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Old 30th October 2004, 10:54 AM   #1
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Unhappy Audio Innovations Series 500 problems

Hi

I've an old Audio Innovations Series 500 that seems to have gone wrong. I've replaced the four EL84s and three ECC83s and am waiting to get my hands on a pair of ecc88s to complete the valve upgrade programme.

However, I was surprised to find that replacing the 84s had no effect as I'd previously thought that they were shot. I'm wondering if there is some other failed component inside it. I have the schematics but I'm not very experienced with tube amps - just love the sound - and would like to ask if there's anything in particular I should check first. I read somewhere that a particular capacitor goes and that leads to tube failure but cannot remember the details..

Symptoms are as follows:
  • Humming (well it's allways done that a little bit, I guess there is a bad earth in there somewhere).
    Very very low output which distorts if you turn the volume up all the way, but stays quiet.
    Phono stage is dead quiet, no output at all.

I guess the first thing would be to test the output from the tape loop. This will provide some indicator for the phono stage.

Can anyone suggest the sorts of things I should look for? What typically fails in these amps? I can post the schematics if it would help.

Thanks

JonB
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Old 30th October 2004, 12:12 PM   #2
SY is offline SY  United States
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If the company is out of business and there's no copyright issues, the schematics would help immensely.
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Old 30th October 2004, 12:28 PM   #3
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The electrolytic capacitors, especially those at the cathode of the 84's are shurely bad.
This is due to the high operating temp.

Dick
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Old 30th October 2004, 12:54 PM   #4
Jeroen is offline Jeroen  Netherlands
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Quote:
If the company is out of business and there's no copyright issues, the schematics would help immensely.
Schematics can be found here .

More links:
Dimitri's website More information on the AI500
Classic 25 homepage Basicaly the kit version of the AI500, with information on modding.

BTW: my AI500 uses el34's, not the el84's.

Good luck!
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Old 30th October 2004, 01:06 PM   #5
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Default AI 500 integrated

There is nothing wrong with your AI 500.

You are just using the wrong type of output tubes, you need EL34's there and not EL84's.

You can get 2 matched pairs of Svetlana EL34's from PMComponents at a reasonable price.
Maybe it would be a good idea to get 3 of those new Svetlana 12AX7's as well (a rumour is spreading around about these tubes) and a new pair of ECC88's.

This is a fine amp and if the 25 watts per channel are enough for you then go on and,
1) replace all electrolytic caps with new high quality ones (esp. the 100uf/100v one between the output tube cathodes to earth in parallel with the 220R resistor),
2) get rid of the cheap selector switches and pots (shop around for ELMA and ALPS, the amp is worth it),
3) check the tube sockets for play and wear replacing them with high quality ceramic ones.

If you cannot perform these tasks yourself, give a call to Mr. Peter Qvortrup of Audio Note UK. He is the man behind Audio Innovations and maybe he will be able to help you servicing the amp.

Good luck,

Nick
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Old 31st October 2004, 09:32 AM   #6
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Default EL84s? Ooops...

Nick

Sorry about that, it was a typo. I do have a matched quad of Svetlana EL34s in there and the ecc83s are (matched) Sovtek 12AX7LPS. Actually, I've got a spare one as I bought a matched quad of them. I'm bidding on a set of ecc88s right now on eBay to complete the valve replacements.

You're right, it's a bloody marvellous sounding amp and that is why I bought it all those years ago. However this particular one has a very chequered past as it was reputedly torched in a factory fire (looks like soot but no heat damage) and has been worked on by some questionable individual who has attempted to put DIY shielding inside it (bent bits of tobacco tins IIRC). It only cost £300 at the time! However, it has worked fine for many years and only got stored away when something even better came along. But I want to resurrect it now for my study, so I'm going to have a go at it.

Thanks for the replies and any other advice would be much appreciated. I don't like the look of some of those voltages alluded to on the power supply schematic... so "proceed with care" is the order of the day for me!


JonB
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Old 31st October 2004, 01:01 PM   #7
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Default Getting access...?

Is it just me or is the bottom plate the only access to the internals of the AI500?Since everything is mounted directly on the top covwer I would expect to be able to lift it off or hinge it back. Well, I can see the allen bolts as well as anyone but is it an adviseable thing to do? Or does all work take place from the underside?
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Old 31st October 2004, 02:09 PM   #8
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Default right, I'm in there.

The circuit off the cathodes on both sides of the amp have been changed relative to the schematics. I've attached a diagram and described it:

Cathodes of both 34s are joined as per normal.
This goes into two resistors (connected in parallel, ceramic, wire wound) with a measured resistance of 220R.
On the other side of this is the fuse resistor, again a wire wound ceramic component. Its value is 39R (as opposed to 15R which I was expecting).
The capacitor is a 220/100 which I assume means "220 uF / 100 V" and it bridges the resistors, all of them, including the 39R "fuse".
The -ve end of the cap is earthed as you'd expect, but the +ve end is connected to the cathodes.
The fuse resistors and 220R pairs are all working (insofar as the meter confirms their expected values).
However, bridging all resistors with the capacitor doesn't look like a very good idea to me.
On the schematic is a measurement point "R" which states the voltage at the cathodes should be 28V. A quick check with the meter confirms that that is the case.

How can I test the capacitor in-circuit? It doesn't show signs of burnout.

Regards

JonB
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Old 31st October 2004, 02:19 PM   #9
PMik is offline PMik  United States
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Jon

yes....useall all service on the series 500 is made from the buttom only, you only lift the hinge at the rear if the tranformers or main capacitors have failed

I dont regard your 500's sickness as any greater prob, just the useall maintaince ...actully i have salvage a lot of AI 500/800's of which a few literally went on fire (lol)

Send a mail where to mail all the info you need
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Old 31st October 2004, 04:20 PM   #10
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I can't send a mail to you throught the bulletin board as your user preferences seem to indicate that you don't want to receive mail from it.

Errm. I would rather not publish my eMail address because some spam bot will harvest it from the site and I'll be innundated! I don't need any Gener1c \/1agra thank you very much...

Tell you what, I'll spell it phonetically. dr underscore zee at en tee ell wurld dott comm

There you are - you can work it out but a spam bot (probably) cannot.
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