Help me refurbish a BK Precision / Dynascan 1601 power supply

[Sir Knight]

After years of wanting a B&K Dynascan 1601, one showed up on Craigslist for $25. It was in excellent shape, no gouges, burns, not even a scuff or a sticky residue from inventory control tape. Mine mine mine mine.

When I got it home I turned it over for a more thorough inspection and noticed the rubber feet were missing. Then I heard something rattling inside so I removed the top cover. The inside looked like the lint trap in a barbershop clothes dryer, so I started vacuuming it out. The rattle was nothing more than a piece of jacketed wire, probably fell in there during a project. The lint and hair was probably keeping the wire from shorting anything.

Then I saw there were a few electrolytic caps in there. A couple of radials on a PCB card, a couple of axials that were floating, PtP wired. A relatively large (probably 40x50) filter can, strapped to the bottom of the chassis. The upgrader in me took over, assisted by the side of my brain which says "it aint perfect - yet". I bet I could just put this thing into service right now and it would be fine, but I'm putting in an order for some other projects anyway, and as long as I have the cover off, well... y'know how it goes.

I've gotten the schematic and gone over the cap list. I see C12 is supposed to be a 200uF 75V, but my 1601 has a 220uF 80v in there. Since caps of the 1970s were most likely -10/+70 I'm not going to worry about it. But I plan on using this supply for my actual design projects, like matching transistors with some precision, so I really want to make sure this is working its best. Given the amount of Dynascan fans here, I'm hoping that at least one of you has refurb'd one of these and can offer some advice on what to pursue.

Things I'm looking for:

- What are the best electrolytic replacements? I see two axials, two radials, and a screw can strapped down on the chassis. (I plan on standing that guy up and clamping him in place, if I find a short enough can that won't rub the leads on the top cover). I'm considering Nichicon PW for the radials, and KEMET PEH169 for the can. I have no idea what to replace the axials with. They're PtP and there's no board to worry about, so unless there's some obscure and arcane reason that require axials, I could invert a couple of radials and hot glue them to the bottom of the chassis, right?

- Are there any service bulletins or updates for the 1601? These things were made for a couple of decades, under two or three different corporate owners. At some point they might have discovered some weak points.

- Are there any tricks/hacks or modern parts substitutions that will ensure smoother output and increase the life of this supply? My first project with this will be matching batches of transistors for audio use. Eventually I'll try getting into small signal amplification - I want to learn preamps and voltage gain stages and a bunch of other things I don't understand. Clean power is the basis for audio. These circuits and components are about 40 years old, I'm sure it could be made "better" by whatever definition you want to assign to that term.

- Anything else I should know? Like the air-speed velocity of an unladen swallow, perhaps?

[bob91343]

I think you are just playing games.  Rebuild the unit, if you must, using the original design or as close as possible.  It is what it is, and substituting parts without knowing the considerations that went into their original selection is asking for trouble.

[Sir Knight]

What exactly did I ask that sounded like I was "playing games"?

[bdunham7]

I wouldn't replace any parts, I would test it and see how well it works. 

[rsjsouza]

I am of the same opinion; leave the parts alone as long as the unit is working as intended.

I have a B&K Dynascan 3030 Function generator that is completely original and, by any of my measurements of performance, it works just as it left the factory. Changing or modifying any of its original transistors without a specific reason is a gamble, as the parts sometimes were better made 30, 40 years ago when the manufacturing process was tailored for excellence and not volume. An example? It is not uncommon to see TO-3 transistors that have copper housings with its incredible heat transfer properties - nobody in their sane mind would think of using copper on a modern part manufactured at a cost. 

[WattsThat]

Let’s not hyperventilate. It’s a power supply from a (sometimes) mid-tier test equipment house. Replace any caps that go pow after you turn it on. If it works, use it. If it doesn’t, fix it. It’s just a power supply. Period.

You previous threads give me the distinct impression you've been exposed to far too much audiophile marketing wank.

[Sir Knight]

You previous threads give me the distinct impression you've been exposed to far too much audiophile marketing wank.

I actually came to this forum to stay away from that, and to build my electronics knowledge. Are you prepared to support that comment?

