Pioneer Spec-4 Part 3: Restoring the Amplifier Module

[@Silent-Lucidity]

Hi Tony, Beyond your incredible intelligence, your personality and demeanor are so genuine! Thank you, from the bottom of my heart, for being a wonderful human being and bringing excellent content to the community!

[@wschuurmans1]

exactly the right words, I can't help but agree completely. I am also a big fan of Tony, partly because of his character, great... And also a thank you from Willem from the Netherlands.

[@isoguy.]

KZbin quote of the year. Xraytonyb: "Sorry my finger doesn't produce a perfect sinewave! Maybe I need to be recapped? Just getting the tee-shirt designed. Really enjoying this series and certainly looking forward to the solder and talk vid. Thank you.

[@poormanselectronicsbench2021]

I thank you for your efforts in mentioning the replacement semiconductors you used, as it helps anyone else that might want to restore a unit that they have. @ 50:38 , the added ambiance of your wife practicing is delightful, but in my mind I can't help to think that its actually Ron Burgundy tuning up for a gig. @ 55:30 MG chemicals does make a solder mask, or "overcoat" pen for small touch up areas. @ 24:05 It's kind of funny that, Pioneer designed that over current circuit to be used as and adjusted for ( Pg 14 of the service manual) a soft clipping limiting function, where most other amp designs I have seen just have them as a set value for over current limiting into low impedance or short circuit protection. I do wonder how they might limit a fast transient on the output being designed and used to be more restrictive than most other designs, but, seeing the amp only uses 2- PNP and 2 - NPN output transistors per channel I guess it takes some stress off of them in general if all is working properly.

[@xraytonyb]

The diodes D8, D9, D20, D21, D2 & D3 set a hard limit for the drive voltage. That is what provides the protection against overdriving the signal. The Power limiter circuit limits the power to the speaker load to about 180 watts. Even though this circuit may have a small amount of latency, it will limit the power within a reasonable amount of time to protect the transistors......at least in theory ;)

[@t1d100]

I really like the printed comments, explaining various functions.

[@wizzkidelectronics]

allen w2aew is an awesome mentor . i didnt have many hands on amateur mentors in my area . he is a great resource

[@toddhoug9713]

Great video Tony, thanks for sharing. The theory videos are my favorite! Your descriptions of how and why the circuit works are the main reason I tune in to your videos. I learn so much from your descriptions, than I can from just tracing electrons to infer how it operates.

[@johnnytoobad7785]

These are the type of vids I love..especially since I have an "amp fab" project I'm doing over the winter.

[@paulschreuder4211]

The circuit looks so complicated but when you explained it i learnd a lot. Tnx for sharing Tony!

[@mistermac56]

Nice work Tony. I love how through you are and explain as you go what the replaced parts function is in a circuit.

[@arsimahmetaj6272]

Excellent work 👌

[@craigm.9070]

Thank you for your time, Tony; the amplifier sections look to be stout and prepped for long life.

[@Amplified208]

Love the theory videos when you explain the circuit operation. I am looking forward to the solder and chat video as well.

[@RBlacklightR]

I really enjoy your videos, It would be super helpful if you could provide links to the schematics, parts lists, and tools and materials you're using (like the Teflon tubing, etc.) in your videos. Adding some affiliate links could be beneficial for you as well, it’s a win-win! Thanks a lot for all the great content!

[@MomirPeh]

Tony, you're not a good antenna apparently... But, you do produce some quality content which is fun and educational. I really appreciate that!

[@poormanselectronicsbench2021]

He just has to be careful to not change his occupation from being a technician to being a conductor, of electrons. 🙃

[@ligius3]

It's the little things in your videos that make me happy. Maybe I'm distorted and also need to be recapped.

[@jeremiahchamberlin4499]

Really enjoyed this one, Tony. I’m still trying to understand how amplifiers work in a detailed fashion. I plan to build a kit when I can make the time.

[@johnm4962]

🎉thanks for all your help. I have been successful in getting to the same point and bench testing the right side ! Now, on to the left. This is my first big repair. Helping my friend fix his broken amp. I couldn't have done this without you. Thank you.

[@Davor-hifi]

Theory is always nice! :)

[@Beexzz]

All Pioneer (and from other manufacturers from that era) carries so many similarities but after watching this video breaking the whole circuit in parts I learned once again many details. What helped me most is Tony's simple and clear explanation how each step in this circuit works, how signal gets slowly amplified in steps, a cascade like, and what many of the steps and components do to the signal, taking care of oscillations, taking care of two halves. For non-professional and layman in this field (I mean me!) I learned quite a lot from this video and from Tony's magnificent teaching and explanatory skills. Thank you very much Tony! As always, great content!

