Need Generic Help

[Gytaryst]

I have a little Hohner CA-12R that's old and needs to be refreshed. In mint condition this amp probably isn't worth $50, (and mine is far from mint condition). I got an estimate from an amp repair place of $150-$200 to recap it and maybe replace a couple pots. I'm pretty good at soldering stuff, I've tinkered with my guitars replacing pots and pickups for years. I've never messed with amps because I don't know what any of those gadgets and gizmos are inside there, and I've heard the capacitors can kill you if you don't know what you're doing. I can clean the pots out and see if that quiets them up - if not they're not a problem to change. The amp is probably 40 years old so I'm pretty sure the caps need to be changed. I'm guessing the three cylinders I have arrows pointing to are the caps I need to change. If that's true I was hoping someone could tell me how to discharge them, (and anything else you can see in this pic that might have to be discharged), so I can swap them out. The second pic is the underside of the guts. It has a 3 prong cord.

[Leo_Gnardo]

Have we not been thru this one before? Why, yes we have! http://music-electronics-forum.com/t43400/ Are you unable or unwilling to accept help from your Phoenix neighbor DrGonz78?

Ground Hog Day is February 2.

[Enzo]

Noisy controls is one thing, you know if you cured it or not. As to the rest. Why don;t you start with what is wrong with the amp? They may be 40 years old, but is the amp acting like they need to be replaced? We want to identify all actual problems and solve them, before we start wholesale parts replacement.

Unplug the amp, MEASURE voltage across the two caps. I suspect there will be very litle if any. In fact, measure the voltage while it is powered just to see what it normally sits at. I doubt more than 25-30 volts. It should discharge on its own in a few seconds. Look at the voltage rating printed on the part, the voltage on it will not be larger than that amount.

If it has more than a few volts, you can discharge it by puting a resistor across it for a moment. 100 ohms 1000 ohms, whatever.

[Gytaryst]

Are you unable or unwilling to accept help from your Phoenix neighbor DrGonz78?

Not at all. I contacted him and we texted back and forth. We set up a time for me to drive over to his place so he could look at it, he gave me some preliminary costs of what he would charge (much less than the $200 quote I got), and told me he even had some amps for sale if I decided not to fix this one. It all sounded great - at the time. But after thinking about it, it's just a cheap little combo amp. My immediate feeling is to just chuck it in the trash and go buy something else. I'm about 99% in favor of that option. BUT . . . before I just chuck it in the trash I wanted to see if it was something I could maybe fix up myself for VERY minimal cost and even LESS effort. I already own the amp and it does work (more or less). I'm relatively handy repairing things around the house, working on vehicles, tinkering with guitars, etc. Currently I'm restoring a 1929 violin, (first time I ever worked on one of those). From my limited searching around on the internet, and talking to both the original guy who gave me the $200 quote as well as DrGonz I'm fairly confident that it probably just needs to be recapped and possibly a few pots changed out. So I signed up for this forum to see if there was someone who could look at the pictures and maybe tell me what they would do and roughly how to do it so I don't get zapped into next week. I greatly appreciate everything DrGonz was willing to do, but he lives quite a ways away in an area of town I rarely ever go to. The idea of fighting Phoenix traffic all the way there and back seems like WAY more effort than I want to put into this thing. And I'm not looking to pay someone to fix it for me - again, I'd rather just throw it out. So I'll keep looking around to see if this is something I can do myself. The worst case scenario is that I gain a little insight into the inner workings of solid state guitar amps, even if I don't get this one working.

Noisy controls is one thing, you know if you cured it or not. As to the rest. Why don;t you start with what is wrong with the amp? They may be 40 years old, but is the amp acting like they need to be replaced? We want to identify all actual problems and solve them, before we start wholesale parts replacement.

Unplug the amp, MEASURE voltage across the two caps. I suspect there will be very litle if any. In fact, measure the voltage while it is powered just to see what it normally sits at. I doubt more than 25-30 volts. It should discharge on its own in a few seconds. Look at the voltage rating printed on the part, the voltage on it will not be larger than that amount.

