UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (https://www.vintage-radio.net/forum/index.php)
-   Success Stories (https://www.vintage-radio.net/forum/forumdisplay.php?f=34)
-   -   JVC JA-S71 amplifier (1976) (https://www.vintage-radio.net/forum/showthread.php?t=82805)

mark pirate 23rd Apr 2012 4:58 pm

JVC JA-S71 amplifier (1976)
 
5 Attachment(s)
I bought this amp second hand over 25 years ago to use as the heart of my secondary system. It was regularly in use until it started to intermittently lose one channel about 5 years ago, when it was replaced by a Quad 33/303 amp.

So I decided it was about time to get it on the bench, it certainly is a heavy old beast!

I blew away 36 years of dust, connected it up to my workshop speakers and CD player and powered up. I was greeted by music from the left channel only, the balance control was very scratchy so this was given a dose of Servisol, along with the tone controls and this sorted that problem. Wiggling the tape monitor switch bought back operation of the right channel, but the only way to get some Servisol into it was via the front of the switch with the amp tilted upwards.

With the amp now working as it should, I left it playing while I went in for a cuppa. On my return the right channel had gone off again :wall:

After some closer inspection, the problem turned out to be some dry joints on the preamp board beneath the muting switch!

I left the amp on all evening with no further problems, so it was given a good clean externally. I then decided to try it out in place of the Quad.

Wow what a revelation!
Connected to my Marantz CD73 and my Tannoy 638's and it blew the Quad into the weeds. The sound quality is simply stunning, far better than I remember, so it is now back in its rightful place.

Amps like these should be cherished, you would have to pay a fortune to match both its build and sound quality.

As can be seen in the photos it has four output trannies on a massive heatsink for each channel, two massive caps and huge transformer.

They just don't make them like this anymore!
:beer:
Mark

robjkmannering 23rd Apr 2012 5:30 pm

Re: Jvc ja-s71 (1976)
 
Nice looking amp Stand, is it Phillips or crosshead?
Nice looking amp too.
Rob

yampy187 23rd Apr 2012 6:12 pm

Re: Jvc ja-s71 (1976)
 
That amp is a beast! I've got a JVC JA-S22 amp and a JL-A1 turntable. They sound better than anything I've ever heard. Now I just need some speakers for it :)

mark pirate 23rd Apr 2012 7:11 pm

Re: Jvc ja-s71 (1976)
 
Rob, it is a crosshead:) It was very useful in keeping the Servisol from running on to the front panel!

Yampy, it is certainly a beast. The power reserves are very impressive, as is the build quality.

I have been listening to it all afternoon and it has not missed a beat and was very pleased that there were no real faults with it. It has never seen any repairs until last week, a real testament to the quality of its components.

Even at full volume with no signal, there is no noise or hum at all.

I really love equipment of this era, it looks great and I am sure it will go on for another 36 years.
:beer:
Mark

Nickthedentist 23rd Apr 2012 7:58 pm

Re: Jvc ja-s71 (1976)
 
Wonderful amp. I love that mains transformer!

JVC made several similar-looking units, but not all were created equal. One very popular model has a much smaller transformer and STK-series "slab" output ICs giving an output of something like 25W x2. Very nice, but not in the same league as yours.

Nick.

mark pirate 24th Apr 2012 8:39 am

Re: Jvc ja-s71 (1976)
 
Hi Nick, this was JVC's flagship model, a quick look on the web revealed the spec.

http://www.radiomuseum.org/r/jvc_ja_s71.html

:beer:
Mark

Herald1360 24th Apr 2012 10:51 pm

Re: Jvc ja-s71 (1976)
 
Quote:

Originally Posted by robjkmannering (Post 523485)
Nice looking Amp Stand, is it Phillips or cross head?

Quote:

Originally Posted by mark pirate (Post 523516)
Rob, it is a crosshead

Err, a Phillips screwdriver is a crosshead type screwdriver, bit like a Biro is a ballpoint pen.

Judging by the colour of the handle, it's probably Pozidriv. ISTR in that style, Phillips had red handles.

robjkmannering 25th Apr 2012 8:43 am

Re: Jvc ja-s71 (1976)
 
Thanks for pointing that out Chris, I was trying (and failing) to be humerous. Phillips because the amp was made by JVC and crosshead because anything supporting that load would probably get cross.
No, it doesn't sound any better trying to explain it!