[@karllangeveld6449]

Thanks Tony, again, that was very educational.

[@rankenfile]

Tony - wishing you blessings for a good life as well. Thanks for this Spec-4 series.

[@greengrayradio1394]

Very nice! Good description of the circuit, and the choice of components as replacement for the refurb!

[@ronbradshaw7404]

Ohhh! Tony!, just a used tootbrush and soapy water to clean-off the heat sinks :) . Loooove your videos!.

[@petertimp5416]

Thanks, Tony! Another interesting video, and also very interesting music in the background.😊

[@wizzkidelectronics]

excellent video as usuall

[@EdwardLehman]

As always, knowledge from experience is the best, love it all and has given me the confidence to start some repairs on my own equipment to start! Just need an o scope!

[@anandadesilva6558]

Thanks Tony. Looking forward to the chat session!

[@rolfts5762]

Interesting&enjoyable video, Thanks for making&sharing.

[@wedoshotz6645]

Thank you, Tony. My ongoing education continues.....

[@braddokken9191]

Thank you for the knowledge and your time. I'm learning something every time.

[@anystereo]

Tony, you might consider a high pass filter on your microphone 😮😅 great work love watching your videos

[@yo3429]

Great explainations! Thank you for this wonderful videos! :)

[@gustavom8062]

Hello Tony, Another excellent video as we are used to. Detailed and instructional. Could you mention the brand and model of the pads for the TO-3 output transistors? They look as something to have handy when doing a rebuild. Thanks

[@Ange1166]

great work neat job

[@t1d100]

FYI... This tip is off-topic, for this video. But, it is the only available means of communication. There is a problem with Relay based Soft-Start Circuits... Well, the problem is actually with relays that use a coil... Relays switch in and out at a voltage lower than their stated rates. In a soft-start circuit, this means that the downstream capacitors/transformers/circuitry are not charged to the full voltage and that there is an additional, smaller in-rush event, after the coil has tripped too early. The KZbinr Haseeb Electronics has a clever fix... {983} How to drive relay at specific voltage. Spoiler... He biases the trip point, to hold it up, with a zener.

[@andydelle4509]

Another approach is to use a 555 timer IC or a 741 OPAMP wired as a comparator circuit. Then the relay sees full coil voltage or not due to the snap action of the comparator. This eliminates the slow pull-in phenomena which leads to increased contact wear.

[@xraytonyb]

Those things are not really a concern. You don't need the relay to come in after the caps are fully charged; you just need it to come in after they are about 70% charged, as that's when the inrush current is relatively small. Relay coils have a very wide operating range, with the pull-in voltage being higher than the drop-out voltage. As long as you use the proper value dropping resistor in this circuit, it works perfectly. Adding the zener will also work, but it's not necessary for this application.

[@zopilotesky]

Thanks for going over the pre-driver stages of these circuits. It clarified some of my questions about how it all works. I've been chasing down a forward side cave in on the positive side of the sine wave issue on a Scott 370 that's driving me nuts...on both channels! It starts at 12k at around 25 watts and gets worse from there. Any ideas? I'm baffled...😑

[@mwhoffmn]

Great video, Tony. Can you please provide a link or other detail on that blue teflon tubing you use? Every time I try to search I end up with shrinkable tubing, or a 100-foot roll of teflon... thanks!

[@dh-_1011]

How much you want for those 424-554’s? I could use those, for sure!

[@jefftobin4034]

Tony, many thanks for this "episode". I'm enjoying this video a lot. I appreciate the discussion of the replacement transistors. I see a lot of people replacing the pnp diff pairs on amp inputs with a matched pair KSA992 low-noise transistors. Is your choice just personal preference or is there a specific reason the KSA992 is not a good replacement?

[@xraytonyb]

That's what i usually use. If i said something different, (as I often do) it was by mistake.

[@TheAmplifierBuildClub]

Nice Video. You have good knowledge demonstrated on the circuit schematic diagram - have you ever thought about designing and building your own amplifier?

[@johnm4962]

love your videos, thank you

[@moisesERP]

Hi, great video. Where do you buy that teflon tubing? and how are those textile sleeves to cover the resistors called?

[@t1d100]

I know that your camera mount has always been difficult to work around, in your small shop. I have no idea of what type of mount/rail/tripod? you are using, but I wonder if flying the mount from the rafters above the bench might remove the mount/riser pole from being right in front of you... In your line of sight/reach/soldering? HTH!