If it has more than a few volts, you can discharge it by puting a resistor across it for a moment. 100 ohms 1000 ohms, whatever.

Thanks Enzo, that's what I was looking for. I've watched a few youtube videos on discharging caps, (one hand in the pocket and all that). I've also heard everyone say the amp "should" discharge itself when powered off . . . then they immediately follow that up with "but you can never assume that."

I was mainly hoping to run into someone on here that has repaired so many amps that they could give me a fairly educated guess as to what I would most likely have to do to get this one working. It doesn't appear to have been abused or tinkered with so I'm guessing the issues are age related. If there's a cheap fix I can try to see if that works - then awesome. If there's not - that's okay too. I'll scrap it and get something else.

Thanks again,

Marc

[Enzo]

We can give you VERY educated guesses, and even advice that is not a guess. But you have to report a symptom or problem description.

MY labor cost you $60 an hour, no idea what your locals charge. YOUR labor costs little or nothing, so if you want to fix it up and learn something, great. A commercial shop has to charge their labor rate, which is not at all based on the value of the item. If I spend an hour on something, it costs you $60, even if the thing is worth only $25. So in that sense it probably is not "worth" fixing, at least not commercially.

I think Leo's point was more about why are there two threads on this amp, rather than Gonz specifically. No big deal.

[Gytaryst]

That other thread was in vintage amps, I probably posted it in the wrong place. It also seemed like I was advised to contact DrGonz, DrGonz joined in, and that was that. I understand charging if that's what you do for a living. I would never walk into you shop and expect you to work on my gear for nothing, nor would I go to anyone's shop and waste their valuable time asking them to give me a free education on how to do it myself. I assumed an online forum like this would be a place where people could ask question and if anyone feels like they want to help they can, if they don't, they don't. If this is a forum where people ask questions and are then pointed to members who charge for their time and knowledge, that's cool too. I just wasn't aware of that upfront. My apologies.

[Enzo]

No, it wasn't us aiming anyone at pay shops. It just happened that one of our members appeared to be real close, and PERHAPS could assist you. The only time I suggest a specific individual is in cases like this - nearness. Speaking only for myself. We assist people at all levels, once we know their level. Some of us are rank amateurs, others are industry veterans, some are in it as a hobby, and some are pros. No charge, no strings. Some of us are more direct than others, some more circumspect.

[Gytaryst]

No worries. Signing up on here and asking a question was just one avenue I decided to pursue before just tossing this thing on the scrap heap. I never have much luck asking questions on forums. I tend to be long-winded, ironically because I want to be as specific as possible so I don't get involved in lengthy threads that don't address my original inquiry. But I think being long-winded actually probably makes my original inquiry less clear, so I end up in lengthy conversations that don't address it anyway.

I really appreciate being put in touch with DrGonz and after talking to him he seems like a real nice guy. After I tried to explain what I was wanting to do we ended up with a meeting set up for me to go to his place and pay him to fix it. When I finally checked out the map it turns out he lives 32 miles from me in an area I never go to, and depending on traffic it's a 45 to 60 min drive. I apologize if I wasn't clear or misled anyone, and no offense to DrGonz, but that's not the kind of help I was looking for. Your last response was,

... MY labor cost you $60 an hour, no idea what your locals charge. YOUR labor costs little or nothing, so if you want to fix it up and learn something, great. A commercial shop has to charge their labor rate, which is not at all based on the value of the item. If I spend an hour on something, it costs you $60, even if the thing is worth only $25. So in that sense it probably is not "worth" fixing, at least not commercially..."

So it seems this thing got off track somewhere and (as is usually the case with these forums), I'm sorry I asked the question. I get that people charge for this stuff and make a living at it - I really do.

