[Jim_E3]

Hi, I've not posted here much, as up until now I've been quite happy with everything about my 55N+! Unfortunately, i seemed to have damaged the amp and need to diagnose what has gone wrong!

I had built & tested the amp successfully in Feb, and it has been running fantastically up until this week. Under a bout of over-enthusiasm i hooked up pair of (untested!) speakers I've just finished building. After initial turn on the drive units gave the usual turn-on pulse, then nothing came out after that! Obviously something about the speakers has damaged the amp, but i'm most concerned with getting the ASKA fixed!

I'm at the level where i can put the Kit together, but not understand how it works, so i could do with some help diagnosing what the problem is!

The current readings i have taken from the modules (roughly the same for each) are as follows:

(with the fuses removed)

Voltage over R21/R22: 6.5V
Offset between output and earth = 45MV, dropping to 42MV after a minute
R1 - Earth: 111MV
R9 - Earth: 127MV
R6/R5 Earth: 18V
across C7: 2.38V
across R4: 5.99V
across R15: 1.195V
across R10: 400MV
across C4: 0MV

I measured the bias which was around 420MV!! - No matter how much i turned down the bias pot this would not change. Also - rather worryingly, when i attached to multi meter probes to make the reading R19 began to smoke!

Some of these readings look low, and some are way off -  does this point into anything in particular?

Help!

Jamie

[multibit16]

Sorry to hear about the amp Jamie.

Are the resistors across the fuses ok?  was the fuses blown? you don't seem to be getting any voltage to the bootstrap cap or much after the resistors across the fuses

BTW  I find it odd R19 smoked when you tried to measure, your meter isn't set to measure current is it?

Cheers,
Leo

[AKSA]

Jamie,

Your readings are odd;  but the high bias and correct Vbe generator voltage (C7) would appear to indicate there has been a shorting accident and the outputs are toast.

So, easy fix, replace both outputs with C5200/A1943, easily obtained most electronics shops, and try it again, with no load connected, fuses out, until you get correct readings.

If the output replacement does not fix it, the next logical replacement is the voltage amplifier, for which you might have to ask me since these are regrettably now an obsolete device (I have plenty).

When you have it going, switch off, check for shorts in the speakers and cable before hookup, install the speaker connections, switch on, check all life signs, then slowly turn up the volume.  If something breaks, you know it's the amp/speaker combination, but these amps are VERY tolerant of different speaker loads so I'm concluding that something amiss happened during hookup.

I'm sorry to hear of your problems, but none of this is insurmountable - and you really do get used to blowing transistors in my game!  (Thank God I don't have to pay a carbon footprint for the ones I've trashed over the years, I'd be a poor man!)

Cheers,

Hugh

[kyrill]

Ah yes  that is the professional atttitude
mine to my Aksa's is very very father to son like, if my C5200/A1943 goes kaput, i would feel very very bad.. and a slight emotional break down.

[Jim_E3]

Hi Leo - The fuses were intact! - It is odd that that R19 smoked when taking the readings -

Hugh - Thanks for the words of encouragement and advice - the outputs are ordered and on the way. I'm taking a philosophical approach - I'm pretty happy that this is the first thing I've toasted in what is my first attempt an anything like this! And a week or so without the 55N+ will let me re-appreciate all its good qualities all over again once it is back in the land of the living!

[AKSA]

Jim,

Since all your bias readings are intact, and only your offset is bad, try removing C4 to see if the offset comes down to <20mV with respect to ground.

If it does, the problem is in fact C4, the 100uF bootstrap cap,, and if it doesn't, the chances are that the voltage amp is toast as well.

I'm a little hampered with it not on my workbench, but I think either C3 or T3 is at fault here and it's quite possible the outputs are OK.  However, it won't do any harm to have replacements, and they are very good devices useful in all sorts of circuits.

Stay in touch!

Cheers,

Hugh

[Jim_E3]

Hugh

 - unfortunately this saga gets more complicated before it get better! - whilst measuring second of the modules without c4, i hooked up + and - rails the wrong way round!

