Audio amp step-by-step repair - Page 1
Forum section: electronics / repair

author: okw
author_rank: Frequent Contributor
timestamp: September 07, 2021, 04:05:22 pm
content: Hi! I've searched the forum but couldn't find any beginners "step-by-step" guide for audio repair. Of course, all repair would greatly depend on the symptoms, but any help would help me. Specifically I have a Ampeg BA115HP (bass guitar amp) which makes a huge pop at power up and does not output any sound when played.Here is the schematic:https://ampeg.com/support/files/Schematics/B%20Series/BA115HP/457SCH_0.pdfI have a Fluke 183, UNI-T UTG962E signal generator, TDS 1012B oscilloscope, soon a 200w 8ohms dummy load (and other equipment) at my disposal.I'm decent in power and digital repair, but have not yet tried audio, so please, any help on where to start and how to move through the circuit would be appreciated

author: keland_uk
author_rank: Contributor
timestamp: September 07, 2021, 04:18:28 pm
content: Ah, you have MOSFET's, they should call them fuses. :-) (Sounds like it's not blowing your fuses tho, unlike my Sony whose ouput MOSFETS had shortened.I'm in same boat ref starting out repairing audio amps.I'm fixing a Drawmer 1960 compressor for a friend. Valve heater power supply suspect, BUT not whats causing a problem on CH2 which doesn't work in bypassI have a Flair1 infrared camera, and it appears that one of the NE5532's has failed, certainlty very hot..... (watch the voltages out there....)Good luck... I'll also be interested in any hints et al

author: Smith
author_rank: Frequent Contributor
timestamp: September 07, 2021, 04:36:19 pm
content: Quote from: keland_uk on September 07, 2021, 04:18:28 pmNot quite, the MOSFET's blow your fuses.I assume you are talking about amplifiers in particular. It's basically just like any other repair.If it blows fuses, you start at the power supplies. Most amplifiers quite a lot of different voltages. It may be a good idea to find the reason you lost the power first, most times just replacing fuses and blowing hem again does more damage than you started with.A common problem the safety kicks in. The output relays will be disabled, and if there is a display it will tend to say something like protection. Some amplifiers have LED's or will blink some LED. This commonly means there is DC on the output, and the amplifier is pretending you to blow up your speakers. Mostly means one or more output transistors (and probably the resistors) are blown, or one of the voltages is gone.If it doesn't start at all check all voltages, and find out why they where gone. Sometimes caps, regulators, bad connections (incl solder joints).If anything is not working, or not properly, check switches, potentiometers and relays first. Specially with older gear. Specifically relays are difficult to troubleshoot the first time. Normally the sound will work, and at low volumes a channel will distort, of stop outputting audio. You can clean potentiometers or switches, relays should be replaced. If it is not your gear check ALL switches and connections. You won't believe how many amps I "fixed" by enabling the right input, plug the input to the right connection or switching the speaker selection switch.If you still have problems, check ALL soldering connections, specifically ones that tend to get hot (output transistors, voltage regulators) Re-solder them is necessary. A plastic stick is great for finding bad solder joints. Apply voltage to the device and just poke around carefully.Sticky brown goo that was used to hold connectors and caps in their place can become conductive after any years.Maybe this helps as a basic guideIn your case I would do the following:1. Do NOT power the device with speakers connected. It has NO protection, and DC will kill speakers fast.2. Check all the voltages3. If they are OK, put some audio in the amp. Like a sine wave from a PC or function generator. Measure down the schematic, follow the path of the audio, until it stops (check AC AND DC on BOTH channels). Use a sine at it is continuous and easy to measure with your multimeter. Audio will go on and off and is terrible to troubleshoot. It's best not to use a load, as a sine is a quite heavy load for any amp.

author: keland_uk
author_rank: Contributor
timestamp: September 07, 2021, 05:35:53 pm
content: Quote from: Smith on September 07, 2021, 04:36:19 pmI meant fuse destroyersbut thanks, thats helped me as well.Just to be clear ref load, if the o/p is via valves then you need a load? ie fine for semimiconductors, not for valves in the o/p stage.