[big_teee]

Hi, I'm just a hobbyist.
Earlier this year, I bought a used Hohner Hi-15B.
It was broke, I repaired it, and it wasn't much to listen to.
I liked the cabinet with the heavy 15 inch speaker.
I scraped the solid state amp.
I salvaged the chassis.
I rebuilt it as a 2 6v6 tube plexi junior.
Now it works and sounds great.
GL,
T

[g1]

Most of the precautions you read about with regard to discharging caps is for tube amps, which run at lethal high voltages.
Solid-state amps are not usually a concern when unplugged. They will discharge very quickly. To be 100% sure, you can measure across them with a meter, and if you find any voltage discharge with a resistor like Enzo mentioned.
I see about 15 electrolytic caps in that amp (the 2 plastic you pointed out and around 13 other metal cans). You could probably buy them all for a couple dollars from somewhere like mouser or digikey. The plastic one on the left looks like the typical AC line cap that is usually removed when a 3 prong plug is fitted.
Pots can be cleaned or replaced if cleaning doesn't help. A control cleaner with lubricant is usually used for this.

[rjb]

I see about 15 electrolytic caps in that amp (the 2 plastic you pointed out and around 13 other metal cans). You could probably buy them all for a couple dollars from somewhere like mouser or digikey. The plastic one on the left looks like the typical AC line cap that is usually removed when a 3 prong plug is fitted.

OK, here's a noobish question:
Can the caps be tested while in-circuit to determine if they need to be replaced?
(Assuming that if you have to desolder an electrolytic cap, you may as well replace it.)
Or is it a given that 40 year old electrolytics (especially in a unit that hasn't been powered-on for years) are "shot"?

And oh yea, I vaguely remember a Bob Pease column- something about powering up antique radios gradually with a variac, to "reform" the electrolyte in the capacitors.... or something like that?

-rb

[DrGonz78]

Also, let's talk symptoms like Enzo suggested. Does the amp hum? Describe all symptoms and behaviours of the amp so we can understand what it is doing.

On a side note... This amp looks identical to the Marlboro amp lines from the late 70's to early 80's.

[big_teee]

Here's some of the schematics, we chased last spring.
With the hohner, it is hard to get real specific.
Most of them are similar.
http://music-electronics-forum.com/t...ghlight=hohner
T

[Junk Dr]

Gytaryst - I'll chip in here, G1 gives an excellent interpretation above of what I was going to say, this is a SS amp and does not have those lethal cap voltages hanging around after the amp is charged. As far as generic help goes I would recommend you go to YouTube and look up Uncle Doug's Vintage Amp pages. This man is a retired algebra teacher and explains things very clearly, I found him instrumental in my learning over the years.

I agree that the cap on the left in your photo is a "safety cap" in conjunction with the old two prong plug, however, they are usually a ceramic disk cap and usually are a special cap - a safety cap.. If your amp is suffering from hum (and there are two types - see Uncle Doug) then it is most likely the other two caps you have pointed out, i.e., the filter caps that take out AC ripple. As stated above, you have more electrolytics and any one of them could be dried out and cause some sort of issue. Also, as stated, you could buy all the caps you need from Mouser or Digikey or, being your in Arizona, conveniently from Amplified Parts in Tempe for a few dollars. Shipping will likely cost you more than the parts!

I am not taking a shot at your intelligence but if you are going to do this make SURE you replace them observing the correct polarity or you are likely to have an explosion when you plug it in. I recommend replacing one at a time and taking your time to double check the new is as the old was. And...rarely will there be a problem with a pot unless it has been subjected to much dust / dirt or has had a beer spilled into it! Here in Canada there is a place called Long & McQuaid that sells amps and instruments as well as repair supplies. They have a "contact cleaner" that works excellent restoring scratchy pots but beware, it is not cheap ($25 about 10 years ago). I bought my can years ago and because it works so good and you need so little of it it lasts a long time. Apply a bit of this stuff, work the pot back and forth and do it again the next day - in most cases the pot will behave like new.

Hope this helps.

[52 Bill]

Gonz makes a great point, if you look at the 2nd photo of the original post, the pc board is marked Marlboro.