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Old 31st January 2008, 05:45 AM   #1
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Default PA Power Amp Troubles

Greetings, all:

I have with me now an EV-7300 stereo power amplifier. It was being used as a PA amplifier for a band of musicians. The band has three of these amplifiers, and the last time they got together, each of the three amps lost power in one channel. I do some tube amp repair for the guitar players in the band, and they had me over to check it out.

Well, the place where they were playing at had a guy running a variac to turn his neon lights down to the point where they flickered on and off. It was neglected to ensure that the amps were not on the same power bar (d'oh).

Although I do have tube amp experience, I'm not very familiar with solid state. I was wondering if anybody had some suggestions as to where I might start my inspection.

Again, the problem is one channel is not working. Fuses check OK, and the XLR inputs are on the circuit board without any physical damage. . . The big blue caps test ok. . . I'm wondering if this is one of those "crowbar" protection systems I keep hearing about. Seemed to me that "crowbars" were intended to avoid power spikes, not power sags (I went to the location afterwards and the guy had his neon lamps and variac down to 90VAC). Anyhow, what do I know!

I have a link to the owner's manual, in PDF format. This contains the only schematic I was able to find for this amp, as well as performance specifications. The skeemos are on pages 9-10-11, or therabouts, right near the end of the PDF:
http://archives.telex.com/archives/E...s%20Manual.pdf


I imported the PDF into photoshop in an attempt to sharpen/contrast and make the schematic clearer, but it was futile, and nothing I did made the original PDF look any better

I also have an album of 19 pictures of this amp (one of three, each with the same, one channel only problem) uploaded to Photobucket:
http://s2.photobucket.com/albums/y21...rts/EV%207300/

Any help would be much appreciated!

Will
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Old 1st February 2008, 02:45 AM   #2
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It doesn't do much good to put a link to your album that requires a password to get in.
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Old 1st February 2008, 06:26 AM   #3
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Thanks for letting me know. I was wondering about that. First time for everything.

I did find the Photobucket control settings for the album and it is now public.

Sorry about that!
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Old 1st February 2008, 10:01 AM   #4
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Will,

Do you get nothing out of one channel?

Is the bridge mono switch set to stereo and both gain controls

set the same?

Are there 2) jumpers in each of the octal sockets on the back of the unit?
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Old 1st February 2008, 11:23 AM   #5
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does the protection relay for the dead channel click on and connect the amp to the speaker terminals?

If it does, Is there signal at the + term of J7? and after the op-amp.

If the relay does not click on. is there DC offset at the amp output before the relay? If there is heavy DC offset there. IE rail voltage. then there are blow output transistors. check drivers too.

If there is only half wave signal there. check drivers.
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Old 1st February 2008, 08:44 PM   #6
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Quote:
Originally posted by burnedfingers
Will,

Do you get nothing out of one channel?

Is the bridge mono switch set to stereo and both gain controls

set the same?

Are there 2) jumpers in each of the octal sockets on the back of the unit?
Hi Burnedfingers,

Yes, nothing out of one channel.

There are two correctly positioned jumpers in each of the octal sockets, the bridge/mono switch is set to stereo, and both gain controls were tested in unision up to 12 noon (half gain).


Quote:
Originally posted by Zero Cool
does the protection relay for the dead channel click on and connect the amp to the speaker terminals?

If it does, Is there signal at the + term of J7? and after the op-amp.

If the relay does not click on. is there DC offset at the amp output before the relay? If there is heavy DC offset there. IE rail voltage. then there are blow output transistors. check drivers too.

If there is only half wave signal there. check drivers.
Hi Zero Cool

The protection LED for both channels (the dead one as well) clicks on at startup, and then the protection LED at the front goes off. The band members say that when it was working properly, this is what the lights would do--protection on briefly at startup, and then off again fairly fast. Beyond what the LED is showing, I'm not sure.
I intended to start getting some testing done this weekend.

As far as testing the amp is concerned, I have a question.

Because I work on tube amps, I always keep a big brute of a speaker connected when I do my testing. Do I need to do this with this transistor based unit? If so, just one channel, or both?

Burnedfingers and Zero Cool - - thanks a bundle to both of you for your replies. Much appreciated.