author: keland_uk
author_rank: Contributor
timestamp: September 07, 2021, 05:46:36 pm
content: Quote from: okw on September 07, 2021, 04:05:22 pmI've done a dummy load based on Pete Milletts design, using an old BOSE SA2 case, was a bit of fun.https://www.diyaudio.com/forums/tubes-valves/284351-audio-dummy-load-2.htmlThinking of doing a Reactive load based on this design to fit same said Bose SA2 Shell(SA3 are useful as standalone amplifiers so I wouldn't iuse one of those, seem to be a lot of people throwing these Bose amps out, the B&O ones are silly money)https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/Is your one commercial load, and is it reactive load? (Since Guiter Amp)

author: Smith
author_rank: Frequent Contributor
timestamp: September 07, 2021, 07:14:48 pm
content: Quote from: keland_uk on September 07, 2021, 05:35:53 pmYou don't need a load on most amplifiers. Amplifiers MAY become unstable without load. Never happened to me though.The only thing you need to remember when using a load, is that an amp will play audio, and the average power to the output will therefore be quite low. When using a load with a function generator you can generate a lot of heat within minutes depending on the volume. This will stress both the output stage AND the power supply. Most power supplies will have reasonable capacitance as a buffer, the power supply might not be able to generate this continuous load.Not to mention the load resistors can become very hot.

author: keland_uk
author_rank: Contributor
timestamp: September 07, 2021, 10:35:01 pm
content: Quote from: Smith on September 07, 2021, 07:14:48 pmYep, basically think of them as an ovenHad to blow on mine to keep temperature reasonable.....Albeit I'd never run the dummy load at that wattage, and anyway I was overegging these, should have limited the total to 100W, since 50W resistors in series, hence current goes through bothMine is so I can test the THD et al after I've "fixed" said amplifier, my ears aren't what they used to be...

author: okw
author_rank: Frequent Contributor
timestamp: September 27, 2021, 11:24:12 pm
content: I've gone through the whole circuit now, and all voltage rails are ok, signal levels (test points) are more or less within the values from the service manual. Except this:TP10 (NE5532 out2) should be 2.0VRMS, which is 10mV. I suspected the op-amp and replaced it, still 10mV. I measured in2- and it is also around 0V. Even though TP12 was 950mVRMS (should be 800mVRMS), which is connected with a 1k between them (and a lot of other stuff).TP11 should be 26.3V, but i 31.6V.The schematic is here:https://ampeg.com/support/files/Schematics/B%20Series/BA115HP/457SCH_0.pdfAny help would be appreciated.

author: Grandchuck
author_rank: Super Contributor
timestamp: September 28, 2021, 11:19:36 am
content: R22 open? Or joint or PCB problem?

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2021, 11:42:02 am
content: TP10 signal will be low if the amplifier's output isn't loaded.What is happening at the output?TP 11 should be 0V DC.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 02:52:48 pm
content: Quote from: Grandchuck on September 28, 2021, 11:19:36 amR22 is 1k (0.998k ish) . I've resoldered most of the solder joints.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 02:56:13 pm
content: Quote from: xavier60 on September 28, 2021, 11:42:02 amI left the output open, to not damage the speaker. Should I connect an oscilloscope on the output? No risk of damaging it (it is a TDS 1012B with x10/x1 probe)?In the schematics TP11 says 26.3V. Do you mean it should be 0V without load. And 26.3V with output loaded?

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2021, 03:36:30 pm
content: Quote from: okw on September 28, 2021, 02:56:13 pmProbing the output will not damage your DSO. The schematic shows TP11 as being 26.3V RMS. Im certain that it means 26.3V RMS AC.Don't connect the speaker if there is significant DC voltage at the output.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2021, 03:48:11 pm
content: The ouput should be close to 0 V DC level, but of cause with the music / tone, which is AC.Similar TP12 should be near zero ( < 10 mV) DC.With a 10:1 probe the scope is safe. Most scopes should also be OK even without the probe, but no need to try.The first part should be up to TP12 - that is the low power part. So try with some signal from the function generator and check with the scope.A slightly higher amplitude like 950 mV instead of 800 mV should be OK. For easier comparision maybe reduce the amplitde at the input to get back to the nomal 800 mV.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 05:08:30 pm
content: Quote from: xavier60 on September 28, 2021, 03:36:30 pmYes, at all test points (TP) I've only measured AC. I'll go through the circuit again and measure DC, and see where DC is introduced. If this is the correct way?

