Dull Hum Amp Sound Question

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Solid State PA amps have been common for about 45 years. I was replacing wo rn out tube amps in the early '70s with new Bogen solid state equipment. I' only saw one wall mounted tube amp in all that time. It was built for indu strial use by CW Thompson in Middletown Ohio. It was designed for use in mi nes, chemical plants and factories. They also built the first sound system that was approved for use in a nuclear power plant.

I knew him, and he took me int his home workshop to show me the prototypes for many of his products back in the '70s. When he started, Altec refused t o sell him their products for use in hazardous locations so he would buy th e transformers and drivers from them, and build his own. He used sand casti ng to make explosion proof speakers, and the custom junction boxes for the wiring. They were 1/2" thick, and used a large O-ring to seal the covers. T hey were installed with rigid conduit with fire stops at each opening.

He had the custom, prototype of the switch used to turn the multiple subsys tems at a nuclear power plant into a single system in an emergency. It requ ired Federal approval for the design before it could be sold.

He did pretty well for someone who started his career building and repairin g radios as a teenager. He went from that, to owning one of the first radio shops in the area and then into manufacturing as local factories and a lar ge local steel mill added PA systems.

** Really ?

Bob must have just awoken from a 40 year long snooze.

.... Phil

You don't keep them handy? Somebody else might be able to get away with hiring them when he needed them, but yours would get regular use.

--
Bill Sloman, Sydney

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If you had a 1500 lot sample and had one failure per year, in 1500 years all 1500 units would have failed.

On the other hand if you had a 1500 lot sample and a 1500 year mean tim e to failure, in 1500 years , half of the sample would have failed. At least that is how I understand it.

Dan

Dan

uffering severe derioration in almost every component as well as the PCB it self. Even soldered connections are no longer trustworthy and push on types only functional by sheer luck while on borrowed time.

s before failure. This is why Motorola and most others changed to steel pak s sometime in the 1980s.

lure.

In your first explanation the failure rate falls from initial 1/yr since the number in service falls over time.

NT

** Try reading what I wrote. MTBF numbers are not predictions of huge life spans, that is the dumb the mistake naive folk always make.

.... Phil

That sounds a little 'fishy' to me. :-)

--
Johnny B Good

====snip====

I should imagine that that small closet is now somewhat warmer than it once was when the amplifier was first installed which lends all the suggestions about dried out caps being prime suspects additional weight.

If you can take a lightweight 'scope to the job (modern cheap bench DSOs are pretty well all lightweight), you can probe the line output with the 'scope trigger setting switched to "AC Line" to positively identify whether this mystery hum is related in some way to the mains frequency or not.

PSU ripple is typically twice the mains frequency (100 or 120 Hz) whilst hum induced into badly screened signal lines will usually simply be a copy of the AC supply frequency waveform.

It's worth keeping in mind that electrolytic capacitors have a limited lifetime. Such lifetime ratings are part of the specification data, typically quoted for a specific operating temperature and qualified with statements like "halved for every 10 degrees C above the rated temperature". I can't recall what the actual lifetime reduction factor with temperature rise actually is so it could be even worse.

Life ratings on electrolytics can range from as low as 2000 hours to a high of 50000 hours (usually at an elevated temperature). If the caps run significantly cooler than that, they can last maybe as much as ten times longer which is why a good designer will specify 105 deg C rated caps and aim for as low a temperature of operation as possible (avoiding placements next to hot running components and provide good ventilation, supplemented with fan cooling if needed).

With all the activity related to maintaining the other kit in that closet, there's every possibility that the connections to the amp have been disturbed sufficiently to compromise the screening on its inputs making a visit to the amp a high priority not just to hook a 'scope up to it but also just simply to check the state of its cabled connections.

There's little point in further speculation. You need to actually get your hands dirty and do some basic tests. In this case, a 'scope is a very handy bit of test kit, especially when your hearing is as compromised as you say it is.

--
Johnny B Good

Thanks Johnny,

Very good input and nice writing.

The closet temp is not high in any way. There is a completely open ceiling above. Very good ventilation.

Sorry, I don't have a scope, unlikely ever will. The present diagnostics is completed and a new Amp is on order.

Will receive and replace next week. Would like to have sooner but shipping is shipping.

Again thanks for the input. Please be prepared for objections.

Have a good day.

Les

For those that are interested.

Replaced the Amp today with a New Bogen TPU100B Amp. Swap took about 30 minutes mainly due to the location. Would have liked to reposition but was not possible due to the limited space. The Amp works great and all is good.

I opened up the old University Sound Amp and there was only one Cap. 50V 10000uf 1.125" Dia. x 2" Long There were no signs of any leakage.

There is another smaller alum. can that had no markings. The end wire leads had a silicon sealant filling the end. Not sure what it is or was.

For me there is really no good reason to repair this Amp.

Therefore it is going to trash.

Thanks again to all the input even though I posted in the wrong group.

Happy Easter.

Les