Safely testing a recently repaired amp head

SuperBassSam · #1 05-16-2011, 08:23 AM

Hi all

I recently repaired a faulty Hughes & Kettner amp head (power transistors were totally missing so replaced them). I plugged the amp in today into a surge protected outlet after reassembling it and nothing went bang. However I have done a lot of research and I know that amps can destroy cabinets if DC voltages travel down the speaker out sockets (the head has no Speakon connectors unfortunately.) Is there any way to safely test the amp with my cab and bass? Or as nothing has gone bang, is it safe to assume that the amp is functioning properly again? I flicked some switches etc and lights seemed to come on when they were supposed to.

I have a horrible old no name bass I'd be willing to test it with but I don't really want to put my Markbass 104HF at risk.

Thanks a lot

BassmanPaul · #2 05-16-2011, 11:04 AM

So ya fixed it Sam - good for you. However you still have to set the bias and zero the voltage on the output terminals. There should be a pot for each function on the PCB. It should be identified on the silk screening of the board.

Measure the voltage across the output terminals - it should be close to zero. If it is you should be safe to test the amp into a cabinet.

SuperBassSam · #3 05-16-2011, 11:10 AM

Haha yeah I did Paul, thanks a lot for your help! Ah okay I was unaware I had to do that. Presumably the measuring across the output would have to be done while the amp is switched on?

I have had a look at the board and it seems that the Hughes designers have left holes in the PCB to reach the pots to adjust these things, definitely knew what they were doing!

Thanks again

BassmanPaul · #4 05-16-2011, 11:30 AM

The bias is set with no input. The value HK should be able to supply you with or should be on the schematic if you have it. The zero is set to get the minimum across the output terminals. The amp has to be on for both operations. Some topologies can be a little touchy in that a small amount of rotation of the pot causes a huge change in bias. Others can be interactive so you should recheck each setting several times.

Edit: I'm assuming you don't have a scope.

SuperBassSam · #5 05-16-2011, 11:55 AM

Okay, I do have the schematic, the two pots I can adjust are labeled TR1 and TR2. TR1 has a value next to it of "PT10v500k" and TR2 has "PT10v10k". I know that to check the zero I need to measure across the speaker output but where would I measure to check bias? And how would I know which pot affects which, would it be to do with their position in the circuit?

No unfortunately I do not own a scope but if this repair goes to plan and I do some more I will definitely be investing in one, they seem invaluable from what I've read.

I really appreciate your help in this, thanks Paul!

SuperBassSam · #6 05-16-2011, 03:58 PM

Well I just measured the DC voltage across the speaker output and it comes in around 0.02V which I think I can remember reading somewhere was okay. If anyone can confirm it would be safe to run this through my cab I would be extremely grateful!

Thanks

SuperBassSam · #7 05-19-2011, 05:29 AM

Bump & update:

I plugged my bass in and some headphones into the headphone socket just to test it. Good news is, the amp works fine! (and sounds pretty good actually.) However bad news is, I had been playing around with it for around 5 minutes and I noticed a kind of burning smell coming from the poweramp section. No smoke or anything, just a kind of plastic-y melty smell. What would this be likely to be? I thought maybe it could be the thermal paste I applied but would that then mean I put too much on? I have checked all the fuses and they are all the correct rating so I'm not sure if I should be worried or not. As I say, the amp seems to be working fine.

Thanks a lot

OneWayPunk · #8 05-19-2011, 11:34 AM

I would be careful, you probably need to bias those replaced power transistors. They can easily blow if overheated. Contact the manufacturer to find out where they should be biased at, or it might be on the schematic.

JimmyM · #9 05-19-2011, 12:05 PM

Quote:

Originally Posted by OneWayPunk

I would be careful, you probably need to bias those replaced power transistors. They can easily blow if overheated. Contact the manufacturer to find out where they should be biased at, or it might be on the schematic.

+1. it could also be industrial solvents burning off, but it's possible your transistors need biasing. at any rate, since you're this concerned, it certainly can't hurt to have a good amp tech take a look at it.

SuperBassSam · #10 05-19-2011, 02:11 PM

Thanks for the replies. OneWayPunk I am having trouble finding out exactly how you bias them - I've found the trimmer pot for adjusting both bias and zero voltage however I am not sure which is which, how would I tell? I also have the values next to each one from the schematic but again I'm a little confused with the bias where I would actually measure the voltages? Presumably across a resistor somewhere?

