Request repair help JVC AX-211 stereo amp

[peterlo]

Hi everyone,
I was unsure exactly where to post my question so hope this is forum OK.
My mid 80's amp appears to have satisfactory power supply & caps (2 x 6800uF, 50V).
It's said to be a "Two amp block configuration".
I measure +/-47 & 94 VDC variously across the 4 rectifying diodes, so I guess this is a two rail design? The transformer is centre tapped.
I cannot locate a schematic, service manual, or circuit diagram & have had a bit of trouble getting datasheets for the Sanken A1489, & C3853, there are two of each fixed to a very substantial heatsink running most of the length inside the housing.
The source selector switches don't light up & the unit has no output.
I'd appreciate a bit of guidance on what to do now to progress up the diagnostic ladder, or maybe where to dircect my attention on the likely suspect components.
Also there are a number of quite small diodes that I don't know how to test "in place", unlike the rectification diodes they all fail to show continuity in either direction???
Hope this is not too elimentary, I know folks here are extremely talented & I don't wish to degrade the learned exchanges.
Pete

[sakis]

not an easy task unless you have some basic skills

you may take a look in the link bellow but from the way youwrite things it seems that your skills are not enough .... take a look a the link , search for a schematic , then if you need more help post here ...

if you are not sure about what are you doing you may consider getting some help from a professional ....

beyond all that be adviced that leathal voltage exists in some areaw of your amp ....so please take one step at a time and carefully what you have there is a comercial amplifier that has a potential to work happily for many many years more

regards sakis
vintage amplifier repair/upgrade manual

[peterlo]

Sakis,
Thanks for the reply.
I have read & re-read your tutorial on refurbishing older amps.
I guess the issue is what are basic skills, I formed the impression that not many pros would be able to conduct unaided the kind of meticulous & detailed approach you outlined.
Certainly I do not have these skills, & have no prospect of developing them in this lifetime, worse I suppose, I can't afford to have any pro do the kind of thorough work you detail. It would be much more cost effective to simply buy good condition used equipment that was demonstrably working.
I understand your warning re mains AC, I am familiar with (single phase domestic applications) mains work.
I have just ordered an ESR meter so I should soon be able to test electrolytics much more thoroughly. I do not have a 'scope however.
I need to tell you I felt a bit put down by your reply, justified as it may be. But I'll soldier on. As far as I can see I have learned the little I know by stripping failed gear & searching the net for material that may help in pinpointing faults. I will continue, though I don't suppose you will not be surprised to hear I have a long list of binned gear that I could not repair, or in most cases fault find. Latest count includes 3 CRT TV's, one LCD TV & numerous audio amps, etc.
OTOH I have made up quite a few nice simple packages, PSU's remote light switches & the like.
If you have, or know how to obtain a schematic for my unit, that would greatly help, I have of course done the usual searches.
Peter O

[sakis]

many things is just a question of approach

for example the cost of replacing all the electrolytics in your amp ( at least all the smallones ) will be far mucj lower than buing a capacitance meter ( cause this is what you need not an ESR meter )

now if you can actually follow procedure and have simple soldering skills and time you can actually do this ....

finally many of the so called ''pro repair shops " will bothe to replace what is broken and not the all capacitors ( if needed ) eventhough the cost to get the caps is very low and the beneffit is very big this is a time consuming job and of course you will be charged for it beyond other fault finding ...

restoring vintage could be done by a hoobyist since is not really cost effective and especially for a unit like that

regards sakis

[peterlo]

Sakis,

Again thanks for taking the trouble to reply.
Yes I can solder but have much more problems desoldering.

I am as confused now however as I was when I posted:
I did my research & the definitive approach to electrolytic cap evaluation was said to be an ESR meter. The Russian design I ordered will cost US$60 delivered & will also measure capacitance.
Clearly there is a problem checking caps so I've made a small hobby investment which I can use on all equipment just as I presently use my DMM (which measures capacitance only to 2000 uF).
I have a friend who has been in electronics most of his life. When I first asked for some help (another non-working audio amp) he seemed to have some difficulty also, & said he really needed a circuit/schematic. You don't impose on your friends so I have not asked again except the odd phone call.
My current position is that I will recheck the power supply & caps when the meter arrives.
If I cannot find a fault there I will check what is easily accessible & re-evaluate whether to continue.
The replacement smoothing caps alone will cost around US$20, not much less than the unit is worth although your description "commercial amp" has me a bit puzzled, I would have said it was merely "mid level home audio gear" & worth only about US$40 when working?
Can you help with a schematic?
Do you recognise the Sanken components I mentioned in my first post, I can find very little on these & if a replacement is needed I am unsure what to use.

Finally on the subject of caps you talked about "better quality" how do you establish which types & brands are superior. I wish to avoid mail order components, so I will be restricted to whatever my local electronics shop stocks.
Would you take a different approach on this?
Rgds
Peter O

[sakis]

this will be not easy to solve or to work out through posts or e mails .... but lets try ....

---- you need to read whta i write carefully
---- main psu caps if dried out will present high distortion and not enough power if the amp is pushed to 100% .if working at 10% with half dead caps you might not notice anything
---- on the other hand a small electrolytic leacky or dead in the signal path might cause so much trouble and stop the all amplifier

if you need a priority list on were to look first here is how it goes
--- first is the "mechano" meaning dirty pots or switches / dry solders and solder joints
--- then is tha small electrolytics all over the amp
--- then is is broken or bad trimmers
--- finally is the big caps

let me know

[peterlo]

Thanks Sakis,
I will follow your guide when I get back to this repair project.
Right now a couple of things have come up, & I'd still like to have the meter I ordered to hand when I start again.
I'll let you know how I get on, but it will be several weeks away unfortunately.

Pete