Audio amp shorted. Which component failed? - Page 1
Forum section: electronics / projects

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 28, 2015, 08:10:24 pm
content: Hello there,The diagrams below are taken from this schematic:Full schematic. Today, I was adjusting the output amplifiers. Unfortunately the test probe slipped off a test pin and shorted a few of the test pins highlighted in this diagram:I am pretty sure it was pin 2 and 3 which were shorted for a moment, (clearly connecting the -45V rail to ground). Immediately after, I noticed that other power rails were low and two massive input resistors in the fuse board (R601 and R602) were starting to smoke so I immediately pulled the power cord. Heres the board with the resistors:This is what I've found so far:> I made a few other measurements, and I've noticed that right now, got 0.5 ohm between pin 2 and 3 marked in the top-most diagram.> I didn't blow the two fuses on the fuse board, they measure .2 ohm and the smoking resistors seems to have the correct value. Since the resistors are smoking, the -45V must be lower than -24V, which is what the relay RY601 bypassing the resistors on the fuse board is rated for. So I assume something is shorted.Using this information, are you able to suggest what needs replacement or which other measurements it is wise to make?I have some extra material, here's thefull schematicandthisis the service manual for it. Hoping to get some helpThank you for your time.Kind regards,MariusEDIT: I measure .2 ohms between the collector and the emitter of Q354, could it be shot? How would we explain that happening?

author: Kevman
author_rank: Regular Contributor
timestamp: December 28, 2015, 09:33:50 pm
content: If I'm reading the schematic right (probably not), it looks like Pin 2 is connected to the speaker output on the channel and pin 3 is the -45v main power rail. Ouch. Am I right?Were there speakers connected at the time? I THINK there's a DC detector driving Relay 301 ("muting"), which would have triggered. It should have made a loud pop followed by Relay 301 opening in an attempt to keep your speakers from going up in smoke. Which they would have if it didn't open. Check your speakers...Relay RY601 appears to be a soft-start and sure looks like it runs off of the -45v rail for some reason. When you shorted it, the rail may have sagged enough for it to open, causing the full amplifier power to go through the two resistors that got hot on you.You might have taken out the negative power transistors Q354 or Q363 causing one to fail short and that's why you read .5 ohm? I'd check them and replace the soft start resistors.Shorting pins 1 and 3 would not have likely caused any sort of significant damage as it would just bypass the transistor balancing resistor R391, which I think the transistor would have survived for a while, so that's not what happened.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 28, 2015, 10:12:51 pm
content: Thanks for your reply,Luckily, I had disconnected the speakers. Regarding Q354 or Q363, how come you think that they have failed? According to the manual, the2SC1986was chosen. I thought about it, but perhaps I've mistaken: when i shorted pin 2 and 3, that means i effectively lowered the potential across the transistors? Is that true? And that isn't harmful is it? Is there something I've misunderstood?It be great to have some idea what is shorted before I start stabbing the pcb with the soldering iron!Thanks

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 28, 2015, 10:47:11 pm
content: I would check Q353 and Q362 because shorting TP2 to TP1 or TP3 would result in -45V to suddenly appear at their collectors.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 28, 2015, 11:02:13 pm
content: Quote from: MagicSmoker on December 28, 2015, 10:47:11 pmThank you for your suggestion, perhaps I will do that. But I'm measuring .5 ohms between pin 2 and 3. Suppose Q353 or Q362 failed, do you suggest that this contributed to the short between pin 2 to pin 3? ThanksI measure .2 ohms between test collector and emitter of Q364. Could we explain that the accident may have caused it to fail?EDIT: I got rid of Q364 because it was shot. But I still have a short between my -45V and 45V