And JimmyM, after I've done all the work I definitely plan to take it to get properly safety tested and such by a decent amp tech. This is a learning project so I really want to do the work myself!

Thanks a lot

BassmanPaul · #11 05-19-2011, 03:20 PM

Is your schematic in PDF or printed? Can you post a link to the schematic?

Sneakypete · #12 05-19-2011, 03:57 PM

For those who want to get deeper into the art of electronic repair there's a piece of equipment that I simply cannot recommend highly enough - especially for work on output stages and PSU's where many hours of work and some expensive components can easily go up in smoke as soon as you switch on. I've had mine for forty years and it's brand name is Variac but there are other makes. It's basically a mains transformer with an adjustable secondary. In the UK this means I put 240Vac into my Variac and plug the repaired equipment into the secondary with the control set to zero. I can now slowly increase the output voltage - 10, 20, 30 and so on. I have one multimeter in series to measure the current and another (they are so cheap these days) across the secondary to measure voltage. If there's still a short hiding in there I can see early on that the actual voltage isn't rising as it ought to be as indicated on the Variac scale and pull back before any damage is done. Used carefully it means you should never have that horrible experience of seeing your efforts go bang at the instant you throw the switch! I'm sure someone will say that under-voltage will destroy your amp and to be honest although I've used it countless times in the day-job (and never, ever caused a problem) I've only very rarely ever used it with MI gear. Anyway, I simply can't believe that under-volts can hurt anything - unless my understanding of Ohms Law is mistaken. If you put low voltage into a circuit then not enough current can flow to cause damage. The idea that low volts makes the equipment draw dangerous amounts of current in an heroic effort to function, I believe anyway, is a pure fallacy. Switched mode power supplies do nothing until you reach their threshold (usually about 80-90V for a universal supply IME) and then they start up. Linear power supplies output will pretty much track the rising mains input. I'm standing by for howls of derisive laughter!

BassmanPaul · #13 05-20-2011, 12:31 PM

If you don't have the means to purchase a Variac You can make a series lamp box. You take an AC cable, wire the phase to a lamp socket. the other side of the socket goes to a mains socket from your area along with the neutral and ground . Thus the power circuit for the mains has a lamp in series with it. By installing various wattages of lamp you can control the current going to the mains socket. Even with a full short across the mains socket will only pass enough current to light the lamp.

I do have a variac it's a twenty amp brute that used to be part of a theatre lighting console.

Sneakypete · #14 05-20-2011, 01:02 PM

Good idea Paul and cheaper than a variac although I found a couple on the bay for around £30 to £40. I should have mentioned the obvious though and said PLEASE don't anyone try any of this unless you know what you are doing when it comes to mains safety! Also, my bench runs on an 1:1 isolation transformer which I would also strongly recommend - much safer than just raw mains out of the socket, I stand on a thick rubber mat and there are no earthed surfaces anywhere nearby. So please , anyone thinking about 'getting into' amp repair - spend a hundred or two making a safe place to work first.

SuperBassSam · #15 05-20-2011, 02:55 PM

Hi all and thanks for replying.

Paul I've attached what I believe is the relevant part of the schematic showing both TR1 and TR2, I couldn't find any way of uploading the PDF without exceeding the size limit so please let me know if you need a different part of the schematic.

Pete I am looking into getting a variac, they seem invaluable and I have just found a decent used one on eBay which I think should be within my price range. I am sure that if underpowering equipment with a variac was a problem they wouldn't manufacture them! Unless of course this is extremely sensitive equipment that would no doubt mention it could be damaged by underpowering I would have thought.

Thanks a lot for all your help! I am really looking forward to when this amp is working perfectly

BassmanPaul · #16 05-21-2011, 10:56 AM

there's not enough there. I've sent you a PM.

SuperBassSam · #17 06-08-2011, 07:53 AM

Hi all,

I just had an email from Hughes & Kettner informing me of the bias values. They said it should be 14-21mA across a set of resistors, adjustable with a bias pot. After measuring and tweaking a tiny bit, I got it so that 2 resistors measure 14 and the other 2 measure 21. Is this correct? I assumed the values they supplied were an acceptable range, though the amp wasn't that far off those to begin with.

Thanks