Question About Amp Schematic - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: September 11, 2025, 02:29:02 am
content: Currently I have another thread regarding the transformers in an amp I repairing (or trying to repair), but have a different question regarding a schematic I found online (see attached).The person who traced the circuits doesn't guarantee its accuracy, but either it has many errors, or I'm not understanding it correctly. Hopefully someone can shed some light on confirming my analysis.The output of the transformer measured 74V with 100V in; approx. 81V for 110V (I'm going on the lower side of 115-120 in the US). With a center tapped transformer, that places 41V (rounding up) on the cathode of the Zener. If the Zener is rated for 25V, approx. 17V on the input of the 15V regulator which seems too close for the rated 2V dropout voltage on the 7815. Even at 115V still cuts the 2V dropout voltage very close.Am I overlooking something because it seems it should have more headroom.Also, the main section at the top (with all the transistors) doesn't seem to show any of the voltages. I see ground points, but no voltages. The red LEDs both light during normal operation, but it seems like an odd configuration. Regardless of the LED configuration, I don't get how the collector of the transistors are getting any voltage.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 11, 2025, 05:19:13 am
content: The output stage’s collectors should be on be V+/- rails. Seems to be missing from the schematics. V+/- is marked on the power supply on the bottom.Yes, 78/79 series “LDO” needs more headroom. Perhaps the real circuit have some other better spec parts? Or the Zener voltage is not read properly? What are the voltages on the real circuit?

author: moffy
author_rank: Super Contributor
timestamp: September 11, 2025, 05:20:16 am
content: Quote from: bostonman on September 11, 2025, 02:29:02 amIt is unclear whether you are quoting DC or RMS voltages, the electros are rated for 75V DC, if RMS because your mains of 110V is RMS then 41V RMS is about 58V DC.

author: bostonman
author_rank: Super Contributor
timestamp: September 11, 2025, 12:02:07 pm
content: QuoteUnfortunately the entire system is shutting down due the problem I'm trying to troubleshoot.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2025, 03:31:44 am
content: QuoteActually, that's a good point and believe you're correct; clearly something I overlooked.Measurements on input/output of the transformer was measured with a Fluke either a 112 or a 179 (I have both, but don't have access at the moment to confirm which one - but RMS measurements).Therefore I measured 100V AC RMS (with a VARIAC) on the input side, 74V AC RMS on the secondary, and from either phase to CT 37V AC RMS. At 110V AC RMS, you're correct, it's about 58V DC leaving plenty of headroom for the regulator.The 75V filter caps seem underrated for a 58V line (and even higher if the voltage is the typical 120V AC RMS.As for the missing voltages on the schematic, I'm assuming the collector of the power transistors are their locations.

