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Old 16th January 2013, 10:19 AM   #1061
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Join Date: Mar 2007
Location: Oxfordshire
Hi Ian . About slipping probes when measuring . I solder mine on sometimes . I bought some cheap probes which are fine ( $3 US ) . The plating takes solder nicely . It also gives you a free pair of hands to adjust things with . The clip on probes are expensive and often loose grip . I have melted a few ( load measuring ) .

I am surprised that R29 and 30 are different .They should be the same .
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Old 16th January 2013, 11:47 AM   #1062
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Hi Nigel
Yes, I mention the slipping because those new to testing their work only think of the instruments but of course, when you have to concentrate on the probes, measurement and documentation and signs of trouble simultaneously, your juggling luck runs out and ZAP! - no output stage and usually no spares, money or clue how to troubleshoot either. I think a small bag or box full of assorted leads, small jumper leads, clips and connectors is a must for your audio workbench in order to stay out of this trouble.

It's a shame for the low cost of IC clips, alligator/croc. clips or jumper leads, considering the likely number of times DIYs need them. I splashed out for "Parrot" branded hooks for my benchtop DMM a few years ago. They look like having inadequate grip but the quality is very good and they actually don't often fall off. Before that I used cheap, ($2.00) medium sized IC clips for everything but as you say, too much current eventually melts the plastic and they do short out close pitched connections. The baby IC clips are not for much current of course, but for only $1, they sure find a lot of uses.

One of the best type to make is a good set of power leads to test the project. I made several from colour coded 5 and 10 amp flex that are terminated at one end with slotted spade terminals to suit the power supply and best quality shrouded alligators at the work end. A cable-tie keeps them together - No more shorts or drop-offs!

Speaking of Radio Shack (earlier post); they marketed a pair of nice Taiwanese IC clips that simply slid onto the probes - a great idea for infrequent use and if I could find mine.......
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Last edited by Ian Finch; 16th January 2013 at 11:56 AM.
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Old 16th January 2013, 12:02 PM   #1063
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Absolutely Ian .

My favourite UK supplier Rapid Electronics have 4 mm to clips at less than $1 US . As you say a moments carelessness and disaster . I now have Parkinson's ( think I always did ) . My friends say to me how can you work like that . I can and I do . I especailly have to be careful of these things . I laugh mostly . I have to almost like I am diving under water hold my breath sometimes to do a measurement . It will be when a friend says can you look at this and not in my workshop I get nervous , that makes it worse . Seldom do I ask them to do it for me . The point I make is with a little thought risks can be avoided . My worse thing is soldering anything . I cope . My work is not good enough to sell due to this . I have others do it so no problem .
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Old 16th January 2013, 01:50 PM   #1064
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Nigel, I'm impressed by your determination in such adversity. I have no family history of Parkinson's but live in an area where there are many sufferers who need this low-stress environment for their well-being. Some are tragically young.

Electronics has been a hobby and therapy for generations in my family - I can't imagine not being able to tinker with things that entertain and inform or be able to show others how to get involved and plan their own projects. It's so much fun but eyesight (my avatar) has become a bane. It stops me completely when I can't read tiny component markings or benefit from lens aids. There are a few disorders being treated, yet they pale in significance to Parkinsons. You have my deep respect for your contributions here and sincere wishes for an easing of the symptoms.
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Old 16th January 2013, 03:49 PM   #1065
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Ian , thanks . Just in case others need to know for themselves I have made almost a full recovery and they let me drive now ( I'm 56 ) . What is has done is made me so happy to be alive . I wrote some Christmas cards which is as hard as it gets . The sad bit is I didn't build stuff for myself in the past . I am just about able to do that again. I feel a bit like the prodigal son , it is so good to be welcomed back to a life I thought had gone . I dare say it's not forever I get to do things again . I will go for whatever I get . Two Christmases ago I couldn't do up my shoes or shave ! My eyes also , I think that worse .
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Old 17th January 2013, 10:44 PM   #1066
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Quote:
Originally Posted by Ian Finch View Post
Just questions that have to asked to check measurements:
When powered,
First, is the meter set to low or autorange volts DC range and not some other range that appears to give readings?
1. Measure the voltage between the output connections. (have no other connections or input leads fitted)
There should be some DC voltage < 40mV typically.
2. Is Q5 replaced now and does the voltage across it (E-C) not vary when you adjust the pot?
3. Does the voltage across the pot (top to bottom on sch.) not vary when you adjust it? If voltages there
don't change, then remove the pot and test it out of circuit. Since the amplifier works, presumably
you must have tried audio satisfactorily with it, the output transistors must then be OK and correctly
installed so that leaves only Q5 and/or the pot. in doubt.

I'm following your posted schematic, which is different to your actual PCB and schematic so you'll need to cross-check.

4. Measure the voltage across R12 (68R) It should be ~0.4V for 6mA current through the VAS/bias generator circuit.
If the current is there, the previous checks must show voltages across Q5 and the pot. too.

At the output stage,
5. Measure between the emitter of Q11 and collector of Q12. This is the same as across both output "emitter" resistors.
It should be twice the required 5.5 mV for each resistor or 11 mV when the proper bias is set. Try adjusting again.

6. Measure Vbe (The voltage drop from base to emitter) for the output transistors and drivers (Q9-12).
It should be in the vicinity of 0.6V. Unless a solder joint has failed, the bias current must be there if the voltage
is being supplied by the bias circuit.

7. Is there a heatsink fitted and does it get even slightly warm after 10 mins or so?
1. Output to ground reads -40 vdc.
2. Q5 is replaced. No voltage across it (E-C), reads 0. Could it be bad cap at C6?
3. I removed the pot, checked and it works fine.
4. 0.003 vdc

I stopped measuring at this point.

