´75 Orange OR120... in rough (!) shape

[0 2339]

hey gang, hope you're all doing alright! couple weeks ago i found an amp i've been dreaming about for a long time. i had the money and a quick research about the shop selling it looked promising, so without many questions and blinded by gas i took the opportunity and purchased it. I didn't have a matching cab when it arrived so I just quickly plugged into my underpowered 2x12 to make sure everything's working. And man, it did work and sound beautiful and with all possible self discipline known to mankind I disconnected the amp before overloading the little cab.
It's been busy around here, so I didn't find the time to check bias and have a quick look under the hood... until today, and it seems the next firing up will be further out than imagined
The saying goes a picture says more than a thousand words, so without further ado I present my beaten, through hell and (hopefully soon) back 1975 Orange OR120

I'm now in the process of writing down a parts list of all things I'm gonna replace and figure out a plan of how to procede. still not sure if it wouldn't be best to rewire the entire amp and replace anything that looks suspicious. I would definitely go that way if I had a layout plan, so I might attempt to draw one - or I'll be going through the amp section by section and replace where necessary along the way. I'll be ordering pretty much all passive components there are (tube sockets, pots, switches, wires, fuses, capacitors, resistors...) as I don't mind stocking up on parts.

So far I already did some mod & maintenance work on amps and built 3 kits (that came with nice schematics & layout plans though...). I know about general safety when working on tube amps and have what I would call decent basic equipment.
I really hope you guys can help me out with this project as there are no reliable amp techs that I know of around, plus I like the challenge. I know I'm a bit in over my head at the moment, but I'm positive it'll make a mighty fine amp one day! any tips, guidance, gutshots of your OR120s, schematics, layouts... really anything, might help big time and would be much appreciated!!

thanks & have a good one

 

[Mongo Park]

If it works, check the bias and see where you sit. It looks burned on the metal and some wires, maybe I am wrong. The resistors around those two sockets aren’t much to replace the stuff around the two sockets. Make a assessment to see if the big caps need replacement and you should be good to go. If you replace everything you kinda loses your purchase price. If everything needs replacing I would take it back to where you bought it or sell it as is and get one that is up to your quality expectations. Or build one yourself.

 

[Lowerleftcoast]

With vintage stuff I always find it difficult to decide whether to restore the item or keep the scars/patina and make it functional.

My initial thought for this Orange is to not go overboard. I would like to see it as a good runner rather than a museum piece. That is just my opinion and you may want to go a different direction.

It probably needs a cap job. It looks like it may not have the original e-caps. Many times you can find date codes on the caps which will give an idea on when/if work was performed.
I would like to see the octal sockets match.
Chances are the jacks may need to be replaced. (Cliff jacks on the Marshall products many times have switch contact problems. I would think Orange would have the same kind of trouble.)
Usually resistors and poly caps only need to be replaced if they are malfunctioning.

Anyway that amp looks like a beast. Great find. I can see why you had GAS for it. I bet Bass players want it even if they don't want to move it.

 

[CV Jee Beez]

Congrats! That's awesome.

I am a big fan and would add my worthless opinion. If to sounds good, biases well, and the caps aren't leaky, leave it alone.

I would assume it sounds great now and you;dnt want that to change. Anyways, I like relic guitars so it follows relic amps are cool too.

 

[0 2339]

hi again, been some time... Thanks for the input guys! I finally was able to do some progress on this one. Initially, I decided to just replace caps, tidy up the wiring and clean through, but the closer I looked at how things have been done in the past the more obvious it became that I wanted to give it a proper overhaul. There have been just too many small issues that added up and before I knew it I arrived at this point...

Things have been busy around here again but some days ago I could start to rewire that whole thing. And after enlarging the output tube socket cutouts I could even fit in shiny new sockets...

little by little it starts to look like something that's gonna do lots of wall shaking some day

I decided to come back to the chassis after taking care of the PCB - seen enough orange the last 2 days
So if anyone has some advice on how to take care of the PCB I'm all ears. I will definitely remove (dremel) all burnt areas around the rectifier circuit but I'm still a bit undecided how to go from there. I was thinking about rebuilding the cutouts with some epoxy, redrill & resolder but not really sure. Maybe also renew the original circuit tracks underneath? Would be great to get your opinions!

couple additional info:

- I didn't yet check all wiring I did so far but missing parts/wires are noted and will be addressed after the PCB is taken care of

- some wires appear quite thin - they aren't output filament wires are AWG18 for instance... bought some thinly insulated wires to still have some room as I decided to stick with the original layout

- I did draw up a layout plan. Since I haven't got this amp up and running and it's not tested I didn't want to post it. If somebody would like to have it feel free to pm me


I think that's all for now, hope to hear some opinions!
Cheers

 

[sds1]

Wow that chassis cleaned up real nice!

 

[0 2339]

Wow that chassis cleaned up real nice!

yea, quite a difference

 

[Lowerleftcoast]

So if anyone has some advice on how to take care of the PCB I'm all ears.

