J212 Amplifier help

[andybarrett1]

Hi All

I have an amplifier… A Subzero J212…Everything looks ok but I am seeing and hearing the enclosed distortion… the trace was measured at the speaker…. one trace is a sine wave (really) at 1k…. the second is a square wave at 1k.

I  am attempting to find the circuit…. early days… but has any one seen distortion similar ??

Thanks for looking

Andy

[pattyrice]

All I can find while searching for "Subzero J212" is a speaker. Got the right part #?

Some people I know fixed one of those speakers by replacing the lowpass filter inside, it got dropped and broke a ceramic resistor inside.

Have you looked for bad electrolytic caps (bulging, fluid leaking out top) in the amp and the speaker?

[andybarrett1]

Hi

Yes it is a J212A….. Circuit diagram enclosed. I had another look today… All Caps are fine and LCR says they are  in tolerance . The supply rails are fine at +/- 60v DC minimal ripple.. The bases of all power o/p transistors are all reporting the same biasing.

I have taken the crossover circuit out and still the distortion is present ….. The strange thing is if I probe the input (pin4, cn1) it an be seen there…. (My generator op is clean and dc decoupled).

Confused…. !!!

[mij59]

Start by checking dc voltages e.g. output of the amp, zener diode ZD1, voltage across the resistors R9, R26.

[dom0]

Yeah, check the DC operating points first. Check if the AB bias is correct (C-E of Q7 should be about 2-2.5 V or so). Check that Q8 and Q6 don't come into conduction. Check that R19-R24 have good current sharing. Check that R9 and R26 have about one Ube accross them. If not, check Q5/12/15/9.

[andybarrett1]

Hi thank you…

All DC points look ok… 60vDC and 15vDC Rails….ZD1 has a healthy 15v…. It is a 15v Zener.

Transistors are all good both static and dynamic…. A couple have low resistance in the area but fine.

What is confusing is the distortion is present at the input… This can not be right

Thank you for help

[dom0]

No... no that can't be right.

Unless your signal source is the culprit, or the input stage is amiss. Is the GND connected nicely and all?

[andybarrett1]

Hi

Signal source is clean…. looks pretty on a scope  Ground all good :-)

Shack is trashed right now so will revisit tomorrow… I may try another sig source anyway …. maybe put a cap / transformer  in between sig and amp just to see what going on….

To make things harder second scope lead gone faulty now !!!!!!!

Andy

[andybarrett1]

Amp is now sorted 

A track from Q8 to the PA Bases was not good…. Not there….. Missing !!!

A production fault … So much for thermal conditioning / Soak tests !!!

So….. Got it working on bench…. Put back together tomorrow !!!!

Thank you for all suggestions……Appreciated