J212 Amplifier help - Page 1
Forum section: electronics / repair

author: andybarrett1
author_rank: Regular Contributor
timestamp: July 25, 2015, 09:03:46 pm
content: Hi AllI 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 lookingAndy

author: pattyrice
author_rank: Newbie
timestamp: July 25, 2015, 10:25:23 pm
content: 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?

author: andybarrett1
author_rank: Regular Contributor
timestamp: July 26, 2015, 12:55:39 pm
content: HiYes 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…. !!!

author: mij59
author_rank: Frequent Contributor
timestamp: July 26, 2015, 01:15:06 pm
content: Start by checking dc voltages e.g. output of the amp, zener diode ZD1, voltage across the resistors R9, R26.

author: dom0
author_rank: Super Contributor
timestamp: July 26, 2015, 01:19:40 pm
content: 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.

author: andybarrett1
author_rank: Regular Contributor
timestamp: July 26, 2015, 07:14:41 pm
content: 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 rightThank you for help

author: dom0
author_rank: Super Contributor
timestamp: July 26, 2015, 07:17:00 pm
content: 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?

author: andybarrett1
author_rank: Regular Contributor
timestamp: July 26, 2015, 07:25:03 pm
content: HiSignal source is clean…. looks pretty on a scopeGround 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

author: andybarrett1
author_rank: Regular Contributor
timestamp: July 30, 2015, 08:14:56 pm
content: Amp is now sortedA 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