Zen and the Art of Hi-Fi Repair

[birchoak]

I've been approaching audio repair the wrong way. Rather than an efficient workman, I need to be a curator. More of a restorer than a fixer. A mindful, cautious surgeon, vs. the Dr. Frankenstein my younger self has presented.

Here is my new approach, and I will use a 50 year old Sherwood S-7100A as an example. The Sherwood came to me in very good condition and fully functioning. Here is where age and experience pay off. I now know this: Do not let the magic smoke out. Do not do anything to this unless you are 100% sure it will NOT injure the patient. Do not let the magic smoke out; something in damned good working condition is better than irreparably broken because you were chasing perfection. I realize I am preaching to the converted, and that most of you know far more than I ever will, but I think this is a good approach for other amateurs such as myself and it will save many an amplifier and receiver.

Before I even plugged it in, I examined it carefully inside and out, noting that absolutely everything appeared to be original and in good shape, except for a dead tuning meter bulb and new blue LEDs for the dial lighting. I cautiously turned it on via a dim bulb tester, with speakers hooked up, then switched to regular power. FM strong in both channels; CD piped in via AUX sounded strong and clean without a touch of static.

After a few hours of listening, the right channel dropped out in STEREO, so I flooded the controls with CRC, then Deoxit D5,* working them thoroughly during the process. Younger me would have waited a total of five minutes before powering up the receiver to check my work; older me brought the unit upstairs to my heated office, where I let it dry out for 24 hours. Indeed, it was closer to three days before I plugged it in again and that kind of patience is what's needed with these fragile electronic masterpieces of yesteryear. I've always had difficulty reigning in my eagerness to "get going and fix it" and I've paid the price for it.

*I've since learned that these particular Sherwoods employ a fragile plastic tab that does the actual ON/OFF and that what I did should have destroyed said switch! Lucky for me it seems to be just fine, but, whew!

The Sherwood continues to work remarkably, delivering strong, punchy music that no 50 year old amp with original parts should be capable of, but I know it should get at least new filter caps to stay healthy. Here is my thought process:

Younger me: "Let's order every cap in this receiver and replace them all, preferably tonight."

Older me: "We will do the filter caps and stop there for now. This isn't a race. Also, we will listen to the Sherwood with its new filter caps and evaluate the sound and success of the repair. Also, we are taking photographs, drawing detailed diagrams of the pertinent wiring, and consider doing a narrated video prior to any tricky work to be on the safe side."

Younger me: "Huh. Wish I'd done that before I took apart that Scott 420A."

Older me: "Yeah, me, too. I thought we weren't going to mention the 420A ever again."

Younger me: "Sorry!"

Older me: "We will triple check polarity before removing any cap, and we will employ that wonderful dim bulb tester before we even think of plugging this in to raw power. We will solder carefully and mindfully, looking for solder drips, wire clippings, solder whiskers, dropped screws in between components, etc."

Younger me: [knows not to say anything].

Older me:
"And no, we will NOT be replacing all the caps, not tonight and probably not ever. We will work at a pace appropriate for a museum piece, and a complex electronic piece at that. If we think like an art restorer, doing as little damage as possible, working as calmly and gently as possible, we just might keep this work of art intact."

Younger me: "Where have you been all my life?"

Older me: "Right here, grasshopper. You were not ready before."

Younger me: "How will I know when I am ready?"

Older me: "When you can snatch this 2200uF 50V Nichicon Gold Tune from my hand." [begins levitating]

Younger me: "Holy sh--!"

 

[spark1]

Why does it need new power caps (filter caps) to stay healthy?

 

[Binkman]

Older me: "When you can snatch this 2200uF 50V Nichicon Gold Tune from my hand." [begins levitating]

I've got my dad's late 60's S-8600a to be attended to.

Reminds of my first shock from an AC outlet. At 6yrs old I was curious of how my mothers iron could get hot pluging it into this strange device.. So I started playing with it. My mom said "don't play with that" So I ask how does it work? she says "just keep playing with it and you'll find out" and I did. Been addicted ever since.