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 05:10:25 pm
content: Quote from: Kleinstein on September 28, 2021, 03:48:11 pmI reduced the amplitude to 700mVrms (sine 440Hz).I'm measuring 37.5V DC on the output (J23) with my multimeter.Oscilloscope on same output: Pk-Pk 17.6V, Period 2.264ms?, Cyc RMS: 38V, Freq 440Hz.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2021, 05:14:27 pm
content: 38 V DC and some 17 V_PP would suggest that one side never turns on. If the FETs get really hot it may be 1 side never turns off.A next point to check is the gate voltages / BJT collectors.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 05:32:44 pm
content: Quote from: Kleinstein on September 28, 2021, 05:14:27 pmI'll do that. What voltages am I looking for?I used a thermal camera and the FETs stay cold. Seems the two 330R (R66 / R68) gets really hot (~90'C), fast. Along with R6/R7, but they're not nearly as hot.I DC measured every test point, all within 0-4mV DC. Except:TP4: -1.4mV.TP6: 158mVTP7: 127mV.TP10: -262mV.TP11: 2.25V.I also discovered, the amplitude on the oscilloscope jumped from 16-18V pk-pk (output J23) to almost 90V immediately when I touched TP12 (pin 1 of U5 / NE5532). And it stayed 78-90V after i let go of TP12.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 05:47:30 pm
content: Quote from: Kleinstein on September 28, 2021, 05:14:27 pmI DC measured the FETs relative to ground.G: +/-48VD: 37VS: +/-51VThe N-channels (Q5/Q8) G and S are negative, the P-channels (Q2/Q7) G and S are positive.The other transistors relative to ground:Q1 gate: -3.614VQ3 gate: -2.891VQ4 gate: 3.282V

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 28, 2021, 06:30:50 pm
content: Quote from: okw on September 28, 2021, 05:32:44 pmIt doesn't make sense that you are reading 37.5v at TP12, the output of op-amp U5A, when this op-amp has a supply voltage of +/- 16v.Maybe you have a ground missing somewhere or your measurements/references are flawed.Providing a temporary ground reference to this part of the circuit by touching it and then stuff happens might be a clue.Edit: Perhaps the +/-50v supply does not share the same ground as the low voltage stuff.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 06:42:14 pm
content: Quote from: Audiorepair on September 28, 2021, 06:30:50 pmSorry, my bad. I meant J23 (output), was 37.5V DC.TP12 is 940mV AC, 4.0mV DC (but fluctuating quite a lot between 3.5-4.3mV DC).

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2021, 07:22:51 pm
content: While there is 37V DC at the output, the waveform must also be distorted?When you said"I also discovered, the amplitude on the oscilloscope jumped from 16-18V pk-pk (output J23) to almost 90V immediately when I touched TP12 (pin 1 of U5 / NE5532). And it stayed 78-90V after i let go of TP12."Sounds like something is going intermittent open, Q1 possibly. Are Q1 and Q4 on a heatsink?Can you show us the output waveform when the 37V DC is present? A small load might better show up the problem, like a household light bulb.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 28, 2021, 07:32:16 pm
content: Remove the mute JFets Q6 and Q9, and bridge D-S of Q9 to maintain the signal to the op-amp, and put them back later.These are so often faulty, but partially, or intermittently, or just don't help when you are fault finding as they mute stuff because there is a fault.The Opto-coupler OC1 also seems to be a limiter, which would also reduce the AC in DC fault conditions.You could replace the resistive part with a similar fixed resistor to eliminate it while testing.These aren't nearly as unreliable as a J112 however.