I've only started one thread here, and that was to ask about adapting a transistor to a different pin package. I've only posted in one other thread, helping someone who was refurbishing a vintage NAD amp.

Where's the audiophoolery there?

[bdunham7]

Where's the audiophoolery there?

This forum is great but a thick skin is helpful.  Don't be deterred by small kerfluffles.

You made the comment "Clean power is the basis for audio".  Actually, that's sort of not true and unfortunately is the purported basis of a lot of audiophool products.  The main power supply of a 'traditional' audio amplifier is actually very noisy--it is unregulated and usually only has one stage of filter capacitor after a bridge rectifier.  If you saw the power supply rails of a typical Class AB amplifier or receiver under load you would see a 60Hz sawtooth, a mirror of half of the output signal and all sorts of distortion and hash.  In this case, the "basis for audio" is Power Supply Rejection Ratio, or PSRR for short--which is a design feature.

Now that part really applies to output power stages, the earlier stages and separate components like preamps will usually have much better regulation and filtering on their power supplies.  So in those cases it may matter.

As for your power supply, unless you can measure the components and measure the before and after performance of the unit, I wouldn't recommend replacing any parts at this point.  I work on a lot of this stuff and I do have the capability of measuring all those things.  I don't know of any failures that are common enough to recommend just replacing any parts in a working unit.

[floobydust]

If I was going to refurbish it, I'd replace all the electrolytic capacitors, given the age and importance of having no drama with a bench PSU, there are only 6. I outright replace can and axial types they don't always make it past 40 years age.
I would redo the power transistor heat-sink thermal compound, put some DeOxit in the potentiometers and LV switches, and do a calibration.

C13 1,000uF 16V
C12 200uF 75V
C10, C11 470uF 25V
C8 1,500uF 75V
C6 4.7uF 16V

The design is fine and it's a linear PSU, rated 5mVpp ripple which is fine for audio work. I wouldn't do upgrades or mods until using it for a while, if it has some limitations that are a problem. Upsizing C12 from 200 to 220uF is just rounding up to EIA standard values.

BK Precision 1601 Instruction Manual
edit: forgot to attach schematic

[Sir Knight]

This forum is great but a thick skin is helpful.  Don't be deterred by small kerfluffles.

You made the comment "Clean power is the basis for audio".  Actually, that's sort of not true and unfortunately is the purported basis of a lot of audiophool products.  The main power supply of a 'traditional' audio amplifier is actually very noisy--it is unregulated and usually only has one stage of filter capacitor after a bridge rectifier.  If you saw the power supply rails of a typical Class AB amplifier or receiver under load you would see a 60Hz sawtooth, a mirror of half of the output signal and all sorts of distortion and hash.  In this case, the "basis for audio" is Power Supply Rejection Ratio, or PSRR for short--which is a design feature.

Now that part really applies to output power stages, the earlier stages and separate components like preamps will usually have much better regulation and filtering on their power supplies.  So in those cases it may matter.

As for your power supply, unless you can measure the components and measure the before and after performance of the unit, I wouldn't recommend replacing any parts at this point.  I work on a lot of this stuff and I do have the capability of measuring all those things.  I don't know of any failures that are common enough to recommend just replacing any parts in a working unit.

Thanks for the clarification on the audio power. I want to know everything about it so your info is appreciated.

When I first started I just looked at power and said, "Is it transformed and rectified to the VDC needed? Great, lets get to the amplification". Now I'm kicking myself for not getting more into that.

As to skin thickness, I'm fine with snark, smart-ass comments, or just BS in general, so I'm not going anywhere. But if Wattshatt wants to do the whole "so what you're really saying is..." thing, maybe thats better suited to Twitter, not a hobbyist forum.

[Sir Knight]

Thanks for checking in and contributing, Floobs. In my original post I mentioned using UPW for those radial at C6, C11, and C12, and a Kemet PEH169 for the can. It matches the dimensions and specs on the short list, I just didn't know if the deeper specs were appropriate for this power supply. Or any power supply, for that matter. Also, do you have any advice on what series I should look for in the axials? Or should I just use radials and glue them in place?

