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Author Topic: New Member and a question  (Read 8330 times)

BrianS

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New Member and a question
« on: June 29, 2014, 01:00:07 PM »
Hello SSGA folks, new member here.

My name is Brian and I am a luthier/guitar tech/amp repair tech.  I went through a 2 year "electronic music technology" course about 20 years ago here in Red Wing, MN.  I also took a guitar repair course and have been a luthier building folk harps for Stoney End Harps for the past 17 years.  Recently I've decided to quit the harp business and focus on guitar/amp repair.

My main focus has been on tube amps, but I'd like to get better at repairing solid state devices, as there are not too many folks around my region that do that.  Needles to say, I'm a little rusty on my solid state theory, although I have all my notes/text books from school and had good teachers, so I'm hoping to get quickly back in the saddle so to speak.  Also, I've always been into tube amps, so I have decent trouble-shooting skills/equipment and know how to solder quite well.  I have fixed several pieces of solid state gear over the past several months as well, and have some power amps coming in this week.

Anyway, yesterday I picked up a couple old, not working guitar amps just to mess around with.  The one I will ask a question about here is a Heathkit TA-16, and it mainly concerns getting some new parts.  So...

1.  The power transformer seems to be bad.  The 120vac is getting onto the primary, but I only have 3.4vac across the secondary.  I know I can't get a direct replacement for this PT...or at least that would not be cost effective at all, so is it possible to find a suitable substitute?  How would I go about finding this?

2.  As you probably know, these amps came as kits for the customer to complete.  This one is put together pretty well, but the bridge rectifier circuit is pretty messy.  I think I'd like to replace it with a chip, if possible.  What do you think of that idea?  Would I be better off just buying the correct diodes and rewiring that way?  What chip or diodes would you recommend?

Once I get power on the board, we'll see where I can go from there.

I have accounts with Mouser, MCM and Parts Express, fyi.

I've really enjoyed browsing through the forum and I look forward to being a contributing/learning member.

-BrianS

BrianS

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Re: New Member and a question
« Reply #1 on: June 29, 2014, 01:08:42 PM »
I got to thinking a little bit and decided to disconnect the primary wires of the PT from the amp and measure that way.  Doing this, I get 29vac.  The highest DC voltage that the amp uses is 39 volts, so I guess my PT is probably good and I have a shorted component in the power supply?

Also, this amp has circuit breakers instead of fuses, and they seem to be a little suspect.  I'm thinking of jumpering across them, one at a time, to see if that helps.  Sound too dangerous?

Thanks again!!

J M Fahey

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Re: New Member and a question
« Reply #2 on: June 29, 2014, 01:37:33 PM »
Quote
I got to thinking a little bit and decided to disconnect the primary wires of the PT from the amp and measure that way.  Doing this, I get 29vac.
What does that mean? :duh
Please draw a diagram of what you are doing.

g1

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Re: New Member and a question
« Reply #3 on: June 29, 2014, 02:23:40 PM »
I got to thinking a little bit and decided to disconnect the primary wires of the PT from the amp and measure that way.  Doing this, I get 29vac. 
I think you mean the PT secondary?  So when it's connected to the board it reads 3.4VAC, but when disconnected it reads 29VAC?
Is the fuse blowing?

BrianS

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Re: New Member and a question
« Reply #4 on: June 29, 2014, 02:24:09 PM »
Whoops, typed too fast.  I disconnected the SECONDARY wires from the amp!!  So, I'm basically just measuring what the PT is doing.  120vac on the primary & 29vac on the secondary.

That makes more sense, right?!  Sorry... xP

BrianS

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Re: New Member and a question
« Reply #5 on: June 29, 2014, 02:25:55 PM »
There are no fuses, just circuit breakers...and they are not popping.  I can't be certain that they are functioning correctly however.  This amp is in pretty rough shape.

Jack1962

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Re: New Member and a question
« Reply #6 on: June 29, 2014, 04:31:08 PM »
test the diodes in the rectifier and the filter caps.

J M Fahey

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Re: New Member and a question
« Reply #7 on: June 29, 2014, 07:20:39 PM »
OK, you must test step by step.
1) You started fine, testing the transformer.
Hope you downloaded the schematics shown in http://www.ssguitar.com/index.php?topic=1411.0
2) now with amp off and unplugged from the wall measure PSU diodes, D102/3/4/5 .
Build a lamp bulb limiter (search SSGuitar) , plug the amp there and turn it on.
I *guess*  it will shine quite bright, which means a short :(
Measure voltage across C104 , Q9 and Q10 ,  in each case CE (collector to emitter) and BE (base to emitter).
Post results.
Remember to include polarity, "+20V" and "-20V" is not "20V".

