Scott R75s rebuild questions

The reason to use a DBT is if something should cause a short (say a solder bridge or you installed an electrolytic backward) the excess current is taken up by the bulb rather than whatever components are in the way of the short. This can save you major headaches as a shorted component rarely only destroys itself. I've been in many amps and unless the design says no DBT I'd never consider starting one up after repairs without it. One silly mistake can have you set back a few days and out a pile of components (ask me how I know).

Ok, I have the DBT up and running. The receiver is 60 wpc out, back says 375 W max AC input. I mistakenly bought 73W halogens (sold as 100W...I didn't read the box closely.) Should I start this with a lower wattage first--snag an appliance bulb or something? Do I need to get a real 100W bulb to check? Or is 73W enough to see if it has a short in the actual signal path boards?
 
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Thanks for the debate on the transistors. It's helpful to see both sides. Since I'm new at this, I'm going to go slowly. The pre-amp transistors one NPN, one NPN darlington give very different readings on the ohmmeter (comparing equivalents in each channel. EC on left darlington is 25M, right is 3M). I know there is something going on in this circuit b/c a resistor (R6 in schematic is open--left side). I think C1/C101 are likely affecting the readings (the transistor resistances move through a rather wide range once I connect the meter). So I'll check these transistors again once I get the C1/C101 out of circuit for recapping. Thankfully these transistors are still in production.

Preamp.jpg
 
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As I said, I'm still working up a parts list. Caps are more bewildering than the transistors in some ways. I've been advised by two knowledgable folks to consider non polar electrolytics for the signal path. Is it safe to do that for the everything on the signal path boards? Or are there some caps, that do non-signal work that I should make sure stay polar. I'm thinking, for example, that C601 on the preamp circuit (pic above) is an example of a non-signal path cap: 220uf across the +power input and ground.
 
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RE bipolar electrolytics in the signal path, I have read those recommendations before too. At least one amp where, IIRC, all polar electrolytics (maybe just signal path, not sure) were replaced with bipolars . But in any case there were a boat load of green Nichicon ES caps in those pictures. The amp owner reported excellent results, as is most always the case when anyone does most anything. May be true, but who can really say...

There is not universal consensus on the qualities of bipolar electrolytic caps used in positions where they were not used originally. Some recommend them, some strongly dislike them. This is something that actually is a subject of debate at DIYaudio.com, which means that various people state their firm opinions and never change, and nothing gets resolved, lol. Can be interesting reading though.


General consensus on signal path caps:
(1) use film where possible. This often ends up being 4.7uf or smaller. Perhaps 10uF if there is room on the PCB. Wima MKS2 series is very popular, is readily available at Mouser, and it is(as far as I know) the smallest available film cap for this type of application. The lead spacing on most of the Wima MKS2 caps is 5mm but the leads are very short, so PCB lead spacing matters. There are some Wimas with 2.5mm lead spacing, not sure at the moment if those are MKS2 or a different series name. These are polyester, not polypropylene (which would be larger), but any film is better than any electrolytic. And all film is bipolar

(2a) if film is too large physically or the needed uF values are too large, then "audio grade" caps are next choice. Elna Silmic II is the most popular and is my personal favorite. Nichicon KZ, FG, KW and maybe a couple others are similar and also well liked. As with anything, there are some (a minority) who will say that "audio grade" is nothing but marketing hype and who would use standard cap grades in signal path.

(2b) If the original signal path cap was a low leakage type, then Nichicon KL caps would be used by most, but not all, instead of audio grade electrolytic caps (with film still being 1st choice). There are different indicators of an original low leakage cap, the most common one being an orange colored sleeve. I would have to search some notes to tell you other colors or other indicators (i.e. "LL" in the parts listing or schematic, or "low noise" ...there are others)


Again, above is pretty much the consensus for signal path. IMHO better to stick with what is known than the relatively seldom used approach of bipolar electrolytic. I am curious, and open minded, about the use of bipolar electrolytics, and would enjoy reading any information that you have come across (links?). But what is stated above is what the vast majority of people do. I really like the Elna Silmic II because I find their build quality to be a notch above (same thing can be said of KZ as well).

For non-signal path stuff, my favorite is Panasonic FC, which is also very popular among AK recappers in general. Other popular caps are Panasonic FM and FR, and Nichicon PW, HE, PM. All of these are 105°C rated which is good to extend service life. And all of these are interchangeable and would be used in the same types of roles.
 
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Re your other comments

I don't know what kind of bulb to suggest. I have only ever used incandescents in my DBT. Don't if those are still available where you live, or you can find some in use in a lamp or fixture somewhere. If so I'd try a 60w, or close to that, to start. No idea if one of the new LED or halogen bulbs would function correctly in a DBT.