author: Whales
author_rank: Super Contributor
timestamp: September 12, 2025, 03:45:40 am
content: Zenersin serieswith the voltage regulator inputs? Huh???To stay within the 0.5W rating of those 1n5252b zener diodes they would have to be passing 20mA or less average current. There is no way that circuit uses less than 20mA per voltage rail.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2025, 04:35:53 am
content: I said the same when I saw the 500mW on the data sheet.I think the voltage to the translators is taken before the zener and the 15/-15 powers the diff input amp along with a few other low power circuits.It’s my assumption though.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2025, 11:23:45 pm
content: Maybe someone can shed light on my discovery.First off, the input of the unit has a MOV (I believe this is the component type) in series with the main fuse. The hot comes in, goes through a 12A fuse (accessible from the rear panel), into the "MOV", and the other side of the "MOV" goes to the relay including one smaller fuse, the other side of the fuse goes to a resistor, another small fuse, and then to neutral.When I measure the resistance of the "MOV", it's almost a short. I thought these start with higher resistance to limit the inrush current. I wasn't able to find the part number online, so I'm uncertain, but seems it would be blown if it were bad.The major break through with the right channel blowing fuses could be regarding the 10ohm 1W resistor on the bottom left of the schematic that connects Earth GND and chassis GND.From chassis to Earth was 74ohms so I lifted one side of the resistor and the resistor measured 74ohms. Wondering what the other two channels measured from chassis to Earth (without removing the resistor or the board from the unit), I got 225 ohms on the left channel, 132 ohms center, and, as stated, 74 ohms right (the channel blowing fuses).My thought is this is causing a grounding issue, however, it's odd the other two channels don't measure 10ohms either.Does anyone know if this could cause the fuse to blow when all three channels are measuring higher resistance?Give me a moment and I'll post pictures of the "MOV" and the resistor. Although the resistor clearly reads 10ohms, I want to post the picture to confirm I'm not completely crazy and this resistor actually increased in value.The big question is: would this cause the fuse to blow on one channel only when the other channels Earth to chassis GND seem damaged too - I'd expect at some point while the resistor(s) were losing their value that they'd cross the 74 ohm point, much like the right channel, and blow the associated fuse.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 12, 2025, 11:30:12 pm
content: An MOV connot not be in series, it can only function when in parallel with the supply. So either your circuit description is incorrect or it’s not an MOV.Unsure of the purpose of the 10 ohm R in series from Gnd to earth.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 12, 2025, 11:38:02 pm
content: A resistor going high in value does not blow a fuse. It went high in value first, before the fuse blew, because it was over stressed (excessive wattage) so either earth floated up or something’s shorted causing excessive current through the resistor.Could have been some kind of incoming power problem with an improperly ground receptacle or who knows what. Too may things to speculate.

author: bostonman
author_rank: Super Contributor
timestamp: September 12, 2025, 11:45:25 pm
content: I thought maybe it was causing a grounding issue that may result in the fuse blowing.All three channels seem to have this issue since all are much higher than 10 ohms.So I need to keep looking at what is blowing the fuse.

author: floobydust
author_rank: Super Contributor
timestamp: September 13, 2025, 02:00:14 am
content: I would stop applying power to it, and just diode-test the output transistors and see if any are shorted. Because there are 4 transistors almost in parallel, any one failed would upset the reading for all of them. You can compare readings between channels. Also using Ohms as well. Work backwards to the driver stage etc. Look for shorted/failed semi's.It's not a MOV, looks like an NTC inrush limiter possibly 10 ohms cold. Typically in series with the power transformer primary, and a relay will later short it out once the rails come up or a timer is done. If the rail does not come up, these parts usually sit there very hot. as the soft-start never finishes.The 10 ohm resistor to earth and chassis GND is to prevent ground loops.It's hard to fix an amp like this because it has extra complexity with the op-amps involved with the DC servo and amplifier input.To repair with power, this I'd use a dim bulb tester. Latger... probably need a dedicated +/-15V lab supply for them (op-amps) to work while you undervolt the main power amp and find the faults there.The schematic is only of the basics and by/for someone who knows power amp architecture. You can leave out many wires that are ordinary such as DC feeds to the output transistor collectors etc.One danger is there are many junction dots missing so the schematic is not for people who don't know how these are typically wired up.