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2021, 07:55:38 pm
content: TP12 should normally be within a few mV of 0V DC.

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 07:56:34 pm
content: Quote from: xavier60 on September 28, 2021, 07:22:51 pmWaveform is a beautiful sine at 440Hz, same as going in just very amplified.Q1 and A4 are on a heat sink. Those clip on, mickey mouse heat sinks.I'll see if I can find a normal light bulb. I mostly have LEDs.See images for input signal (721mVrms, 440Hz sine with probe set to 1x). Oscilloscope probe is set to 10x.And one output image untouched, and one output image where I touch TP12. The crazy amplification did not keep after I let go of TP12 (like the last time, last time it was also slighly distorted).

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 28, 2021, 08:06:28 pm
content: Get those Fets out.

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2021, 08:12:01 pm
content: Yes, it sounds like a high - DC offset is occurring on TP12,

author: okw
author_rank: Frequent Contributor
timestamp: September 28, 2021, 08:43:11 pm
content: Quote from: Audiorepair on September 28, 2021, 08:06:28 pmQ6 and Q9 are out. Q9 D-S is bridged.TP10 (U5, pin 7) is 246mV DC.TP12 (U5, pin 1) is finally 6mV DC.Output is 160mV DC, 30V AC (DMM).Pk-pk is 88V on the scope, Cyc RMS 30.6V. Not sure how to interpret the pk-pk vs. cyc RMS in the audio context.Touching TP12 only slightly increases pk-pk to 91V.Put a 2K resistor over Q6 D-S (VTL5C7 datasheet says 5 kΩ @ 0.4 mA and 1.1 kΩ @ 2 mA). This drastically lowered the output AC (30V->1.7V). However output DC is up (160mV->559mV).New readings:TP10 (U5, pin 7) is 270mV DC, 0V AC.TP12 (U5, pin 1) is 2mV DC, 48mV AC.Any clues?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 29, 2021, 10:01:22 pm
content: The Optocoupler seems to be a peak limiter, and will only operate when the output is near +/- 50v.At all other times it is switched off, so the resistor you should substitute is its off state resistance, maybe for all intents and purposes open circuit.The scope photos do seem to show some DC offset, but presumably this is without a load, so may be misleading.Run the tests again with some sort of dummy load, (Don't need full power, just check the dc offset).it may just be that the only problem is the Fets.I would first make sure, using a meter on ohms mode with the amp off, that the 16v power supply ground, the 50v power supply ground, the ground on the op-amp, and the ground on the speaker output are all connected together by zero ohms.Also, the speaker ground return runs through the headphone jack, which is a switched type, so that inserting a jack plug cuts off the speaker.This switched jack is notoriously bad at failing, so don't put your dummy load on the speaker cable, put it directly on the power amp, and replace the headphone jack later.

author: okw
author_rank: Frequent Contributor
timestamp: September 29, 2021, 10:17:07 pm
content: I'll try that. Measure each grounds points, and see if they differ. If not, then remove optocoupler and put a 1M instead.I have 5pcs of this (100W 8R):https://www.aliexpress.com/item/1005002749358940.htmlI would think one suffice as load?I always measure output on J23. Is this the point you mean for output?By the way, I didn't see the test point conditions note in the schematics:1kHz sine 170mV, vol+level@10, tones@5.I used 440Hz 721mV, all pots at unity (@5).It probably won't make much difference, since I adjusted the sine amplitude until the first test point showed the spec 700mVrms. Most TP after this was within spec.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 29, 2021, 10:35:26 pm
content: Yes, J23 is your load test point, thus eliminating anything past this from the equation.One 8 ohm resistor is perfect to test, you don't need to run it full power or for long to see whether it still has DC offset or not.The Otpo and the fet are in parallel with R23, 47k.Mute/gain reduction is achieved by reducing this 47k considerably, so I think you can just take the opto out, 1meg isn't going to do anything in parallel with 47k.I wouldn't worry too much about precise frequencies, levels, etc, you are just finding a fault at the moment, and service manuals do sometimes have incorrect data for the particular revision you have on your bench.

