Luxman SQ38FD Tube Amplifier Repair

BG117 - A very good troubleshooting walkthrough for a Luxman SQ 38FD preamp / amplifier. This one was a tough one to chase down. Amazing amplifier but it uses some scarce 50CA10 output tubes.

[@1999zrx1100]

Thanks for this, my Dad would spend hours in his shop in the basement working on TV’s and audio gear for a hobby, wish I had taken the time to learn from him. The satisfaction when finding the problem must never get old. 🤓

[@deadfreightwest5956]

Back in the 70s, an older brother of mine worked at Speakerlab. One Xmas he bought a Luxman receiver for our parents. The thing was gorgeous in its rosewood cabinet. All the controls had heft and precision, especially the huge tuning knob. Did excellent service for over 30 years.

[@15743_Hertz]

Up until now, it never entered my mind that a well engineered and meticulously wired amplifier would qualify for circuit porn. But this one is breath-taking!

[@walkertownvet4108]

Nice logical progression on tracking down the problem. Thanks for the walk through.

[@rickg8015]

Lovely construction and point to point wiring.. The transformers are top notch.

[@retro1970style]

I have just purchased one of these. The one I have uses a mains transformer type 8A70B. Your amp uses the same mains TX as can be read clearly in your video at 22.40 minutes. It does not show on the rear of the amp (only says 100v 50hz and 60hz only) but there are tapping’s on my mains transformer for primary inputs of 100V (Japan) and 117V (USA). Using it on the 117V taping the secondary heater supply correctly reads 6.3v ac so the transformer can definitely be used on both with a slight wiring change.(be sure you know what you are doing !!) Sadly it doesn’t help me as I am in the UK (240v ac) but hopefully this info might help someone.

[@Blueglow]

I'm going to investigate this more... thanks for the heads up

[@frankgeeraerts6243]

I always take the correct filament 6.3 V as a reference for line voltage adjustment ......Very important and often not respected..

[@stillastillsfan]

@@Blueglow I have a Luxman LX-38 Ultimate 100V model - same family as the SQ38FD. Having a similar symptom that this amplifier had and I am hoping that this is the same problem and can just wire in jumpers. What did you find out about the transformer because I will have tech use the 117V if it is there that will eliminate the need for the stepdown transformer?

[@Blueglow]

It seems some do and some don't have transformers that can do 117v. Take off bottom and send me pic. @@stillastillsfan

[@stillastillsfan]

@@BlueglowSorry for the delay in response. I asked my tech and he said no the LX-38 Ultimate was only 100V. And this probably makes sense since this one was never made for US market. Love your videos.

[@TheOpenAirGarage]

Great step by step video. Appreciate your hard work.

[@Trex1268]

Great work Mark. Very informative!

[@HipocratesAG]

This videos are awesome, very informative... thanks!

[@Kaxlon]

The strange Japanese speaker jacks are found on the "modern" DENON POA-4400 & POA-6600 power amps too. But the US versions used the traditional jack type. Odd. Great video! Thanks. =)

[@kevinbeckenham3872]

Very good documentary & plenty of info.

[@SoddingaboutSi]

What a great amp. Hope the customer will fit a new switch. I would!

[@Blueglow]

They are. Found one but it wasn't cheap, $250 + $40 shipping from Japan... but when you are dealing with hens teeth, nothing is cheap.

[@SoddingaboutSi]

Thats a rediculous price. But for such a nice amp I would pay it if the switch was irreparable.

[@RobertKohut]

Nice!! Premium amp!!

[@stillastillsfan]

Dear Blueglow - Do you know the model bulb for the power ring indicator? Also, do you know the procedure for precision bias after replacing the output tubes? Thank you.

[@diabolicalartificer]

Those switches look like those used on Japanese 70-'s era reel to reel tape decks as made by Akai. I have a few NOS wafers and SW's and might have a replacement. Drop us an email with wafer/SW spec's and I'll have a look.

[@ABC-rh7zc]

Nice video but don't keep us on the edge of our seats like this! How did the story end?

[@fernandocoutinho6661]

Hi! Thank you for your video and knowledge sharing. About a year ago I purchased a Luxman MQ-88uC and really like the sound but I have experienced one problem that is distressing me...suddenly there's no sound from the left channel. In one year of intense use it has now happened four times. I am using a single source connected directly to the Variable input and after turning the volume down and up a few times the sound eventually comes and no more issues for a few months. Since it works well most of the time and I can't replicate the problem it feels awkward to send it to repair service. On the other hand it is very disappointing that such an expensive amplifier has this kind of issues. I have checked the bias current and all seems fine, reading near 480mV in all tubes. Anything I can do? Any suggestions would be much appreciated. Best regards.

[@frankgeeraerts6243]

You mean ..480 mV measured over a 10 Ohm resistor = 48 mA bias current in each tube.

[@greaterbayareahero1401]

Hi, I have a Luxman CL35 and it is causing distorting on high frequencies. Do you have any advice on what I need to look for. Is this a tube problem?

[@BernsteinOmega]

Very fun to watch this particular problem get solved. Next project, might I recommend my Yamaha B-2? ;-)

[@DieselRott]

what's wrong with your Yamaha B-2?

[@DAVIDGREGORYKERR]

Would the double triode tubes be ECC88s if so then try new ones they don't normally go wonky but only very seldom do they do go bad or what about Slavetna equivalents that might solve the problem.

[@robertjermantowicz-uw3iw]

Defective source selector switch not able to pass voltage.

[@scorpven]

Why is only one of the 12AX7 shielded?

[@dieselrauch1733]

Take a look at the rectifier diodes in the schematic.

[@5argetech56]

Holy Scabolly.... Get ready to spend at least $450 dollars for those 50CA10 tubes. They are sold in matched pairs.

[@gregjones3660]

So their are Russian made nixie tubes. Maybe there are also amp tubes for sale?

[@董民基]

好機器就是怕故障 頻頻送修可是受不了

[@moodyga40]

good amp i have one

[@moodyga40]

output transformer go on theses amps too

[@robertjermantowicz-uw3iw]

It was a defective source selector switch. All the tubes and transformers tested good.

[@htc345youyou2]

Blablabla