Author Topic: Power Designs 2020 Repair  (Read 15222 times)

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Offline motocoder

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Power Designs 2020 Repair
« on: October 11, 2014, 02:54:54 pm »
Buoyed by the success on my Power Designs 2005 repair, I've acquired a 0-20V 2 amp model, the Power Designs 2020 Precision Power Supply. Note - this is not the same as the 2020B, and unfortunately, it seems no one has a manual or schematic for this sucker, so repair may be a bit more difficult.

The unit powers up OK, and the oven comes on. However, voltage regulation is not up to snuff. The first thing I discovered is that there is no semiconductor control of the oven heating element; It's mains voltage wired straight up to the thermostat, and relies on this mechanical thermostat to switch on/off mains voltage to the heater. Second thing I discovered is that the thermostat starts switching on/off almost immediately (~40C), but the temperature does not stabilize until around 85C! That was measured with the cover partially removed (so I could insert my thermocouple), so in actuality I suspect the temperature is higher than that with the cover on. Too hot? I think so. I got better results running my 2005 at 50C.

So, disconnecting the heater and just running the thing without the temperature stabilization, it initially looked like I was getting pretty good regulation - around 50 microvolts. However, while I was out of the room, something caused a disruption in the output voltage, as when I came back a bit later, the statistics on my meter showed a drift of several millivolts. The unit looks like someone might have been monkeying around with it. I did check all the caps for short/open, and didn't see any issues there.

I am going to start by rewiring the heater and converting it to use a lower voltage off the secondary. Then I will clean the contacts on the switches and recalibrate, and see where we are at.

Pictures below.

« Last Edit: October 11, 2014, 05:32:09 pm by motocoder »
 

Offline SeanB

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Re: Power Designs 2020 Repair
« Reply #1 on: October 11, 2014, 03:49:41 pm »
Side of the transformer as well so we can see the thin wiring used inside.
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #2 on: October 11, 2014, 05:55:01 pm »
Side of the transformer as well so we can see the thin wiring used inside.

Here you go. I totally missed that there were some taps on the underside of the xfrmr. I had ohmed the ones on the top out, but let me ohm those on the bottom before I post the results.
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #3 on: October 11, 2014, 07:27:23 pm »
The attached screen capture shows the AC voltage between all the different secondary taps. I have arbitrarily labeled the taps on the bottom A, B, C, and D. Tap A is on the same side as taps 0 and 5, tap D the same side as taps 4 and 10, etc.

For the primary side (taps 0, 1, 2, 3, 4) are connected as follows:

Tap 0, 2: Mains neutral (white wire)
Tap 1, 3: Mains hot (black wire)
Tap 4: Chassis ground / mains ground

I'll continue to do some investigation into where the secondary taps connect.
 

Offline valvedude

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Re: Power Designs 2020 Repair
« Reply #4 on: October 12, 2014, 12:00:00 am »
i would start to replace and or check the caps too
just a tech who is addicted to electronics what more can i say so much fun nothing better then a challenge find a fault in a piece of an electronic equipment
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #5 on: October 12, 2014, 03:01:57 pm »
i would start to replace and or check the caps too

I have ordered replacements for the electrolytic caps. There are a number of low value orange caps in there that I wasn't sure about, but I can't see any sort of polarization marking on them, so I don't think they are electrolytic.

I've got the transformer secondary tap wires all traced to where they go (all onto that bottom board). Just need to look at the circuit there a bit more to understand what each is being used for. This board seems to handle rectification and filtering as well as driving the blinking of the current limit lamp.
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #6 on: October 13, 2014, 04:50:25 am »
Ok, I have reverse engineered enough of this to know how to achieve what I want to achieve. The heater is going to be driven off the output supply taps on the secondary using the same approach that SeanB suggested and which worked so well on my 2005. What I will do differently is to use a microcontroller based PID loop to control the temperature, and some thermal epoxy as the insulation between the tube and the heating coil. This should eliminate the oscillating oven temperature that I believe is the limiting factor on the voltage reference stability. I'll get the PID loop working outside of the power supply, and then just solder it up to the connections I've already identified once I have verified that it works.

I'm kind of regretting ordering those caps now. I have the ability to measure the capacitance value and esr, but I'm not sure what is an expected good range for these. And after replacing the inadequate diode someone had bodged in as a replacement, and then doing a rough calibration (bearing in mind that the oven is disabled), this thing seems stable within about 20 uV. Hard to imagine doing much better than that.