 

[VYNULADIKT]

Have done nothing but deoxit and this one sounds great for a under $10 thrift find. It was nothing but static prior to the deoxit. May open it again one day and replace dead lights and possibly a few caps. It does have a little transistor noise but its intermittent and a very low amount. Have resisted doing anything thus far. Its been on my desk for at least 8 yrs now.
It is powered on 24/7 so the switch will not go bad.

 

[birchoak]

Have done nothing but deoxit and this one sounds great for a under $10 thrift find. It was nothing but static prior to the deoxit. May open it again one day and replace dead lights and possibly a few caps. It does have a little transistor noise but its intermittent and a very low amount. Have resisted doing anything thus far. Its been on my desk for at least 8 yrs now.
It is powered on 24/7 so the switch will not go bad.

8 years! Holy thrift find, Batman! Your story encourages me to not even replace the filter caps. I like "have resisted doing anything thus far," because you do have to resist going in there and "making it better."

 

[birchoak]

Why does it need new power caps (filter caps) to stay healthy?

From what I've gathered, as electrolytic capacitors age they build up an increased resistance to current and don't let as much power through as they did when they were new (anyone, jump in and correct this unwashed amateur!). They can look perfectly fine from the top but can be leaking through the leads, right at the board where you can't see (excellent write up with photos, from Pacific Stereo guy over on Tapeheads). It seems to me the filter caps see pretty hard use, due to their job description, and because they are not in the signal path, replacing them should not significantly change the unique sound of said amp/receiver (other than perhaps making the bass tighter). Hey, if the air filter was clogged on your car and it was bogging down on the highway, you'd change that air filter, right? I don't know if the analogy carries over logically, but it seems to me those filter caps do a similar job and will eventually wear out. Maybe not tomorrow, but they can't last forever.

That said, the caps in the Sherwood are probably still performing within, say, 30% of factory specs because the thing would sound flabby and anemic otherwise, methinks. I would not attempt it unless I had done it successfully several times on other receivers, which I have. The key is going very slowly and carefully, not lifting any solder pads off the board from too much heat and tip pressure, and stopping at those two caps before doing anything else. Power up cautiously via dim bulb tester (after triple checking proper polarity on replacement caps), then move onto raw house current.

There is a YouTube video featuring a master guitar amp repairman. This guy repaired amps for some famous guitarists (can't remember any right now, of course), and he said not replacing aged capacitors was like not replacing 50 year old tires on a sports cars. No, the machine is no longer "original," but now you can use it without 1) killing yourself or 2) destroying the machine it's attached to. That stuck with me, and there is probably just as much superstition and black magic surrounding vintage guitar tube amps as there is vintage home stereo equipment.

All that said, if you don't know what you're doing you will make mistakes, most likely turning a working amp/receiver into a boat anchor. I was willing to take those chances because I am a risk taker by nature, and also because there is no one near me that works on vintage hi fi. Also, the cost of proper repairs would far exceed the actual value of any of the modest components I have.

 

[birchoak]

I've got my dad's late 60's S-8600a to be attended to.

Reminds of my first shock from an AC outlet. At 6yrs old I was curious of how my mothers iron could get hot pluging it into this strange device.. So I started playing with it. My mom said "don't play with that" So I ask how does it work? she says "just keep playing with it and you'll find out" and I did. Been addicted ever since.

I put a bobby pin into the outlet under my bed when I was 4 or 5 and I swear blue balls of lightning shot across the floor! Before that, I bit the cord to my vaporizer (hey, it was right there on the ground, where I was crawling) and electrocuted my mouth.

What is that Sherwood like? Bet it sounds awesome. It seems the late 60's/very early 70's are the true Golden Age of Audio (the stuff just didn't look as impressive as the mid-70's and later).

 

[RobertCO]

This made me laugh out loud and smile thoroughly. Nice write-up.

 

[birchoak]

This made me laugh out loud and smile thoroughly. Nice write-up.

Thank you, my good man. I had fun writing it and sincerely hope it will help other amateurs braving the electronic deeps for the first time.

 

[BLAH BLAH]

Yes, same here, made me think if I needed someone to repair my vintage pieces...I would contact you...
But then I read the amount of time you took just to detox switches!!! I thought I can't wait that long to have a unit repaired!!!

