Hello everyone, this is my first post on here. Sorry for it being long winded, but I want to try to give you as much info as poss.
I would appreciate some help/guidance for my HH MA 150. The problem is, it blows both fuses instantly. I have checked the output transistors, they are fine, and I even disconnected them from the main board in case they were shorting to the chassis some how, but fuses still blown. I am confident filter caps are good as I disconnected the power leads, (the wires from the caps to the main board) and this time the fuses didn't blow. So I assume everything upto there is fine. I then checked all components on the main board, There were many problems, most transistors were faulty,I replaced Q2,3,4,5,6,7 ( these were all faulty when checked out of board too), also many resistors were out of spec and there had been a burn out around R 22,24,28, one of those 3 was totally burnt black but I can't remember which one. I replaced R4,13,19,20,22,24,26,27,28. I was confident the problem would now be solved, but no, the fuses still blown. I also checked all diodes and they are fine. Any help would be very much appreciated as I am now stumped.
I used to have that schematic some 3 computers ago, it´s probably on some old hard disk.
If anybody has it, please post it here, otherwise "R4,13,19,20,22,24,26,27,28." and others mean nothing to us.
In any case start building a lamp bulb limiter, you´ll need it , search this very same Forum for it.
HH MA150 schematic:
JM Fahey, you are right the numbered resistors are no use without schematic, doh sorry about that, I actually have the schem too and didn't think to put it up. Also thanks Jazz P Bass for posting it. Any ideas at all. Also yes JM Fahey I have read about the light bulb tester, I will build one, but am a little concerned that if I use it, there is still current getting to the rest of the board etc, wouldn't this still do some damage to components?
That is the point of the bulb, most of the voltage is dropped across the lamp under failure conditions. It certainly puts less current through a circuit that just plugging it into the wall.
Grogg say "lightbulb limiter goooood" ;)
Ok, thanks Enzo and Galaxiex, and indeed Fahey, I realise how the lamp works and that there is a lot less current going through the circuit, I was just concerned that it still may burn something out after a while. However if you boys say it's ok then I'm with you all the way and will defo use the lamp limiter. I really appreciate all help given here. I have the lamp limiter and am keen and ready to go!
Help!! :loco
I don't have the head to wrap around it at the moment, but do not assume it has to be a pad part, a broken trace or solder joint can be just as damaging to its function.
If it isn't the power supply, then the amp circuits are bad in some way. If the bias circuit opens, then both polarities will turn on at once, blowing fuses.
Rightio Enzo, for now then,I will check all traces and solder joints on the power supply board and wait patiently for you or other replies. Thanks for checking in.
Quote from: Melv1 on September 30, 2017, 08:07:49 AM
I am confident filter caps are good as I disconnected the power leads, (the wires from the caps to the main board) and this time the fuses didn't blow.
This leads me to the exact opposite conclusion, that one or both caps was causing the fuse to blow.
I might be wrong, but I read that as the caps are in the power supply, which he disconnected from the "main board" which I interpret to be the amp circuits. So if I am right, the caps and power supply alone will hold a fuse, but connect the amp circuits and it blows.
That's right Enzo, When I refer to the "main board", I mean the board with all the transistors on, Q1 to Q7 on the schematic. So I disconnected the wires that lead from the filter caps to the board (I de-soldered them at the board). The fuses didn't blow when I did this, so I'm pretty confident everything is good to that point at least.
G1, I am confused why you say that "leads you to the opposite conclusion", as the fuses are before the caps, so if the caps were shorted, surely the fuses would still blow?
I THINK g1 was imagining you pulling the transformer wires off the power supply.
Yeah probably. Sorry if I don't use the correct terminology here and there, I am English and guess you good people are American? Therefore I may explain things a little differently. :-\
Sorry I misunderstood, I thought power supply and amps were all on one board and you simply disconnected the main caps.
Carry on. ;)
Hey no probs G1.
:tu:
Bit of an update. I have checked the tracks on the board and some of the tracks have had repair work done and are a bit shabby, one even has a wire soldered to it and is soldered further down the track to make continuity. However, that same track is shorted to it's adjacent track where an attempt at repair work has been attempted. Hope everyone's following this lol.
Anyway, one track is connected to the Collector of Q7 and the Base of the main output Q2. The other track is connected to the Ground wire terminal in which one of the filter caps connects to and the other end to R32. I'm pretty sure they are shorting to each other where the attempted repair is, as there are a few dodgy blobs of solder, but it's hard to see them touching even with a magnifying glass. How I've established a short between the two tracks is I did a resistance test between the two tracks and there is continuity. Am I correct in assuming there shouldn't be or are these connected to each other further down the line anyway? Either way I'm going to try and desolder the dodgy area to separate the tracks there. This just could be the whole problem! ;)
Well I've done all I said above and no traces are touching each other now. So I tested it with the lamp limiter and the bulb lit up ::) Once again I'm stumped! Can someone give me any ideas? Those two traces I was talking about in my previous reply are now getting a 33ohm resistance across them so no longer directly shorting together but are connected some how, does that seem right?