So I think I may swap out the big input cap, and leave the rest in place. Clean the switch contacts, make my oven mods, and should have a nice 20V 2A power supply.

BTW - if anyone else has one of these without the schematic, and wants the data from my reverse engineering, let me know and I will figure out a way to post it here.
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #7 on: October 13, 2014, 10:33:41 am »
How are they doing the over current lamp blinking? Is it a 555 like the B model? (By the way, I experimented with replacing the neon lamps with LEDs on my 2020B; adding a 1k resistor allowed them to run off the 125VDC just fine, unfortunately they wouldn't blink since the transistor triggered by the 555 never drives the voltage to zero!)


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Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #8 on: October 13, 2014, 04:14:13 pm »
How are they doing the over current lamp blinking? Is it a 555 like the B model? (By the way, I experimented with replacing the neon lamps with LEDs on my 2020B; adding a 1k resistor allowed them to run off the 125VDC just fine, unfortunately they wouldn't blink since the transistor triggered by the 555 never drives the voltage to zero!)

On my 2005 they are using a transistor. I looked at this part of the circuit on the 2005, and didn't fully understand how it was working. I replaced the lamp with an LED there, and observed the same effect you describe - no blinking. I calculated the current through the LED, and it was fine with the existing resistor. However, I did put a rectifier diode in parallel with the LED, in opposite polarity. I did this because I observed some transient when the lamp was shut off, and the large reverse-biased voltage could blow the LED.

On the 2020, it appears to be a transistor as well. There are no integrated circuits anywhere in this design. And in fact, I would not recommend others get a 2020. The whole design appears a bit bodged. There are 2 main circuit boards instead of 1, and the terminals on the back have a bunch of capacitors bodged onto them, and crowding up against the mains wiring. Unless this was a modification added by an owner (I did find at least one of those for sure), it's a shitty design.


 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #9 on: October 13, 2014, 04:15:59 pm »
After discovering the three problems with this supply, which was listed on eBay as in 100% working order, I left negative feedback on the eBay transaction. The seller contacted me. He refunded the entire amount and is letting me keep the unit in return for updating my feedback.  :D

The seller is aerospacesurplusnett . Impressed with their standing behind the sale like that.
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #10 on: October 15, 2014, 04:10:07 am »
I am starting to have second thoughts about this micro controlled PID loop. It is going to be PWM the current through the heater. The power for the heater is coming from the same transformer taps (albeit out of phase) as the regulated output. Isn't that going to introduce switching noise?
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #11 on: October 16, 2014, 04:45:08 am »
Swapped out some of the electrolytic capacitors this evening. I tested all of them, and they were actually all fine, so it seems this was a waste of time and money...
 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #12 on: October 16, 2014, 07:59:08 am »
well, i blew the thing up. it was all,working fine after swapping out some caps, and i came back later in the evening, and turned it upright, powered it on, and boom: multiple parts smoking.

I'm going to salvage what parts I can and send them to SeanB, toss the rest.
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #13 on: October 16, 2014, 04:36:05 pm »
If you want to send it to me, I'll give you a few bucks for it. I'm in the process of restoring a number of these and could use it.


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Offline SeanB

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Re: Power Designs 2020 Repair
« Reply #14 on: October 16, 2014, 05:38:48 pm »
Send it to timb, that front panel is too nice to simply toss away as scrap.
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #15 on: October 16, 2014, 07:20:06 pm »
Yeah, it looks like it's had very little use. I might even be able to get her going again!

And didn't mean to step on your toes there SeanB.


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Offline SeanB

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Re: Power Designs 2020 Repair
« Reply #16 on: October 16, 2014, 07:24:02 pm »
Not at all, you are more than welcome to all the parts you want from it. I would have to either rewind the transformer or get another one to use it on 220V so you should use it for parts if Motorcoder is willing to send it to you.
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #17 on: October 17, 2014, 12:03:52 am »
A 2005A I ordered on eBay cheap came in. I noticed something rattling around inside the unit and black “dust" all over bubble wrap. I opened the case to find it like this (I pulled the red cover off, as it was already bent like that and not being held on anymore):





 :palm:

There’s burn marks on the top cover, so it looks like it was like this *before* shipping, since I know the seller had it plugged in.

