Help A Dummy: in simple language please.

[seebert]

Firstly A Happy Thanksgiving to all - I hope it was good.

I've got a problem with my newly "repaired" Audire 2 power amp. Long story short: original MPSU07/57's too expensive (and fragile) so I used CEN MPSU07/57's in their heavy duty package on both the original good board as well as the blown board. The specs are the same apart from being able to handle higher current. The 10K 1/2W trimmer pot was blown on the bad board and (using questionable wisdom) I removed the good trimmer pot, measured it out of circuit at 2.46K ohms and made a resistor pair to 2.4K as seen in the photo; the original pot didn't have any series resistor which I'd have thought would have been a good idea and I reckoned that 2x1/2W R's would be a safer bet than a single trimmer - and FYI there is only one trimmer pot on the board. The blown OP transistors (pre suffix G) were changed to a matched pair of MJ15003G/4G from Mouser. These are the only changes to the original that I made.

Now the 'repaired' amp works on both channels BUT (and it's a big but - I'm not shouting) when a signal is input the repaired board and OP's run cold whilst the original once good channel now runs the OP transistors hot enough to fry eggs after only 15 seconds and the transistors and components on the PCB itself stay cold. When there's no signal input everything stays cold. It seems that the OP transistors are now not biased correctly but I don't know what I've done to mess up.

Guys I'm new at this SS circuitry and don't have any gear apart from a DMM. Please tell me how I messed up and better yet please tell me in simple (like ABC) terms how to put it right. I did try a search first as I don't want to hang in like a leech but I just don't know enough to make sense of 90% of the theory and I don't have the schematics for this amp. The DC offset measures -6 to 8 mV on the repaired board and -10 to 12 mV on the original. Please help.

Thanks, Mike.

[janneman]

Hi,

I'm assuming that the trimmer is the one that sets the quiescent current in the output stage. This trimmer normally should be adjusted to the output (and sometimes driver) transistors used.
If I understand you correctly you also changed out some transistors in the good channel. That would mean the trimmer is not at the right value anymore, and the quiescent current is too high.
In the repaired channel you replaced the trimmer with the two resistors which again aren't exactly at the right divider ratio but in this case the quiescent current is too low.

In cases like this one would put a multimeter across an emitter resistor in the output stage, measure the DC voltage (which is a measure of the quiescent current) and adjust the trimmer for the correct value.
So in your case, first thing I would do is replace the two resistors with a trimmer that has the same end-to-end value of the original (10k?).

Then you need to find an output stage emitter resistor, put a multimeter across it and adjust the DC voltage such that it carries say 30 mA. It may not be optimal but it will then not blow up and the crossover distortion wont be too bad.
The best is of course to find a service manual and actually adjust as specified.

One final thing to note: before switching on the amp after replacing the trimmer, set it to minimal quiescent current. Almost always that is when you turn it such that the in-circuit end-to-end value is close to zero. That minimizes the Iq and you can then turn it up while measuring the DC drop across that Re.

Hope this is helpful,

jan didden

[AndrewT]

Have a look at any schematic that shows the output emitter resistors. See they connect the upper emitter to the lower emitter.

There should be one emitter resistor to each output device.
All BJT power devices have bce pin order. The rightmost pin is always Emitter when looking at the package at the labeled face. To220 are the same. To126 are reversed, i.e. ecb.
When you look at your PCB you should be able to see the trace and the emitter resistors that connect the upper emitter PIN to the lower emitter PIN. Those are the emitter resistors that are used to measure the output bias voltage.

[seebert]

Thank you both gentlemen for giving me the details in an easy to understand way. The Audire is a really old amplifier with all of the components except the OP transistors located on two identical PC boards; then there is a 'bunch' of tied together leads (wiring) that travel from the boards to the OP transistors which makes it hard to see which wire connects what to what and also because of their location it's virtually impossible to separate them out to get a better view of the connections.

There are two 0.22 ohms 5 Watt 5% wirewound resistors per board that I assume must be the emiitter resistors since they are the only ones of such low R and high W. These were blown on the board that I repaired if that helps - I believe these may act like a kind of fuse in the OP?

I didn't want to change out the original good transistors but they were very fragile and carried glued-on heatsinks - about 1/2" x 2" of alloy 'wings' that made handling and manipulating the board for desoldering without damaging it impossible for me. The legs on those original transistors were very very thin and snapped off at almost a touch; it's a pity that I had to replace the good ones because of my own lack of handling skills but I did as best I could.

I tried to find a replacement for the original 10K trimmer pot but couldn't find a vertical pot 1/2W like it listed anywhere except one seller in the UK on eBay, and I did try a multiturn Cermet but it was physically too small: do you think it would be okay to use a Cermet with 'extension' legs and (say) epoxy glue to give it some support?

Again many thank you's for your great help that I truly appreciate.

Best regards, Mike.

[AndrewT]

If the 5W wirewound 0r22 emitter resistors were blown, then there will be much greater damage in the semiconductors around the output stage. The fragile legs of the driver transistors is of no importance. They were almost certainly damaged from that "mishap".
Are the output devices in To3 packages (metal) with 2PINs underneath and no apparent third PIN?

[seebert]

Hello Andrew, and thank you again for your help. I did see that you offer a lot of help and advice here on DIY ... most kind. Yes the output devices are exactly as you describe ... To3 I believe.

It's so odd (at least to me as a newbie oldie) that the amp now works and doesn't sound distorted in any way even though the bias is so far off. However I'm looking forward to getting it right - it's hard work for me, but I do get a lot of satisfaction in trying.


My best, Mike.

[seebert]

Ian Finch can you please contact again please - I received an email from DIYaudio showing your kind reply but it doesn't show up here in the thread.

You asked:

BTW, just how loud are you playing the amp. when listening? It may not be from the main outputs at all if playing softly.

I played it softly at low volume when I tested it. I apologise for my ignorance but I don't understand the quoted words - how could it play without using the main outputs - I'm lost here. Could you elaborate please.

Many thanks, Mike.

[AndrewT]

Ignore that post from another thread.

[seebert]

Thanks for chipping in Andrew, it's appreciated.

I see you are listed as living on the Scottish Borders ... may I ask (even approximately) where in the borders you are: I spent some time in the way past days in both Northumberland and Scotland (on the coast of Scotland and in a tiny place in Ayrshire) and have very fond memories of both the places and the people I met.

My best regards, Mike.

[Ian Finch]

Hi Seebert
My post only detailed the TO3 connections which Andrew had already posted but I did not see. I deleted the unnecessary post asking that question, since there was doubt about the rest of the output stage being OK.

A number of faults allow Emitter follower design power amplifiers to pass audio from the preceding driver transistors or earlier in the amplifier through faulty transistors. As the volume is increased, a point sometimes arises when harsh distortion occurs, confirming that not all the silicon is well. Then it's DMM time and first verify all Base-emitter junction voltages as around O.6V. This needs a steady hand, not slipping to short something else into oblivion.

However, as you replaced the output transistors and the DC offset is OK, you have clean sound, then it remains to set the quiescent current as Janneman has suggested then assuming the "cold" amp remains functional as quiescent current warms it a bit, gradually raise the volume to check that the drivers are actually intact. It may seem extravagant, but I would replace drivers regardless in that circumstance as they will certainly have had a rough time and maybe cause havoc sooner than you'd like.