• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Model 18 fire!

Richs_Trains

Richs_Trains

Well-Known Member
Hey all.

I was trying to re-cap my Model 18 that was just repaired and working fine. Well I got through most of it without an issue until the last section I did. I replace C-20, C-21 and C-22, unfortunately I installed one of these backwards. My question, I dbt'd the unit it passed fine, I put it on main power and everything was fine and it played at low volume for about 15 minutes and all of a sudden poof, smoke and then it caught fire. I pulled the power and blew out the flame, it took out R24 a 68 ohm 1/2 watt and smoked it gone. The fire managed to cut the dial cord also, but that's a different issue. It looks to me, and I am really not any good at understanding circuits ( I have recapped and adjusted amps, but I usually have a real tech work on them when something else is wrong), that this section is a rectifier/power supply for 36 volts DC. I have since replaced the resistor and the unit is currently playing on the dbt, and all seems well. What else may have been damaged as a result of my error and why did it take 15-20 minutes of playing time before it went poof?

WP_20170209_19_35_41_Pro.jpg
 

Attachments

Register to hide this ad
Ouch. I wouldn't have 3 or more Posts on the same topic. Try to condense them to make it easier for others to respond and provide guidance!
 
You might have gotten away lucky.
If all the cooked and/or reversed parts were replaced you might be good. I would make some effort to clean out any carbon residue with denatured alcohol and whatever you need to access it (qtips, toothbrush?)
Sounds like you need a dial string too.
Edit
Wait until alcohol has completely evaporated before you power back on.
 
Hey - I think we have all put in caps backwards at least once, at least I have (got smoke but no fire). If it is operating and sounds fine I would check the bias and DC offset but would also check the schematics to check any transistors downstream and measure there to make sure all is as it should be. Too bad about the dial string - those are a PITA.
 
bryans12v said:
Ouch. I wouldn't have 3 or more Posts on the same topic.
Click to expand...
Well, the first post was supposed to be a success story, but when it failed after 20 minutes and caught fire, I was pretty desperate for help and I posted the resistor question which got some traction and I was able to work out the problem I created with help. I posted this one because I thought the title would attract more views and I might get the answer to my questions for my understanding.
1. What circuits was I affecting with the short I created?
2. Am I correct that the circuit in question is a rectifier/power supply for 36v?
3. Why did it pass the dbt and then take 20 minutes to blow?

The answers are really for my understanding for the future (learn fro your mistakes, and I learn a lot).

I have been running it on the dbt for about 1/2 hour and all still seems fine. Thanks for everyone's help! Rich
 
ringading said:
Too bad about the dial string - those are a PITA.
Click to expand...

I actually had re-strung it last night and it was working, but the string wasn't the right kind, it was designed for old tube radios and was too thin and twisted, not braided. I ordered some thicker, braided cord and I hope it works, I'm just not sure what I should use. Thanks Rich
 
nj pheonix said:
I would make some effort to clean out any carbon residue with denatured alcohol and whatever you need to access it (qtips, toothbrush?)
Click to expand...

The circuit was actually built between two terminal strips and I scraped it back to brown with a razor knife and stripped of the burnt wires and re-terminated them. Thanks for the help! Rich
 
The cap took its time shorting out. I have also made that mistake. Mine started bulging and i shut it down before it melted together inside.
Its a great lesson and shows how important it is to wear glasses when firing these up with the top off and your head in there.

It did not draw enough current to brighten the light at first because even though it was pissed off, it took a ten minute rest before blowing.

It really comes down to knowing exactly how to read your dbt. I had an assbackward cap in one board of my 300dc and it passed the dbt as far as i was concerned. It fooled me until i fixed the unit and used a higher watt bulb.
The lesson for me was to be more careful and not to talk to people when i'm working. I call it "
Satans happy time" when my hands are off doing something stupid while my minds on something else. I should have known better. I never work on transmissions with a customer in the shop.
That is when i pull out the. " Who the hell did this" & "The guy that did this, must have had his head up his ass" expressions, ....knowing it was me.

The difference between a boy and a man is a man fixes his mistakes and nobody can look down on you for that.

Keep pushin till you break through. This is an ongoing lesson everytime i touch an amp or receiver.
It takes years of schooling to learn this stuff Rich, and experience comes from working through mistakes and problems.

Use a 100w bulb and see how it acts.
Jk
 
KutzlerTrans said:
Use a 100w bulb and see how it acts.
Click to expand...

Thanks! I have had it on a 100w for about two hours, it is playing fine, but I'm afraid to put it on main power after my little episode. The 100w bulb has a soft glow and gets just a little more glow when I increase the volume, what do you think? If it's okay, I will certainly feel like I dodged a bullet, the dial string will be a PIA, but gotta get it done! Thanks Rich
 
KutzlerTrans said:
even though it was pissed off, it took a ten minute rest before blowing
Click to expand...

and it was quite pissed, I work on them on the kitchen table (my wife has alzheimer's, so no one to sit at the table) and I was at the sink, it was just playing along and wham, a fizz, smoke and the flames coming through the mesh screen, surprised I didn't break something running over to unplug it. Anyway thanks for all your help in the past, present and hopefully future. Rich
 
Maybe you should keep a fire extinguisher handy or at least some marshmallows
Sorry, I couldn't resist.:dunno:
I'm reasonably confident you're good to go now.
Don't sweat it. The only way you never make mistakes is if you never do anything.
It's a good lesson.
 
I would recommend removing the dbt if its playing. The dbt will knock your voltages out of whack and underpowering the unit while playing it can't be good for it if it isn't balanced.
If your worried, put a smaller fuse in it.
 
Well, I connected it to main power, only for a few moments as I noticed a hum through the speakers as soon as the unit powers on and before there is sound (it takes 5 or so seconds for the sound to 'come up') and when going through the switches the hum is very noticeable when the 'low filter' is depressed. Any thoughts or recommendations?
Thanks! Rich
 
I am glad to hear that you have it powered up again!

Since you smoked R24 you may have also damaged the cap that was in backwards, probably C22. It may be a bit leaky now. As a test you could install the original cap for C22 and see if it improves. I guess the rectifier diodes CR6,7,8 and 9 may have an issue as well. I think they are the big ones near the back wall.

In any case I think the parts that could be bad are limited to the power supply in the -36V section.
 
txturbo said:
In any case I think the parts that could be bad are limited to the power supply in the -36V section.
Click to expand...

Thank you Txturbo for the advice, I will check it tomorrow when I get a chance. Can I test the diodes in circuit with a DMM? Thanks for your help with the cap list and hopefully fixing my mistakes. I hope you are right about it only affecting the power supply. Rich
 
As i mentioned earlier
I'm thinking you replaced everything that was reversed. Is the hum in just one channel or both. Based on the event, its possible something up or down stream took a hit
 
nj pheonix said:
Do you have a scope?
Click to expand...
No scope and really not a tech, I did not replace the cap that was reversed as it was an expensive vishay axial and I only ordered what I needed and did not 'expode'. I am going to try the old cap in it's place when I get a chance today and see if it has any effect. The hum is present before the power amps power up ( as soon as you turn it on, it is in both speakers, doesn't increase with the volume, but does when you press the 'low filter' button. Thanks for all your help everyone! Rich
 
Not to rain on your parade. If you can't isolate the noise to a part or an area, you'll be stuck replacing things until it goes away. (Truth be told i'm not a tech either. I have enough knowledge and tools to get myself into trouble and usually (sometimes with assistance) get myself out of it).
 
You must log in or register to reply here.
Back
Top Bottom