Any transistors in your amp that can still be obtained, same original part numbers, then I would get 'em. For back-ups if nothing else. But I wouldn't solder them in now. The way you are going about this it seems that you want to solder in a bunch of parts before ever testing the amp. That is not a typical approach. But it is good that you are probing around in there. And if you find parts of any type that you are fairly certain are bad, then by all means...

And to your one specific caps question, there is no argument that I am aware of for using bipolar caps in non-signal path locations. So, no don't do that.
 
My go to caps....Nichicon Fine Gold, Nichicon Audio Grade.

Never tried switching to bipolar for signal path. Never had the urge.

United Chemicon for filter caps have been my go to.

100 watt incandescent. I once tried a bulb that was not incandescent and did not work.
 
In the signal path, my preference is to replace electrolytic capacitors with miniature film capacitors (e.g., Wima MKS2 or Kemet R82) whenever possible, i.e., for capacitors that are ≤4.7µF. Film capacitor are superior to electrolytic types wrt ESR, ESL and leakage. For larger values I will typically install Nichicon ES bi-polar types. This approach is largely derived from the published work of Cyril Bateman examining the impact of capacitors on sound quality: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles. He tested electrolytic capacitors extensively and demonstrated that in AC coupling (signal path), bipolar electrolytic capacitors have the lowest distortion/noise of all the electrolytic types (even the presence of a polarizing voltage) and that film capacitors are generally lower distortion/noise than electrolytic capacitors of any type. The relative absence of bi-polar capacitors in vintage electronics is typically due to their larger physical size and higher cost; if used at all, it was often in the more expensive TOTL models (e.g., the power amplifier feedback loop in the Sansui AU-999 has a bi-polar electrolytic capacitor whereas the AU-555 uses a polarized electrolytic).
 
On the transistor testing front, I tend to avoid measuring in-circuit - the more reliable method is to remove & test them separately, if needed. There's a good sticky at the top of the DIY forum for transistor testing: http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Caps-wise, the others seem to have pretty much covered everything I'd say - personally, I tend to normally use Wima & Pana films, Pana FC & FM, and Nichicon KL, KW, PW, ES. Don't forget to watch out for the dimensions as well - including pin spacing.

One of the things I found most confusing when I started, was identifying & understanding the function(s) of each electrolytic cap - that's essentially what guides capacitor choice. I can strongly recommend studying some of 'Leestereo's' restoration threads (http://audiokarma.org/forums/index.php?members/leestereo.1051/) - where he provides superb background information & recommendations for exactly how & why he makes the choices / decisions for any given part. There are certainly other forum members who provide similar information, just I've personally found Leestereo's to be some of the best 'study guides' for me.

Edit - I was actually writing this when Leestereo answered.... decided to leave it anyway, can't hurt.
 
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Re: DBT, the halogens are a form of incandescent. Just rated higher lumens per watt. So these are just 73W electrically. If I'm going to need a true 100W i'll get one.
 
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I continue to work through the boards and chassis: checking for open resistors, shorted transistors, and bad diodes.

I have a question about a Double Diode in the driver circuits. [Note that with the daughter boards out this part is completely isolated. So no in circuit confusion is involved.]

It's mounted off board on the heatsinks with the big power transistors. D3 in the middle of the schematic. When I check them, one seems very intermittent. I've checked the physical connections repeatedly, they are fine. I notice the "good" one takes a half second before returning a .996 v reading on the diode check setting. My meter is very low power -- 2 AAA batteries. So I decided to heat the intermittent one a bit with a high lumen flashlight. It switched on with a voltage reading similar to the other.

Is this double diode functioning to temp track the bias of the outputs as this part discussed here does in a Kenwood, et. al?

If so, I guess if it switches on with a quick bright flashlight blast it's still ok. Ideally the two channels would have very similar response curves I guess, but that's not crucial. Service manual only gives the useless Scott # (012-1022-002) -- without any other specs (can't read the part itself in the mount). It's one of the few parts on this chassis that is physically hard to replace. So, can I assume this one is ok?

Driver circuit copy.jpg diode.jpg
 
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Ok, I should also share that I learned how NOT to clean the faceplate.

It's a massive extrusion. Looked like utilitarian matte finish that was anodized. I've read other's accounts of soaking it in mild dish soap. I started with very warm water. Then mild dish soap. I thought this was conservative. Turns out the it was covered in a lacquer that had oxidized/uv degraded. In a minute or two, it started to slough off revealing bright aluminum underneath. Argh. I didn't lose any markings, but now some ugly patches. This happened so fast the dirt I was cleaning still remained in places. Turns out denatured alcohol works fine. (Nice to know!) Now the remaining lacquer is even cloudier from the moisture.