When I did the initial test run, heat sink never got warm even after on for more than 10 minutes.
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Old 18th January 2013, 12:52 AM   #1067
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Hi, good to see you have some results.
Unfortunately, 40VDC on the the output is real bad, particularly for a speaker.
Did you hear audio at any point after building it?

You can temporarily remove C6, it isn't essential to operation. I think you'd realise if that didn't
restore voltage across Q5 that it is either wrongly oriented or shorted again. I would look
around the other parts there at the bias generator circuit around Q5 for shorts, open solder
joints or PCB problems. Something is wrong there. Check R15.

A) Lets check the rail voltages at Q4,6 emitters. Approx. +/-40V to ground should be there.
B) Now check the voltage between both collectors. This should read as the bias voltage applied to
the output stage or Vbias. It should be around 1.8V when adjusted but could be between 0-40V
or more if things are wrong or unadjusted. (C) If this is the case, measure the individual C-E
voltages across Q4,5 to find which is the problem area. Let us know what you find.

'Need to know those Vbe readings in the output stage (Q9-12) to check the transistors are OK.
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Last edited by Ian Finch; 18th January 2013 at 01:02 AM.
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Old 18th January 2013, 09:12 PM   #1068
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Quote:
Originally Posted by Ian Finch View Post
Did you hear audio at any point after building it?
Yes, during initial test. It also had lots of audible radio signal due to exposed board.

Quote:
You can temporarily remove C6, it isn't essential to operation. I think you'd realise if that didn't
restore voltage across Q5 that it is either wrongly oriented or shorted again. I would look
around the other parts there at the bias generator circuit around Q5 for shorts, open solder
joints or PCB problems. Something is wrong there. Check R15.
R15 (27 Ohm) is fine.
Quote:
A) Lets check the rail voltages at Q4,6 emitters. Approx. +/-40V to ground should be there.
Yes, + and - 41 vdc.
Quote:
B) Now check the voltage between both collectors. This should read as the bias voltage applied to the output stage or Vbias. It should be around 1.8V when adjusted but could be between 0-40V or more if things are wrong or unadjusted.
0 between collectors of Q4 and Q6. I think Q6 (2SD667) is defective because I pulled it out and ran some test but no tone from my multimeter no matter which pin is touched. If local Radio Shack doesn't have exact one, what are acceptable substitutions?
Quote:
(C) If this is the case, measure the individual C-E voltages across Q4,5 to find which is the problem area. Let us know what you find.
82vdc from C to E of Q4.
0vdc from C to E of Q5.
Quote:
'Need to know those Vbe readings in the output stage (Q9-12) to check the transistors are OK.
82vdc from C to E of Q9.
0.5vdc from C to E of Q10.
81vdc from C to E of Q11.
2.5vdc from C to E of Q12.

Last edited by Evenharmonics; 18th January 2013 at 09:17 PM.
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Old 19th January 2013, 04:37 AM   #1069
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by Evenharmonics View Post
82vdc from C to E of Q4.
0vdc from C to E of Q5.

82vdc from C to E of Q9.
0.5vdc from C to E of Q10.
81vdc from C to E of Q11.
2.5vdc from C to E of Q12.
Hi,
I don't know about Ian, but I don't trust your measurements.
You can't have 82V across Q9 and additional 2.5V across Q12, if your power supply total is 2x41V.

I think the best way to get some meaningful help with this circuit is to write the voltages at all nods on the schematic and then to scan it and post it. That way would be easier to troubleshoot.
I'd suggest that you connect the black probe of your meter to the ground, and leave it there. With the red probe take all the measurements and don't forget the positive or negative voltage sign (take DC measurements only).
By the way, hearing radio is very bad sign. Something is very wrong with your amp.
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Old 19th January 2013, 08:15 AM   #1070
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Hi Ruwe, yes I agree that the voltage measurements are inconsistent. It would be better as things get more complex, to only use the supplier's schematic and post the node voltages on that, if possible. (the nodes are the the junction points on the schematic representing the PCB trace junctions). I actually asked for B-E voltages (Vbe) at the output stage, which should have given a quick idea of where to look next.

I think now that applying power is just going to take out transistors if they aren't already blown. I should have insisted first up, fitting a bulb tester but was satisfied by the indication (#1045) that it worked ok.

Can you safely build yourself a bulb tester that connects a 60W incandescent lightbulb in series with the the active mains lead to the amplifier, Evenharmonics? It's elementary wiring but it's at mains voltage and dangerous unless you take great care with the proper insulation, wiring and mains fittings. Even a table lamp can be used as the bulb and socket and wired into the power supply if need be but the idea is to limit the current such that parts don't get enough to blow, yet there is enough voltage to fault-find. There are literally hundreds of posts here urging its use with all new construction and mods. A glow indicates excess idle current and a fault.

The 2SB647/D667 transistors will be hard to source other than through Ebay which means they will be dodgy anyway. The original Naim parts are ZTX653/753 from Diodes Inc. Mouser sell them for $1 or so. Unfortunately, there are no cheap and easy substitutes there that supply the current at the speed and voltage. BC639/40 can actually work but would be unreliable as Vceo is only 80V. In all substitutions, pinouts will likely be different. Google the relevant data sheets for simplest info. from Mouser or Digi-key on-line catalogues.

If you replace transistors, be sure then to use the "tester" to check that it is not about to repeat the same fault. Just replacing failed parts still doesn't eliminate the reason they failed. This is a DC amplifier - a nudge at the output stage can even cause the input transistors to fail by the domino effect so be aware that problems can keep coming back.
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