The result was a rather spectacular explosion from C10 and C12! 

Before i get round to determining the original problem, i need to fix this first!

I've sourced some NCC 120uF 63V cap. The markings on these read LXF, 8(2) 1 E. These differ from the ones supplied with the kit. Are they suitable? Given this type of incident, are there other components than may have been fried in addition to the caps?  - i'm hoping C10 and C12 will have taken the brunt of the damage!

Cheers,

Jamie

[AKSA]

Jim,

Ah, reverse polarity is a trick for young players.  You only do it about ten times, then you learn.....

You will not have damaged anything else except C10 and C12;  the sensitive parts of the circuit are protected by a diode.

The caps you mention will be fine;  LXF is the premium grade, so no problem.

Nearly there......  every disaster is one step closer to fixing it!

Cheers,

Hugh

[Jim_E3]

With fresh C10 and C12 caps installed, and the polarity double and triple checked, here are my readings for both modules with C4 removed!....

Voltage over R21 and R22:   1.56/1.87
Offset between output and earth: 11mv/7.8mv
R1 - Earth: 119mv/122.5mv
R9 - Earth: 126mv/130mv
R6/R5 Earth: 17.4/20.1
across C7: 2.23/2.29
across R4: 0.6/0.6
across R15: 1.05/1.08
across R10: 455mv/460
across C4: 20v/20.2
bias 4.1mv/6.6mv


Would i be correct to assume that since offset is down below 20mv, then the voltage amp is ok and the C4s are fried?

[AKSA]

Hi Jamie,

This looks right to me, though the bias is way too low, should be 55mV across the emitters of the output devices.

I think you conclude correctly;  it looks like the VAS is fine, the output and driver devices are fine, input stage is functioning as it should.

I'd simply replace C4, that should do it.

Good sleuthing!

Cheers,

Hugh

[Jim_E3]

Great! - this is good news - i had turned the bias right down as a result to re-following the setup steps in the instructions.

So new C4s it is and then were back up and running! - Thanks for your help Hugh - i'd have been lost otherwise!!!

Has any one used 'type n' black gates for C4 in place of the standard type? - a quick search doesn't seem to find anything on these forums about them. I might as well give them a shot in the search of better performance, but i wouldn't want to loose any of the 55n+'s great qualities!

Cheers,

Jamie

[Jim_E3]

Hugh - Still not quite out of the woods yet! Whilst setting up the bias i'm finding some strange readings! I can set it up to 55mV, only for the readings to suddenly jump to anywhere in between 280 - 320mv. This voltage level will persist no matter how much i wind back the bias pot - until randomly returning to 0 - 55mV (depending on how far i wound the pot back).

This is suggesting to me that either the pot is damaged - flipping between being on 'full' and the back to normal, or it is occasionally being bypassed somehow. Any ideas?

I'm really keen to sort this out as my old Marantz just isn't doing the new White Stripes album justice !!

Just a thought whilst trying to figure what went wrong in the first place -  based on what little i know (which really is little, so please humor me if the following is vastly wide of the mark!) - Since the bias is related to the voltage across the voice coil - could a change in load - or type of driver contribute to damaging the amp? My new speakers have ribbon tweeters which i understand provide much less of a resistance than a traditional type? Could these have allowed too much voltage to flow, resulting in damaging components? The speakers themselves have sustained no damage, and are working fine!

Jamie

[AKSA]

Hi Jamie,

This is still not right.  A jump of this magnitude indicates oscillation - damaging to the speakers, so test without a load until you are confident all is well - so we must consider if somehow circuit conditions have changed from the original design.

Can you take five or six quality digital pics, send them to me, giving me a view, top and bottom, so I can eyeball all components and their values?

Better to email me.....  I'm sorry for all this angst, but normally these amps are easy to set up, there's something strange going on here.  It's possible the pot is defective, but I doubt it.....  with everything switched off, measure the resistance from wiper to active end UNDER the pcb for two or three successive adjustments;  the measurements should indicate proper progression to 100R.

Hope this helps, get a camera on the case!!

Cheers,

Hugh