Quote from: Melv1 on October 02, 2017, 03:59:09 PM
The other track is connected to the Ground wire terminal in which one of the filter caps connects to and the other end to R32.
Hum?? That's not what the schematic shows. ???
The Centre Tap of the main filter caps (which is circuit common) goes to *G* on the PCB board.
To get to R32 it has to pass through R31 first. The junction of R32 and R31 is the speaker negative.
R31 is a 5 watt OR1 (.1 Ohm) which is a *current sense* resistor (shown as S on the pcb connection)
These circuits have 2 types of feedback,, being Voltage FB and Current FB, R32 is the current FB
The speaker neg is lifted off ground (circuit common) via that CS resistor and the output sockets have to be isolated from ground. If those sockets get grounded back to circuit common in some cases it can burn out the power stage.
Others here will know far more but meantime if you can post some hi res pictures of what you have in front of you then it may speed up the process. :tu:
Phil.
Thanks for your reply Phil. I will check those traces on the board again against the schematic and make sure everything is as it should be, I may have followed them wrong. Also thanks for explaining what each of those resistors do especially the R31 as I was wondering what that S stood for. I will have a good look at it in a couple of hours time.
When working between schematic and PCB it's easy to get lost,, It took me quite a few years to master the art and I still get lost with more complex circuits. :duh :duh :duh One trick is to turn the PCB around so you are not looking at it from the same direction all the time. this forces the brain to take a different view of the track layouts and can help avoid the same mistakes. same for Schematics turn them upside down and often you find it's not what you thought. and take a break if you get lost and the brain starts to fry with frustration. xP
Phil.
Wow! Good advice Phil, I will certainly do that as I do struggle and get lost at the mo. I do have an NC in electronics, but that is only the basics really. But I do have a massive appetite to learn. Infact most of my learning has came from books and the internet, so I will certainly take on board what you've said. Thanks.
PS. I will try toput some photos on later.
Don't forget Rod Elliot's pages which are aimed at folks just like you who are keen to learn.
http://sound.whsites.net/index2.html
Look at project pages for some explanation of how these things work.
There are many other pages on subjects that will help you grasp how to trouble shoot amplifiers. It's a massive site with a massive amount of info.
Rod helped me out a few times and got me out of a few jams even though I have never purchased any of his projects. :dbtu:
Phil.
Thanks Phil, I will defo check that link out when I've got time.
I still have the same problem folks. Any ideas at all please?
The reality here is that you have just shot gunned a whole lot of components without FIRST establishing what is at fault. That approach actually makes it harder to find the real issue and now you may never know the cause? :duh
Put the lamp limiter on the amp and measure some voltages.
Start by reading the 2 supply voltages (which will be lower while lamp limit is on) and measure the DC voltage at the speaker terminals?
Maybe start with a 40 to 60 Watt bulb which will glow bright but keeps it all from blowing up in an instant.
If R28 is burnt then it's likely blown Q7 and Q2.
If R22 and R24 have burnt then Q4 and Q5 may have a problem.
That whole middle section around Q4 and Q5 is the current limit circuit which is supposed to save the amp from overload but just like all electronic circuits,,, what happens if the backup system fails?
That works for intermittent shorts and overloads but NOT meant for continuous shorts. :trouble
If you do not know the history of the unit then it may have been flogged to death with a low Z speaker load and finally blew up. Sadly a lot of muso folks often have very little understanding of the limits of their equipment and as long as it SEEMS to work and makes sound it must be alright. yeah it runs really hot and blows a fuse every now and then but it keeps going :loco :loco :loco
So get some pics and report the speaker voltage.
Phil.
Okey doke phil, will do.Thanks for your advice. Although I wouldn't say I've just shot gunned a load of components. I did actually use a structure as I always have when I've successfully fixed other amps. I always first check the power supply and then the signal chain. But like I say this just instantly blows fuses so I first checked the obvious and took it from there, and then discovered all the blown components listed earlier, so replaced them. I don't really see what I've done wrong there, or what I could've done differently?
Yes you replaced some parts which may well have been faulty but without first establishing the fault it just instantly produced the same fault and blew the new parts. :'(
These are DC amps and if something is not right then as above,, instant destruction. :trouble
A DC (Direct Coupled) circuit means that ALL the transistors are directly coupled so if one blows then more often than not it will take out other parts,, as you noted it had at least 3 burnt resistors. You can't reset these suckers without a Variac or Limiter of some kind and bring them up to voltage and current softly.