BrianS

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Re: New Member and a question
« Reply #8 on: June 30, 2014, 01:18:58 PM »
Two of the diodes in the rectifier were shorted, so I need to rebuild that before I go on.

I do have a bulb limiter and use it religiously.   I also have the schematic for the amp in question.

I will post results as soon as I have them...

J M Fahey

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Re: New Member and a question
« Reply #9 on: June 30, 2014, 01:51:45 PM »
Way to go.  :dbtu:
Try to see if you can "separate" the +38V source (the + end of the big filter cap) from the rest of the circuit.
Don´t know if there´s a wire joining them or you have to cut a track, the idea is checking that you can reach those +38V without load.
Add a 2K2 or 4K7 1W resistor across that cap so it self discharges even without external load.
I suspect you have 1 or 2 dead power transistors, let´s see.

BrianS

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Re: New Member and a question
« Reply #10 on: June 30, 2014, 06:19:55 PM »
Well, I replaced all the rectifier diodes with a single bridge rectifier "chip"; got rid of the circuit breakers (one was bad) and added a mains fuse and installed a 3 prong power cord.  Plugged it into the leak light, which did not light up, and checked my voltages.  All were just a few volts off thanks to the leak light. 

I also measured for DC voltage across the speaker leads and had around 5 vdc, but that steadily dropped down to less than 3 volts, so I decided to plug a guitar in...and the amp worked! The normal channel works fine and the reverb channel won't work unless you jumper the footswitch jack.  I could not get the tremolo to work, and there is no reverb tank, but I did get a dry guitar signal with the jumpered jack.  I don't have a footswitch, or a "stereo" plug that the footswitch uses, so I'm totally sure that the tremolo doesn't really work, or if it is just that I don't have the proper plug.  More to tinker with I guess.

Once I get the reverb channel working, I think I will add a speaker jack & build a head cabinet for this thing, as the original cab was basically disintegrated when I received it...and I finished the process getting the chassis out :trouble.

I'll post some pics and sound clips if anyone is interested.  Thanks for your help!

Oh, I forgot to ask, is there an easy/quick way to determine the correct fuse to use in this amp?  Right now I just have a 3A fast blo fuse in there, but I know that is probably too large to do any good.
« Last Edit: June 30, 2014, 06:21:44 PM by BrianS »

Enzo

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Re: New Member and a question
« Reply #11 on: June 30, 2014, 11:29:22 PM »
Is the fuse not on the schematic?

J M Fahey

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Re: New Member and a question
« Reply #12 on: June 30, 2014, 11:44:45 PM »
Not in this case, it used circuit breakers.
They must have been on sale, it has a 2.11A one in the secondary which is probably fine, and same 2.11A on the primary which is ridiculous.
It´s an around 25W amp (30W if optimistic) so I guess a slow blow 500mA or 750mA would be fine there.
So T500 or T750 .

Roly

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Re: New Member and a question
« Reply #13 on: July 01, 2014, 11:32:04 AM »
Hi Brian, welcome.


Quote from: BrianS
DC voltage across the speaker leads and had around 5 vdc, but that steadily dropped down to less than 3 volts

This is a bit of a worry that needs to be explored.  Please disconnect the speaker until we have this fully explained - it implies that there is still something wrong with the output stage (perhaps the cause of the rectifier failure?).  Output residual DC must be less than half a volt absolute max, normally only around 100mV.

Is it sounding a bit weak and more distorted, not fully clean?

A failed transistor in the output stage may put a high DC voltage and resulting high current on the speaker, cooking its voice coil.
If you say theory and practice don't agree you haven't applied enough theory.

BrianS

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Re: New Member and a question
« Reply #14 on: July 01, 2014, 01:11:51 PM »
^Thanks.  I was pretty sure there shouldn't be much, if any DC on the speaker, but this is not something that I deal with when repairing tube amps! 

Yes, the amp distorts fairly quickly...maybe around 5 or less on the volume knob.  The distortion is a decent "crunch", though, so I' wasn't sure if this was normal or not.

The amp is fairly noisy, too...I'm sure it needs some new caps and maybe some of the old carbon comp resistors should be replaced.

I will test the output transistors today...

 

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