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Messages - Roly

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31
Quote from: blueboo
I checked the pre-amp out with the speaker disconnected, put it in my mixer and your right, the distortion's gone on both on clean and gain channel through the pre-amp out.. So does this mean anything in particular?

Yes, it means the preamp is in the clear and your fault is after the Fx point, i.e. in the power amp.

Did you try it with a known good lead between Fx Send and Fx Return?

Sorry, I should have been more specific, we are looking for the voltage across the speaker while the amp is idle, un-driven.

32
Amplifier Discussion / Re: Fender RocPro 700 (hard clipping)
« on: May 19, 2015, 02:51:14 PM »
{ogg! One of your strings is flat :-\ }

Well something is clipping alright.  The CRO Y vernier looks like its uncal so I can't tell what the actual peak to peak voltage is.  Swing it around to "cal" (far right) and see if you can measure the actual peak-to-peak voltage you are getting when it clips.  A sine source is much better for this; you could try recording a 10 sec 500Hz .WAV in Audacity or similar.  There are also test tone downloads.

If the p2p at clip is around 80V, rail to rail, then what you see is what you get and you have discovered what some of us call "the wall" with s.s. amps - you just avoid OP stage clipping 'coz it sounds nasty.

On the other hand if the clipping is occurring at a much lesser voltage that might give us a clue to a problem.

While we don't yet know what the actual p2p output voltage is at clip, I have to agree with JMF that that looks and sounds pretty normal.  {head scratch}

Have you checked it patched to a different cab to eliminate a speaker fault?  Frayed speaker braids can give some very nasty intermittent farts.

33
Quote from: PhredE
b). burned power resistors  (mine was a nice tone of brown! - supposed to be ivory white)

And when they get really hot they turn snowy white again.  I've seen these glowing red hot, carbonised the laminate and burned a hole right through a PCB.  Pretty hard to kill these ceramic wirewounds.  ;)

34
Amplifier Discussion / Re: Output impedance of tl072 input buffer
« on: May 19, 2015, 07:43:59 AM »
Quote from: mladenu
Very appreciate your willingness and commitment to help me.

And well worth the price...   :lmao:


...um... title block/board ID - project/your name/date?

35
Amplifier Discussion / Re: Fender RocPro 700 (hard clipping)
« on: May 19, 2015, 07:34:55 AM »
Hi Amp1, welcome.


Quote from: Amp1
If I hook guitar (vintage strat) into Return of FXloop I get same problem

So it's in the power amp.

Quote from: Amp1
when you hook a scope to such output – where to put ground of scope?
Minus of speaker is not at the same potential as ground!

CRO ground to amp ground.  With the load disconnected there is no signal on the current feedback point "CS2", so we should be able to simply deal with this as a normal voltage amp on "CS1".

Quote from: Amp1
scope photo, it is kind of oscillation problem?

Don't think so, the wobbles on the peaks are mains ripple on the supply, normally not an issue, but the flattening off of the waveform coming to the peak is interesting (but without a ground reference...).

The idle bias is set by CR15-CR18.  100mA idle does seem a tad high.

Quote from: Amp1
output of driver chip MC1436 (pin 7) is clean, problem is in power amp (TIP142/147), I think.

It looks that way.



With conventional transistors I'd be pulling them for an out-of-circuit test, but I don't have much experience with Darlingtons and their failure modes so I'll wait for the Brains Trust to weigh in on this one.

36
Hi blueboo, welcome.


Yep, you're in just the right place, and that information is a flying start.  :dbtu:  {and that circuit is not so bad when you zoom in.}

Measure and post the DC voltage at the speaker connection.  This should be no more than +/-0.25V.  Anything more than this, leave the speaker disconnected.

Wild early guess; it sounds like you might have AC ripple on one of your supplies (notice something is beating with your guitar notes), and a typical cause is a cracked solder joint where the big power supply capacitors fit to the board.

Another stock test if it has Fx Send and Fx Return, or Pre Out/Main In, is to jumper these with a known good lead and see if that makes any difference.


{techs with more experience of Princetons than I will be along shortly.  ;) }

37
Amplifier Discussion / Re: Output impedance of tl072 input buffer
« on: May 18, 2015, 01:10:36 PM »
An important point that needs to be stressed; just before you transfer give the copper a really good scrub with steel wool or similar so it is nice and bright, and don't touch the surface after cleaning (or you might get a lovely etched thumbprint).

38
Schematics and Layouts / Re: Simple Preamp please check my work
« on: May 18, 2015, 01:06:40 PM »
Ha!  Yeah, I know; "You call that snow?  This is snow!"  Aussie doesn't have much snow worth talking about.  I lived in rural England for a while and getting snowed in was a winter reality.

You can tame a kangaroo but you can't make them act.  The claws-on-a-stick that were used so Skippy could fly helicopters, turn speedboats away from certain disaster, dial phone numbers, etc, are still a source of great hilarity today.  They are lovely critters, but they just ain't that smart.

The nine (or was it fifteen?) 'roos they used for each episode (they have one hell of a Union) were female Eastern Greys, but the northern morph which is only about 3-4 foot high.  My guy is southern morph and over 6 foot.

Thankfully they try to avoid trouble, but if cornered they sit back on their tail, making them even taller, and lash out with formidable claws on very formidable hind legs, and even if you are not opened up neck to knee you will go flying with broken bones.  Thankfully this is extremely rare, but when you almost fall over one unexpectedly it's something to keep in mind as you rapidly retreat.   ;)

39
Amplifier Discussion / Re: Output impedance of tl072 input buffer
« on: May 18, 2015, 06:35:59 AM »
Quote from: mladenu
Something like this?