90% of power amps all work in a similar way, they are just High current opamps.
The old Valve circuits were AC coupled,, meaning they had a DC blocking capacitor between each stage so each stage was isolated from the others. Any faulty valve was just replaced and all was good,, but DC SS Amps are a very different beast.
When working properly these SS Amps will keep the input and output as close to 0VDC as possible hence the middle of the split supply is Zero volts DC which should also be the DC at speaker,,expect +/- 100mVDC at the speaker when working right.
When a transistor blows it often pulls the output to one of the supply rails and the output will sit at Pos or Neg rail voltage and burn out the speaker VCoil.
You have not mentioned the ua741 opamp and if one of those Zener diodes have blown it would be a major issue. They regulate pins 4 and 7 at +/-15 volts.
I'm only a hobby geek so others here will have far more experience.
meantime fire up through the limiter and measure voltages.
spk out,, supply,, and 741 pins.
Phil.
You are a top man Phil, and that was some really useful info, thankyou. Yes, funny enough, most amps I've worked on successfully have been Valves, as I love the old valve amps. I'm not so clued up on ss so that is a good explanation for me. When I found those dodgy components, I presumed I had found the fault, I guessed one had failed due to just old age and then took the others out, but clearly there is another problem. I really do appreciate your help Phil, so please keep it coming. Also, sorry about the delays in some of my replies, I am rather busy at mo, but really do want to get this thing back up and running again.
Melv
Troubleshooting is a skill, an approach, and it doesn;t really matter whether it is tube or SS you are working on. Yes or course the details are different, but it is all about isolating the problem. Once you know WHERE a problem lies, it is usually pretty clear what needs to be done.
CLues are clues, resistors burn up in SS for the same reasons as in tube gear - excess current.
MY point is that you probably know more about troubleshooting SS than you realize.
Cool! Thankyou Enzo, that was a bit of a confidence boost, and a clue I think. which are both rather needed in this case ;). I will get to the bottom of this fault if it's the last bloody thing I do. :duh
Right then, I 've found the short causing the fuses to blow, it was R11 and R13 were shorting where they had been badly soldered.After rectifying this I Tried the lamp limiter test and this time the bulb flickered for couple of seconds and went out, so all good there. I then turned on full power and no blowing fuses :tu:. However I have full rail voltage at the output, that's 60vdc. Any ideas please?
Post DC idle voltages for Q6&7, Q1&2. No lamp limiter.
Ok g1, will do.
Ok I have read the voltages. Let me just point out there are two different sets of Q1 and Q2 , one set are on the board, and one set are the output transistors and are off board. I put the black lead of multimeter to ground and red lead at the Base, Collector and Emmitor at Q6,7 and Q1,2 (that is the output Q1,2), and all readings are the same at 60vdc. I then put the red lead to Q2 Collector on the board and suddenly R32 burned up and smoked. ???????
Yes, I meant the output transistor Q1&2.
I think you must have had a burst of oscillation probing the Q2 on the board as R32 is a feedback loop component.
You say there is 60V on all those transistors. I assume you mean positive DC?
Where is your -60V supply?
Sorry I forgot to check if it was - or + volts, I think they were all plus but I will check in a mo.
Right g1, I've took some more measurements and here they are all VDC
Q6 B=-58 C= +58 E = -58
Q7 B=-58 C=-58 E= -58
OUTPUTS
Q1 C=+59 B=-58 E=+58
Q2 C=-58 B=-58 E=-58.
I also checked the other transistors
Q1 B= -10(this was falling with time), C=+59 E=-2(steady).
Q2 B=+59 C=-58 E=+59
Q3 B+59 C=+59 E=+59
Q4 B=-57 C=-58 E=-58
Q5 B-57C-58 E=58
Also when I touch red lead to Q6 C something smoked around q1,q3,q2,c6 area but was hard to see what component as didn't smoke after that.
Any help would be much appreciated.
It's all gone quiet! ???
I'm not sure if you are making typos or what but it makes things confusing.
Q4 emitter and Q5 emitter are the same point. How can they be different polarity voltages?
Same for output Q1 emitter and Q2 collector, unless R26 or R27 are open.
I'm assuming they are typos and the output is stuck to the negative rail.
The feedback loop puts DC to the 741 IC which makes it impossible to chase DC readings.
I'm not familiar enough with this type of amp to say whether the feedback loop can be disabled safely.
Perhaps someone else may have a suggestion.
Well thanks for your help g1. Hmmm I'm pretty sure it wasn't a typo (I guess that means typing error?) So I will check those voltages again. Very strange.