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Messages - Tann-74

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Amplifier Discussion / Re: My Problem Amplifier And Symptoms...
« on: April 18, 2012, 05:40:50 PM »

Also the tail pair can be mpsa92 matching them give's performance and lower's offset voltage..

the driver can be BD139.. also the current limiter transistors and components can be removed for pre-fight checks.. large offset voltages can happen if the tail transistor legs are fitted in the wrong way round when using different replacements.. make sure the legs are fitted the right way in...

I would agree about different ranges of offset voltages.. but these amps should be about 100mv or so.

o/p pair can even be 2n3773's etc etc..

these amps can be tested on supply rails as low as +- 15 volts to 47+- dc..

Amplifier Discussion / Re: My Problem Amplifier And Symptoms...
« on: April 18, 2012, 05:25:34 PM »
Just a quick look over this amp schematic... it look's the same as a laney or carlsbro amp..

same ltp pair with bootstrap cc in the vas stage..  the only thing a miss is the zobel 10 ohm and 100n cap on the o/p end..

 straight forward amp to solve...

Tubes and Hybrids / Hiwatt
« on: April 18, 2012, 03:54:33 PM »

General repair of valve HI watt valve head...

This one came in dead!  1amp ht fuse o/c plus valve fault fuse's o/c.. main problem was rectifier diodes s/c.. so after replacement and tube checks all ok... then move on to soak test for a few hours...

So after a two days of checks to ensure things run smooth then back to the local studio...

A month later I got a call...'that head is not working can you sort it?  sure, be round to sort it...

Turns out the studio has been have power surge's and different amps have been acting up...

There getting the sparky's to check the place over ;)


Amplifier Discussion / Laney RGB500 combo..
« on: March 19, 2012, 02:36:14 PM »

This oldish combo amp came in for repair last before things got under way..The question I asked, was what's the main fault?  such as, blown fuse..odd sounds..low gain etc..etc.

The owner said he'd brought it 3rd hand for a cheap price due to it not being able to power up!
On inspection of the power amp stage it had been so badly reworked that the tracks started to break loose from the pcb on one side.  for those who've worked on these's amps would know it's a double sided pcb with on board psu.   

On checked it's schematic the power amp is 250w @ 4ohms use. I normally have spare module that would do the job..mmm..not this time so after getting the go a head to rebuild a replacement..

Next came fitting all the parts in and power up..all's well there...till feeding a signal through the front end and hearing distortion so a quick scope showed a over heated tl072 opamp at the main in put stage. possible damaged due to over driving it's in put with effects pedals.. to much i/p signal and the poor op-amp gets it!!

Anyway that's a happy customer with his amp back in!


The Newcomer's Forum / Hi watt 50 repair..
« on: February 10, 2012, 07:19:15 PM »
This less than a year old Hiwatt 50 had a no sound fault? 

So it was down to checking it on wards.. hook up to test speaker and bring it up on the vr transformer, all is well front panel light's up.  now apply signal..mmmm, no sound?  so removal of the chassis for more checking.. in the end it was a reflow with 60/40 solder on all valve base's and she sprang back in action!!

that lead free solder as a lot to answer for.

Amplifier Discussion / Re: Dean Markley back in the Amp Biz
« on: February 10, 2012, 07:05:20 PM »
I worked on a early 80's dean markley cd60.. this on had worn o/p valve and preamp socket's plus needed a overhaul due to someone's tinkering finger's that made a right mess of all the wiring not sure what they was trying to do here ::)...
In the end it needed some sorting out and fresh power caps etc then refitting in the case ready for action..I must say it was straight forwards to under take the task all in a afternoons work!   

Amplifier Discussion / Re: Light Bulb Limiter
« on: February 10, 2012, 06:57:13 PM »
Hello to all...

we used to do the bulb limiter thing when working on tv's to check the ht feed to the loptie..

It's a handy plus cheap way of limiting the ac voltage in to a peace of equipment being worked on.

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