 

[birchoak]

Yes, same here, made me think if I needed someone to repair my vintage pieces...I would contact you...
But then I read the amount of time you took just to detox switches!!! I thought I can't wait that long to have a unit repaired!!!

Ha ha. You don't have to wait that long. I think 30 minutes would do it, depending on how much spray one used and whether it pooled up somewhere. Per the title, I'm trying out a gentle, restrained approach after making many, many mistakes. I used to wait five minutes!

 

[birchoak]

I lost a Realistic SA-1000 when a filter cap blew and did way to much damage to repair. It sounded and was working great then bang and smoke. On older equipment I recap the power supply as a minimum requirement these days it's cheap insurance.

Thank you for telling us about that, painful though the memory must be. That's two votes for at least a minimal recapping. I really do think it's like putting new tires on a vintage sports car. And I've found that if you go slowly, makes notes and take pictures of the wiring beforehand, it's really not too tough a job to install new filter caps. Sometimes they're in an awkward spot or have multiple leads soldered in, but if you take things step by step it can be done. Obviously, remember that these things can store a painful electrical charge and do not ground yourself to said charge in the process.

 

[spark1]

Perhaps it would be better to discharge them before undertaking any work on them.

Did you ever get your KA7002 working, birchoak?

As for undertaking a "minimal recap"...what would this entail, and why only some, vs all (electrolytics)?

 

[Omegaman]

If you want to repair vintage audio, then you must FIRST invent the Universe

 

[Binkman]

Haven't had a chance to work on it.. been in storage and orig. rescued from family who have no appreciation for audio. Cab enclosure needs some finishing but did dbt test it a few months ago and passed was working okay last listen long before storing/ rescue, just have other stuff to divest before proceeding. LOL my maturity exempts me from "grasshoppa" foolishness. like treading on rice paper.

 

[birchoak]

Perhaps it would be better to discharge them before undertaking any work on them.

Did you ever get your KA7002 working, birchoak?

As for undertaking a "minimal recap"...what would this entail, and why only some, vs all (electrolytics)?

Alas, I finally gave up on the 7002. Sometimes you have to move on from a difficult relationship! Even when I had it working, it did not sound particularly amazing to my ears.

To me, a minimal recap would be as follows (wiser people, correct me!):
1) Replace the filter cap(s). Put everything back together, power up via DBT. If amp passes DBT, power up normally, with speakers attached (I try to always power up with speakers connected), then play music through it to check for sound, then sound quality. Also to see if I can actually hear anything different now that the new filter caps are in. Check the values of the existing caps, then check the service manual specs. When in doubt, keep the capacitance exactly the same (voltage rating can be higher but I think the closer one stays to original design parameters, the happier the patient will be). From what I understand, if you cannot find a replacement cap with identical capacitance, you can exceed its value by 10%. Do not ever replace a cap with one with lower voltage or capacitance values or bad things will happen and magic smoke will be released.
2) Next, replace the caps on the power supply, following the same precautions as in step one. I've been finding it helpful to listen to the amp before doing anything at all to it to establish a baseline. I would only do two or three caps at a time, then check my work. That way, if the receiver doesn't pass the DBT, I can narrow my mistake down to a maximum of three components I just put in. If I was a beginner, I would probably only do one cap, then check my work. Was younger me calm enough to do that? No, and I ruined a few receivers because of that.
3) If I'm feeling confident and haven't destroyed the receiver yet, I might consider replacing caps in the pre-amplifier section; however, if the music sounds clean and strong I would probably NOT go in there. This is surgery, and any time you or a receiver goes under the knife there is the possibility of an accident or death.
4) If a capacitor is bulging, leaking, has scorch marks on it (from living next to a hot transistor, for example), or has any other visible signs of damage/wear, I would replace it.
5) I would touch nothing in the AM board (don't listen to AM anyways), nor the FM section (a tuner guru, much wiser than me, said that these rarely go far enough out of spec to be a concern--doing an FM alignment is what's needed, something way, way out of my skill set). Also, if a cap is too small to read the values on it or is soldered to a lot of other things in a really, really complicated way (like the underside of a Scott 380R amp board), I am inclined to leave it alone. If the replacement caps exceed the value of the entire unit, I don't do them: for example, those four, huge caps in the Proton D1200 power amplifier are something like $400 to replace. I love the Proton D1200, and if it was my only amplifier source I might consider it, but that's just too much money for this mere mortal. I would rather invest in $400 of new music or a new set of Wharfedale Diamond 10.1 speakers.
6) When in doubt, do nothing if it plays music nicely. Consult with the many generous, wise, experienced people here on AK or pony up the money and take it to a reputable shop (I just found out about a place in Massachusetts, and I will gladly pay him if I think a piece is worth it).

