Re: 475 questions

Leanna L Erickson <lle@...>

You may need to bend replacement FW Bridges to more horizontal after replacement.

On May 23, 2020, at 9:57 AM, Harvey White <> wrote:

I'll give this a try, so my replies are in line

On 5/23/2020 12:48 AM, ciclista41 via wrote:
Hi Harvey,

So frustrating! I had a reply nearly as long as your post above, but accidentally hit some button and the entire thing evaporated. Oh well, maybe I will be less wordy and more to the point this time.
I used to have that problem, and then I took the entire "old" message, copied it to a word processor (notepad is fine), then did my editing, then pasted it back. Useful if you anticipate a long reply. As for emails, I use Thunderbird, and haven't had that problem.

Thanks for such a thorough set of suggestions and explanations! This sort of advice is so valuable to me. I understood the part about the rectifiers right away. The part about the linear voltage regulators will take some research on my part to get up to speed on that.
Ok, think of it in plumbing terms. You have a bucket from which you syphon water, that's the load. You have a water supply with a valve, that's the series/pass transistor. You've got a mechanism for measuring the water height in the bucket, and you set it for a certain level.

The bucket gets water removed, which we know. Think of that as the power going to the load (rest of circuit) and doing its job. So we have to adjust the valve to keep the amount of water going in equal to the amount going out. That's all the complicated circuitry. Measure height of water in bucket and adjust valve. Suppose we take less water? Then the bucket starts to get too full, and the mechanism closes the valve. Take more water? Bucket gets too low and we open the valve a bit more. Turn over the bucket, and we "ought" to go valve full on. However, if we don't want that, the little extra resistor and transistor act like a flow limiter so that you only get so much water going out.

I think I almost understand the part about NPN emitter followers, though. I'll be using all of that as well as everyone else's advice as I figure this thing out. Glad the advice has not been conflicting! Gives me confidence that I'm headed in the right direction.
You are. There's likely some good books on transistors out there, but one of the absolute best sources is the service manual itself. Tektronix (and HP) are noted for the quality of their service manuals.

Some on this list have been involved with electronics for years, and this kind of thing is familiar territory for them.

So, tacking a question on to this reply that's meant for anyone following the thread, since you've spent so much time already:

How do I know what generic components are best to replace the original parts? For instance, the tantalum caps come up a lot, but I assume when folks replace them, they don't use the same as the originals. What DO they use? There are so many types of capacitor, even in this one scope, and it's more than 37 years old. The large power supply caps are replaced with newer, smaller electrolytics, but I've heard that some brands are more reliable and last longer than others. What are some good choices?
Many tantalum capacitors can be replaced by aluminum ones. (standard aluminum electrolytics). Depending on the power supply, there are some parameters you want to follow. Others will want to amend these thoughts.

Capacitance, I'd personally use as much or a little more, but not double.

Voltage, going a bit higher is good, depending on where the part is. Early capacitors were thought to be capable of withstanding more of the voltage rating than not, so a 12 volt circuit might have a 15 volt rated capacitor. Time and evidence proved this wrong. Now, I'd use a 25 or 35 volt capacitor on the 12 volts. 25 being somewhat preferable to the 35 volt part. Get too high, and the capacitor doesn't work as well.

Temperature rating, most capacitors are rated at x thousand hours of operation at y degrees. You may not get to that temperature limit, but the typical temperatures are 85 degrees C and 105 degrees C. I'd use the 105 degrees C part.

Nichicon is a good brand, Panasonic is a good brand. Most people steer clear of clearly Chinese made parts which may not be made to appropriate standards.

I looked up one of the original rectifiers, an MDA960-3. I found a NOS of it on Ebay for almost $9 plus shipping. At that rate, this is going to get expensive very quickly. So what can I use instead? I assume that by looking up the specs on each original component that I end up replacing and finding something that meets or exceeds those specs, I'll probably be okay, but I could really use the voice of experience here.
Ask on the list. I think that a 960-3 is a bridge, 600 volts, and 3 amps. Any decent equivalent will work. A 5 amp bridge would work, but more than that and you may find that it's too big to fit properly. You could make an equivalent with (IIRC) 1n5400 series diodes, but that's awkward in a physical sense. The advantage of the bridges is that it's all one unit, and the case provides a heat sink. Any part with current (AC or DC) flowing through it will get hot. It may not be a measurable amount of heat, but there will be some. Generally, the higher power parts (semiconductors and resistors) have some method of dumping that heat, frequently into a chassis.

I'm aware of Digikey and Mouser. I ordered some diodes from Digikey several years ago to build a rectifier that I used to convert the power from a hub generator on my touring bike to charge my phone as I rode. It even worked, although my buddy said the one I built for him killed his phone. Mine had some issues that I never resolved. I'm hoping to soon have the ability to design my own circuit, rather than copying someone else's, and have it be rock-solid! But, I digress...
There are some places you can also buy parts. All Electronics, Electronics Goldmine are two of them. They operate as "dump surplus/excess inventory" and as such, if you see it, buy it and enough for future needs. No guarantees that they will have it again once sold out. James Electronics operates more as a hobby distributor, and has a more consistent inventory. You may find older parts there as well.

Mouser and Digikey are, of course, industrial distributors. If I buy resistors from them (I use surface mounted parts), I would consider buying in lots of 100 for the best price, perhaps even 1000, depending on my history of using the part. You'll see the economies of scale if you go to building things. Replacement parts are another matter, though, since you are not likely to need 100 each of 47 uf/200 volt capacitors. (certainly not at what I think they go for).

You may also find in your area, depending on where it is, some surplus electronics stores that have stocks of parts. Florida has at least two that I know of. Those stores are hard to find, though.


Thanks again, Harvey!


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