author: bostonman
author_rank: Super Contributor
timestamp: September 13, 2025, 01:46:38 pm
content: That (possible) NTC component looks like it's in the circuit as I previously described, however, I'm not putting much into analyzing the "Main Board". I looked at it out of curiosity to see if a possibility existed something could be causing the "Power Board" to blow a fuse. All I did was shine a light behind it to see the underside traces. As expected, all three fuses are fed by the same trace and each fuse goes to each transformer; so they are isolated from each other.Measuring across the NTC is about 3.3 ohms room temp, and with a hair dryer used for a short time, dropped to about 2.1 ohms. Although I called it a MOV, I knew it couldn't be in series, so thought maybe it was some variant of a MOV.In any case, thanks for the advice. I've already loosely measured every transistor and believe every diode multiple times. Before removing another board, I was trying to approach it methodically and sort of a shot gun approach. Usually stuff like this has an obvious failure and why I go after anything on a heat sink, and look for blown higher wattage resistors, but this isn't the case. My other attack was thinking the sil pad developed a crack and one of the transistors was shorting to the heat sink, however, I didn't measure any shorts (pin to heat sink).Besides the higher resistance resistor (I'm still amazed all three boards have this failure and the resistance drops by almost 30% from one to another), the TIP 120 measures low resistance (without yet diving into the datasheet to tell which pin is which, I'm getting 120 and 230 I think one pin to another and another pin to another).Since the schematic doesn't show any parallel lower value resistors,I thought this could be the culprit, but the other channels measure identical values.Ironically, this amp was bought new (I know because I've been friends with my friend since) and never repaired, but the TIP 120 has solder work indicating it was either sold as "new" but was a repair, or some sort of rework at the factory. Since I haven't removed the other boards yet, I don't know if they too have solder work.In any case, I was hoping the TIP 120 is the culprit, but comparing measurements and seeing the parallel resistors/pot, it doesn't appear to be.Although, assuming the schematic is accurate (which I'm taking with a grain of salt), I don't understand whatit'sQ18 is doing. It has positive on top and negative on the bottom without a clocking signal on the gate. All it tells me is it's somehow trying to balance the two rails since it would see zero volts across it (or at least I think - I'm not the best "design engineer").As for the schematic, you made me feel better because I think the schematic is simple, but confusing too (assuming it's drawn accurately). Simple in the sense that bottom part of the circuit is a mirror of the top, then as you move left, the other parts of the circuits are mirror images. At the same time it's confusing because, as you said, not knowing amps doesn't make understanding easy. Plus, I tend to get overwhelmed when I see a combination of positive and negative voltages with op-amps feeding back to other parts of a circuit.What do you mean about the dim bulb tester? I know this is usually done to test transformers (although I only did it once some time ago to figure out the wiring - with the help of some individuals on here). The 15V and -15 in my assumption is for theTIP 120Q18 and op-amp(s), but guessing the power transistors take the entire approx. 58V.

author: bostonman
author_rank: Super Contributor
timestamp: September 14, 2025, 01:33:52 pm
content: Just an FYI, I made two corrections to my previous post.I accidentally confused the TIP 120 with Q18.

author: bostonman
author_rank: Super Contributor
timestamp: September 26, 2025, 01:56:50 am
content: I wanted to post an update because I'm slightly confused about something (schematic is in my initial post).After measuring both diode and resistance through out the board and comparing it to a good channel, I didn't see anything odd (ignoring the frustrations of caps charging causing negative and increasing resistance).Using my multichannel bench top supply, I began by setting both channels to 5V with somewhere around 200mA current limit. Unfortunately unless I connected two supplies and carefully moved both simultaneously, increasing the voltage was limited to one channel at a time forcing me to turn off the output each time I wanted to increase the voltage.Eventually I kept increasing the voltage (5V at a time) until the supply maxed out at 31V, and it didn't experience any current limiting. Knowing the board needed more voltage due to the 24V zeners, I shorted the two zener diodes which gave me the voltage the board needed at the regulators; which was close to what the board would see powered to AC.With the zener diodes shorted, the board seemed to power fine drawing about 75mA max on each rail. The only anomaly was the bench top supply would quickly go into current limit at turn on, or seemed to quickly display current limit at turn on but then was normal. The most I set the current limit to was 2A - the inline fuse inside the amplifier is a 2.5A SB and figured 2A is more than enough. Most likely due to the two capacitors on the amp board, but figured I'd mention that anomaly.Taking random measurements, I saw some of the following:TIP120 (Q17) BCE, 49mV, 1.092V, -1.063V respectively.2010 (Q18) DGS, -11mV, 16.44V, -19mV, respectively.The power transistors had approx. the same voltages (only opposite polarity): 0V on the emitters, 0.548V on the base (negative on the other power transistors for the other rail), and the collectors are tied to the rail (approx 28V DC on the bench top supply connected to the board).At that point I removed the shorts across the zeners, connected it to the transformer inside the amp, connected a VARIAC, etc... and the board powered fine.When the amp blew the fuse at my friend's house, it was on and connected to the speakers. I'm uncertain if he left the stereo on too, but the system wasn't being used as in playing music.Having said this, the only other differences in connections is the feedback circuit (the bottom right of the schematic) isn't connected - doubt this would play a role in blowing the fuse, but wanted to mention it - couldn't connect it because I don't have a spare 2.5A SB (I took one from another channel) and the unit would be in fault state as a result of the other channel not turning on.This is the baffling part and maybe I can get some input. My only guess is a capacitor inside the speaker shorted, drew high current, and blew the fuse, however, the datasheets for the power transistors state 1.2V B-E, so it seems the output transistors would have been turned off throwing my theory out the window.

