amplifier loss of signal strength

Several questions. Why were you having to 'install' the Tx in the first place ? Was it a replacement for one that had failed ? If so, in what way had it failed, and was there any apparent reason for the failure ? If this Tx is a replacement, is it an exact manufacturer's original, or a 'ringer' that you've sourced yourself ? If a ringer, is the output voltage exactly the same as the original ? Have you checked if any of the power supply rails are dropping as the output diminishes ? If the power supply uses monolithic regulator ICs ( 78xx / 79xx series ) then if these are subjected to excessive input voltage, compared to the amount of heatsinking that they have, they will rapidly overheat, which will cause them to retreat into their SOA, by reducing their output voltage. Also, if the mounting bolts to the heatsink have come loose, or the heatsink compound behind them has dried out and powdered, the same thing will happen, even with the correct level of input voltage.

Does anything mounted on a heatsink ie regulators, output IC or transistors, 'feel' or even smell like it's getting too hot ? Very often in small or cheap amps, the heatsinking is only just about adequate for the job, and the devices will run very hot even in normal circumstances. Never-the-less, they should still not be so hot after 30 seconds, that they are so uncomfortable to touch that you can't keep your finger tip on for more than a couple of seconds.

If you suspect that you have found a device that's getting too hot, if you can find enough exposed metal on the heatsink, you can try clipping on an office bulldog clip to temporarily increase the amount of heatsinking to se if that makes it stay on longer.

Arfa

What kind of amp? Why did you replace the transformer? Replace it with exact replacement? Schematic?

Are you by any chance sine wave testing it for those 30 seconds ?

Graham

I took it he wasn't, as he says that "when I test it with a variety of input

Arfa

I was a bit dozy at the time.

I've seen that effect though.

Graham

Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature.

Arfa

I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. :)

You should send me the files !!

Arfa

Yes, guitars, lots of guitars (a variety) were used to test the amp. The tx was an exact replacement. It seems we have narrowed this down to heat-related.

In what way had the original Tx failed ? Might have some relevance to the problem, which although may be heat-related, might not be a 'heat' problem - if you see what I mean ... For instance, it may be that one of the supply rails is dropping due to a monolithic regulator IC going into heat foldback, but that could be because something on the end of it is drawing too much current. I have also had this occur due to open circuit decoupling caps on the regulators, allowing them to hoot, and even open circuit decouplers around the output IC, or open circuit Zobel networks on the output, either of which can allow the stage to hoot ultrasonically.

Arfa

Or the screw holding it to its heatsink may have come loose.

Basically it needs proper fault finding ! Monitoring the supply rail(s) is the obvious starting place.

Graham

Probably could in MP3 form. Would have to dig it up tho. These days when someone brings one I just get out a guitar :). I won't work on any SS stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my services were pretty heavily sought. Tried to train a couple young apprentices because I had so much work but it was too hard to train and do top quality repair at the same time. Nobody wanted to do tube repair and I didn't want to do SS guitar, still don't unless you come groveling on your knees with a wad of green in your hand. I used to do my fair share SS of high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey. Those I did have some help from a friend who did Yamaha warranty work but he didn't know shit about a tube :)

Ok one last time, why was the transformer replaced? What led up to that diagnosis?

Thanks - the tx was replaced because there was 0V at the secondaries.

So the secondary was 12vac? Just trying to picture what kind of amp this is and what the PO is. Why don't you tell me more, you've been fairly vague so far.

I did detail the mounting bolt problem in my initial reply, Graham

Arfa

Know what you mean. None of the kids seem to have any aptitude for high voltage stuff any more. I far prefer working on tubed stuff to SS. At least it's well behaved, and fixes - and better yet *stays* fixed - with a minimum of fuss. The circuitry is straightforward, and understandable. At a pinch, you can pretty well fix any tube amp without schematics, just by having an understanding of the principles of operation. Unfortunately, the music shop in my village only has one person doing their service work - me ! So I have to take on all-comers, although it is a proper music shop, always having several long-haired 'musos' hanging about talking, or jamming in the play test room, so a large proportion of the gear is tubed anyway.

Back in the old days, I knew Jim Marshall. Their works is just down the way a few miles from where I live. Jim himself custom built an amp and console for a very good friend of mine who ran a mobile disco, when such things were new. I used to run the light show for him. We were out on the road 6 nights a week, and I reckon that it's got to be one of the best periods in my life ...

Arfa

Arfa

Sounds like one of the shops I did work for. I wasn't the resident but was the only Fender and Marshall specialist around. I really tried to stick with a couple brand names but always got tossed others. Back then I had long hair tied in a pony and a face full of beard so I had the rocker image plus I was that rocker too :)

I used to run lights and sound in between gigs for friends or customers caught in a jamb. Lots of times I would get up and fill in on a set too. Those were some great days of my life.

That sounds great. Did Jim do much custom work, outside of Marshall? I would love to learn about tube repair - my father did quite a bit of that, as well as in organs. Speaking of that, Emerson's organ technicians must get quite a workout!

Back to the amp in question - it's rated at 10watts RMS. I did a few more 'listening' tests with it yesterday, and made more comparisons. I was able to have the amp on for about 15 minutes before the volume 'cut out' occurred (no variation in the input source

- same guitar). I then turned it off and then back on - the signal was strong again. This isn't consistent, though, as I could turn off, wait about 10 seconds and then turn on again, and it might or might not have the full volume. I tested the line out and headphone jacks, and they each produced a healthy audio output. One item of note: when plugging into the line out, the amp's internal speaker went back to that 'cut out'/ half volume sound. This would occur, even if just plugging in an unconnected guitar cable. When the internal speaker was producing that weak output, the amp into which the line out was plugged was producing a healthy signal. The headphone jack had the same effect. After I had turned it off and on several times, I would hear a 'scratchy' sound (like turning dirty pots) just after turning back on. Now I'm wondering if speaker wires/connections might be suspect?

Thanks