Most of the board looks OK. I think the repairs look fairly typical. I would check them and maybe redo them if they look too dodgy. The rectifier area looks pretty shabby. Of course it has not been cleaned up yet so the actual damage may not be as bad as it looks. Even if it were not damaged, I would consider ditching the individual diodes in favor of an *all in one* style bridge rectifier.
So, my thoughts are to modify that part of the board with eyelets/turrets if the damage is not reasonably repairable. There seems to be enough room to mount an *all in one* style bridge rectifier on that section of the board so I would be trying to figure how to incorporte a mounting point with sufficient cooling space to make that happen.

 

[mountainhick]

So, my thoughts are to modify that part of the board with eyelets/turrets if the damage is not reasonably repairable.

Duh, moment for me. The simple things I mighta never thoughta.

 

[0 2339]

Most of the board looks OK. I think the repairs look fairly typical. I would check them and maybe redo them if they look too dodgy. The rectifier area looks pretty shabby. Of course it has not been cleaned up yet so the actual damage may not be as bad as it looks. Even if it were not damaged, I would consider ditching the individual diodes in favor of an *all in one* style bridge rectifier.
So, my thoughts are to modify that part of the board with eyelets/turrets if the damage is not reasonably repairable. There seems to be enough room to mount an *all in one* style bridge rectifier on that section of the board so I would be trying to figure how to incorporte a mounting point with sufficient cooling space to make that happen.

thanks mate! yea, the rectifier area is burnt through on both sides so that has to come out. I already thought about the enclosed bridge rectifier but reading about issues people had with them made me shy away. so I guess I'll stick with the 4007s I have lying around...

Duh, moment for me. The simple things I mighta never thoughta.

lol, same here didn't think about turrets either

 

[0 2339]

hi there, quick question: upon checking the wiring of the amp I noticed that I switched the 2 OT primary wires coming off the output tube's plates - stupid, but happened and so i could expect nasty positive feedback... which I'm not too fond of

as I already had to splice and solder the OT wires, they're neatly dressed and unfortunately too short to just switch between the 2 output tube pairs I was wondering if it was possible to also switch their feeds? The two tube pairs get fed by the V2B's chathode (V2B-3) and anode/plate (V2B-1) respectively so I was wondering if I could just swap these 2 wires as well... EDIT: they'd be switched starting after C24/R22 and C23/R23 - that's where the wires leave the PCB...

Schematic attached

thanks again

 

[sds1]

Yep that will work.

 

[NTC]

Is therw feedback around the output section? I don't see it, though I am probably just missing it. If there ISN'T feedback, It doesn't matter which ot wire goes to which tube...

 

[zeppelinofled69]

Is therw feedback around the output section? I don't see it, though I am probably just missing it. If there ISN'T feedback, It doesn't matter which ot wire goes to which tube...

The boost circuit (containing VC4 - right below v2A) is the feedback for the power section. No line drawn directly to the output section, but it is labeled as feedback and terminates at the 16 ohm OT tap.

 

[0 2339]

thanks guys! yes, there's a NFB loop fom the 16ohm tap fed back to V2A via R18, as zeppelinofled69 pointed out... "boost" in orange speak

edit: sorry about the crappy quality schematic I posted - updated with a neater one

 

[wangdaning]

Can you swap the 16 ohm and common on the OT?

 

[0 2339]

would be possible, but switching the 2 wires coming off the PCB and feeding the output pairs is easier as they're not hooked up yet. besides, I think the impedance switch and output jacks are the last original parts that haven't been messed with in the last 50 years

btw, i didn't know about that being a possibility and I can't quite wrap my head around it... Thanks for the idea mate

 

[wangdaning]

Not sure how it works with multiple taps, but it is easy if you only have two secondary wires. Larger minds would know better with the switch thing going on.

 

[0 2339]

aaand back with another point I'd like to hear your opinions:

PCB is pretty much done, there are a couple spots that look a bit dull and I'll adress that before reinstalling, but in general it's not too far away from being mounted again.
As the traces were showing their age, I started cleaning them with loads of isopropanol alcohol, flux & solder. Some spots were easier than others, but they all cleaned up eventually. I then reapplied quite a thick layer of solder back on the cleaned tracks as I didn't know for how long the traces would have been sitting unprotected.
Now I'm a bit undecided if I should remove all the solder again and seal the traces with some PCB lacquer I have laying around or if I should just leave the layer on and call it a day... Probably not a huge deal one way or the other, still I'd be curious what you think!

edit: traces are in good shape, had only 2 or 3 spots where they lifted from the board - those areas have been adressed already. They are also quite thick and I would guess they'd have more overall area than any of the cables I'm using... just wanted to add this if it makes a difference for someone

here's the PCB with new (and correct values!) electrolytics installed - btw, don't hold back on criticism if you think something's off

right clicking the pics and "open in new tab" should give you a much bigger picture

cheers & thanks

 

[0 2339]

deleted double post