Scott R75s rebuild questions

Scott75s1974

Well-Known Member
Hi, I’m rebuilding my Grandfather’s Scott R75S from 1974. I’ve had it for about 30 years. Used it intermittently up until about 8 years ago.

***I’ve read a lot of posts but haven’t found any general newbie refurb FAQs. So if I’ve missed that, please direct me there.***

It’s symptoms at last use were:

-It was always pretty muddy sounding. Not much high range, maybe not a lot of bass.
-A build up hiss at power-off in both speakers, kind of like a waterfall for a second.
-Some combination of speaker switch channels didn’t work (schematics suggest this can only be a dirty switch. No electronics involved on that board.)
-Cracking and popping on the volume and tone controls

-The last time I used it, one side seemed to be going out. I haven’t plugged it in since. I don’t want to toast anything.​

I’ve looked at all the boards. I can’t see any obvious burnt or blown parts. There are some braided looking high wattage resistors on the driver boards. One looks darker. But check’s ok.

So, I’d welcome advice about how to proceed. It’s a very robust design with removable daughter boards. I have unsolder and replace skills, but not much knowledge of amp circuits. I also have an oscilloscope and a signal generator. (But those are much older and prob need recapped too!).

Here’s my plan:
-Remove the boards.
-Clean all pots and switch banks.
-Clean/deox all the contact pins for the daughterboards.
-Replace the caps in the PS. Measure that for proper output.
-Replace the caps on the regulator board. Reinsert. Measure that for proper output.
-Replace caps on the other boards.
-Resolder any suspicious looking spots (they all look very good...it is really a finely built piece of equipment).

All pretty straightforward.

Should I remove, check and reseat the big T03 transistors? My assumption is that these need mica insulators and non-conductive grease. (I have some arctic silver which I assume I cannot use.)

Are there any other component checks I should do on the boards? I don’t really know much about how to check components in situ. I have a transistor checker with probes. Do these work on a board?

I’ve read there are certain transistors/diodes that are likely fails. Is there any guide to those somewhere?

Any other obvious thing I should do?

Thanks for any help. And again, my apologies if I'm missing a FAQ.
 
Hello Scott75s1974, Welcome to AK. The T03 's mica insulators should be good to reuse if removed without damage. Maybe regreased. http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ Check out the testing of the transistors thread. You have the basics covered already for starters. Make sure you take plenty of photos as you go. Mark each cap to keep sure of which is positive and negative as placed in the board. Sometimes the board markings are wrong. And the Service Manual is not perfect either so check the parts in the unit against the SM. Also check the SM to see if there are "Fuseable" resistors (resistors that are a safety feature), if there are check the value to see they match the SM. They can age badly and go up in value by quite a lot. Pics would by nice to post as to the state of the receiver. And it helps to see the actual PCB/parts for anyone helping. Or going to help anyone in the future doing the same R75s. Oh the SM is on HiFiEngine.com. https://www.hifiengine.com/manual_library/hh-scott/r-75s.shtml If you need it, just register and download for free.
 
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Thanks. I attach pictures of major boards. I suppose those cooked looking resistors on the left driver board are a symptom that should be diagnosed (R20 and R21 pairs--numbers hidden but match the right board). They still read properly. But clearly a lot of current has been going through them.
 

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Thanks again. Here is the schematic for the driver board. I would welcome any ideas what parts I should check upstream that might be causing the overheating of R 20 and 21.
 

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I would add...

Replace power relay and filter caps.

Test resistors. If testing in circuit, they can give a false low when in parallel, but not a false high. If they read high, there is a problem.

Replace transistors.
 
Thanks for this. I'll check the resistors.

As far as the transistors go, I've read references to having them "matched." Is this simply a matter of getting two from the same batch? Since transistors are commodity components, are there certain brands I should try to find?
I have Heathkit's quicky transistor checker - the late version of the IT-10. It just tests leakage and gain using 3v from C cells. It will find shorts or open transistors. But is it enough to evaluate the big power transistors? Or are there problems that will only appear with higher voltages/currents?

I finally got the filter caps out and was amazed that the 4700uF caps read only as 5nF now. So they've been out of commission for a long time. As far as replacing those, the larger 75 watt version of this receiver had 10,000uF in the filter caps. [The only schematic difference I can find is a 5v higher secondary output and the larger filter caps. The PS supplies 44v (as opposed to 40v on mine) to the driver amp boards. Both use the same regulator board to supply the other boards 28v.]

Is there any advantage or caution to increasing the size of the filter caps? There is plenty of room (4 big holes in the chassis). Both models use the same bridge rectifier--so it can handle charging bigger caps. Is bigger smoother with more reserve to avoid clipping? or asking for trouble in some way I can't see?
 
Certain transistors need to be gain matched to get desirable dc offset readings in the event that dc offset is not adjustable. This is more that just getting transistors from the same batch. Search for "gain matching transistors". Again, this is only for certain transistors.

