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Author Topic: New Member and a question  (Read 8529 times)

BrianS

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Re: New Member and a question
« Reply #15 on: July 01, 2014, 04:31:02 PM »
Both output transistors test bad using the ohm setting on my DMM.  One seems to test "worse" than the other, but that's kind of speculation on my part.  The PNP transistor fails all tests:  Emitter to base, base to collector, Emitter to collector, reversing the leads...I get conduction with all these tests.  The NPN transistor passed the emitter to base and base to collector test, but not the emitter/collector test.

Guess I need to hunt down some new transistors...

Roly

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Re: New Member and a question
« Reply #16 on: July 03, 2014, 11:37:12 AM »
Protip: remove and test both the driver transistors for the output pair.  With s.s. power amps whenever you find a dead semicon, particularly the very common dead output one(s), you really need to look at its driver as well.  If either of these are dead go for a snoop back to the Voltage Amplifier Stage/bias network, protection transistors, and so on.


Normally what happens when an output transistor fails is that the amp suddenly starts producing a very loud mains hum, it is generally switched off in panic within a few seconds, and this generally saves the rectifier (and power transformer), but not the reccy in this case, so it may have been flogged or left on for some time post-failure and we have to be more alert for other collateral damage, e.g. a driver transistor or two, the occasional roasted resistor, &c.

You must use the limiting lamp until you are quite certain that the output stage is again re-balancing the output half-rail to zero (or very near zero) volts across the speaker connection.  Together with only a dim lamp this balanced condition is a pretty fair indicator that the power amp section is working as it should.

Being DC and direct coupled s.s output stages look more like a hairy-legged op-amp than a valve output stage, and they are less forgiving of mistakes, but the use of a limiting lamp is the main difference in servicing.  Unlike a valve amp the output of a s.s. under service should be left open until you measure less than half a volt on the output.  There may be some other tips but really it's just more electronics (and you already know how to deal with that around valves, right?)

There is some physical stuff about insulating washers when mounting semicons on heatsinks (and sometimes live heatsinks, e.g. a fat Ashdown I worked on).

A while back I knocked this page up to help people deal with s.s. amps (particularly those who may already have some valve experience);

http://www.ozvalveamps.org/repairs/solidstateamprepair.htm

HTH
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: New Member and a question
« Reply #17 on: July 03, 2014, 11:46:09 AM »
Pity this thread was called new member and a question which means absolutely nothing.
I don't even remember what amp are we talking about and it definitely will not show in a future search for somebody looking to repair a similar one.
It would have been much better if it were labelled : New member and a problem with xxxyyy amp or whatever.
In fact you can skip the new member bit ... we *all*  were one once.
Just sayin'

Loudthud

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Re: New Member and a question
« Reply #18 on: July 03, 2014, 11:50:03 AM »
Roly, Please look at the schematic. This amp is a single rail design with an output capacitor. The 3V on the output is just leakage from the output cap.

Brian, check the voltages around those output transistors and compare to the schematic.

Roly

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Re: New Member and a question
« Reply #19 on: July 03, 2014, 12:19:06 PM »
Roly, Please look at the schematic.

Circuit?  {rummagerummage}  Circuit??? {rummagerummagerummage}

Ahh ... there it is ...

http://ctgelectronics.weebly.com/uploads/3/1/6/6/3166248/ta-16_schematic.pdf

 :tu:

Oh, it's only a baby; yeah a new OP pair should fix that (but I wonder what the basic cause was?).

With that level of leakage the OP coupling cap might be worth a close look as well.
If you say theory and practice don't agree you haven't applied enough theory.

Loudthud

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Re: New Member and a question
« Reply #20 on: July 03, 2014, 01:03:12 PM »
I'm worried about those output transistors. My data says the 2N2148 is Germanium rated at 12.5W. It won't look good on most DVM diode test functions. I have no idea on the TA2577. There isn't much drive current available to Q10 so it needs high beta.

Roly

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Re: New Member and a question
« Reply #21 on: July 03, 2014, 01:22:37 PM »
Quote from: Loudthud
Germanium

Oh oh.  Well spotted.   :dbtu:

{We really must get that repair form thingy going so (I'm) on the same page.  ::)
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 #22 on: July 03, 2014, 03:45:49 PM »
Quote
I'm worried about those output transistors. My data says the 2N2148 is Germanium rated at 12.5W. It won't look good on most DVM diode test functions. I have no idea on the TA2577. There isn't much drive current available to Q10 so it needs high beta.