[@NovaluxStereophonic]

1:00:00 I have not encountered an issue yet replacing the .22uF cap in the overload detect circuit with kemet stacked film on multiple pioneer output boards. Should be good.

[@pglick123]

Do you know why the manufacturer would have put those white tubes over all the resistors?

[@Parkhill57]

Heat wrap. Likely fiberglass. Protect surrounding components from temperature.

[@apollorobb]

You have a Harmonic in your finger sir. Where are you getting the 21194/93 to-3's I havent been able to find them in stock

[@richardchuray329]

Hi tony. I particularly enjoyed your discussion on the current source on the differential pair and about the first class A transistor. I am working on a Sony Str-7065 and have 30-45 mV on each speaker terminal but there is no adjustment for that. Only for bias. Is there some of the area of the circuit I could look to reduce it? I don’t expect a detailed answer but pointing me to a general part of the circuit would be helpful. As always, you’re a treasure for the community

[@xraytonyb]

I you only have 45mV of offset, you are OK. A lot of the amps out there don't have offset adjust. As long as it stays under 50 mV, you'll be fine. If it gets too high, sometimes the manufacturer will have a fixed resistor that you can change or jumper out to bring it back in. Beyond that, you would have to check to see if a transistor has gone down in gain, causing a bad mismatch.

[@danhorton6182]

Would you be able to do a video showing the path of the signal? Say using the schematic you have there for the amplifier boards. I’m not too savvy on the path it takes through the Feedback circuit or anywhere else. That would be incredible.

[@johnnytoobad7785]

I read somewhere that you should NOT use wire-round resistors in the audio signal path due to their inherent inductance. But since you have many "oscillation squelch" circuits on that board it make not make a difference.

[@jdmccorful]

Would like to hear more discussion on this subject. Good question.

[@xraytonyb]

A 0.5 ohm wire wound resistor would have extremely low inductance, with reference to these types audio circuits. I would avoid them when working with RF circuitry and could also see how they might affect a class D amplifier, if the switching frequency were high enough.

[@hoobsgroove]

Was hoping to get an answer from my other comment your feelings about split foil capacitors. I was wondering if you'd get a benefit as well using bipolar capacitors wouldn't be as good but better than the standard electrolytic I would have thought.

[@xraytonyb]

I only use bipolar (electrolytic, not film, etc.) capacitors where there is a signal that actually changes polarity. Most single ended circuits don't actually change polarity, which is why normal polarized capacitors are used. Bipolar capacitors are more expensive and usually don't perform as well as a low noise/low ESR polarized electrolytic. In addition, I have read that bipolar capacitors really need to have a true AC signal on them (where the polarity actually changes) in order for them to stay properly formed. I'm not sure where the idea of using bipolar caps in this type of circuit came from, but I am skeptical of it for the above-mentioned reasons. As for the split foil capacitors, I have never really used them, as they are expensive and they are more difficult to find in a lot of different values. I have not tested to see if they would make any difference in the sound quality and I'm always a bit hesitant to believe someone's opinion, just based on their listening experience. On the plus side, split foil caps are supposed to be more durable than a standard electrolytic as long as you don't subject them to high transient voltage surges. That's about all I know about the shrimpin' business ;)

[@robnic52]

I use a blob of blue tack to hold very awkward components still while soldering thier little legs in. I loved the jest about getting yourself recapped. I hear refurbishing and calibration of an electronics tech is horrendously expensive. Maybe just get that finger done? 😊

[@robertduvall7392]

I'm beyond recapping; I need a new transformer...

[@johnadams9041]

WHY you replace 40 Mhz druvers with a 4 Mhz driver subs? There are a lots of other choices with an ether faster transistors

[@xraytonyb]

I explained this in an earlier series. You have to look at the test parameters when you read the datasheet for a transistor. The beta of a transistor changes with frequency, as well as voltage and current. Some manufacturers will list the Ft of a transistor when tested at very low voltage and current, which will make it look like a much higher frequency device. If you test all of the transistors at the same test parameters, they will look a lot closer in spec. In addition, you will never hear the difference in a circuit like this one. As long as the device doesn't add too much latency to the negative feedback loop, it will work just fine.

[@jdmccorful]

Very cool! Didn't hear if you prefer using the 10 turn to the single turn pot for stability of calibration. Would love to hear more on this. Thanks for the time you give us!

[@xraytonyb]

10 turn is easier to adjust. Sometimes the single turn pots are very touchy.