author: floobydust
author_rank: Super Contributor
timestamp: September 26, 2025, 09:43:10 am
content: Not 100% following the threads. On the bench, this channel blows fuses with mains power?The VBE multiplier TIP120 voltages look reasonable, it needs about 4*VBE across C-E which you've got. This is good, implying the driver and output transistors E-B junctions are OK. It also means the muting mosfet is biased on, turning Q19? MPS8599's off. If they are on = muted and low idle bias, all for power on/off anti-thump.I would measure and record the voltages at the op-amp pins as a last check.Put a sine wave or music into it then, with the lower supply voltages. You can use a resistor in series with a speaker for a light load. Looking for gross distortion.So far I don't see reason for the amp to (normally) crowbar the supply rails and blow fuses. This is usually a blown VBE multiplier or bad connection to it, Q17.The circuit at the bottom right with Q14 is protection. DC detect (on C3) OR the thermal switch opening will turn on Q14. There is no overcurrent sensing in this amp schematic.I noticed the amp appears DC-coupled at the input. Is this true?

author: bostonman
author_rank: Super Contributor
timestamp: September 26, 2025, 01:29:49 pm
content: QuoteNot a problem, I'll give a brief overview. The other thread was regarding the Toriod transformer and it got answered whereas this one is more specific to the schematic. The schematic was created by someone who traced the board, so it's uncertain if it's accurate (I haven't found any discrepancies so far).This amp was left on 24/7 (not sure if the stereo was powered too and volume down). One day a loud pop was heard, and then about a minute later a second pop. The amp only has a power button and red/amber LED, nothing else on the front panel. In normal mode the LED is red (I think), but in failed mode it's amber; the LED was amber after the pops. No smell, no smoke, nothing, just an amber LED and the amp wasn't working.I opened it and saw the three channels (R/L and center) are identical boards each fed by a separate 2.5A SB to each Toriod transformer dedicated to each board (there is a main fuse too - I think 12A). The right channel fuse was blown indicating a short.Moving forward, I powered the board with a DC bench supply and didn't find an issue. I shorted the zeners so I could get more voltage into the circuit as it would normally have (my supplies only go up to 31-35V), and still didn't see an issue. Now I've placed the board back into the unit (well laying outside the unit) and it powers fine with AC feeding the Toriod directly. The only difference is: I'm not going through the main board because I don't have another 2.5A SB so currently that feedback line is disconnected - the main board only feeds 120V to each Toriod and has the feedback line, nothing else (except it feeds the front panel power switch and LEDs). If I try powering the unit, the main board will shut down the system because whichever channel is missing the 2.5A SB will tell the main board to shut down - hence why I've bypassed the main board for now.My thoughts are: since I didn't find anything shorted, the only way the 2.5A SB could have blown was if the speaker shorted the output (maybe the capacitor inside the speaker shorted), but that would mean the driver transistors would have had to feed current (maybe the stereo was powered on thus causing the transistors to be biased).Another thought (and I've checked this a hundred times without finding an issue) is the sil pad failed causing a power transistor to short to the heat sink. The only oddball thing I saw are some of the transistor mounting screws aren't shorts to the heat sink, but have some resistance - but the good channels have similar measurements. Seems odd a metal screw touching a metal heat sink (maybe the holes have paint inside them since the heat sink is black and it's not making direct contact) wouldn't be zero ohms. Personally I'm not a fan of the sil pad being the deciding factor between the transistor shorting to a grounded heat sink and being isolated.Also, the only issue I found was the 10ohm resistor (bottom left on the schematic) that connects chassis and ground together reads 75ohms (center channel is 175 ohms I think and left channel is 250 ohms I think). Initially I thought that had something to do with messing with grounds, but believe someone said that wouldn't be the case.QuoteI'm not sure, but can check if need be. Whoever took the time to trace this and create a schematic deserves credit as it's been a help.

