H303a Harmony

wetordry

Member
Messages
4,950
At work yesterday, i was given a harmony h303a from someones attic. Looks like it will clean up well.
It turns on and makes sound with a guitar plugged in but has a static sound as loud as the notes, which happens with every chord/note, and rings out past the chord/note.
Poking around the net I've heard some nice sounds from these, and read about the power chord needing to be swapped for a three prong....

But where do you start with the typical, lower value, non working yard sale or barn find amp, replacing all the caps, or isolating the bad component?

Any comments about repair or amp itself appreciated.
20180928_052135.jpg

20180928_052102.jpg


A video that got me wondering....if the first section is representative of amp alone...
 
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Tron Pesto

Silver Supporting Member
Messages
920
Assuming you have the ways and means to do the checking/repairing itself, I would track down the issue and repair the specific problems. That's because I enjoy that process greatly (I learn something everytime I work on an amp), and I would prefer to keep that beautiful old amp as intact as possible.

The first steps I would take would be:
  1. Check the speaker (test amp with another speaker)
  2. Check the tubes (tube tester or replace with known good tubes)
  3. Check/clean all connectors, sockets, jacks, pots, and check solder joints. Also a general review for damaged components and bad connections.
  4. Replace the two prong with a properly installed chassis grounded three prong plug and defeat/remove any polarity circuit (this is the only "must" modification I will make to an old amp for safety reasons - in fact, in many amps, I will make that change BEFORE I ever fire the amp up, so this might be step "A"). Once again, that's if the amp is a personal amp of mine. I will give the owner the option of that replacement, urging them to do so.
  5. Fire up the amp and do a quick check of voltages and electrical performance (by "performance" I mean drifting/inconsistent/intermittent readings). TAKE NOTES (applicable throughout the process) for future troubleshooting process (or depending on findings, isolate issue and address right then and there)
  6. Once those items have been addressed/eliminated, if this was a personal amp for me, I would replace what looks like an original cap can - there really is little chance that those capacitors are viable - if this was for someone else, and it was easy enough, I would test those caps to double check, then let the owner decide if they want to spend the cash on the cap can (they can run $20-$40 so the bill would start adding up on a $250 amp).
  7. Check/replace bypass and bias caps as applicable.
  8. Check/replace B+ resistors and power tube cathode resistors (as applicable).
Once all of the above is done and the amp is not functioning the way I want it, then the "next level" trouble shooting process begins. That varies greatly depending on the symptoms being displayed - it could be anything from checking how well coupling caps are performing (checking voltages to see if they are "blocking"), to checking resistor values, to tracing the signal through the amp with an oscilloscope.

I hope that helps and enjoy the process and that cool little amp!!
 
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Tron Pesto

Silver Supporting Member
Messages
920
By the way - for me, on this "budget garage sale amp" - if I got to step 2 and the problem was fixed, I would NOT continue, barring installing the three-prong.

Actually, that's not true - I would still go through the amp and fix/replace obvious issues. Probably the only thing I wouldn't do, assuming the amp is running quiet and sounding strong/good, is replace the cap can.

Actually, that's not true either - if there was a "direct" or "close" value replacement cap can for the amp, I probably would replace it too!

It might end up costing more in parts and time than the amp is worth, but I know that's what would end up happening!!! :D
 

DonP

Member
Messages
3,682
My friend (back in the early 80's) had that same exact amp. I've used that power tube compliment (50C5 + 35W4) in some of my home brew amps, usually with a pair of 12AX7's.

Good luck with the restoration. Don't know for sure, but if it hadn't been used much the tubes might still be good, but if the attic was hot it was likely bad for the caps. Caps are the first place I'd check.

Post a picture of the guts.
 

Jeff Gehring

Gold Supporting Member
Messages
7,419
What an odd amp. It is like they didn't want to re-think the design at all when they moved from a line-derived power supply (ie, no power transformer) to the same amp but with a (unusally tapped) power transformer added. So you CAN get away with just adding a 3 conductor AC cord and grounding the chassis (you could not do that if it was a line derived supply). May be a bit hummy, since it is single ended with a half wave rectified HV supply.

You WILL need to add a fuse for it though, the 303A doesn't have one.

(edit) it gets odder: the 303B apparently went back to a line derived power supply.
 

Tron Pesto

Silver Supporting Member
Messages
920
@Jeff Gehrig brings up a great point that (shame on me) I didn't zero in on: in my Step 4 (replace the two prong chord) is NOT appropriate for ALL amps. Jeff mentions the "line derived supply" - those tranformerless amps and others that may have serial house-supply filament circuits. Those amps are dangerous to begin with (sometimes referred to as widow-makers) and require special care and approach.

I just wanted to point that out if for some reason anyone would use my steps as a guide to working on an amp. It is not a "one size fits all" method.
 

wetordry

Member
Messages
4,950
Been rushed, and had to pop one of the Phillips head screws that had rusted up...lots of bugs had nested in there, wouldn't surprise me if they had cobwebbed some new schematic paths....
20180928_195351.jpg
 
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wetordry

Member
Messages
4,950
Getting the bugs and dried flowers?? out of there reduced the static by about 70%.
Tapping the chassis also produces that static, so maybe the vibration was part of the issue too.
I have a nos 12au6 from ge, and one rca....popped the rca in there and the amp seems to be fine.

