How to proced with this ?

[Audio Al]

Hi

A recent ebay purchase £5.50

Not working , I changed the fuse , still not working

Took the case off and boy o boy what a surprise :stalks:

Resident spider and web

Thick dust and crud over all components , I think its the first time the case has ever been off

How would you go about cleaning the inside components ?

http://img.photobucket.com/albums/v240/bigboobs/IMG_2496.jpg (http://smg.photobucket.com/user/bigboobs/media/IMG_2496.jpg.html)
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Yes I know I am supposed to be decorating , but couldn't resist a look inside :eyebrows:

[Firebottle]

Well done that man! For taking the top off I mean, well it would have been the first thing I did anyway.

To clean the crud from the inside just use a softish brush, one of the paint brushes you're supposed to be using elsewhere would be ideal :)
If there is any more solid gunk that needs to come off I usually use an old toothbrush.
Have a good look around at the leads on the components to check if any are corroded, if they are all clean it has probably been in a dry environment, all the better.

The most obvious fault if it doesn't power up is the on/off switch. You could try shorting it out, make sure the amp is disconnected from the supply beforehand!

:cool:Alan

[Alex_UK]

one of the paint brushes you're supposed to be using elsewhere would be ideal :)


:lol:

[Barry]

This (http://theartofsound.net/forum/showthread.php?7659-On-the-cosmetic-refurbishment-of-used-classic-or-vintage-audio-equipment) might be of some help/interest to you Al.

Did the replacement fuse blow? If so then the spiders web probably caused a short. If it didn't and the amp still doesn't work then the problem lies elsewhere - cleaning the dusty interior, although desirable, will not effect a cure.

Keep us informed as to how you get on.

Cheers

[istari_knight]

Nice little project. I use a 1" paintbrush & isopropyl alcohol to clean... Be sure to do it outdoors or in a well ventilated area !

If there is no power & the fuse is blown [or blows when replaced] you need to check all four rectifier diodes, both reservoir capacitors & all 4 output transistors. The fault will be with one or more of those components, short/open circuit mains transformers are very rare but you should check continuity just to eliminate the possibility.

You use the "diode test" function of a multimeter to check the rectifier diodes: If you get zero both ways, the diode is short circuit. If you get infinity both ways, the diode is open circuit. If you get infinity one way but some reading the other way (the value is not important) then the diode is good.

You can test the reservoir capacitors for short circuit with a multimeter [I've seen this quite often.]

For testing the output transistors this is quite a straightforward guide: http://www.stickycarpet.com/pinx/test.html The output transistor datasheets showing pinouts are here: http://www.jmnic.com/pdf/2sc/2SC2578.pdf & http://www.svntc.com/TPDF/1390.pdf

I don't know how familiar you are with servicing / repairing old gear so please forgive me if I'm telling you what you already know !

[Audio Al]

James

ALL advice is good advice

I am a novice at " Trying to repair items "

This is the wording from the add

" If you're looking at this then you probably know what it is! This trio kenwood amp has served my family beautifully since the late 70's, then one day I came back from work and my wife had rewired the speakers and it had stopped working. I've changed the plug fuse but still no joy.

May be a clue as to what went POP :scratch:

Thanks for the advice

Al

[istari_knight]

James

ALL advice is good advice

I am a novice at " Trying to repair items "

This is the wording from the add

" If you're looking at this then you probably know what it is! This trio kenwood amp has served my family beautifully since the late 70's, then one day I came back from work and my wife had rewired the speakers and it had stopped working. I've changed the plug fuse but still no joy.

May be a clue as to what went POP :scratch:

Thanks for the advice

Al

From that description its almost certainly the output transistors but you'll need to change the drivers too as if they are even slightly iffy the new outputs will die on power up.

I would also check the emitter resistors [white ceramic coffins] for open/short circuit... The repair wont be a quick or easy I'm afraid but if you can solder it needn't cost more than £15 :)

So yes, first port of call: measure those output transistors ! [... Be sure to remove any components before measuring as you will get incorrect readings "in circuit."]

[Firebottle]

Al, it might actually be a very easy repair. Now listen very carefully and I shall begin......