author: bostonman
author_rank: Super Contributor
timestamp: September 30, 2025, 12:37:27 am
content: The only parts I found bad (which I've already mentioned) is the 2.5A SB (it's marked Buss MDL - datasheet confirms it's SB) and the resistor from Earth GND to chassis GND on all three boards.The resistor clearly shows 10ohms (with a J at the end). The schematic someone created (which could be incorrect) states it's a 1W.Since the resistor doesn't state anything about wattage, the schematic could be incorrect, and the amp is over two-decades old so the physical size of components could have changed, does any other way exist to confirm it's 1W?I planned to go higher since I'm only buying three (probably more to keep on hand), but sometimes I worry that the component is sized so if something were to happen, it would burn and open whereas if I hypothetically put a 3W, it would keep passing current.Any suggestions on power selection?

author: floobydust
author_rank: Super Contributor
timestamp: September 30, 2025, 05:12:23 pm
content: If you can give part's size (dia, length) I can check with many manufacturers I have data for and see what power it ballparks to.It looks like Panasonic/Yageo/Fukushima etc. brand, should have more markings to say what series it is.Were these fused open circuit? That would imply a ground-fault happened. Mains hazardous live hits the amplifier ground and takes out the resistor. A little math and it's a 1.4kW until the mains fuse blows. I hope you did not see this. It would also mean melted PCB traces.In newer "audiophile" designs, they have a 25A bridge rectifier for clamp diodes across the resistor and it's a 1W part. So you can have audio ground up to +/-2V away from earth ground before the diodes hard clamp. But even then, that would take >200mA and where is that coming from on audio ground?

author: bostonman
author_rank: Super Contributor
timestamp: September 30, 2025, 06:28:59 pm
content: I removed one completely because I saw writing on the underside, however, it's a repeat of the top (10 > ohm symbol > looks like a dot > letter J).Obviously the "ohm symbol" is ohms, but, since I'm asking to decipher the power, thought I'll be specific in case it points to a specific manufacture that writes values in such a manner.The dimensions I got are L x W: 0.465" x 0.14"As the picture may or may not show, the ends are flared a bit, so the width on the ends is about: 0.155" (i.e. 0.015" larger than the main body).The picture shows it as somewhat pink, it must have been the lighting because they are grey in color. Also, it's relatively smooth, but, rubbing my fingernail seems like it's a bit rough coating.To answer your question, no burn marks or any indication of an issue anywhere on the PCB or components. The resistors are not fused open, but are higher in resistance. The channel that blew the fuse (right channel) measures 74 ohms, the center channel measures 134 ohms, and left channel measures 227 ohms). Personally, with the exception of SMT resistors having resistance issues due to the end caps, I've never seen a TH resistor change resistance by a tens of hundreds of ohms.In any case, the right channel isthe only one blowing fuses. the only one that blew a fuse, but isn't now while being powered directly by a VARIAC (but no input signal, no speaker connected, and feedback not connected).Besides not fully understanding the schematic, one thing I'm puzzled by and maybe it's a hint at something else being an issue: each board has its own fuse, the right channel is the only one that blew. When power is turned on, under normal conditions, the unit remains with an amber LED regardless if it has an input signal from a stereo or not. Currently it's red, and I've assumed it's red because the (what I call) feedback line isn't sending a signal to the main board due to the blown fuse. In turn, the main board doesn't detect a signal and puts the unit in standby.Looking at the schematic, that feedback circuit (bottom right) has two paths to send a signal to the main board. One is if the thermal fuse opens (a fault condition), that turns on Q14, pulls current on the PNP above it (no ref des on the schematic), and sends a signal to the main board. Obviously this isn't the case since the fuse is short (I measured it) and the unit is cold.The second path seems to be if DC is present on the output, that would send voltage to the bridge rectifier, turn on Q14, etc...The confusion is: what is the second path actually doing (C3 doesn't help reduce my confusion), and have I been wrong to assume the red led on the front panel is due to the right channel having an open fuse?