The other tubes, I don't have.
 

teleman1

Silver Supporting Member
Messages
16,010
At work yesterday, i was given a harmony h303a from someones attic. Looks like it will clean up well.
It turns on and makes sound with a guitar plugged in but has a static sound as loud as the notes, which happens with every chord/note, and rings out past the chord/note.
Poking around the net I've heard some nice sounds from these, and read about the power chord needing to be swapped for a three prong....

But where do you start with the typical, lower value, non working yard sale or barn find amp, replacing all the caps, or isolating the bad component?

Any comments about repair or amp itself appreciated.
20180928_052135.jpg

20180928_052102.jpg


A video that got me wondering....if the first section is representative of amp alone...


This was my first amp with my Gibson es 125 tc. I got rid of this amp specifically because it distorted. I wanted clean solid state. I was 13 and it was 1968, who knew? I could probably only play 6 chords at the time of sale.
 

wetordry

Member
Messages
4,950
I'm not sure if I'm getting the real deal because of any capacitor degradation, or the 50c5 and 35w4 tubes. They are Westinghouse with yellow script.
It really does sound good, and even better goosing the input a little.
Think I'll clean the pot, jacks and tube sockets, put it back together properly, clean...
And the power cord....
 

Jeff Gehring

Gold Supporting Member
Messages
7,419
I think I'd make a little eyelet board and replace that can cap with discrete axial caps, as well as the cathode bypass cap. The 50C5 and 35W4 are really cheap to buy, so you don't have to agonize over that -- you can drop a sub in really affordably. And of course the 3 conductor AC cord and added fuse.
 

JJman

Member
Messages
994
Old wax caps like that are often leaking DC. Interesting how they added a power tranny but kept the higher filament-voltage tubes by ground-tapping the secondary. Much safer yet still lower cost with only one secondary.
 

Tony Bones

Member
Messages
1,212
Getting the bugs and dried flowers?? out of there reduced the static by about 70%.
Tapping the chassis also produces that static, so maybe the vibration was part of the issue too.
I have a nos 12au6 from ge, and one rca....popped the rca in there and the amp seems to be fine.

The other tubes, I don't have.

Have you cleaned the tube sockets and tube pins thoroughly? Input jacks and speaker jack too, if there is one. And the volume pot.

Purpose made electrical contact cleaner is best, but I have cleaned tube sockets and pins with rubbing alcohol in a pinch. Use a toothpick in the sockets and a Q-Tip on the pins.
 

wetordry

Member
Messages
4,950
Have you cleaned the tube sockets and tube pins thoroughly? Input jacks and speaker jack too, if there is one. And the volume pot.

Purpose made electrical contact cleaner is best, but I have cleaned tube sockets and pins with rubbing alcohol in a pinch. Use a toothpick in the sockets and a Q-Tip on the pins.

Haven't done all that yet, but have deoxit etc and plan too when I get caught up and have some time to put it on the workbench. Also needs a good cleaning and some regluing of the contact paper/covering.

So if I look at the schematic as linked by Jeff Gehring above, a safety fuse in line before the switch, and the cord's ground wire to the chassis, is the 3 wire conversion needed?
What value for the fuse?

All help appreciated!
Just haven't been able to dive into it yet because of work.
 

Tony Bones

Member
Messages
1,212
So if I look at the schematic as linked by Jeff Gehring above, a safety fuse in line before the switch, and the cord's ground wire to the chassis, is the 3 wire conversion needed?
What value for the fuse?

A 1 amp slo-blo should be safe.

The only wire from the power cord that should be connected to the chassis is the green (or green and yellow) earth ground. The black wire should go to the fuse and then the switch. The white wire is connected to the other PT primary and ONLY to that wire, not to ground.

A 3-wire cord is easy to install, why not do it? So many of the 2-wire cords on amps this age are rotted and dangerous and need to be replaced anyway. Might as well do it right.
 

Tron Pesto

Silver Supporting Member
Messages
920
I'm not sure if I'm getting the real deal because of any capacitor degradation, or the 50c5 and 35w4 tubes. They are Westinghouse with yellow script.
It really does sound good, and even better goosing the input a little.
Think I'll clean the pot, jacks and tube sockets, put it back together properly, clean...
And the power cord....

Catching up on the thread - I think I have dozens of 50C5 and 35W4 pulls in the "Semi-neglected" portion of my tube stash (i.e. the big rubbermaid box in the garage that has the less-used varieties of tubes I haven't gotten around to throw out!!!). I should get a chance in the next day or so to grab some - I'll PM you when I find them to see if you still need any. Their yours for cheap as free.
 

Tron Pesto

Silver Supporting Member
Messages
920
So the first jack seems louder than the others...

That makes sense according to the schematic. The first jack has an effective grid resistor of 33K with a 1M resistor to ground. When you connect to the second jack, you are going through a 33K resistor, it is routed through the first jack via the switch, which is connected to the first 33K resistor to ground. It's about a 50% drop in input signal hitting the grid of the 12AU6.

The third jack has no effective grid resistor (it could potentially be marked as "microphone") and uses the same 33K resistor to ground that the second jack uses.
 




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