In the era these amps were made a separate small board was used to hold the internal fuses; If you look at your 2nd picture you can just see it at the very top of the picture.
Once you have cleaned the dust off you should find the fuse values are marked on the board. Most times these fuses are glass 20mm so you should be able to see if the fuse wire is intact or blown. I think from the sales description this is the area to look at.

Good luck,
:cool:Alan

[Audio Al]

Cheers Alan

I did notice the 2 small fuses , Then spent over one hour trying to find my meter :scratch:

Will pop them out and have a look

[Stubies]

Al,

A friend fixed up an old macintosh amp, it turned on, but no sound - during tests he realized the signal stopped at the volume control, so before replacing it, he 'soaked' the volume control in contact cleaner while turning it, as a last resort - waited until it dried (more than a day i think) and then the amp worked ok. Seems the contacts inside the volume were so gunked .

Not sure if that helps, Good luck!

[Audio Al]

I got bored of the wallpapering so stopped to do something more fun :D

Found my meter and had a prod around on the 2 inside fuse's

Bingo :D both 6.3amp fuses blown

now I need to go to Maplins to see if I can obtain some replacements

Hopefully / fingers crossed it may just work again :eyebrows:

Will update when the new fuses are fitted

Back to the decorating :(

[istari_knight]

I got bored of the wallpapering so stopped to do something more fun :D

Found my meter and had a prod around on the 2 inside fuse's

Bingo :D both 6.3amp fuses blown

now I need to go to Maplins to see if I can obtain some replacements

Hopefully / fingers crossed it may just work again :eyebrows:

Will update when the new fuses are fitted

Back to the decorating :(


I have my fingers crossed for you but those fuses sit on the transformer secondaries, they are not speaker protection fuses. There is a protection IC & relay for that. Generally they wouldn't both die for no reason [and shorting the speaker wires like that blokes wife did wouldn't pop them.]

[Audio Al]

Opps !

May need to look deeper then

[istari_knight]

Just be sure to buy more than 2 fuses ;)

If the new ones blow at power on check the output transistors as said before. When his wife shorted the speaker terminals the relay should have disengaged thus protecting the output stage but its my experience that in these old amps that almost never happens ! The output transistors will short circuit & the line fuses will pop.

[Audio Al]

Just be sure to buy more than 2 fuses ;)

If the new ones blow at power on check the output transistors as said before. When his wife shorted the speaker terminals the relay should have disengaged thus protecting the output stage but its my experience that in these old amps that almost never happens ! The output transistors will short circuit & the line fuses will pop.


OK 1 pack of 100 fuses should do then :thumbsup:

[Audio Al]

Just be sure to buy more than 2 fuses ;)

If the new ones blow at power on check the output transistors as said before. When his wife shorted the speaker terminals the relay should have disengaged thus protecting the output stage but its my experience that in these old amps that almost never happens ! The output transistors will short circuit & the line fuses will pop.

James
As you predicted

I fitted 2 new fuses power on and POP both fuses blown , Now need to find the output transistors :scratch: will clean it up and have a look for them ( when I finish the decorating )

[StanleyB]

Looking at the picture you posted I would say with a 99.9% certainty that the output stage transistors are OK. I'll keep the technical reasons to myself ;).
If you can find the power supply regulator and rectifier circuit have a check to see if there are any shorts there. Check the rectifier to see if it has gone short.

[Firebottle]

I repaired a friends old Technics amp, just one of the 4 rectifier diodes in the bridge had gone short circuit.

Replaced the bridge with an encapsulated uprated one and voila :D

:cool:Alan

[istari_knight]

Sorry, only just seen your update :doh:

I would love to hear Stan's technical reason why the outputs aren't toast from a visual inspection ? Would be good info to have as I've had amps on the bench that looked fresh out of the factory inside, powered up as per normal yet had shorted output transistors.

The output transistors are the most time consuming parts to test so I would recommend testing the rectifier diodes and reservoir capacitors first. You will need to remove them before testing as you'll get false readings "in circuit"...

Rectifier diodes are labelled: D7, D8, D9 & D10 ~ Reservoir capacitors are labelled: C45 & C46. Hopefully you will find a short, if not you need to test those pesky outputs!

The output transistors are labelled: Q17, Q18, Q19 & Q20.

I've attached a picture showing the location of each if your still not sure. Shout if you need help :)

[Audio Al]

Thanks James

Will have a go at it when time allows , hopefully not too long now the decorating is finished :D