author: floobydust
author_rank: Super Contributor
timestamp: October 01, 2025, 01:53:46 am
content: Oh it's a Mark Levinson? Did you know he was married to Kim Cattrall.How are the fuses set up in this amplifier?Usually there is one for each power transformer primary (mains), and two more for DC (+) and (-) rail.Which fuse are you talking about that blows? If one DC fuse is blown then the amp is (powered) out of balance and DC offset at the output will happen, Q14 on.I'm assuming the front panel orange/red LED is showing protect. See if P4 has +ve voltage meaning Q14 is on. Or unplug it and see if the front panel LED acts different.The resistor is most likely a 2W reminds me of KOA SpeerMOX2or MOSX2 series, they are pinkish MOX.YageoKNPwirewound 1W or 2W looks OK for a replacement too.I haven't seen power resistors increase in value unless they are getting transient overloads. Seems strange.

author: bostonman
author_rank: Super Contributor
timestamp: October 01, 2025, 03:14:04 am
content: I don't get the Mark Levinson comment?The amp has a bunch of fuses. It has a main fuse on the rear (12A), two small fuses just after the input on the "main" pcb, and three 2.5A SB on the "main" PCB. I don't know what the two smaller fuses are for, and the 12A on the rear is obvious (it's the main fuse).Also, the power boards themselves don't have any fuses except the thermal fuse that's mounted to one of the positive rail transistors.Each one of the 2.5A SB fuses on the main board power each Toroid transformer for each channel; so one fuse for the right, one fuse for the center, and one fuse for the left. The 2.5A SB for the right channel is the one that blew.Currently I'm powering the right channel from a VARIAC with a 2.5A SB installed, into the right channel Toroid, and the same right channel board that initially blew the fuse. I've powered it several times in this setup and it hasn't blown the fuse. The only difference is: the channel doesn't have an audio input connected, the feedback isn't connected to the "main board", and no speaker connected.You're thinking that resistor is 2W wirewound? This is valuable and appreciate it.As for the front panel, all it shows is amber or red, nothing else. Currently it's red, so I don't know how the main board knows the right channel has a blown fuse since the feedback (from my understanding) needs to send a voltage to the main board when something is wrong.I'll take measurements, and posted some of my previous measurements before, but I'll take more.

author: floobydust
author_rank: Super Contributor
timestamp: October 01, 2025, 08:39:52 pm
content: Proceed was from Madrigal Audio Labs who bought the Mark Levinson trade name. Interesting trademarkLawsuitover that. Past decades it's owned by Harman Int. now I believe. There's politics with high end gear. Never mind Samantha Jones lol.Front panel LED red = standby mode, amber = on.It's got 3 power transformers? I would expect two DC fuses per channel, as well as a transformer primary fuse.You can get an idea of output stage (idle) current by measuring DCmV across their emitter resistors. But the manual says mains consumption is 210W at idle so it's biased heavily, more into Class A I think.It's not making sense so far- the amplifier channel is drawing high current yet you measured decent voltages as if there are no failed semiconductors or high bias problem due to losing the VBE multiplier.

