First time build- backwards filter cap

[goldenmonkeycolor]

I bought an amp kit from ampmaker.com a few years ago and just looked at it again after it not working and some resistors burning up on my first power on. Ampmaker has NEVER answered my emails (even tried sending from different addresses to trick him... he'll respond once then not provide support- terrible business for real)

Anyways, I noticed I had a filter cap installed with the polarity orienteded incorrectly. ON the schematic it is C18. i also see that resistors r28 and r29 are burnt up.

my question is... will this mistake have ruined any other parts of the amp like the transformers? i didnt turn it on for but a couple seconds and saw smoke from the resistors and flipped it off.

should i try to replace these parts and give it a go or will it be more complicated than that?

I appreciate the help for someone that doesnt know hardly anything- because id hate to scrap what could be a decent practice amp for no good reason.

[xtian]

Replacing the burnt resistors and the incorrectly oriented cap is a good start. But we have no way to know if it will work after that...that's why we use a careful start-up procedure, using a light bulb limiter to prevent disasters like this from happening.

[printer2]

If you could not smell the transformer smoking it probably is ok. With a short they take a little while heating up and seems like your other parts took the brunt of the mistake.

[ampgeek]

I don't think that you have anything to loose but a couple of quid in replacement parts to find out!

Personally, my gut feeling is that it is unlikely that you have completely lost your investment if you didn't hoover too long with the reversed filter cap...but...exactly how much has been lost really remains to be seen.

If it were me, I would buy replacements (and maybe a couple of spares) for everything from the UF4007 diodes down to C-17, replace the obviously failed resistors and C18, study the "Light Bulb Limiter" sticky here (build it and use it!) and fire it back up to see what you have.

For "extra credit" (and maybe the elimination of head aches later), just rebuild the whole power supply section with the new parts and start over with confidence that everything there is fresh and probably good-to-go.

Interesting....I don't think that I see a fuse anywhere in the schematic. Is it in the "L" leg off of the AC power input? The schematic is too small for me to tell.

Good luck!
Dave O.

[Littlewyan]

Bit odd that Barry hasn't got back to you, hes always been great with me. He was on holiday not too long ago I know that. What email address are you using?

[martin manning]

If R28 and R29 are toasted that suggests something drawing too much current from the "B" node. Check the resistance to ground from there, and check your wiring in the power section carefully.

[Tillydog]

I had a filter cap installed with the polarity orienteded incorrectly. ON the schematic it is C18. i also see that resistors r28 and r29 are burnt up.

C18 reversed would overheat R28 & R29. I very much doubt that any other damage would have been caused, especially as you switched it off quickly (R28 would have limited the current somewhat).

Replace C18. You can check R28 & R29 with a meter and if they measure OK, you could leave them there for now. (If you have to order the replacement for C18, replace the resistors anyway - they only cost pennies).


Sound advice regarding the next startup above.

Again, I'm surprised that Barry hasn't got back to you...

[Structo]

I think if it were me, I would rebuild the power supply.
Caps, resistors and diodes.

Unless you are pinching pennies.

You may blow more components if they are compromised by the earlier mistake.

Just my 2 cents

[tubeswell]

Polarised caps need to be installed observing the correct polarity w.r.t. the (positive or negative) voltage-potential . The dielectric compound quickly (read 'instantaneously') breaks down if the polarity is reversed, resulting in a shorted cap, whereupon the HT supply ends up going straight to ground at that point in the circuit, thereby putting more demand on the PT to supply current, causing the latter to heat up (and burn up if the mains fuse doesn't pop in time). Like what the others said, replace burnt resistors and the shorted cap, and see if the thing goes without letting any more smoke out.

[Structo]

Yeah we used to blow up electrolytic caps to scare the neighborhood kids.

120 VAC across a cap will make it go bang every time.

[goldenmonkeycolor]

I appreciate all the responses! I will definitely read up and build the light bulb limiter and go for replacing the power supply components.

thanks so much for all the help! ill report back when i try a safer start up!