author: xavier60
author_rank: Super Contributor
timestamp: September 29, 2021, 11:02:37 pm
content: When dealing with offset problems, it's useful to note the polarity of the voltage. It can indicate whether a stage is causing a problem or is just responding to a problem elsewhere.Sometimes the act of taking of a DMM reading can temporarily clear a fault. In cases like this, a DSO with a X10 probe should be used.I sometimes tie a 100K resistor to the end of my DMM probe to reduce circuit disturbances. Not the safest idea.

author: okw
author_rank: Frequent Contributor
timestamp: September 29, 2021, 11:39:48 pm
content: Measured all grounds to be equal. Except opamps (pin 4), but they are connected to -16V if I understand the power table correctly.Removed opto and the 2k resistor. Q6 and Q9 is still removed, Q9 D-S jumper'ed.Hooked the 100W 8R on output. Still 500mV DC.

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 12:47:51 am
content: If the power amplifier stage is working properly, signals including DC offset at TP12 will appear multiplied by 33 at TP11.Some offset at TP10 could possibly be normal.Also check for distortion when the output is loaded.Edit: added "at TP12"

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 09:49:05 am
content: With load, but without signal (forgot to connect the cable):TP12: 0.1mVTP10: -236mVTP11: -492mVWith load, with signal (170mVrms 1kHz, all pots@5):TP12: 0.1mVTP10: -294mVTP11: -515mVOutput: -520mV.(all DC).I attached images with the 8R load. load1 are three different zoom (not sure if that helps). load2 is with touching TP12 with my DMM set at VDC (causes offset of 0.5V -probably normal).I'm clueless at this point.Could it be caused by some capacitors not filtering the DC? I have not replaced any caps yet.

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 10:11:12 am
content: Although not a massive amount, there should not be that much offset at the output. "TP12: 0.1mV" rules out an input problem.Measure pins 5 and 6 on the opamp for now. Use a resistor attached to the DMM probe so as not to disturb it too much.Im beginning to suspect board leakage. Has there been any yellow/brown glue on it? Any signs of extra flux having been used which could be the wrong type?

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 10:30:43 am
content: You mean 100k in series with the positive DMM probe?U5:pin 1: 19mVpin 5: 4.45mVpin 6: 1.5mV(all DC).There is this transparent/light milky colored silicone (acoustic vibration) around some caps and the 10W resistors, but I measure absolutely no ohms in them (Fluke 183). I tried removing some, but it is extremely elastic and hard to remove.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 10:49:20 am
content: I tried with a dummy jack in the headphones to break the connections:U5, pin 1, 0.0mVDCOutput still 520mVDCJW51: 80mVDCJW52: 134mVDC (which(In my early measurements before removing Q6/Q9/opto, etc, TP6 was 158mV and TP7 127mV.)I also measured resistance between gnd and headphones L+R (with amp OFF):TP6: 45Mohm, very slowly rising.TP7: 3Mohm and slowly rising.Does this mean something? JW52 is pulled "closer" to ground which results in more DC on this side?

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 11:54:32 am
content: Quote from: okw on September 30, 2021, 10:30:43 amYes put a 100K in series with the DMM probe if needed. You can monitor the output with your DSO while taking voltage measurement with your DMM. No need to use the resistor if there is little disturbance to the output.I think you are chasing a moving target. In this post you have Pin 1 which is TP 12 at 19mV which would cause the output to be 19 x 33 = -627mV. The opamp's 4.5mV input offset would alter it a bit.That adhesive sounds safe.You need to monitor the DC offset at TP 12 and TP 11 at the same time.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 12:12:05 pm
content: Wow, measuring TP11 while touching TP12 makes TP11 drop under the magical ~10mV.TP12: -0.1mVDCTP11: 8.3mVDCWhat does this tell you? Is it the internal DMM resistance that pulls TP12 down? What bad component does this lead you to?PS. no DSO reaction when touching only TP11. When touching TP12 it offsets about 500mV up.