Will
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Old 1st February 2008, 10:10 PM   #7
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Will,

The protection leds should both come on for roughly 4 seconds or so and then go out. This is the same basic circuit as the EV2600 which is a later model amplifier. Unfortunately I cannot read the schematic at all.

I would re-check the amplifier by putting a sine wave into it and looking at the output with a scope. I usually look at the output without a load and then attach a load and see how it behaves. If the sine wave is good check the cap in the input and the feedback cap to ground. Just replace them as they are cheap. See where you are at this point and post again.

Joe
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Old 2nd February 2008, 01:57 AM   #8
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Quote:
Originally posted by burnedfingers
Will,

The protection leds should both come on for roughly 4 seconds or so and then go out. This is the same basic circuit as the EV2600 which is a later model amplifier. Unfortunately I cannot read the schematic at all.

I would re-check the amplifier by putting a sine wave into it and looking at the output with a scope. I usually look at the output without a load and then attach a load and see how it behaves. If the sine wave is good check the cap in the input and the feedback cap to ground. Just replace them as they are cheap. See where you are at this point and post again.

Joe
Hi Joe

Indeed, it is a 4 second period of light from the protection LEDs at power-up, and then out.

The schematic in the PDF is really terrible. I probably shouldn't even have linked it, but I wanted to share all that I could provide.

However, I've gone ahead and ordered a large schematic for the 7300. It should be here in the following week. I have a great flatbed scanner. When it arrives I will add the scans to my Photobucket account with a new link.

I will go ahead and check the amp as indicated. The caps you mentioned will be investigated.

Again, I am grateful for your help. It is much appreciated.

I've spoken with the Neon sign/variac owner, and the variac has been boxed up!
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Old 2nd February 2008, 10:42 AM   #9
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Will,

You are dealing with an amplifier that is around 20 years old. I sincerely doubt that the variac had anything to do with the problem in one channel of the amplifier. I have run a number of amplifiers on the bench hooked to a variac running at lower voltage than normal in order to track down problems. Basically if the amplifier will turn on and the protection circuit latch up it will run at lower voltage without any damage to the amplifier.
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Old 2nd February 2008, 11:47 PM   #10
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Quote:
Originally posted by burnedfingers
Will,

You are dealing with an amplifier that is around 20 years old. I sincerely doubt that the variac had anything to do with the problem in one channel of the amplifier. I have run a number of amplifiers on the bench hooked to a variac running at lower voltage than normal in order to track down problems. Basically if the amplifier will turn on and the protection circuit latch up it will run at lower voltage without any damage to the amplifier.
Right!

The amp is dated "Recieved 2-11-88" Just about 20 years to the day when the store (or whomever) first got this thing.

The power came from the variac, then went to a short white extension cord with only two prong outlets (with the little plastic nub cut off so a three prong power bar could be plugged into the end of it), which had a chain of neon signs plugged into it, as well as a large 10-outlet power bar made from wall outlets (looked like a construction site item that had been seriously battered and as a result, retired). The 10 outlet power bar was plugged into the last of the three two prong outlets on the short white extension cord attached to the variac. Although the 10 outlet power bar had a ground prong, this ground prong was taking the spot of the removed little white nub on the short extension cord. . . So, no ground either. I'm not sure what the deal is with this setup, but it is changing, or never being used again. From what I hear a Furman power conditioner was just purchased today, and is to be added to the rack setup, along with a tester to check for wall voltage levels and grounds.

Anyhow, I'm posting again because I've scanned in some better schematics that I went hunting for today.

http://i2.photobucket.com/albums/y21...300_pt1of2.jpg
http://i2.photobucket.com/albums/y21...300_pt2of2.jpg

These are limited by the photobucket size; the scans are quite good, high res, and around 4.5 MB each (part 1 and 2).

It's a bit of a mess to pull apart. I did check over the input board,
and everything seems to be running properly there.

Going to investigate the Potter-Brumsfield T90S5D12-24 relays.

Edit: Also added two pictures of the back side of the input board:
http://i2.photobucket.com/albums/y21...0/IMG_2798.jpg
http://i2.photobucket.com/albums/y21...0/IMG_2792.jpg

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