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Messages - aruption

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Amplifier Discussion / Re: Ampeg SVT450
« on: June 27, 2019, 08:09:46 AM »
Finally got around to replacing the op amps after dealing with a divorce lol. Unfortunately this did not fix anything (with the amp). What's the next thing I should take a look at?

Amplifier Discussion / Re: Ampeg SVT450
« on: February 23, 2019, 08:19:07 AM »
Bad op amps then?

Amplifier Discussion / Re: Ampeg SVT450
« on: February 21, 2019, 05:43:30 PM »
Both 16v rails are right in the 15-16v range.

If I am reading the pinouts correct, then on IC1 there's -14v on output 2/pin 7 (0v on the other three outputs), on IC2 there's -14v on output 1/pin1 (0v on the other three outputs), and no voltage on the output pins of the IC3 and IC4.

At the test point near D4, I am reading 14v on one side of D4 and 0v on the other side. Is that correct? Because it looks like the side that is reading 14v is the side that the test point should read 0v, because that is the side that connects to R13.

Amplifier Discussion / Re: Ampeg SVT450
« on: February 19, 2019, 07:48:35 PM »
Sorry, that was unclear. I was saying that the problem is definitely in the preamp, because I did the tests as you described, and had a clear undistorted signal from the power amp input. The rest was still super distorted. There's no issue with hum, just distortion of any signal being input.

Amplifier Discussion / Re: Ampeg SVT450
« on: February 19, 2019, 11:48:58 AM »
In that case, it's definitely the preamp. Changing the EQ settings or limiter/pad switches don't do anything to make the sound better or worse (although the pad will still reduce the overall volume). Engaging the other EQ section doesn't change anything either.

Amplifier Discussion / Re: Ampeg SVT450
« on: February 16, 2019, 10:53:31 PM »
Thanks for the schematic. My testing setup is limited to a DMM. Do you think the issue is likely to be in the preamp or power amp section?

Amplifier Discussion / Ampeg SVT450
« on: February 14, 2019, 07:21:55 AM »
My friend just picked up an Ampeg SVT450 that's definitely got some issues. Plugging either speaker output into any speaker, any notes played on any instrument are super distorted, and do not sustain. Using the line out post, and note played is distorted and sustains infinitely. Only the line out pre sounds normal, but that of course is because it's bypassing pretty much everything in the amp. What should we first take a look at under the hood?

Amplifier Discussion / Re: Another Hohner Electric Piano - Cembalet CF
« on: February 14, 2019, 07:16:16 AM »
I ended up taking it to a vintage keys specialist in NYC. Turns out there were three things wrong with it. First, I installed the e-caps backwards  :-X Fortunately this did not kill anything else! Second, the bridge rectifier symbol on the original schematic was backwards. Third, it needs a working vibrato lightbulb to produce any sound, even with the vibrato off. So it works, aside from the usual hum and hiss. The main problem now is what sound like a loose connection in the cheekblock where the mains switch is. Playing the highest octave of the keyboard will set off these noises. So that's what I'll be looking into next!

Amplifier Discussion / Re: Another Hohner Electric Piano - Cembalet CF
« on: December 04, 2018, 08:36:38 AM »
When he finally got around to it, he found that the bridge rectifier was indeed dead, and replacing it got what he though are appropriate voltages to the entire board...except for the pickup, which still has 0 volts. He is pretty sure that the oscillator circuit is working properly, but is stumped about why there's no voltage on the pickup (and this guy's a savant with amp electronics). Any ideas about what I should look at next? I'm probably gonna go pick it up from him later this week.

Amplifier Discussion / Re: Another Hohner Electric Piano - Cembalet CF
« on: November 06, 2018, 02:26:25 PM »
Hi there!

I actually just learned that from another forum, and it makes sense now about the continuity that I'm seeing. Thanks! I brought it over to a guy who's much more knowledgeable that me. He hasn't had a chance to actually fix it yet, but he is pretty sure that the bridge rectifier is dead, and that the entire preamp is not getting the 12V it needs from the transformer. He's out of town for a couple weeks, but when he figures it out, I will report his findings back here!