I have good luck with Fairchild transistors from mouser. They are so cheap and so reliable, just replace them all.

As for filter caps, go a little bigger or keep it the same. If you go too big you could cause other problems, such as premature failure of the on/off switch, amongst other problems. I would shoot for 4700-6000 uf myself depending on what is available in the size, style needed.
 
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Thanks for the reminder about the switch.
I had spent many hours reading of debates on recapping in general. But I found the threads on changing values of filter caps. So I thought I'd post the links of conservative views here, since they helped overcome my newbie-ish desire to install bigger caps. (Although in my defense the R75 is basically the R77 with smaller caps and a slightly lower secondary voltage.)

Recap, restore, or modify: What, why, and how (an Engineer's perspective)
going up with capacitance
going up with capacitance
 
No problem in increasing PSU reservoir caps as per larger model. With the main supply voltages at +/-40Vdc the output power will not change, the increased cap sizes may well provide higher peak capacity and the output transistors will remain safe. The resistors R20/R21 are the output transistor emitter follower resistors. These are liable to get hot and in most amps are rated at 5 to 10 watts capacity (higher the better).

Good luck with the rebuild.
 
I would add..

Replace transistors.

Thanks again for all of the advice. Both here and on the wider forums.

Ok, I've looked at replacing the transistors.

The T03 output transistors are obsolete: 2n6029/2n5629 pairs.
-I can find NOS on Ebay to get the 6029s, 5629s are still in stock on Mouser. (Ok, the 6029s are also available if I want to order 220 at at time and want to wait 12 weeks.) But this seems a very likely way to get pretty unmatched pairs. Are matched pairs important for these NPN/PNP output pairs?
-From what I've read here and been able to find, there is a contemporary match for the next size up, the 2n6030/2n5630 pair: MJ15003/MJ15004. Those are 120v rather than mine which were 100v. The new one's have an fT of 2 mhz, double the specs on my components. Will this be a problem? What are the specs I need to compare to make sure they can work? Should I plan on buying 2x the number I need to try to match pairs? (I'll need to find a local friend with an appropriate meter to do so.) Should I consider going to more recent T03 packaged drivers? (MJ21193, MJ21194).

[Answering my own gain matching question here after staying up very late reading many long threads: NPN/PNP output pairs aren't really a matching concern b/c a mismatch would just affect the relative amplitude of the push/pull output, not do something destructive like DC offset.]
This amp has only single stages, no ganging. Are there any crucial pairs that should be gain matched?
The differential pairs on the driver boards? [This seems to be the key pair to match, after reading that long and classic thread on DC offset started by Echowars. But reading that thread, the advice is often just replace the differential pairs, not necessarily match them. I guess I could buy 10 anyway and match some if it becomes an issue.]

The TIP31C and TIP37C that feed the output transistors? [Same answer as for output pairs above.]

The IF amp and the Mult/Demod board have JFETs on them: 2n3823. When I look these up they are both expensive ($30) and have 4 leads, whereas mine have 3. I have no idea what to do. My inclination is to just focus on the pre-amp and driver boards and assume those JFETs are working, circle back later if FM is bad.
[Ok, checking data sheet closely...Lead 4 is the "Case" so it's the same thing inside.]
Is their a cheaper substitute for this JFET?​

One final transistor question. On the transistors, do the digits after the model # matter or are they just batch/date indicators? They aren't on the schematics.
E.g., TIP29 7336, or 2N3644 340, etc.​
 
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And while I'm asking about transistors, in that DC Offset thread, EW often recommended the
Zetex ZTX795A/B for differential pairs. My driver boards have 2N5087s in the differential pair. The specs seem similar. Can anyone tell me the key specs I need to make sure match to do this substitution? What should I exceed or stay below?
 
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Test resistors. If testing in circuit, they can give a false low when in parallel, but not a false high. If they read high, there is a problem.

Thanks! I've found an open resistor on the preamp board. One of the one's tied to the output in parallel with a capacitor. I'm sure there is a name for that.
 
And while I'm asking about transistors, in that DC Offset thread, EW often recommended the
Zetex ZTX795A/B for differential pairs. My driver boards have 2N5087s in the differential pair. The specs seem similar. Can anyone tell me the key specs I need to make sure match to do this substitution? What should I exceed or stay below?
These are very good transistors, but the main problem is that you need to have them pretty much hFE matched in order to have good/acceptble DC offset, since the circuit doesn't have DC adjustment, thus you need to order a few and try to match the 4 ones you need (2x for each channel).

Key specs are ok, 2N5087 has Cob of 4pF, whereas the Zetex has 15pF, not a big deal.

Check R4 (10K?) and replace it with a Dale MF resistor of a higher wattage.
 