[floobydust]

Nichicon PW are a very good capacitor, I use them a lot. Had Panasonic FC leaks so I am no longer a fan of them.
I use radials (with spaghetti on the leads) and tack them down with silicone. Axials are hard to find and just oddballs nowadays. The modern radial parts are so much smaller.
Take pics before, I double-check polarity afterwards and red felt pen mark them.

The main filter cap 1,500uF 75V I think could be upsized to 1,800-2,200uF without penalty if you need to match physical size or a more popular value. Today it would be a snap-in but these are not great for ripple current rating. The 120Hz ripple current is something to look at as a figure of merit, needs to be higher than the 2A DC output current of course which weeds out the small parts e.g. LGY series. ESR is not as important as it is for SMPS applications.
As long as you wire it to the same spot (right at bridge rectifier output) and not somewhere else which can cause extra hum. The PSU should be using a single-point (star) grounding scheme.

Kemet's website goes offline for datasheets on weekends lol so I can't check out the PEH169's but any decent can will be great here.

[w9gb]

B&K Precision Model 1601 Regulated DC Power Supply
INSTRUCTION MANUAL (includes Schematic and Parts List)
https://www.mrrenwick.ca/media/B&K 1601 Power Supply User Manual.pdf

B&K Precision Model 1601 Regulated DC Power Supply
First manufactured in 1973 (Carl Korn, president), when B&K was a Division of Dynascan Corp.
At this time (1970s), B&K was located at: 1801 W. Belle Plain Avenue; Chicago, IL 60613.
==
This DC Bench Supply landed on my electronics bench (2017) with a nice layer of dust and a "rattle".
This unit also had a burned out NEON bulb.  Easy NEON Replacement (schematic and Parts List in Manual)

The "Rattle" was one of the two P.C. Board Supports, Plastic (Part number: 380-202-9-001),
that Support had a broken "snap-in" that secures and supports this "Vertical Card Guide" to bottom of metal case.

After 40+ years many e-bench techs would assume that this part is obsolete or unobtainium.
However, from 1950s through the 1970s (before Asian migration) most Chicago-based manufacturers sourced their parts (OEM) from local area manufacturers and distributors.

This specific OEM part, 380-202-9-001 : P.C. Board Support, Plastic
was manufactured by the well-known plastics mfg. : Richco, Inc. of Forest Park, IL
This part was used in other similar B&K bench DC supplies, like model 1602.
http://bama.edebris.com/download/b&k/1602/1602 Parts List.pdf

ESSENTRA (global HQ in United Kingdom)
Three (3) companies: Alliance, Richco Inc., and Micro Plastics
were joined together to form Essentra Components.
https://www.essentracomponents.com/en-us

CBG-3-01 Vertical Card Guide, Product Page (former RICHCO part, Forest Park, IL)
You can Request a Part Sample on the Product Page
https://www.essentracomponents.com/en-gb/p/vertical-card-guides/cbg-3-01

Technical Drawing for CBG-3-01 , Vertical Circuit Board Guide
https://essentracomponents.bynder.com/m/68044e48e22c06a8/original/1279499-pdf.pdf

[Chris56000]

When I repair an old piece of T & M I concentrate on getting it working with as many of the original parts as possible and only replace anything if it clearly IS defective!

I would much prefer to have a unit in good condition outside cosmetically and "virgin" condition inside than have to try to resurrect something that someone unknown to me before I got the thing had "recapped" or "upgraded" and added goodness–knows–how–many unknown faults on it!

Modern electrolytic capacitors are made in much smaller sizes than the old original components were and characteristics such as ripple current rating were often quite a bit greater in the old component, therefore the heat generated in the original will be less compared with a small modern component, and the same applies to semiconductor devices, as the many discussions on here regarding "fake" replacements testify!

Original 1970s and 1980s transistors often work in old circuitry with absolutely no problems for years, and then you get oscillations/instability or noise when you replace a faulty device with a new one, because the new replacement part doesn't match the junction capacities, or the lower beta or transition frequency (fT) of the original, this is another topic that has been extensively discussed on here!

Chris Williams