Marantz 250 - Repairing the Right Channel

[@xraytonyb]

0:00 Intro 0:55 Comments from the previous video - Soft Start Circuit and ICL thermistors for Solid State Amplifiers 10:08 Testing the right channel of the amp - component testing 19:50 Testing the Output Transistors and finding an interesting problem 23:00 Testing the inverter/VAS stage and finding another faulty transistor 27:19 Testing the amp after replacing faulty components - Still some issues 28:07 Output transistor replacement - Using the Triplett transistor analyzer for testing/matching outputs 49:50 Trying the matched outputs - Amp works, but offset adjustment is still not correct 53:11 Finding a better replacement for the Inverter & CCS transistors 1:03:00 Testing the Diff Pair, finding and matching a suitable replacement 1:06:11 Final test and sound demo - The amp works!

[@electronicsoldandnew]

Well Tony, I must say I really enjoyed watching you work on the this amplifier. I did a 250 M last year which probably took a few years out of my life. It reminded me of the dog I had when I was a child, which was all cuddly most of the time, and then would suddenly turn around and bite. That’s exactly what used to happen every now and then with my amp, except the bite would manifest itself as smoke. So it was actually quite a relief to watch someone else do a good job on this, knowing that I couldn’t get bitten from afar. Thanks again.

[@jeremiahchamberlin4499]

Wow! You did a lot of work just to get it running. I’m sure I’m not ready to tackle anything as complex as this Marantz receiver. I learn a lot watching you, though. Thank you.

[@19janiboy96]

Thank you for being a such great teacher. I've learned so much watching your videos. :)

[@rankenfile]

"you have to remember".... And yet, many won't or can't. Glad you keep teaching the points Tony.

[@jameswortley6716]

Thank you Tony for your time in this project. Has made a very interesting watch and has given me insight in to how much transistors can play a big part even if they are out and not matching. Very interesting and thank you once again for your fantastic videos.

[@jeremiahchamberlin4499]

I’m not sure if it’s just me, but I’m getting a strong ground hum over my new HiFi headphones while listening to your audio. I think it’s 60 cycle, but I wouldn’t swear to it. It went away after the 13 minute mark. I was getting it while you were discussing the soft start circuit. I’m glad you took the time to explain the soft start again, because I was getting a bit confused myself. The hum came back at about the 31 minute mark when you came back after a couple weeks to check the output transistors and found a small DC offset. It went away after you brought out the fancy transistor tester. The buzz came back at the 50 minute mark, when you went back to the bench. Went away at 59 min when you came back with four transistors laid out on a piece of paper. Comes back at 1:02 when you’re back on the bench trying to adjust out the DC offset. Disappears at 1:04 back at the tester. Returned at 1:07 when it was all back together.

[@xraytonyb]

You are hearing the isolation transformer that powers part of my bench. It is very large (70 Amps @ 125VAC) and has some considerable transformer buzz. This is not picked up by any of the devices, but is the result of the laminations of the transformer vibrating. Lesson learned - I should have mounted the transformer further away from the bench so the camera mic doesn't pick it up. ;)

[@jeremiahchamberlin4499]

@@xraytonyb Thank you for responding. I hope you understand that I was only trying to help, I wasn’t criticizing your content in the least and thought you ought to know what we were hearing. Plus, I was using my fancy new headphones, I didn’t try without them or with other headphones I have around here. Others may not have heard anything at all. I do feel better knowing what the cause is, though, again, thanks for responding.

[@questionmark9684]

Thank you, Tony, another great and informative video. Really enjoyed it. Cheers, Mark

[@johnnytoobad7785]

That old school transistor analyzer is awesome ! Looks like a prop from a 50's Sci-Fi movie.

[@erikdenhouter]

Very nice and informative trip down that rabbit hole.

[@oldavguywholovesRCA]

Our high power 20kw broadcast transmitters all have soft start circuits similar to these. Works fine and for the same reason.