And while I'm asking about transistors, in that DC Offset thread, EW often recommended the
Zetex ZTX795A/B for differential pairs. My driver boards have 2N5087s in the differential pair. The specs seem similar. Can anyone tell me the key specs I need to make sure match to do this substitution? What should I exceed or stay below?
Finding proper transistors to replace old, unavailable transistors is not my area of expertise. When I am in this position, I do a search for the transistor number I have, and interchangeable transistor part numbers come up in the search. Then I give mouser my part number, and I see what they come up with. This has worked well for me historically. i am not knowledgeable enough nor do i have the time to learn to find my own replacement transistors to replace discontinued units.
 
Thanks. I was getting nothing on Mouser to match the 2n6029. But then I learned that the TO-3 package has been replaced by the TO-204 designation. When I put that into the search criteria, many more came up. I'll work through those.
 
welcome Scott...

First projects always involve a lot of enthusiasm. Being eager to get started, and having a long list of tasks to do that makes one feel the need to get things moving now, could perhaps result in rushing ahead too fast.

The first item on your list is "Remove..."

I think the first item on your list should be to build a DBT (and buy or borrow a variac if you can) and see if you can get her running as is. You'll use the DBT later, so might as well go ahead and build one now. Instead of recalling symptoms by memory, thoroughly explore and document the operational condition of the amp in its current condition. Then fix what you are able to before replacing caps.

There are two benefits to a conservative, methodical (yeah, slow) approach: First, you will avoid potentially mucking something up that is not currently broken. Troubleshooting when multiple issues exist becomes much more difficult. Let's say that the amp does have issues. But you rebuild boards before ever testing. Perhaps you will have added new issues? How would you know? This is your first project. Taking it one small step at a time is much better than retracing your steps if (when) things go wrong. Another benefit of the slow approach is that when you make changes that improve sound, you will have a fresh reference point having heard the amp recently. You will be able to better appreciate the effect of changes you make doing it in small steps than if you do it all in one fell swoop.

The things on your list, and many of the things suggested, are good things to do. Do a little, test, listen, repeat. Everything doesn't have to be done all at once.

RE transistors: Unless there is an indication that a transistor is faulty, IMHO leave it alone. There is far too much transistor shotgunning going on and it is more of a trend than it is established wisdom. Some transistors are known by the AK community to be trouble makers. Okay, change those out if your amp has any. But at least wait until you are very familiar with the amp's functionality before doing that. And if the amp does have functional issues that point to specific transistors, again, change those out. But be careful and do your research. Changing out all transistors in an amp because it was suggested on the internet is not wise.

my .02¢ FWIW
good luck with your Grandfather's amp :)
 
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welcome Scott...

First projects always involve a lot of enthusiasm. Being eager to get started, and having a long list of tasks to do that makes one feel the need to get things moving now, could perhaps result in rushing ahead too fast.

good luck with your Grandfather's amp :)

Thanks, that's very helpful advice. I've been headed in that direction. Still doing on board eval of components and working up parts list. I've worked through the easy stuff: diodes and resistors. And will now do what onboard transistor checking I can. For starters I will only replace any that test as shorted out. (Transistor wiring adds an exponentially greater level of potential error...so I won't shotgun those.) I do plan to replace all the caps. But will do it in an ordered way. The Scott design is great to work on. The daughter boards just pop out, so it's easy to work through step by step.

I don't think I need a DBT, but I'm open to correction. The transformer works. By taking the daughter boards out, I have ability to check the rails (if that's the name for big, unregulated dc power output for the drivers). Then I can place the regulator board in and check that function. Once those are cleared (and I take it that those are the potential big, bad smoke all things downstream problems), I can replaced the various audio boards after I've checked them as best I can for any bad parts.

If that makes sense, here's a process question: I'm not sure how to proceed with the signal path caps. I can't check the caps on board. If some of those are destructively out of spec, will powering up risk damaging other components? Or is that the sort of thing that would take while?

For other newbies, I've learned that the cap check function on my multimeter has some unspecified ceiling. So it's fine for lowish uf, but above that it doesn't read properly. So the big filter caps read in the nf range, but a battery check shows they definitely have the ability to hold a significant charge.

Your point about learning the circuits is well taken. I'm slowly coming to understand what's going on in the pre-amp and driver circuits. (And hence the newbie desire to upgrade those drivers!) I don't think I'm up to understanding the front end or demultiplexer. I "get" bi-polars, JFET's not so much. So I'm hoping they are fine (especially at $30 per unit). They were working last time I used it.
 
It's doubtful you'll have a capacitor that's shorted to the point where it would cause damage on power up. If it was a 1950's television it would be a different story but with your Scott I'd bet most caps are at value or just a little below.
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).

As far as actually fixing it, I'd go with Rogers advice. Fix it first then do any maintenance work you want afterward.
The crackling is likely dirty pots while the hiss/waterfall noise can be a bad transistor or resistor. Shotgunning either is not a good idea as it's time consuming and gives you plenty of chances to make mistakes, plus is a much better practice to diagnose rather than replace.

Once you get it running well with clean pots I'd start to do some signal tracing and see where the noise comes in.
 
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