I tested both transistors using a basic "transistor test" using the ohms function on my DMM...maybe there is a better way to test these.  Also, I seem to remember learning something about "leaky" transistors, meaning that they sort of work, but "leak" current where they're not supposed to?  Could these transistors be "leaky", but still function somewhat?  I mean, I know they function, as I played my guitar through the amp and actually got some decent tones out of it...

Quote
Pity this thread was called new member and a question which means absolutely nothing.

Sorry about the title faux pas.  Some forums expect a formal introduction from new members before questions get answered.  I didn't see any kind of rule or introduction page, so I just went with what I thought would be the most effective.  I also searched this forum before I joined, and found several references to the TA-16 in a number of different threads, so I assume that this thread will pop up if someone searches the right terms.

If there is a moderator that could edit the title for better search-ability, that would be fine with me.

Thanks to all that are participating in this thread.

I'm off to measure voltages...


BrianS

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Re: New Member and a question
« Reply #23 on: July 03, 2014, 04:26:34 PM »
DC voltages on the output transistors:

Q9: Base: 19.15
      Emitter: 19.3
      Collector: 0 (connected to ground)

Q10: Base: 19.83
        Emitter: 19.3
        Collector: 33.8 (This is my "B+" voltage (rail voltage?))

So, my main "B+" voltage is about 4 volts less than the schematic, but my base and emitter voltages are about 1 volt higher than what is listed on the schematic.  However, the voltage drops across the base/emitter junctions are just about spot on with the schematic.

If this was a tube amp, I'd say these parts are functioning as they should!

Oh, the amp is also still plugged into the limiting bulb, so I suppose that is affecting the B+ voltage.  Is the correct term "rail voltage" here?  There is no negative rail, so...

Roly

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Re: New Member and a question
« Reply #24 on: July 04, 2014, 06:17:14 AM »
When testing germanium devices (which we hardly ever see these days) there are two significant differences to the common silicon devices we normally deal with.

The forward cut-in voltage of a silicon junction is around 600mV +/-100mV, depending.  The same voltage for a germanium junction is about 100mV.  If we test a germanium device thinking it's a silicon device we would then say "ooh, shorted junction" but it isn't - it's normal for germanium.

The second trap is leakage.  A silicon junction in reverse, or transistor between Collector and Emitter with no Base injection, is as near an open circuit as makes no never mind.  A germanium junction or transistor in contrast normally has some leakage when its supposed to be off; that as healthy as they get.

This makes testing germanium devices a bit harder, but in your case if these devices are giving reasonable transistor action in circuit, that is they seem to be amplifying the current between the input and output, then that is a good enough test.


{yeah, "rail" is a bit of a buzzword, but it has been around since at least the 1920's when perhaps power was distributed around equipment by large exposed metal bars mounted on elegant insulators.}
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: New Member and a question
« Reply #25 on: July 04, 2014, 06:43:19 AM »
DC voltages on the output transistors:

Q9: Base: 19.15
      Emitter: 19.3
      Collector: 0 (connected to ground)

Q10: Base: 19.83
        Emitter: 19.3
        Collector: 33.8 (This is my "B+" voltage (rail voltage?))

So, my main "B+" voltage is about 4 volts less than the schematic, but my base and emitter voltages are about 1 volt higher than what is listed on the schematic.  However, the voltage drops across the base/emitter junctions are just about spot on with the schematic.

If this was a tube amp, I'd say these parts are functioning as they should!

Oh, the amp is also still plugged into the limiting bulb, so I suppose that is affecting the B+ voltage.  Is the correct term "rail voltage" here?  There is no negative rail, so...
Looks good  :tu:
If the top transistor were shorted or horribly leaky, the mid rail would be practically +B , +38V or whatever.
If the bottom one were so, you would have close to 0V
But you have roughly half +B there, so they are proving themselves fine without further testing.
As of voltages on schematics, usually 10% away is fine and 20% away acceptable, so no problem there beither.
Now what was the remaining problem?

BrianS

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Re: New Member and a question
« Reply #26 on: July 04, 2014, 04:08:22 PM »
Thanks for all the advice thus far, folks!  Here's a couple recordings of the amp.  The first one is the amp with no production effects.  I do turn on an OCD to give the amp more treble, as it is a little "dull" on its own.  The recording is rather long, so skip around to hear various tones.  I get to the overdriven tones near the end:

https://soundcloud.com/brianstewart-1/heathkit-ta-16-dry

Here's a recording with some reverb/compression/somethingelse that I put on with my DAW, just for fun:
https://soundcloud.com/brianstewart-1/heathkit-ta-16-with-effects



Quote
Now what was the remaining problem?

