Harmony h-304

hollisdevillo · 10-27-2008, 09:55 PM

I have an old harmony h-304 that i purchased in new york. The amp worked (barely) but was in need of some servicing. The schematic is on the inside of the amp and upon opening up the amp discovered it was nothing like what it was supposed to be. Wrong values and extra capacitors. Upon further investigation i discovered that it seemed to be infused with the h-304A amp, but it was neither the closer to one as to to the other. So i decided to build it back to spec. there is very little sound coming from the speaker. there is a .02 cap (C2) coming off pin 7 that has no voltages on the pot side. is this right? going through the first preamp tube with a signal tracer, it becomes very quiet. coming off the output it is very loud, but then almost nothing going through the output transformer. less sound than the initial signal. there is also a 4 mf cap (C5) that seems to be grounded on both sides. what is the purpose of this cap?

thanks

schematic is on schematic heaven in bargain amps section.

Dave Curtis, dB AudioTech · 10-30-2008, 03:59 AM

Quote:

Originally Posted by hollisdevillo

I have an old harmony h-304 that i purchased in new york. The amp worked (barely) but was in need of some servicing. The schematic is on the inside of the amp and upon opening up the amp discovered it was nothing like what it was supposed to be. Wrong values and extra capacitors. Upon further investigation i discovered that it seemed to be infused with the h-304A amp, but it was neither the closer to one as to to the other. So i decided to build it back to spec. there is very little sound coming from the speaker. there is a .02 cap (C2) coming off pin 7 that has no voltages on the pot side. is this right?

Safety first. Does it have a 3-wire power cord? Has C9 (death cap) been removed? There should be no DC voltage on the pot side of C2.

Quote:

Originally Posted by hollisdevillo

going through the first preamp tube with a signal tracer, it becomes very quiet. coming off the output it is very loud, but then almost nothing going through the output transformer. less sound than the initial signal. there is also a 4 mf cap (C5) that seems to be grounded on both sides. what is the purpose of this cap?

That's a filter cap (smoothing; decoupling). and it seems to me that the circuit is wrong. The positive side of that cap should connect to R4 & R9. It looks like the line drawn from R4 (plate resistor) to ground is a mistake. BTW, if the electrolytic caps are original, you might want to change them.

Quote:

Originally Posted by hollisdevillo

thanks

schematic: http://www.schematicheaven.com/barga...rmony_h304.pdf

Hey, no problem! Welcome to the group!

hollisdevillo · 10-30-2008, 04:47 AM

great! thanks, i figured there was something wrong with that ground on R4. but why would i not trust the schematic?
yes i installed a 3 prong cord. all the caps and resisters are new. does C9 need to be removed now, is it of no use? right now it's still there.
so C5 needs to be polarized. because of the double ground i figured it was non-polarized. so could i put in say an 8mf cap instead of the 4mf? just because there doesn't seem to be a great many 4mf caps. or should i stick as close as i can. from my understanding that will give more bass? or will it lose too much tone?
thanks so much again for the reply, i'm dying to find out what this amp sounds like!

Dave Curtis, dB AudioTech · 10-30-2008, 12:14 PM

Yes clip C9 out. For C5, you can use an 8uF or more. It won't matter much AFA tone goes; it might tighten up the bass or may not make a noticeable difference.

About the 3-wire cord: you should rewire it so the hot AC (black) goes through the fuse and then the switch. One leg of the power tranny primary goes directly to the neutral (white). The way they did it back then isn't done anymore, for safety reasons.

hollisdevillo · 10-31-2008, 10:06 PM

Thanks so much Dave, it works! And it sounds great!
As it is, the white of the 3-wire cord is going to the fuse and then the transformer, and the black is going to the switch then the transformer. if i wire it as you have suggested, the black goes through the fuse, then to the switch, then to a leg of the tranny, and the white hooks up to the other leg. is this correct?

Dave Curtis, dB AudioTech · 11-02-2008, 02:51 AM

Quote:

Originally Posted by hollisdevillo

Thanks so much Dave, it works! And it sounds great!
As it is, the white of the 3-wire cord is going to the fuse and then the transformer, and the black is going to the switch then the transformer. if i wire it as you have suggested, the black goes through the fuse, then to the switch, then to a leg of the tranny, and the white hooks up to the other leg. is this correct?

That's right. I'm glad I was able to help you get it working. (yippee!)

10-27-2008, 09:55 PM	#1
hollisdevillo Member Join Date: Oct 2008 Posts: 27	Harmony h-304 I have an old harmony h-304 that i purchased in new york. The amp worked (barely) but was in need of some servicing. The schematic is on the inside of the amp and upon opening up the amp discovered it was nothing like what it was supposed to be. Wrong values and extra capacitors. Upon further investigation i discovered that it seemed to be infused with the h-304A amp, but it was neither the closer to one as to to the other. So i decided to build it back to spec. there is very little sound coming from the speaker. there is a .02 cap (C2) coming off pin 7 that has no voltages on the pot side. is this right? going through the first preamp tube with a signal tracer, it becomes very quiet. coming off the output it is very loud, but then almost nothing going through the output transformer. less sound than the initial signal. there is also a 4 mf cap (C5) that seems to be grounded on both sides. what is the purpose of this cap? thanks schematic is on schematic heaven in bargain amps section.

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10-30-2008, 04:47 AM	#3
hollisdevillo Member Join Date: Oct 2008 Posts: 27	great! thanks, i figured there was something wrong with that ground on R4. but why would i not trust the schematic? yes i installed a 3 prong cord. all the caps and resisters are new. does C9 need to be removed now, is it of no use? right now it's still there. so C5 needs to be polarized. because of the double ground i figured it was non-polarized. so could i put in say an 8mf cap instead of the 4mf? just because there doesn't seem to be a great many 4mf caps. or should i stick as close as i can. from my understanding that will give more bass? or will it lose too much tone? thanks so much again for the reply, i'm dying to find out what this amp sounds like!

10-30-2008, 12:14 PM	#4
Dave Curtis, dB AudioTech Senior Member Join Date: Jun 2007 Location: USA Posts: 301 Member's Picture Albums	Yes clip C9 out. For C5, you can use an 8uF or more. It won't matter much AFA tone goes; it might tighten up the bass or may not make a noticeable difference. About the 3-wire cord: you should rewire it so the hot AC (black) goes through the fuse and then the switch. One leg of the power tranny primary goes directly to the neutral (white). The way they did it back then isn't done anymore, for safety reasons.

10-31-2008, 10:06 PM	#5
hollisdevillo Member Join Date: Oct 2008 Posts: 27	Thanks so much Dave, it works! And it sounds great! As it is, the white of the 3-wire cord is going to the fuse and then the transformer, and the black is going to the switch then the transformer. if i wire it as you have suggested, the black goes through the fuse, then to the switch, then to a leg of the tranny, and the white hooks up to the other leg. is this correct?

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