Rob

Herald1360 25th Apr 2012 1:03 pm

Re: Jvc ja-s71 (1976)
 
Well, you just tickled my funnybone, sort of ;D

Back with the amplifier- the RM spec is rather vague... 260W output. Did you get any decent info with it?

I see they went for wirewrap in some of the power supply stuff at least. I guess that's typical of the period. Hopefully it will live up to the claims made for its longterm reliability as a connection method.

mark pirate 25th Apr 2012 2:03 pm

Re: Jvc ja-s71 (1976)
 
Quote:

260W output. Did you get any decent info with it?
Looking elsewhere on the web, it appears to be rated at 80 watts per channel.
As for reliability, this has been the first time any attention has been needed. I was relieved that it was only a couple of dry joints and dirty switches and crackly pots and not as I had feared, the output stages.

I have certainly given it a lot of use over the years, it even got pressed in to use as a disco power amp back in the 90's.
:beer:
Mark

Silicon 30th Apr 2012 12:43 am

Re: Jvc ja-s71 (1976)
 
What are the markings on the power transistors?
They look like Hitachi MOSFETs.

mark pirate 30th Apr 2012 8:15 am

Re: Jvc ja-s71 (1976)
 
There are 2 x 2SA753V and 2 x 2SC898V transistors per channel, so the rating of 80 WPC looks correct. It certainly delivers more power than I will ever need!
:beer:
Mark

tri-comp 9th May 2012 4:49 pm

Re: Jvc ja-s71 (1976)
 
1 Attachment(s)
Mark,

Nice amp and no doubt nice sound.
Here are the spec's.
They came from the full service-manual. PM me for more if you need it.

rgds,

/tri-comp

JoshWard 13th May 2012 2:39 pm

Re: Jvc ja-s71 (1976)
 
Nice looking amp. I love Jap equipment from the 70s/early 80s. Used to have a Luxman amp and then a Yamaha CR600, both completely blew away any more modern IC amps I have heard.

Nice to see someone else with some decent 'speakers (Tannoys) too 8-)

bgood2bme 13th May 2016 7:42 pm

Re: Jvc ja-s71 (1976)
 
Hi all, wonderful reading in here. I just bought a JVC JA-S71 for 80 dollars. The ad says no power up, should get it from UPS in a few days. Are there any fuses that would cause this unit to be completely dead?

Ted Kendall 15th May 2016 8:20 pm

Re: JVC JA-S71 amplifier (1976)
 
If it blew a 33/303 into the weeds, methinks the Quad is next in line for the bench...

Radio Wrangler 15th May 2016 8:53 pm

Re: JVC JA-S71 amplifier (1976)
 
The reputation of the 33/303 is somewhat larger than the reality. It was probably the first transistor amplifier which overcame thermal instability and reliability issues. Back in the day it was a very desirable amplifier indeed, but it pre-dates the fixing of a few problems created by peaks in music.

I did a serious revamp job on one a few years ago and tried it out into my big transmission line speakers and I found it generally disappointing compared to their usual driver, but then it's a substantially lower power unit and the coupling capacitors in the outputs create a roll off which can be detected with these speakers.

So I'm not surprised that one of the Japanese monster amplifiers from the period before they went mad on STK modules, comes out well in comparison. They were very good amplifiers and were designed after a few limitations had been recognised and overcome. I find it a bit amusing that they can be picked up for a better price than the Quads, but then hifi is all about brand worship anyway. I'm also somewhat amused by the Japanese liking for sticking the heatsinks on the inside of the cabinets.

David

mark pirate 15th May 2016 10:10 pm

Re: JVC JA-S71 amplifier (1976)
 
Quote:

The reputation of the 33/303 is somewhat larger than the reality.
I agree, IMHO the power amp never sounded as good as my Quad II's (which I will never part with).

The JVC does sound better than the 33/303, it really makes my Tannoys sing ;D
There is a wonderful clarity and depth from the JVC that the Quad just could not match, it has not missed a beat since I serviced it four years ago, no doubt it will outlast me!

The 33/303 was swapped for a Leak stereo 50 last year, another restoration project awaiting attention...
:beer:
Mark


All times are GMT +1. The time now is 9:43 am.

Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.