[@garygranato9164]

thanks for making this vid. that is a cool transistor analyser

[@joetaylor5078]

As a former technician I always replaced the diff pair with a hand matched set along with a new feedback cap to ground. This alone seemed to help eliminate a lot of the front end problems these older amps have. With respect to your constant current source...it will probably make no audible improvement. Inrush limiters.... My experience is they work fine when used in a circuit that takes them out of circuit after say 30 seconds or so on a solid state amp. In tube gear I simply leave them in circuit. The 2N5210 (NPN) and 2N5087 (PNP) make great diff pairs in the low voltage front ends. They are low noise and high gain.

[@ReyciclismoMTB]

Great video as always, amazing you still have time to feed the trolls. The 5v arduino generation or maybe even 12v. I been considering the soft start for my marantz 1200b, which I bought two years ago not working. Got it going, plays very well, but is 100% stock so a full restoration is due. Again thanks for all the awesome content.

[@mikepxg6406]

Wow that was tricky. Well done.

[@MrProvinspoul]

Your relay system is also used in the Bang and Olufsen Beomaster 8000.

[@jeditoto3441]

I think you just gave the hint I needed to fix my 510m, waiting to find or buy the parts - might make a video if fixed….

[@t1d100]

Excellent investigations, diagnoses and repairs. Is that a new set of dedicated soldering glasses that I see in the case next to the amp?

[@xraytonyb]

Why, yes it is! 😀

[@jameszimnicki4846]

Yes, I am finding similar issues in a 3rd generation 250.

[@johnnytoobad7785]

"Leakmaster 2000" -- Oh that's funny 😂 I designed and built the "Noisemaster 2012" phono pre-amp a few years ago.

[@BjornV78]

The Leakmaster 2000 , love that quote 😅

[@erikdenhouter]

Well so many comments on the delay system, I would want to add now, and I am thinking about a MOSFET in the DC line, with a cap to ground on its gate to delay its opening. Easy to find one with an Rds-on of a few mΩ, so no cooling required, and no moving parts and contacts. Just theory, have not tried it yet.

[@xraytonyb]

Soundcraftsmen did something very similar to that in some of their amplifiers, only they used SCR's in place of the rectifiers and pulsed them on and off to limit the inrush current. This is not the same as what Carver did with the triac, but they could turn the power on and off by gating/not gating the SCR's at each half-cycle.

[@markweingart7636]

Holy Mackerel , Andy !

[@jeditoto3441]

Would the DC offset problem re appear if you put the MJ15003/4 in? In my last couple of repairs I have always used those - but the SM did not mention any matching requirement and the amps had no // xtors. But i guess now that the amp works I guess you leave the power xtors alone 😀

[@andymouse]

There are many ways to skin this cat and who's to say what is the best, I'm here because I want to know how to do it and how you do it, this is your preferred method and it makes sense to me and I'm sure it would to Mr Georg Ohm as well, so cheer's !

[@MrMaxeemum]

Could you use a high gain transistor where a low gain is required by adding a resistor between base and ground thus making a potential divider on its input to scale the output?

[@iceberg789]

can you replace that relay with some solid state arrangement ?

[@2packs4sure]

Man you must have at least $200 of labor in that thing so far... :)

[@TB-nz4kf]

Look at addendum 1 note 6 about output matching.

[@xraytonyb]

Thanks for mentioning the area in the schematic where it shows this. As I said, it is more important that the outputs are in the same gain range than if they are all perfectly matched. That said, some complimentary pairs have different gain ranges for the PNP vs the NPN. The trick is to find ones that share similar ranges for both compliments. Thanks for the comment!

[@worroSfOretsevraH]

Have you tried some 2n3055's?

[@DAVIDGREGORYKERR]

I would have thought that the supply voltages should be +35V,GND,-35V nothing more maybe even less depending on the output power of the unit also preamp should be +5V,GND,-5V.

[@jdmccorful]

That's some great sleuthing Tony! Thoroughly enjoyed this session. And of course the proof is in the pudding, it does sound good. Looking forward to the continuation of this project. Thanks for your time.