author: bostonman
author_rank: Super Contributor
timestamp: October 01, 2025, 10:45:26 pm
content: QuoteYes, three transformers (one for each channel) and only one 2.5A SB for each transformer. There are two smaller fuses near the input, I assume for the electronics on the main board; and the 12A main on the rear panel - see attached picture of the "main board".Since there is a pot (only one pot) on each power board, does the possibility exist the DC bias voltage needs to be adjusted? Unfortunately I don't know what this pot adjusts (I assume the emitter voltage on the power transistors), but not sure what it should be.Most likely I can measure a good channel, but maybe another channel has drifted too.I'm somewhat out of options at this point and plan to order new resistors (the ones that went high in resistance for some odd reason that connect Earth and chassis GND) and a fuse, but it doesn't make sense to assume nothing is wrong since it blew a 2.5A SB.

author: floobydust
author_rank: Super Contributor
timestamp: October 02, 2025, 01:44:33 am
content: The amplifier uses a DC servo, one op-amp making sure the output is at 0V. The trimpot is for setting bias (idle) current which is measured as the voltage drop across the emitter resistors.If the amplifier was putting out at high power and loudspeaker (+) touched earth ground, it would cook those resistors. But they show no signs of heating up or long-term overload for them to change value.It's like a ground-fault/surge or short-circuit happened making the loud pop. I wonder if the loudspeaker survived?If you can power up the amplifier even low on the variac, maybe measure the op-amp pin voltages, or some more transistor voltages and post here. That will narrow it down.

author: bostonman
author_rank: Super Contributor
timestamp: October 02, 2025, 02:04:59 am
content: If I get time tomorrow, I'll power it and take more measurements. If not tomorrow, this weekend.I appreciate the assistance so far as this would baffle me due to not seeing any obvious failures; or reason for the "pop".

author: bostonman
author_rank: Super Contributor
timestamp: October 02, 2025, 11:56:10 pm
content: I took more measurements.I noticed the op-amp on the schematic is OPA2604AP but the part is marked OPA2132P. Also, if my measurements have a discrepancy in the area of tens of milli-volts (i.e. the collector on the power transistors versus another component connected to the rail), I noticed positive and negative rails changed as the unit heated during my measurements - uncertain if this could be due to old main caps.The measurements are:Positive power transistors (BCE): 0.563V, 50V, 3mV (but increased to 8mV when I remeasured after the unit was on for >15min)Negative power transistors (BCE): -0.559V, -50V, -4mV (but increased to 8mV when I remeasured after the unit was on for >15min).Also, now that I'm looking at my measurements for the negative rail power transistors, maybe I should recheck base and emitter because I didn't write negative, but would assume all three legs would be negative. I was focusing on not shorting anything or getting zapped, so I may have accidentally forgot to include the negative. I'll recheck tomorrow if need be - but trying to avoid too much live measuring to avoid accidentally shorting something.Q17 (BCE):-51mV51mV, 1.11V, -1.08VOp-Amp (1, 2, 3, 4, etc...): 10mV, 0V, 0V, -15.2V, 0V, 0V, -45mV, 14.92VPos 15V regulator: 19.94V input, 14.92V output (as expected)Neg 15V regulator: -18.25V, -15.20V output (as expected)Q18 (SGD): -8mV, 8.45V, -23mVQ16 (BCE): 1.07V, 49.26V, 0.574VQ2 (BCE): -1.04V, -49.25V, -0.559V

author: bostonman
author_rank: Super Contributor
timestamp: October 03, 2025, 06:17:17 pm
content: Just an FYI, I remeasured the negative power transistors. As expected, the voltages are negative.Also, when modifying my previous message, I noticed Q17 had the incorrect polarity on the base (I also fixed it).