(Laquer is fine on top and bottom of the extrusion. It's only the faceplate that so vulnerable. So I think its' just UV exposure over the decades.)

The plan:
1. Scan the faceplate so I have graphics available for corrective decals or toner transfers down the road. Esp. if #2 ends badly.
2. Give it a light spray coat of lacquer and hope that dissolves the finish and lets the moisture dry out. (Furniture folks do this with lacquer sometimes.)

I really didn't want to do this sort of thing. I'll prob punt these concerns for a long time.
 
Is this double diode functioning to temp track the bias of the outputs as this part discussed here does in a Kenwood, et. al?

That's my read on it, yes, except in this case it's a 'double diode' type.

If so, I guess if it switches on with a quick bright flashlight blast it's still ok. Ideally the two channels would have very similar response curves I guess, but that's not crucial. Service manual only gives the useless Scott # (012-1022-002) -- without any other specs (can't read the part itself in the mount). It's one of the few parts on this chassis that is physically hard to replace. So, can I assume this one is ok?

Not sure what you mean by "intermittent" ? It's certainly possible that your meter's not up to measuring them (try another meter ?), but if they're both behaving in a similar manner when heated slightly, and both then measuring as diodes, I'd be inclined to leave them, for now at least; as you say, they don't look to be the simplest to physically replace.

Bad luck on the faceplate (photos ?) - it's not a problem I've had (yet). Your plan #1 sounds very good. Plan #2 I'm afraid I find much less convincing, but hopefully someone who's experienced similar can chip in....
 
That's my read on it, yes, except in this case it's a 'double diode' type.

Not sure what you mean by "intermittent" ? It's certainly possible that your meter's not up to measuring them (try another meter ?), but if they're both behaving in a similar manner when heated slightly, and both then measuring as diodes, I'd be inclined to leave them, for now at least; as you say, they don't look to be the simplest to physically replace.

Thanks. By intermittent, I mean that it didn't give a reading on the diode test either direction. ".0L" -- both ways. I thought it the diode was open. Then suddenly it read like the other (.998v). (I must have touched it and heated it up a bit I think.) I returned later and it was giving no reading again. So I decided to try the flashlight to heat it up. It consistently responds to that: No reading either direction on diode test when cold. Kicks in with just a second or two of flashlight heating it up. (This is just a very bright LED flashlight. So it's not like a IR heatlamp or something.) Then reads exactly like the other. Similar readings for both in response to the light. As they heat up they drop (.998->.991). But the "good" diode is a bit ahead of this one. (There is a brief pause for the good one to give a reading when cold as well. A half a second or so.)

I can try another meter.

I'm not eager to mess with this since I don't have a part number for it. I don't like to unsolder semis if I don't have to. The schematics list voltage points near where these feed into the circuit. So I can compare those and monitor as things heat up once I get it plugged in.

As I read the schematic, if it fails open, nothing horrible should happen. But I'm not really confident on that reading.
 
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In the signal path, my preference is to replace electrolytic capacitors with miniature film capacitors (e.g., Wima MKS2 or Kemet R82) whenever possible, i.e., for capacitors that are ≤4.7µF. Film capacitor are superior to electrolytic types wrt ESR, ESL and leakage. For larger values I will typically install Nichicon ES bi-polar types. This approach is largely derived from the published work of Cyril Bateman examining the impact of capacitors on sound quality: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles. He tested electrolytic capacitors extensively and demonstrated that in AC coupling (signal path), bipolar electrolytic capacitors have the lowest distortion/noise of all the electrolytic types (even the presence of a polarizing voltage) and that film capacitors are generally lower distortion/noise than electrolytic capacitors of any type. The relative absence of bi-polar capacitors in vintage electronics is typically due to their larger physical size and higher cost; if used at all, it was often in the more expensive TOTL models (e.g., the power amplifier feedback loop in the Sansui AU-999 has a bi-polar electrolytic capacitor whereas the AU-555 uses a polarized electrolytic).

Now, a bipolar cap is formed when new. it has two foils. After many hours in a polarized circuit, one of the foils will have been "wrong way around"all the time, so I wonder if the comment on bipolars would be the same if the bipolars are older.
 
Now, a bipolar cap is formed when new. it has two foils. After many hours in a polarized circuit, one of the foils will have been "wrong way around"all the time, so I wonder if the comment on bipolars would be the same if the bipolars are older.

AFAIK, with modern bipolar capacitor construction, the AC signal voltage is enough to maintain both oxide layers. Installing a bi-polar capacitor in a DC-only situation will indeed degrade one of the foils since it is always reverse biased. Eventually, this bi-polar electrolytic capacitor becomes a polarized type (but not as good as a real polarized capacitor since the non-working oxide layer just adds more ESR).
 
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