Post by indietech on Feb 9, 2018 21:32:47 GMT -5
Hello GuitarNutz!
I have an Ovation-made 100w amp head from the late 70s that's having some trouble. I would love any and all assistance the hive-mind is willing to share. Details below. I'm giving as much info up front as possible, but Solid State Amp Circuit Repair is new territory for me, so I may fail to mention glaringly important information. My apologies.
Preface/ disclaimer: I am aware that amp repair can be dangerous, and is not for the faint of heart.
I am a professional Technical Director for a small regional theatre, which means I build scenery and install/operate lighting and sound gear as well as most power and hand tools available, and can hold my own behind a digital or analog console as a sound engineer for live performances.
I have training in low voltage electronics, mostly motors and the control of them. I have also successfully made basic repairs to many electrical components of electric guitars over the past 12 years, and installed humbuckers in guitars not originally fitted for them (Tele-Star H1 Hollowbody, Danelectro Hearsay in neck position). I understand a bit more than the basics of waveforms and how most audio controls/effects affect them.
I know my way around a soldering iron and multimeter, and can usually identify individual components in a schematic given some time. I can read a resistor.
All of this to say: I'd love practical repair advice. I've come across too many forum threads where the initial request for help is met with some variant of "Don't open it up. You'll hurt yourself. Take it to a professional." I believe GuitarNutz has this knowledge, and I was reminded of this forum's existence by a longtime friend who originally got me lurking on the original forum way back in the day.
On to the problem at hand:
I bought an Ovation K6424 head (100w 4ohm) for $50 from a local music store's going-out-of-business sale last summer with the power plug cut off the end of the hardwired 2-wire power lead. I purchased a 200w 8ohm speaker for $2 from a thrift store for amp testing purposes.
You can see front views of these in the final picture in THIS IMGUR ALBUM. Enjoy the gear pics that precede it, or ignore them. I'll happily talk about the othergear in a thread elsewhere if you desire.
After wiring a power plug on (wired good 3 prong Edison plug to 2 existing leads (I know I should ground the case to the pin, haven't gotten to it yet) and trying it with various guitars in front of it (Epi T-310 w/ single coils, Hondo II SG w/ possibly aftermarket humbuckers, and the Norma EG-403 with gold foils shown in the picture) I've discovered the good and bad qualities listed below. Issues are consistent across all instruments.
The good:
Reverb tank inside is almost the entire width of the head, and sounds beautiful.
EQ is super flexible and wide, Tremolo is super adjustable and has burst (more square shape) option and has enough depth to go completely silent in valleys, with a wide time range.
Channel A works great.
The kinda bad:
Channel B works not so great. With reverb turned off, no sound comes out of Channel B. HOWEVER, if Channel B's reverb is turned on, it's volume knob can be used as a sort of gain control to create breakup, that I'm guessing happens in the reverb preamp. This channel is definitely lacking in bass response though.
Using an A/B/Y pedal to split my guitar into both channels, I can use A as a clean channel and B as overdrive, and this works especially well with my Looper on the A side after the ABY.
Channel B's clipping results in a super cool vintage drive sound. Jumping A and B together (each has 2 inputs) can make some monster tones, and the tone is different based on which channel the guitar is plugged into while they're jumped.
That being said, the quirks of Channel B are obviously a result of problems in the circuit, and not a feature.
The more bad:
Engaging the reverb for either Channel A or B will occasionally result in a super loud crack/pop through the speaker. The occasional super loud crack/pop will also happen on it's own accord. I've also gotten it to consistently happen while strumming chords, and it seems to line up with my pick attack on the low E and A anywhere between open and the 5th fret, but that's only when my signal is running super hot (either boosting compressor in front and/or with the "reverb drive" cranked).
I'm aware that this could be damaging the speaker, so I haven't tried the amp with another speaker/cabinet. I have a 150w Crate cabinet ready to test it out once I think I've repaired the amp.
I've heard pops through systems with mismatched speaker/amp impedance before. My crack/pop is significantly louder than those, and does not come with the minute build of static I've noticed with impedance mismatching.
After opening it up, I found some more issues that don't bode well...
Some photos are in THIS IMGUR ALBUM. More are available upon request, it's still opened up.
A white wire and a black wire are capped near the rear jacks. There's some evidence that these got hot while touching each other; there's black residue on the white wire on the last one to two inches. These were capped before I purchased the amp.
There seems to be corrosion on the 3 components clipped alongside each other, shown in 2 pictures in the album. I think they're caps?
My knowledge of the gear as manufactured:
The Ovation K6424 head was part of a PA sold by Ovation, seemingly in the late 70's based on catalogs I can find. It was part of the Cat-III Amplifier System. The manual can be found HERE at ovationtribute.com, where I also found the catalog.
The only source for a schematic I can find is HERE at musicparts.com, listed as "offline", but priced at $20. I assume that $20 would get me a scan of the document, but I have not heard back after sending an email on January 16th.
If Anyone has any ideas regarding where to start, I'm all ears. I'll happily answer any more questions.