 

[birchoak]

Haven't had a chance to work on it.. been in storage and orig. rescued from family who have no appreciation for audio. Cab enclosure needs some finishing but did dbt test it a few months ago and passed was working okay last listen long before storing/ rescue, just have other stuff to divest before proceeding. LOL my maturity exempts me from "grasshoppa" foolishness. like treading on rice paper.

I love that you rescued it. And maturity is a key asset in this particular hobby. Let us know how it sounds when you do finally get to it. I cannot believe how good the tiny, low-tech Sherwood S-7100A sounds. It has become one of my favorite receivers.

 

[birchoak]

Perhaps it would be better to discharge them before undertaking any work on them.

I totally want to do that; what is the best way to do so? I once soldered leads to a 100 watt lightbulb and tried to use that on big caps, but it never glowed, let alone lit up. Is that one way to do it? I know you have to get the stored power to spend itself somehow, in heat or light ideally, and that you have to have enough of a reservoir to do it safely (15 watt bulb would NOT be safe).

 

[spark1]

I use a 5W 200ohm resistor that I had in a box of used parts. Alligator clips and 8 inch lengths of 16 gauge wire, with heat shrink on wires and legs of resistor.

Too bad about the 7002...beautiful piece of hardware, and when in top condition sounds pretty good.

If doing a piecemeal recap, I dont think I would focus on the big money caps. While opinions vary, unless they are measuring badly, I've not noticed any significant improvement from replacing them.

I saw the story by another poster about one of them shorting, but believe this to be quite rare. And if you are worried about those going bad, I would be at least equally worried about electrolytics in the power supply and power amp sections. Problems here can sometimes cause chain effect damage. I would also do the protection board/circuits.

It is my understanding that the work of the filter caps places very little strain on them. If they measure well now, they are very lkely to do so into the future.

 

[birchoak]

I use a 5W 200ohm resistor that I had in a box of used parts. Alligator clips and 8 inch lengths of 16 gauge wire, with heat shrink on wires and legs of resistor.

Too bad about the 7002...beautiful piece of hardware, and when in top condition sounds pretty good.

If doing a piecemeal recap, I dont think I would focus on the big money caps. While opinions vary, unless they are measuring badly, I've not noticed any significant improvement from replacing them.

I saw the story by another poster about one of them shorting, but believe this to be quite rare. And if you are worried about those going bad, I would be at least equally worried about electrolytics in the power supply and power amp sections. Problems here can sometimes cause chain effect damage. I would also do the protection board/circuits.

It is my understanding that the work of the filter caps places very little strain on them. If they measure well now, they are very lkely to do so into the future.

I'll have to look around and see what I have for large-ish resistors. So that setup would convert the stored charge into heat? How long do you touch the parts together?

I have not noticed big differences after replacing caps, and I am willing to adapt my approach if others have had success not recapping. It's a lot of work and I don't want to cause extra problems messing with 50 year old boards. That said, technicians do recap, all the time, so there must some reason for that. The guy at select45rpm.com routinely recaps power supplies, pre-amp sections, etc. and seems to really know his stuff. I don't know of any other site on the internet where so much detailed information is given about so many different amps and receivers, especially the way they sound. This is a roundabout way of saying this guy is the real deal. Truly fascinating reading, albeit with slightly odd sentence structures (perhaps English is his second language? Dunno. Don't care. Extremely bright person nonetheless and every receiver I've tried on his list has been simply fantastic!).