Amplifier Discussion / Another Hohner Electric Piano - Cembalet CF
« on: October 23, 2018, 02:01:16 PM »
Just picked up another Hohner electric piano, this time the Cembalet CF:

It powers up and does nothing but hum - no keyboard signal. Replaced the e-caps and resistors, same deal. The original volume pot had been removed and jumped together. Installing a replacement 250K pot did not make a difference, and does not affect the hum level when turned.

The vibrato bulb is burned out. I have a Cembalet N which does not have a working vibrato bulb, this does not stop the keyboard from working, it just means that I don't have a vibrato option, so I am assuming this one must work the same.

There is no voltage at the pickup. The other Hohner EPs typically have 150-200vdc at the pickup. This one also shows continuity between the pickup and the reeds. Even with the preamp unhooked from the pickup and reeds, their hookups on the board still show continuity, so it's not anything with the harp itself. None of the other Hohners behave this way, if they pickup and reeds have continuity, they are shorted and there is no sound. But this model has a strange coil thing in parallel with the reeds and pickup, which if I'm understanding the schematic correctly, would explain the continuity.

What's the next thing I should look at here? The transistors seem to have appropriate voltage readings. Thanks!

Amplifier Discussion / SWR 550X Bass Amp Trouble
« on: September 20, 2018, 09:46:35 PM »
Not sure if this is the best forum for this, but some folks on here have helped me out with an electric piano amp, so a bass amp shouldn't be too far fetched, right?

Anyway, a friend of mine asked if I could take a look at his SWR 550X bass amp. It had developed a pretty loud hum, and louder notes from the bass would cause a horribly buzzy clipping effect. This is one of those amps with an 12AX7 tube preamp section. We thought that replacing the tube might solve this, it did not.

What should I be looking at to clear up all this noise? I also had to replace the master volume pot because the shaft had broken off the original pot. The level of the hum increases as the master knob is turned up, as was the case before it was replaced. Any help is appreciated!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: June 17, 2018, 01:54:16 PM »

Yes, the volume pot is activated by a leg bar on the underside.  I leave the bar unhooked because it just seems useless given the volume situation.  There's definitely more volume when I plug the unit into an external amp - but that just means there's more buzzing and noise too, which is acceptable when the rest of the band is playing, but just knowing that it's there...well, you know...

J M:

The original amp was capable of 1W (if that much)  and intended for quiet practice at home, at table radio levels, not more.

You are correct about that.  When I'm just playing it at home, it's perfect for quiet practicing.  I like your suggestion about how to replace the amp.  I'll keep a look out for those TDA2003s.  So far the ones I've seen probably wouldn't fit in the unit (it's incredibly tight in there). 

Now, would this mod increase the volume at the external output, or just the internal speakers?  I think I can live with the level of the internal speakers, it's just that (as I was alluding to earlier) I'd like to get more keyboard volume from the external output, without all the extra noise volume. 

So y'all can hear it for yourselves, here's a video of the Pianet in its current state, played with internal speakers only:

Thanks guys for your help!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 30, 2018, 11:04:18 AM »
I ended up taking it back over to the tech who originally worked on it.  He determined that the transformer was perfectly fine, and that the problem might have been in the transistors of the preamp section.  He eventually found that a wire near the voltage regulator for the was crimped and shorting out the circuit.  Uncrimping the wire brought the Pianet back to life!  Strange thing was that this wire was nowhere near the film cap that caused the spark that stopped it from working for begin with.  We did discuss the possibility of building a replacement preamp for everything past the power section of the amp, to maybe reduce the buzzing and humming.  One step at a time though, I'm just happy that she's running again!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 17, 2018, 01:40:15 PM »
That's what I'm afraid of too.  I contacted the tech who worked on it last year, and I think he's leaning towards that being the problem too.  So I'm going to try to bring it over to him soon.  I'll let you know what happens.  Thanks for your help so far!

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