I have an Ovation-made 100w amp head from the late 70s that's having some trouble. I would love any and all assistance the hive-mind is willing to share. Details below. I'm giving as much info up front as possible, but Solid State Amp Circuit Repair is new territory for me, so I may fail to mention glaringly important information. My apologies.
Preface/ disclaimer: I am aware that amp repair can be dangerous, and is not for the faint of heart.
I am a professional Technical Director for a small regional theatre, which means I build scenery and install/operate lighting and sound gear as well as most power and hand tools available, and can hold my own behind a digital or analog console as a sound engineer for live performances.
I have training in low voltage electronics, mostly motors and the control of them. I have also successfully made basic repairs to many electrical components of electric guitars over the past 12 years, and installed humbuckers in guitars not originally fitted for them (Tele-Star H1 Hollowbody, Danelectro Hearsay in neck position). I understand a bit more than the basics of waveforms and how most audio controls/effects affect them.
I know my way around a soldering iron and multimeter, and can usually identify individual components in a schematic given some time. I can read a resistor.
All of this to say: I'd love practical repair advice. I've come across too many forum threads where the initial request for help is met with some variant of "Don't open it up. You'll hurt yourself. Take it to a professional." I believe GuitarNutz has this knowledge, and I was reminded of this forum's existence by a longtime friend who originally got me lurking on the original forum way back in the day.
On to the problem at hand:
I bought an Ovation K6424 head (100w 4ohm) for $50 from a local music store's going-out-of-business sale last summer with the power plug cut off the end of the hardwired 2-wire power lead. I purchased a 200w 8ohm speaker for $2 from a thrift store for amp testing purposes.
You can see front views of these in the final picture in THIS IMGUR ALBUM. Enjoy the gear pics that precede it, or ignore them. I'll happily talk about the othergear in a thread elsewhere if you desire.
After wiring a power plug on (wired good 3 prong Edison plug to 2 existing leads (I know I should ground the case to the pin, haven't gotten to it yet) and trying it with various guitars in front of it (Epi T-310 w/ single coils, Hondo II SG w/ possibly aftermarket humbuckers, and the Norma EG-403 with gold foils shown in the picture) I've discovered the good and bad qualities listed below. Issues are consistent across all instruments.
The good:
Reverb tank inside is almost the entire width of the head, and sounds beautiful.
EQ is super flexible and wide, Tremolo is super adjustable and has burst (more square shape) option and has enough depth to go completely silent in valleys, with a wide time range.
Channel A works great.
The kinda bad:
Channel B works not so great. With reverb turned off, no sound comes out of Channel B. HOWEVER, if Channel B's reverb is turned on, it's volume knob can be used as a sort of gain control to create breakup, that I'm guessing happens in the reverb preamp. This channel is definitely lacking in bass response though.
Using an A/B/Y pedal to split my guitar into both channels, I can use A as a clean channel and B as overdrive, and this works especially well with my Looper on the A side after the ABY.
Channel B's clipping results in a super cool vintage drive sound. Jumping A and B together (each has 2 inputs) can make some monster tones, and the tone is different based on which channel the guitar is plugged into while they're jumped.
That being said, the quirks of Channel B are obviously a result of problems in the circuit, and not a feature.
The more bad:
Engaging the reverb for either Channel A or B will occasionally result in a super loud crack/pop through the speaker. The occasional super loud crack/pop will also happen on it's own accord. I've also gotten it to consistently happen while strumming chords, and it seems to line up with my pick attack on the low E and A anywhere between open and the 5th fret, but that's only when my signal is running super hot (either boosting compressor in front and/or with the "reverb drive" cranked).
I'm aware that this could be damaging the speaker, so I haven't tried the amp with another speaker/cabinet. I have a 150w Crate cabinet ready to test it out once I think I've repaired the amp.
I've heard pops through systems with mismatched speaker/amp impedance before. My crack/pop is significantly louder than those, and does not come with the minute build of static I've noticed with impedance mismatching.
After opening it up, I found some more issues that don't bode well...
Some photos are in THIS IMGUR ALBUM. More are available upon request, it's still opened up.
A white wire and a black wire are capped near the rear jacks. There's some evidence that these got hot while touching each other; there's black residue on the white wire on the last one to two inches. These were capped before I purchased the amp.
There seems to be corrosion on the 3 components clipped alongside each other, shown in 2 pictures in the album. I think they're caps?
My knowledge of the gear as manufactured:
The Ovation K6424 head was part of a PA sold by Ovation, seemingly in the late 70's based on catalogs I can find. It was part of the Cat-III Amplifier System. The manual can be found HERE at ovationtribute.com, where I also found the catalog.
The only source for a schematic I can find is HERE at musicparts.com, listed as "offline", but priced at $20. I assume that $20 would get me a scan of the document, but I have not heard back after sending an email on January 16th.
If Anyone has any ideas regarding where to start, I'm all ears. I'll happily answer any more questions.
to The NutzHouse!
Only when it's in natural forum view for readers does it go T.U.. That's a bug if I've ever seen one. Sigh.