820uF one would be the common choice

--- End quote ---

Awesome insights, thank you. Makes a lot of sense. I think I will try to go for the most similar values but now at least I understand why. Replacing the 1uF for film seems like a nice idea as well. Done that before in other equipment I recapped.

Edit: Just tested the "good" caps with a DER EE DE-5000 LCR meter @ 120Hz.

Main supply
The dual 300uF, 35V
1: Square 352uF 0.2ohm, Triangle 361uF 0.2ohm
2: Sq 338uF 0.2ohm, Tr 351uF 0.2ohm

Dual 400uF, 25V
1: Sq 548uF, 0.1ohm, tr 541uF, 0.1ohm
2: Sq 63nF, 130Kohm, tr 534uF, 0.1ohm, leaky.

Scaler supply
Sprague 500, 20V: 530uF, 0.2ohm although it leaked all over the mainboard.
Mallory 680, 15V: 720uF, 0.1uF.



--- Quote from: jonpaul on November 29, 2024, 05:17:40 am ---
A wonderful and unique instrument, I first encountered at Dolby Labs in San Fran 1976. Needed to test audio noise reduction systems like DBX, Dolby.


The R-C filters are common 1950 design practice. See 1935 RCA rcv tube manual.

We used  similar R-C  filters in our IEEE Spectrum 1942 SIGSALY  ADC reconstruction.

Multisection lytics are  seen  in epoch radios, TVs, instruments to save space and fewer chassis punches.

Vintage Rebuilders   remove and gut the alum lytics,    rebuild the cans caps with small modern axial/radil hidden in the old can.

See Antique Radio forums.

Jon



--- End quote ---

Thanks jonpaul. Very interesting, had no idea about these considerations. I will definitely have a look to that manual.


--- Quote from: coppercone2 on November 29, 2024, 05:37:29 am ---what might be interesting too is a picture of the transformer(s)

the thought about not having a giant capacitor connected directly to the transformer is valid, though it generally seems they can take whatever you manage to throw at them

--- End quote ---

Here you go. Picture attached.

Thanks for pinpointing the diagram error, fzabkar and Harry_22.
barbaroja:
Measuring some good caps I have lying around:

330uF 25V Elna Silmic II: 300uF, 0.2ohm. Spot on with the old dual caps. Got no 680uF at the moment.
470uF 35V Panasonic FR: 506uF, 0.0ohm.


coppercone2:
That transformer looks substantial for like a tone generator, I think it can handle pretty much any reasonable capacitor. I thought it might be some micro sized thing

Those capacitor all have good ESR other then the leaky one, it depends on what you believe. I cut alot of those apart and sometimes they are brown inside and it gave me the creeps so I typically replace them, but 99% of people would leave them alone. I actually saw lower ESR in the old ones then the premium ones I replace them with that are usually much smaller but have equally nice temperature/life specs.

The one that measures as leaky, that is a sign of some kind of flaw, you should definitely replace that one

There is extended tests you can do like charge them up to max voltage and see how they hold charge if you are interested


You can also smell them. They all might smell a little but if one smells alot more that is another good reason to replace it.

However,


Dual capacitors in 1 enclosure are NICE parts. I would not replace that unless you find something wrong with it. This is a special case IMO. So it looks like you should replace two, the physically leaked one and the one that measured bad. I think they have a performance benefit of being in the same package that helps the circuit they are powering, and changing them would make it worse.


And since its a gen-rad equipment.. you might wanna just take the time and wait / search for a true dual cap replacement, unless you need it right now. That is one of those things where 'they don't build them like they used to'.

Dual electrolytics, and tapped foil/paper/oil caps are NICE as hell, from a EM and a manufacturing quality standpoint. They got rid of that for cost reasons. Same with axial electrolytics, you can still get them, but less popular, however from a EM standpoint for certain circuits... its just 'correct' compared to a can. Like for coupling.
coppercone2:
I always hope that eventually some business will come up that can make custom foil and electrolytic capacitors that can come in the form you want. Axial, dual, back to back, isolated foil, tapped foil (dividers), single layer ceramic networks, etc. There is a vast amount of 'premium engineering' parts that are just not available or ever actually sold that can be made IMO. And some other fancy shit like electrolytics with feedthrough capacitors built in  :-DD

Also, case positive caps (reverse polarity) for axial, imagine you can get both polarity axial, with whatever ceramic 'feedthrough' built in, designed to be grounded by a braid for chassis work. So it would combine your ceramic and electrolytic cap into a low impedance ground.
barbaroja:

--- Quote from: coppercone2 on November 30, 2024, 12:07:35 am ---That transformer looks substantial for like a tone generator, I think it can handle pretty much any reasonable capacitor. I thought it might be some micro sized thing

Those capacitor all have good ESR other then the leaky one, it depends on what you believe. I cut alot of those apart and sometimes they are brown inside and it gave me the creeps so I typically replace them, but 99% of people would leave them alone. I actually saw lower ESR in the old ones then the premium ones I replace them with that are usually much smaller but have equally nice temperature/life specs.

The one that measures as leaky, that is a sign of some kind of flaw, you should definitely replace that one

There is extended tests you can do like charge them up to max voltage and see how they hold charge if you are interested


You can also smell them. They all might smell a little but if one smells alot more that is another good reason to replace it.

However,


Dual capacitors in 1 enclosure are NICE parts. I would not replace that unless you find something wrong with it. This is a special case IMO. So it looks like you should replace two, the physically leaked one and the one that measured bad. I think they have a performance benefit of being in the same package that helps the circuit they are powering, and changing them would make it worse.


And since its a gen-rad equipment.. you might wanna just take the time and wait / search for a true dual cap replacement, unless you need it right now. That is one of those things where 'they don't build them like they used to'.

Dual electrolytics, and tapped foil/paper/oil caps are NICE as hell, from a EM and a manufacturing quality standpoint. They got rid of that for cost reasons. Same with axial electrolytics, you can still get them, but less popular, however from a EM standpoint for certain circuits... its just 'correct' compared to a can. Like for coupling.

--- End quote ---

I did want to use it to calibrate the current limiters on some Westrex solid state amps for Vinyl lathe. The manual specifically recommends this machine. It states one cycle on, sixteen off with a certain Peak to Peak voltage, into a 1ohm load, with bigger fuses and then you trim some pots that control FETs wired as current limiters. However, I found a suitable replacement with the software REW. Can create whatever test signal you need.

I will try to get the originals, the 1396B is worth it. However, googling for those pieces has proven them to be hard to find.

As to test the caps, sure, I can do it. Any ideal procedure or should I just charge them and measure the discharge time?

I'm attaching a pic of this beauty.
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