Well, there was dc on the output, so replaced that coupling cap, which solved that problem:


The main filter cap had a bulge in the end, so I replaced that as well.  I used what Radio Shack had on hand, which was (2) 2200ufd, 50v caps that I wired in parallel.  The original was 4000ufd, 50v:


That pic also shows the new bridge recto and the new 3 prong cord.

Now, the only things left are the tremolo and reverb.  I measured the voltages on all the remaining transistors, and they are good, so my guess is that the opto-coupler is bad in the trem. circuit.  Internet sleuthing revealed a good replacement, which I will order soon.   

There is no reverb tank, and I really don't want to spend the money on one yet, so we'll see what comes of that.  I also need to figure out the correct tank to get.  I have to match the input and output impedance of the circuit, correct?  Need advice here.

I also will be building a head cabinet for this at some point, as I have nothing else to put it in.


Loudthud

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Re: New Member and a question
« Reply #27 on: July 04, 2014, 09:53:04 PM »
Bridge rectifiers like what you installed are typically only rated at 1 Amp. That's a little too low. See if you can find one rated at 4 Amps or more. For voltage rating, anything over 50V is fine.

I had a heatsink with a 2N3055 and MJ2955 mounted so I just sky-wired the rest of the output stage, Q8, Q9, Q10. The beta of the 2N3055 was too low, the amp only made about 20W at 4 Ohms and about 18W into 8 Ohms. Your output stage seems to be working so finding replacement transistors won't be a problem.

Roly

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Re: New Member and a question
« Reply #28 on: July 05, 2014, 04:46:38 AM »
Quote from: BrianS
Well, there was dc on the output, so replaced that coupling cap, which solved that problem:

Ah!     :dbtu:

Quote from: BrianS
I also need to figure out the correct tank to get.  I have to match the input and output impedance of the circuit, correct?  Need advice here.

Some things are easier than others.  The circuit shows the reverb unit as "4FB3A1B".  I'l  make a fairly large bet that this is an Accutronics/Belton part number and can also be decoded to find the tank specs. <http://www.accutronicsreverb.com/>

4 = 04 series
F = input 1475 ohms
B = output 2250 ohms
3 = long decay
A = input and output grounded
1 = no transport lock
B = mounting horizontal open side down

Just search "4FB3A1B" for offers, ~$20 +shipping.

HTH

And yeah, 1 amp diodes are a bit light-on, 4 or 5 amp would be safer (and don't worry about excess voltage rating, that's fine).

Quote from: Loudthud
The beta of the 2N3055 was too low

This is their classic problem.  For all their hairy-legged ratings, e.g. "Icmax = 15 amps", the Hfe drops to about 20 at only Ic = 5 amps.  Practically this means that you really can only get about 50 watts into 8 ohms out of a pair (on +/-36V), and that a pair alone won't be happy on 4 ohms loads.  There are far better transistors available these days, but they don't often turn up in the "throwout" bin by the door for 20c each.  :cheesy:   The homebrew Twin-50 I use as my stage synth amp (PA, foldback, etc) uses a pair in each channel and they have served well.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: New Member and a question
« Reply #29 on: July 05, 2014, 07:21:35 AM »
Well, 3055/2955 used *without* drivers in that simple circuit will definitely lack Hfe.
Add a couple Tip 31/32 which multiply it by at least 40 and now you have an easy to drive compound pair, still 15A rated and very usable at 10A.
Roughly same thing comes neatly packaged as TIP142/147 , with guaranteed Hfe of 1000 at 10A  :o
That's why it's practically the new standard industry workhorse, used by none less than Fender, Marshall (in an European incarnation), Laney, Crate, H&K and tons others (including yours truly).

EDIT: got real curious about what speakers were used there, got a couple pictures?
Would love to see what frames, magnets, cones they used.
Also the frames might have some small rubber stamped numeric codes , such as 2207614P12Q which although looking very mysterious actually means easy to decode Jensen week 14 of 1976 model P12Q .
Yours might show a similar one.
« Last Edit: July 05, 2014, 07:38:14 AM by J M Fahey »