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2025, 07:55:09 pm
content: Those voltages look fine to me. No smoke, no short circuit etc. It should play music OK. Nothing to pop the fuse. Unless something is later heating up, or there is a bad connection which tapping a plastic pen here and there can flush out.Bias current rising to 8mV/0.22R is 36mA per pair which is reasonable. You can compare other channels as well (mV across the emitter resistors), it's important to look at each one separately in case one of the paralleled output transistors is fungoli.What about Q14 the protection transistor, and the PNP it connects to? Balanced input U1 SSM2143 voltages OK? I'm not sure if it was being run balanced or unbalanced input.Or even a bad (oxidized) switch contacts on S1 would give quite a pop, according to the schematic.

author: bostonman
author_rank: Super Contributor
timestamp: October 03, 2025, 09:31:47 pm
content: Q7 (not marked on the schematic - the one that connects to P4) from BCE: 49.20V, 20mV, 49.30VQ14 BCE: 0V, 49.55V, 0VU1: all pins 0V with the switch in both positions except 14.92V and -15.2V on the two power input pins.At this point I'll order a replacement fuse (2.5A SB) and the 10ohm resistors that connect chassis and Earth; you mentioned wire wound and 2W, is this correct? The schematic shows 1W, but not sure if that's correct and I'd like to go larger if possible.Two technical questions about this schematic regarding the unit not working. I can measure all the emitter resistors including the other channels to compare, but what voltage should I expect across the resistors while idle?Also, I don't get that feedback/shutdown circuit. As mentioned, the unit immediately turns off the main power relay when trying to power the amp. I thought the feedback circuit was telling the main board to shut down because (in this situation) the fuse was blown. Looking at that circuit, the only time it should send a signal is when the speaker output has DC and/or the thermal fuse opens (as you previously mentioned). Does this board quickly send a signal to the main board at power up to tell the main board it has power or can something else be wrong with the unit?Perhaps maybe another board (i.e. channel) is sending DC voltage due to an issue to the main board (something I'll have to investigate), but wanted to ask if I'm overlooking something in that feedback/protection circuit.

author: bostonman
author_rank: Super Contributor
timestamp: October 12, 2025, 07:03:40 pm
content: I took measurements across the 0.22 ohm resistors at the emitter of all the power transistors simultaneously (see attached) using 55V DC on the rails.The owner of the amp was over yesterday and I was explaining to him how things didn't make sense and no reason the fuse to that board would have blown. Apparently he completely forgot one channel hasn't worked for well over a decade which began shedding light on things (he was only using two channels and switched to the center channel when the right stopped working). The one confusion was: if the right channel board was bad with a blown fuse, how was it sending a signal back to the main board to immediately shut down.Turns out, with a fuse powering the center channel and right channel (the one that had the blown fuse but nothing was wrong with the board - i.e. the one I've been looking at this entire time), the unit turns on fine. When I put a fuse in to power the left channel, that's when the unit goes into shut down.This entire time I was troubleshooting the right channel because it had the blown fuse, but turns out I was chasing the wrong issue.Now I've confirmed, the left channel causes the unit to go into shut down immediately. All three boards have two red LEDs (I guess they are on when the board is working) and they both light for the brief moment at turn on before the unit goes into shut down mode; and the fuse isn'tshortedblowing. This tells me it's not a hard short, but for some reason the feedback circuit is telling the main board to shut down.The interesting thing: immediately when I opened the unit, I noticed yellow dust around the positive filter cap on the left channel and it has a slight bulge on the top (the negative one and the ones on the other boards are quite flat). The caps have black wrapping, so I can't tell if the bulge is due to the cap being bad or just poor wrapping. As for the yellow stuff, I thought it was carpet dust or whatever, but I've never seen yellow dust from a bad cap.Since the fuse to the right channel was blown, I figured the right channel was the culprit and ignored the bulge in the cap in the left channel, but now I'm second guessing whether that bulge in the cap in the left channel is causing issues and/or if that yellow dust is worth paying attention to.These are Phillips 3186 series capacitors (P/N 3186DC213M070HPA3) with solder leads. They are 21,000uF, 70 VDC, and 95 V surge.I don't have a plan at the moment. I'm waiting to see if my friend wants to continue having only two channels. If I attempt to repair this board, most likely trying to power it with a VARIAC isn't safe since the board doesn't want to be powered. I may resort to a DC power supply, but this may not help figure out if the cap is bad.Has anyone seen yellow dust from the bottom of a cap? In my experience it's usually brown crud.