author: deadlylover
author_rank: Frequent Contributor
timestamp: December 29, 2015, 12:26:42 am
content: I think all 4 of the output transistors are suspect, they have a Vceo of -+80V and they've possibly been exposed to -+90V. We know the Q353 and Q362 has a good chance of failing from directly putting -45V at their collectors, but after you took the probes away, if they've failed short then +45V may appear at the collectors of Q354 Q363, exceeding their Vceo as well.You can try buzzing out the diode junctions of the trannies to see if it would give you any more info, but that's a bit iffy to do in-circuit, can't rely on it that much.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 29, 2015, 12:55:43 am
content: Thank you deadlyloverTurns out you and kevman were correct, Q354 was shot, the other shorts I had were transistors which had been burnt out while I was testing them disconnected from their heat sinkI wasnt able to find the original parts,2SA771or2SC1986at farnell or digikey. Do you reckon IMJE15032 and MJE15033would be suitable substitutes, or are you able to recommend others which are better for my purpose?Thanks,Marius

author: deadlylover
author_rank: Frequent Contributor
timestamp: December 29, 2015, 01:27:51 am
content: I'd imagine it'll be fine.Hope it all goes well, that's a lovely looking amp you have there.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 29, 2015, 10:56:58 am
content: Quote from: king.oslo on December 28, 2015, 11:02:13 pmI suppose I should have wrote, "I wouldalsocheck..." to make it more clear thatallof the output transistors should be suspected in such a situation.As for how everything failed, a reasonable chain of events would be that shorting TP2 to TP1/3 effectively bypassed the negative rail output Q's, putting -45V at the collectors of the positive rail output Q's, which caused them to fail short. When you removed the short from across the test points that put +45V at the collectors of the negative rail output Q's, causing them to fail short as well.Which is why it is a good idea to insulate the probe tips with heatshrink tubing or electrical tape, like so:

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 29, 2015, 12:58:55 pm
content: Quote from: deadlylover on December 29, 2015, 01:27:51 amMe too, and thank you for your compliment.Quote from: MagicSmoker on December 29, 2015, 10:56:58 amThanks for your suggested chain of events, sounds very reasonable I think. The probe insulation tip is great, thank you

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 30, 2015, 02:43:10 pm
content: Installed new transistors in both channels, but there seems to be at least one more problem: When I set the DC bias current to adjust to 16.5mV across test pin 1 and 3, as I increase past 10mV, all of a sudden the bias voltage shoots up to 250mV (~ 750mA) (see the photograph)! I can turn the potentiometer further, and the bias current will increase further to about 300 mV ( ~ 1A). If I try to turn the pot back down, it rapidly decreases 0mV (0mA).This happens in the channel where I didn't get the short a few days ago. This time I dont really have a clue what is wrong, so fixing it depends on your suggestions. What are your thoughts?Thank you for your time. Kind regards,Marius

author: dom0
author_rank: Super Contributor
timestamp: December 30, 2015, 02:50:59 pm
content: Could be as simple as a bad pot.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 01, 2016, 02:30:35 am
content: Quote from: dom0 on December 30, 2015, 02:50:59 pmHave I understood this correctly; if i set the RT352 dc bias pot to about the middle position, its safe to drive the IC351 pin 2 with a low impedance 0 to -8.5V power supply overriding the pot setting?And second, is it correct that IC351 does not have a feedback, so that the chip is most likely faulty if all the voltages at all it's pins are nominal?Thanks.

author: dom0
author_rank: Super Contributor
timestamp: January 01, 2016, 10:55:45 am
content: I dunno, just because it goes to a wiper of a pot doesn't necessarily mean that it is a high impedance input that can be driven from a voltage source. We don't have a schematic of that part. You can try, but set the current limit really low.There should be some kind of feedback, either current or thermal (although almost all amplifiers use largely inferior and often borderline unstable thermal feedback), to stabilize the AP of the output stage. Is Q357 on the heat sink?

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 03, 2016, 10:03:48 pm
content: Hello again,No Q357 is not on the heatsink.Say I adjust the dc bias current to 30mA. If I probe any of the output transistors pins with a multimeter probe, sometimes that is sufficient to trigger a dc bias current of 0.75A! In fact I can toggle the dc bias current between 30mA and 0.75A by tapping the transistor leads with a multimeter probe. Tapping the leads of any of the other transistors or diodes or IC301 is also sufficient to set it off. Any ideas?Thanks.M