Advent 300 Rebuild: Let the jaws hit the floor, let the jaws hit the flooooor!

[randyhouse]

If you are going to use a very high quality amp with it, it does make an improvement. If you do the tone control bypass the loudness switch will still work. I have done it in the past it is a mechanical bypass and can be changed back. I like to leave the tone controls active, but that is the choice of the person who owes the unit.

I would like to at least try the tone defeat mod. I am using an adcom gfa 535II as a power amp. Is there a link to exactly how to by pass the tone controls so that it is reversible , or perhaps even substituting the loudness switch for a tone defeat? Seems simple from the schematic but the board layout makes for a fair amount of guess work. Thanks

 

[stereoguy70]

Next to go are these 3 transistors that make up the -15V. Will put in a real regulator as they probably saved 5 cents per unit or something doing it this way. Then can start the rebuild.

While those three transistors are part of the 15v regulator circuit, they are not the regulator itself. RA400, the IC mounted to the heatsink to the far left of the power transistors is the actual 15v regulator.

 

[stereoguy70]

Regulator is in place with some other things that needed to be done: Voltage is checked and is correct. Now to move on to the rebuild.

Can't for the life of me figure out why you went with this mod. Did you also remove RA400, the actual 15v regulator as well? This mod seems redundant to the actual regulator to me. Seems to me like this would put not one, but two 15v regulator ICs in series with each other........can you explain the reasoning?

 

[patfont]

One is the +15V reg (RA400) one the -15V reg. (Q401, Q402, Q403)

I work on quite a few of these and, Q401, Q402, and Q403 do have failure rates.

Look at 2.3 of the service manual

 

[stereoguy70]

One is the +15V reg (RA400) one the -15V reg. (Q401, Q402, Q403)

I work on quite a few of these and, Q401, Q402, and Q403 do have failure rates.

Ok, that makes sense to me now. And, yes I agree with you that those transistors are known issues in the 300. Several have passed through my hands now all needing recaps and regulator fixes, a few with output transistors needing replacement as well. I usually replace those three transistors with the appropriate equivalents, as well as RA400, the usual electrolytic blanket recap, and call it a day.

Thanks for the input & quick response.

 

[special_20]

Patfont, any chance you could walk those of us less tech savvy through this modification? Is it just removal of Q401, 402 and 403 and replacement of Q401 with a voltage regulator? If so, any chance you can suggest a regulator for use or pertinent specs? Are there other cuircuit modifications or jumper leads needed to effect this mod?

I did a recap of a unit last winter and this circuit failed on me soon after. Have another currently on the bench with similar problems. Seems these are not very robust parts and have a failure rate or don't like the voltage bump of higher value caps in the power supply.

One is the +15V reg (RA400) one the -15V reg. (Q401, Q402, Q403)

I work on quite a few of these and, Q401, Q402, and Q403 do have failure rates.

Look at 2.3 of the service manual

 

[stereoguy70]

Patfont, any chance you could walk those of us less tech savvy through this modification? Is it just removal of Q401, 402 and 403 and replacement of Q401 with a voltage regulator? If so, any chance you can suggest a regulator for use or pertinent specs? Are there other cuircuit modifications or jumper leads needed to effect this mod?

I did a recap of a unit last winter and this circuit failed on me soon after. Have another currently on the bench with similar problems. Seems these are not very robust parts and have a failure rate or don't like the voltage bump of higher value caps in the power supply.

I replaced those parts in mine when I restored it over two years ago, haven't had a failure yet. Those parts are fairly robust, but I suppose it would depend on how much you went up in value on the caps you replaced. I kept mine to stock values and its been working fine since being recapped.

 

[CT Jim]

If you are going to use a very high quality amp with it, it does make an improvement. If you do the tone control bypass the loudness switch will still work. I have done it in the past it is a mechanical bypass and can be changed back. I like to leave the tone controls active, but that is the choice of the person who owes the unit.

I have a 300 in the stable waiting to be worked on, but I'm not sure I understand this quote? First, I do not plan on using it with an outboard amp, but as a receiver as it was intended. So, the tone bypass may be a moot point. Second, if the tone controls are bypassed, which means they do not alter the signal, but the loudness works, does this mean it is the same as when the tone controls are centered when applying the loudness? So why bypass the controls?

 

[Blue Shadow]

First, I do not plan on using it with an outboard amp, but as a receiver as it was intended.