Yep.   :dbtu:


I've done a lot of boards so I can get pretty close first time, but I still might spend hours tinkering with exact positions of components and tracks to share the available space around.  For a one-off home etch there is also no virtue in cramming everything into the smallest possible space - laminate ain't that expensive and you just make life hard for yourself with layout/build/debug.

There are a few tiny things I'd tinker, but meh, on the whole that's a pretty good layout  :dbtu:  (we just hope that all the tracks go where they should and nowhere else - I haven't checked it against Phil's circuit).

40
Schematics and Layouts / Re: Simple Preamp please check my work
« on: May 18, 2015, 06:12:55 AM »
Quote from: galaxiex
Winter down there already?

You betcha.  Light frost this morning.  Snow down to 700m last weekend (430m here).  Often get the odd snowfall over winter in this area but it doesn't settle for long.  Sunny today though, and heading into town along the creek I almost fell over a big buck Eastern Grey Kangaroo sunning himself against my back fence - don't know who was more surprised.  :o  Bigger than me and he has been hanging around my garden lately, but thankfully pretty timid.

My phonepix are blurry, but this is how he was;


I rack up a lot of doona time this season.   ;)


Quote from: galaxiex
Not sure how they arrive at 47k input impedance with a 20k volume pot to ground at the input?

Well since the impedance can't be any higher than the resistance, that plumb evades me too.   :lmao:

41
Amplifier Discussion / Re: crate gfx 212 need repair help
« on: May 18, 2015, 05:29:25 AM »
replaced mje 340. no change

So you have now renewed both of the diff pair and the VAS and you still have the same voltages as in post #5?  Is that right?

42
Amplifier Discussion / Re: Output impedance of tl072 input buffer
« on: May 17, 2015, 07:20:27 PM »
Nice thick track widths.  :dbtu:   Better for home etching, and generally there is no point in skimping on track widths where you don't have to.  The more copper you leave, the longer your etchant will last.

I'd suggest moving your jumper to the right, over to between C6 and C11 and nudge C6 left a bit, then you won't have such a squeeze under C6.

Try to get poly-(anything) rather than ceramic's if you can.  It's no big deal but they are better suited.


43
Schematics and Layouts / Re: Simple Preamp please check my work
« on: May 17, 2015, 06:58:20 PM »
Well given that I have only just got up on this cold and frosty morning, nothing leaps out at me, all looks pretty reasonable.

I'll remark that I think you are going to find your "low" sensitivity input is pretty hot, and the gain channel even hotter - it's more normal to find, say, a 100mV High channel and a Low that's between 2:1 and 10:1 less sensitive, e.g. "Line" level, but it all looks basically sound and tweekable - suck it and see, and amend to taste.   :dbtu:

The upside is that if you want to drop one of the usual suspect tonestacks in at R11 with a typical 20dB insertion loss then you have gain to burn to compensate.  {but either lift your volume control to 1M or scale the tonestack impedances down by 10:1.}

Input sensitivity: 100mV / 47K Ohm

Quote from: galaxiex
Maybe an additional buffer after the Vol control?

Yes, with a 100k volume pot feeding a 47k input you are going to get a rather odd control characteristic, but failing redesign of everything after your op-amps for one-tenth the impedance, a simple emitter or source follower, or another op-amp should give you a preamp output impedance that is quite low enough.

Oh, and I'd move C5 to mid-rail to ground.  Where it is it will couple any noise on the supply to the mid-rail.

I'd personally stick a 0.1uF poly bypass right across the op-amp supply pins as insurance, but that may be gilding the lily.

Includes two gold-anodized heat sinks

Gold looks great, but black works better.   ;)

Keep us posted.   :dbtu:

44
Amplifier Discussion / Re: Vintage amp info and power cord?
« on: May 16, 2015, 06:47:37 PM »
Quote from: Roly
I'd approach this one with considerable caution

Just plugging it directly into the mains without a limiting lamp isn't "considerable caution" - it's an invitation to smoke something.

I hope you wouldn't try to drive a car that obviously had a wheel missing - this amp is obviously missing bits it needs to operate properly.  Two highly experienced amp techs have told you that they can see that this amp has problems, and that they need to be investigated before you do anything else.  We can only advise but if you ignore vital advice/warnings we aren't going to get very far.  You have tapped into hundreds of years of collective professional bench experience here, make wise use of it.   ;)

To do;
Until a circuit turns up we need well lit crisp and clear pix of the main power board, both sides.

Disconnect the speakers at the amplifier end (if you value them).

What is the actual value of the fuse fitted to the "2 amp" holder?

Until we get a circuit or pix you can remove the output transistors and test them.  Be careful not to mix them up as they may be different (mark as you remove).  Post their type numbers.

You can also make up a limiting lamp which you are going to need down the track.


To be crystal clear - this amp shows signs of being hacked by somebody who is clueless, and who has abandoned it in a mess.  This doesn't mean it's unrecoverable, only that there may be several introduced faults apart from the original failure that was being chased, so we need to proceed very carefully.

45
Hi juanMotta, welcome.


Quote from: juanMotta
i´m planning to build a hybrid amp

Why?

Quote from: juanMotta
I would like to have a tube sound

...and the most reliable way to get tube/valve sound from an output stage is to use tubes/valves and a mid-fi output transformer ('tho even that is no guarantee).

Generally speaking transformer-coupled s.s. output stages are a thing of the past and you may have an interesting time finding a suitable transformer, particularly if higher power.  Hunt this vital component down first, before you move too far in this direction.  If you can't find one you may have to reconsider.


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