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Author Topic: Holmes Tech T-25, DC at speaker  (Read 4477 times)

joecool85

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Holmes Tech T-25, DC at speaker
« on: February 24, 2014, 09:16:52 PM »
I'm working on this old T-25 amp and am getting 17.3v DC at the speaker while connected to a lightbulb limiter. The Poweramp utilizes a tda2030a. I think it's blown, what do you guys think?

Of note, the mains wiring on this had been an unsafe mess. I've fixed that, now just need to make the now safe amp work...
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phatt

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Re: Holmes Tech T-25, DC at speaker
« Reply #1 on: February 24, 2014, 10:47:05 PM »
Hi Joe,

Have you checked the supply rails?   and also the preamp voltages?

There is often a big resistor pair (drop resistors) before the regulators and these get very hot if the Zener fails short it pulls the main supply rail low and may cause problems like that.
I had something like that happen long time ago to a chip amp replaced the dud Zener and the power chip was fine.

You could disconnect the psu from Amp (if possible) and see if you have equal voltages on the supply rails in case something dead in the rectifier area.
Phil.

teemuk

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Re: Holmes Tech T-25, DC at speaker
« Reply #2 on: February 25, 2014, 04:31:15 AM »
Agree with Phatt. Test supply voltages to rule them out as fault source, and test DC reference bias circuits to the PA chip assuming there are any. Without knowing details of the design its hard to point out what else to test before just shotgunning the chip, but chip failures are rather common and replacements cheap so its likely the path of least resistance to sub in a fresh one while keeping thumbs up. If it fails then it obviously needs some rethinking. That time the bulb limiter will likely save the fresh chip, though.

joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #3 on: February 26, 2014, 01:30:23 PM »
The world will never know. The customer backed out and is taking his broken amp back. Oh well, at least I get my bench fee still.

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joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #4 on: March 21, 2014, 12:10:21 PM »
Well, it came back.  The guy decided to fix it after all.  I replaced the TDA2030A with a TDA2050 I had on hand (new).  It has relieved the DC as the speaker, but still has a very loud hum (sounds to me like when I've had bad IC's on other amps).  I'm wondering if one of the 4558 opamps are bad.  Is this something else to check?  I was getting +/- 14.3v one rail and +/- 40.1v on the other...which can't be right because that is twice what the power amp chip can take I believe.  I was measuring from one end of a diode to ground in all circumstances.  Two diodes measured 14.3v to ground, two measured 40.1v to ground.  I was measuring from the top side of the board.  I think I may need to measure at the supply caps on the bottom side of the board for a more accurate reading...something seems amiss.
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joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #5 on: March 22, 2014, 10:07:28 AM »
I knew I needed sleep that night. I just got voltages straight from the legs. TDA2050 is +/- 18.7v DC and all three RCA4558 are +/- 16.7v DC.  Both are within spec.
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joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #6 on: March 22, 2014, 10:45:42 AM »
So...I decided to play through it and that solved the wicked loud hum.  I think that part was just a dirty jack. All together and working now. Glad I was right on my instinct for the source of DC voltage. Bummed my wife was right that I just needed more sleep lol.
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joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #7 on: March 22, 2014, 10:46:29 AM »
Weird amp BTW, made me feel like a real novice again too.
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phatt

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Re: Holmes Tech T-25, DC at speaker
« Reply #8 on: March 23, 2014, 06:26:38 AM »
Keep up the good work Joe,,
You are not alone.
A while back I had a mesa valve combo with a badly distorting output, Worked normal for 10 minutes then instantly went ugly.

The one owner had looked after it very well and my visual investigations turned up nothing obvious.
A pain to pull apart, and a rather complex double sided PCB.
left me scratching my head for a time  :-\

A good sleep and a new outlook.
This is a heat related issue hum? (as I sip a morning coffee)

A gentle wiggle of power tubes revealed the problem,,,dirty valve sockets/pins.
A quick scrape of pins and sockets and fixed.

Test for 4 hours,, all voltages reading normal.
My advice to owner;
If you live on the coast expect this kind of issue to happen at some point. 8)

The more experienced tech's would have picked it up without the need to pull the chassis ,,but I'll know better next time. lol.
Phil.

Roly

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Re: Holmes Tech T-25, DC at speaker
« Reply #9 on: March 23, 2014, 09:59:17 AM »
I think one of my worst moments was working inside a stereo for about half an hour before I realised that the mains fuse had blown.  :duh :-[
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Holmes Tech T-25, DC at speaker
« Reply #10 on: March 23, 2014, 10:04:39 AM »
I think one of my worst moments was working inside a stereo for about half an hour before I realised that the mains fuse had blown.  :duh :-[

Now I understand your hairstyle ;)



 :lmao:

joecool85

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Re: Holmes Tech T-25, DC at speaker
« Reply #11 on: March 23, 2014, 10:08:18 AM »
Glad to know I'm not alone :-)
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Roly

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Re: Holmes Tech T-25, DC at speaker
« Reply #12 on: March 23, 2014, 10:23:40 AM »
If you say theory and practice don't agree you haven't applied enough theory.