The marketing information back in the day was that the Advent 300 was intended to be used with an external amplifier anytime there was a need for more power than the receiver could supply. Not many speakers play loud with just 15 watts but some do. Additionally, it provided an upgrade path to higher power without making the 300 expensive with 50 or 100 watts built in. This was a strong selling point so although it is a receiver it was intended to be either a receiver, stepping stone receiver to play the stereo before adding a power amp or a preamp new from the box.

 

[patfont]

ok the big confusion about the "Tone Bypass."

The Audio Critic wrote an article where they "bypassed" the Advent 300's volume and tone controls by using the tape out to go into the Mark Levinson
JC-2 preamp's remote phono input, and it was so good it took a very long time to see which was better, the JC-2's internal phono, or the Advent 300's.

So people equate the "bypass term to this.

But many hi-end units have a button that switches in tone, and tone defeat, this bypasses the tone circuit, and makes the signal path more simple, thus
an improvement.

I have done a mechanical bypass of my own, I have also installed a kit Dave Eaton has for sale on his web site which replaces the FM mute switch with a tone defeat switch

Dave Eaton on his site also has the -15V regulator kit and instructions as asked about above.

Of all the units I have coming through here I am always reminded of how good the 300 is when I get one on the bench!

 

[stereoguy70]

Not many speakers play loud with just 15 watts but some do.

All speakers will play loud with "just" 15watts of power behind them.....it has more to do with room size, shape, reflections, than it does the "loudness" factor of an amp or a speaker. In the right size room, 15watts is more than plenty of power.

Advent themselves said in their literature that most real home applications didn't need much more than 15 watts of power for realistic aka normal listening levels. Do you really need 400watts per channel of power and those Klipshcorns to be able to listen to music at a normal level? No. Is it nice to have? Yes. Believe me when I say that the REST of the NEIGHBORHOOD doesn't care what YOU are listening to.

 

[Blue Shadow]

All speakers will play loud with "just" 15watts of power behind them....

Your loud is different than mine so I can go with the idea that there are different levels of loud. Some of my speakers with 15 watts of pure transistor power will not keep the party going without clipping the amp, severely.

 

[Superman541]

Old thread, new question:

I've got a non-working Advent 300 and I am likely to take it to somebody else's bench, but first I will try what I can. Check voltages the +/-22 voltages and the +/-15 voltages, and such. But first, I noticed something. Here is a pic of each end of the two caps near the transformer. First, I see some stuff bubbling from one of the caps. I think this means the cap has failed, right?

I am also surprised that in one of the pics you can see the black end of one cap connected to the white end of the other. Is this correct?

 

[Superman541]

Each cap has a black end and a silver end, though I cannot see "+" or "-" printed on them. They have a + end and a - end, right? Should they be connected, + to -, as they are in the second pic? Or should they only be connected at the - ends, with the + ends going to different circuits? The first pic, with the brown resistor hiding there, should be the + end, I think.

 

[stereoguy70]

Those are the power supply filter caps and yes, they have gone bad. Best if you get a schematic/service manual for the unit first before attempting any work on it.

 

[Superman541]

Yup, I have the SM. Here is a pic of the power supply. Indeed, it looks like these capacitors are basically wired in series. Am I seeing that right?

 

[Superman541]

Oops.

 

[northpaw]

In series isn't quite a proper way to describe it, as current or voltage does not flow from one cap to the other.

It is standard that for smoothing caps coming off the rectifier, as is the case here, one cap takes the positive rail of the supply (ground to +22V) and the other cap takes the negative rail (ground to -22V). The caps do share a ground connection: negative side of the +22V-rail cap and the positive side of the -22V-rail cap are both connected to ground, as clearly shown in the schematic. That is why you see the negative (silver) end of the +22V-rail cap connected to the positive (black) end of the -22V-rail cap in your second photo in post #93.

It is a little easier to see the schematic from a screen capture:

 

[Wire Nut]

Hi, hope guys are still looking at this great post. I am redoing a minty 300 now, do the mods on this post cover the ones recommended by Advent in the service bulletins? I have a service manual from the HiFi Engine but no mention of the updates.

 

[patfont]

You can check Dave Eaton's site he lists the important updates.
Addition of grounding wire on bottom of chassis.
Mica and new thermal grease for the outputs, if yours are the older style.
If tuner is drifting once warmed up, there is a cap swap that can help.