Marshall short

[Valvehead]

ok thanks--

i put the 4558 in, no change for the better ...


also,
i never did try swapping out those .33 ohm 5 watt resistors , the new ones i initially put in..... maybe worth a shot even though they measure ok and are new ?

[g1]

Not that it matters now, but TL072 and TL082 are essentially the same, with the TL072 being the lower noise version.

[g1]

TR5 and TR6 base voltages are the same, just opposite polarity. This is what we want to see also at the emitters, and also at the collectors, equal voltages of opposite polarity. The same should be true for the pair TR4 and TR7.
You said TR8 and TR10 base voltages are uneven, you need to find out why.

[Jazz P Bass]

I would like to see a reading of the dc voltage on either side of R81.

[Jazz P Bass]

Not that it matters now, but TL072 and TL082 are essentially the same, with the TL072 being the lower noise version.

Ooops.
I missed that fact.

[Enzo]

Um, what EXACTLY are we trying to fix at the moment? Last thing I saw was 350mv offset at the output unloaded. A bit higher than expected, but the amp ought to work well enough to work on any other symptoms. And it might go away with a load, have we tried that?

[NZaudio]

you have obviously overseen a defective component when you started playing it. Don't be too overhelmed when it seems to work all of a sudden. Transistoramps can be very tricky if you are no pro-tech.
Now that you have blown it, you have to start from scratch again. Here are some tips how to start and what to avoid in the early beginnings.
1. always perform a static measurement first befor you connect anything to the item.
2. do not use a lightbulb, a resistive load is better, but later on when you will need it.
3. try to limit the powersupply current, if you have, use a bench powersupply where you can reduce/limit the current. I always choose about 1A because most transistors in poweramps can handle that. This is for not to blow all parts up!
If you don't have a powersupply, then make up yourself some power resistors which you can put into the +VE and -VE supply.Use something like 20W/200Ohm per side. As I am missing some fuses here, you might have to desolder some wires or even cut some tracks.
In 90% of all cases, the fault is static. Measure resistors and transistors by taking them out, not when they are in the circuit, the same for diodes and other semiconductors
So wish you good luck first

[Enzo]

Hi NZ, He isn't using the light bulb as a load, he has it in series with the mains. That is a way to limit mains current as your #3 suggests.

Look up "light bulb limiter" to see what we mean.

We recommend no load at all on the output until the amp is stable and not producing DC.

[NZaudio]

Hi NZ, He isn't using the light bulb as a load, he has it in series with the mains. That is a way to limit mains current as your #3 suggests.

Look up "light bulb limiter" to see what we mean.

We recommend no load at all on the output until the amp is stable and not producing DC.

Thanks Enzo,
yes I know what he is using now. First my thought was he was using a bulb as a load.
Sure, sometimes I did use a 100W lightbulb when testing big amps with big shorts but as I have a professional workshop and make a living of that too, I have quite a few measuring instruments and specialized devices which I had to make myself as they are not available on the market.
I am new to a forum like this and if I have time and find my posts and replies back here, I am happy to help wherever I can. I have a few years to go before I can retire, maybe I have more time for forms then,lol.Also, I am a German engineer and still have to "translate" into my native lingo and back for myself.
And surely do not use a load at all before the static measurements are done and fixed, but I meant that under 2...I have to get used to you Americans first...

[52 Bill]

First my thought was he was using a bulb as a load.

An incandescent light bulb as an output load. I hadn't thought of that since reading the Jack Darr books. He used to suggest that.

If I remember it correctly, he could estimate the power output of the amp based upon the bulb wattage and how bright the bulb glowed.

[Valvehead]

I would like to see a reading of the dc voltage on either side of R81.

+ 0.5vdc
and + 3.5vdc

Enzo--there is a cab connected to the amp

[km6xz]

Please do not connect any cabinet or load to the amp until it is stable and has low offset.
Based on the R81 voltages that will rise when you disconnect the cabinet.
There are a few too many suggestions flowing so I will not confuse things further by suggesting additional measurements.

[Valvehead]

Any last thought before i sell it "as-is" ?

In case someone missed it , looking at the output page of the scem, ive checked all resistors, the 5w are new, ALL transistors are new .

[NZaudio]

Any last thought before i sell it "as-is" ?

In case someone missed it , looking at the output page of the scem, ive checked all resistors, the 5w are new, ALL transistors are new .

Hey Mate, I am just reading this. Please don't give up,I don't know what you are doing at present and at all but I am one of the "old Style" engineers. We never give up. I do not have the time to look after each post even if I gave some advice but this sounds bad. Let me have a look at your problem, send me an e-mail and describe all this again please. If you don't mind I'll be on to it and will help you. My mail is info@delta-audio-engineering.com

[DrGonz78]

Two thoughts: perhaps c20 and/or c55 are leaky on the feed back loop. My thinking there is that they could leak some DC voltage onto the output. Perhaps that is not even close, well it was merely a thought...

Another thought involves each current limiting circuit for TR8 and TR10. That would include the R118 and R96 that you already replaced as they were stressed. (I think those are the ones you replaced...? Confused on that a bit)
For TR8 it is R116,R117,R118 > TR12 > D5 that make up the current limiter
For TR10 it is R96,R97,R115 > TR11 > D4 that make up the current limiter

My idea here is that we are shotgunning this thing pretty much all the way so far, why not go all the way. LOL... After all, those circuits were definitely stressed if the outputs blew. Now I am not too sure if stressed resistors or diodes can create small offsets, but that was my second thought.

Plugged it into the wall and turned it on, looking good, then played a little...for about 20 seconds , as soon as i turned it up loud , the fuse blew and there we have smoke.

Several resistors got hot and some smoked... R113 R115 R118 . now powering up on the limiter i have a massive short. assuming i took out one or more of my new output chips ?

I guess I would like to know exactly every resistor that was stressed and replaced.

Perhaps my thoughts are a bit out there, but I just thought to throw them in here to see what others might say. I know you have checked all the resistors, but I wonder if stressed resistors are less reliable when powered up. That may be a stretch to my thinking... Can anyone set me straight in my thinking there...?