So I’ve already got a use for the oven casing and assembly. I’ve got plans for the rest of the unit as well to bring it back to life with a more modern reference and similar circuity to the 2020B.
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Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #18 on: October 17, 2014, 11:24:37 am »
Yeah, it looks like it's had very little use. I might even be able to get her going again!

And didn't mean to step on your toes there SeanB.


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Not likely since I already cut the transformer out... I think one of my next projects is going to be to create my own PS. The case on this thing, and the switches might be incorporated into that, so I will be holding onto those, not throwing them out.

However, everything on the circuit boards is available to any one who wants them.
 

Offline timb

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Power Designs 2020 Repair
« Reply #19 on: October 17, 2014, 11:34:57 am »
If the part that holds the oven socket to the board is intact (along with the rest of the oven components), I'll take that. (If it's still attached to the main board I'll just take everything, the spare parts would be good for the three units I've got.)


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Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #20 on: October 17, 2014, 11:36:37 am »
If the part that holds the oven socket to the board is intact (along with the rest of the oven components) is intact, I'll take that. (If it's still attached to the main board I'll just take everything.)


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Sorry man, the oven assembly itself I am planning on using as part of my improved oven design (heating coil embedded in thermal epoxy along the thermistor).
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #21 on: October 17, 2014, 11:42:17 am »
Damn, okay. Well, I need to figure out a way to fix this assembly then. The heating coil wrapped around the inner tube is still good, I've just got to figure out a way to repair the broken base (see pic 2 above). Also something to re-pack the oven with. (I'm still itching after cleaning out the asbestos or whatever the fuck it is.)

Keep us posted on your oven project.


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Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #22 on: October 17, 2014, 11:51:37 am »
Damn, okay. Well, I need to figure out a way to fix this assembly then. The heating coil wrapped around the inner tube is still good, I've just got to figure out a way to repair the broken base (see pic 2 above). Also something to re-pack the oven with. (I'm still itching after cleaning out the asbestos or whatever the fuck it is.)

Keep us posted on your oven project.


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I do have an extra metal ring and PCB that sit on the underside of the assembly. If those are useful to you, I am happy to drop them in the post. I have an extra red outer tube that you are welcome to as well.

Regarding the insulation, I think it is glass wool. Nasty stuff. I have been using this stuff as a replacement for the outer layer on the tube:

http://www.amazon.com/gp/product/B000E28EH0/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

For insulation inside, I just stuck with the glass wool. If you want to replace it, you could potentially take that exhaust wrap and tease the fibers apart and bundle them up. It actually comes part pretty easily.

I will be doing some experiments with the PID controller this weekend. One of the challenges with control loops for thermal systems is the latency between the heater and the sensor. So I want to try some experiments with the thermistor on the inner tube wall (bonded with TC epoxy), and on the outside next to the coils. My hope is that using a PID loop to regulate the temperature based on the sensor in this location will give stable temperatures *inside* the tube on the voltage ref circuit board even though the sensor isn't located there. The absolute temperature may be different but that doesn't matter since all we care about is temperature stability rather than the actual temperature value. I was hoping all this would be part of my 2020, but I guess now it will go into my 2005.

I will definitely keep you guys updated with the results.

 

 

Offline motocoder

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Re: Power Designs 2020 Repair
« Reply #23 on: October 17, 2014, 11:53:11 am »
Also, do a search on Amazon.com for "high temperature insulation". A number of potential items come up. The fireplace insulation looks promising.
 

Offline timb

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Re: Power Designs 2020 Repair
« Reply #24 on: October 17, 2014, 03:55:26 pm »
Ohhh yeah, the stuff they insulate fireplace and oven doors with? Silica cord, I think it's called. It's what most eCigs use for wicking the juice; normally a small bit of nichrome or kanthal wire is wrapped around it to cause vaporization. It won't melt, so I bet you could just wrap the outer tube in a few layers of it.

Another idea I had was hemp. It's pretty resilient.

Finally, for the bit of insulation around the reference board inside the inner tube, why not just use cotton? I've got rolls of 100% pure USP cotton compresses that are nice and fluffy when pulled apart.

In fact, cotton gauze might work on wrapped around the outside too.

Did we ever figure out what temp the OEM thermistor trips at on these? (I can always heat this one up and get a number if still needed.)


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