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 12:56:29 pm
content: Quote from: okw on September 30, 2021, 12:12:05 pmI can't make any sense of that. TP12 is a low impedance point. It shouldn't matter what it's touched with. What is the voltage on TP12 measured through a 100K resistor?I haven't had much to do with JFETs, Ill leave that to others who know more.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 01:04:02 pm
content: Through 100k: 19mVDC and 165mVAC.Directly: 0.1mVDC and 159mVAC.

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 01:12:10 pm
content: Quote from: okw on September 30, 2021, 01:04:02 pmWeird things like that sometimes are caused by something oscillating like U5A. So it could just come good when the circuitry is restored back to original with new JFETs.Just try one more thing, bridge pin 1 directly to pin 2. and check what effect it has on the output offset.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 01:16:54 pm
content: U5 pin1 to 2?As for Q6/Q9. I can read the datasheet of J112, but I don't know which specs the transistors really need for this amp (many specs of J112 are over-spec-ed for the application, as for all components in all projects, just a few properties is important for each project).I have these. Would any of these replace Q6 and Q9? (short term and/or long term)?2N50882N39042N54572N22222N3053BF245BC548C (old, not sure of condition)C548 (old, not sure of condition)F485 (old, not sure of condition)And for later, what is a better replacement for the J112 (to keep in mind when I order from TME/Mouser/etc./or even cheap from Aliexpress)?

author: xavier60
author_rank: Super Contributor
timestamp: September 30, 2021, 01:28:05 pm
content: Quote from: okw on September 30, 2021, 01:16:54 pmYes, pins 1 and 2 of U5. Before doing this, try to observe the possible oscillation. The trick is to load TP12 as little as possible. Set your DSO probe to X10. If you see nothing, try a small capacitor, a few pF in series.All of those parts appear to be BJTs, ordinary transistors.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 03:43:04 pm
content: Quote from: xavier60 on September 30, 2021, 01:28:05 pmSo I want to 10x probe pin 1 and watch for oscillations, then bridge 1+2, while continuing to look for oscillations? Or do you mean oscillations on the output (j23)?What about BF245B? It seems it is a "JFET VHF/UHF Amplifier".Some discussion have been made here:https://www.diyaudio.com/forums/parts/183603-bf245b-bf245c-jfet-suitable-audio.htmlI'll get the proper components later, just for now.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 30, 2021, 05:57:55 pm
content: The Fet's aren't really part of the amplifier.They just cut the signal in the case of a fault (Q9), or mute the amp on power up, using a slowly ramped version of-16v, so it doesn't thump at turn on (Q6), so you can just leave them out of the circuit until you have sorted the DC problem.At this stage, I would just replace Q1, Q3, and Q4. That's 3 transistors I know I have in stock, it would take 5 minutes, they don't cost much, and if it still has DC, then I know it is not down to these transistors.That's a lot of knowledge gained for 5 minutes work and a couple of dollars of parts, as against hours of pondering and measuring.Sometimes knowing what the problem ISN'T is extremely valuable, as you can then use this fact to test the rest of the circuit.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 06:07:44 pm
content: I'll do that. I have MJE340 and MJE350G. But I don't have MPS-A06. Which in the list above, would you recommend for a replacement?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 30, 2021, 06:14:26 pm
content: 2n3904.It's pin for pin compatible, lower voltage than the MPS, but that shouldn't matter for testing the DC offset.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 30, 2021, 06:25:17 pm
content: Thing is, you can have a transistor like these that test perfectly OK, but in circuit they don't work.This is when you have to resort to just changing them, as you will never find the duff one otherwise.

author: okw
author_rank: Frequent Contributor
timestamp: September 30, 2021, 07:51:05 pm
content: I replaced Q1, Q3, Q4 with output reduced to around 340mV DC.BUT... I accidentally shorted the output pins. That blew the fuse. I replaced the fuse, and it blew again

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 30, 2021, 09:41:48 pm
content: Oh well, at least you know its not the op-amp, the protection circuitry, or the driver transistors.That kind of leaves what remains of the output Mosfets.It could also be that this is a crappy design susceptible to DC offsets that they never bothered to fix.And now, apparently, that's your job.

author: okw
author_rank: Frequent Contributor
timestamp: October 01, 2021, 09:17:07 am
content: As I accidentally shorted the output, it now blows the F1 fuse. Do you think one or several of the mosfets are shorted? Or anything else?I need to sort this out before I troubleshoot the DC further.If it is the mosfets, I would probably be better off removing all 4 mosfets, DMM testing them for shorts, and replacing the bad ones? Instead of replacing one at a time, powering the amp after each replacement (that could possibly harm the new one if the 2-3 others are shorted?).

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 01, 2021, 09:30:00 am
content: You can test for shorts in circuit first.

author: xavier60
author_rank: Super Contributor
timestamp: October 01, 2021, 09:42:16 am
content: The amp can be test run with one pair of good MOSFETs, just keep it lightly loaded.

author: okw
author_rank: Frequent Contributor
timestamp: October 01, 2021, 11:37:51 am
content: All 4 mosfets were shorted.Would x140 and x240 be interchangeable? I don't have IRFP140/IRFP9140 just IRFP240/IRFP9240.

author: xavier60
author_rank: Super Contributor
timestamp: October 01, 2021, 12:33:20 pm
content: Quote from: okw on October 01, 2021, 11:37:51 amIRFP240/IRFP9240 should be ok for testing the amplifier but not at full power, they have half the current rating.

author: okw
author_rank: Frequent Contributor
timestamp: October 01, 2021, 04:39:03 pm
content: Of course, I forgot to check. I thought I bought these to the BA115HP amp we are discussing, but I bought them for a SVT 3 PRO. This amp has a problem it all of the sudden dies. It could happen after a few minutes of playing or it could be good for months. It has under and over voltage shutdown circuits marked in its schematics. Could it be any of these?And would the fan keep running in case of under / over voltage? I don't remember if the fan stops also. Could be a good clue to watch out for that.

author: xavier60
author_rank: Super Contributor
timestamp: October 02, 2021, 09:34:02 am
content: Quote from: okw on October 01, 2021, 04:39:03 pmThe Shutdown disconnects the output only while either or both stress condition is detected, not latching. The fan will continue to run only while high temperature is detected. What voltage is the fan?

author: okw
author_rank: Frequent Contributor
timestamp: October 23, 2021, 12:13:38 pm
content: Sorry for the late reply, but the SVT was put in storage. I have it on the test bench now. The shutdown actually happened after just 4-5 minutes. Sine 1.0Vp-p input, tube gain (plate voltage) @ 10, gain @ 2-3, master @ 2-3. 8ohms 100W resistor over the output, oscilloscope connected.Suddenly the signal died. Quickly turning the tube gain (25 or more, either direction), restored the signal. But the signal dies again after a short time.I measured both output and effects send (preamp out). Preamp outputs signal, while output dies, so I guess preamp is good, but power amp fails?Is this of any help?

author: okw
author_rank: Frequent Contributor
timestamp: May 10, 2022, 10:05:32 am
content: After i accidentally shorted the output of my BA115HP and blew the FETs (3 out of 4 was shorted), I finally got around to replace them, along with Q3, Q6 and Q9. I don't remember if I replaced transistors Q1 and Q4 before or after output short (I think before). After replacing the FETs, they blew again along with the fuseCould Q1 and/or Q4 be damaged in the mishap, and now causing the FETs to once again blow? Or could the replacement of all transistors and FETs make the bias adjustment so way off it blew the FETs? Which direction (CW / CCW) will reduce the bias on AF1?I know on the SVT3 PRO, bias adjustment is extremely sensitive so it's easy to see smoke.I have a lot of all transistors (Q1/3/4/6/9), but only four IRFP9140 and three IRFP140. I wanna make sure they don't blow again (delivery of new ones take time).Before the mishap I replaced C11, C20, C31 and C44.

author: okw
author_rank: Frequent Contributor
timestamp: May 10, 2022, 04:41:42 pm
content: I removed Q1 and Q4 and if I'm not mistaken, these were swapped. Could that explain the blown FETs?