Checked the DC and AC voltage at the four power transistor cases.

Two transistors at one end,
   50.8V DC / 0.6V AC

Two transistors at the other end,
  32.8 V DC / 16.4V AC

Power amp circuit board removed and Filter Caps examined.

One cap was OK.
On the other cap the solder was cracked on one of the electrical connections and two of the four cap supports.
These were repaired.

The amp now appears to be functioning correctly, no hum and the 'COMP' LED is no longer illuminated.

Re-checked the DC and AC voltage at the four power transistor cases.

All transistors,
   50.8V DC / 0.6V AC

The voltage is higher than the 45V you specified but it is consistent.

I would like to thank you for your help, guidance and encouragement without which I would probably have returned the amplified in its original condition.

Many thanks,
John

The amp was delivered last night.
For ease of transportation the amp was removed from the cabinet and delivered on it's own, no speaker.

I contacted customer support at Peavey for a circuit diagram – waiting for a response.

The following tests have been carried out.

Test 1
No input, No Speaker.
Switch on – 
  'COMP' LED illuminated
  DC offset 0V
Switch off –
  DC offset 27V reducing to 0.1V over 60 seconds.

Test 2
No input, Speaker connected to molex posts.
Switch on –
  'COMP' LED illuminated.
  Pulsating hum.
Switch off –
  Load bang.

Test 3
Guitar connected to Input, Speaker connected to molex posts.
Separate amp and speaker connected to 'PRE OUT'.
Switch on –
  'COMP' LED illuminated.
  No Hum and undistorted guitar from 'PRE OUT' Amp and speaker.
  Pulsating hum and undistorted guitar from speaker connected to molex posts.
Switch off –
  Load bang.

Test 4
Guitar connected to 'PWR IN', Speaker connected to molex posts.
Switch on –
  'COMP' LED illuminated.
  Pulsating hum and undistorted guitar.
  Lower output than test 3.
Switch off –
  Load bang.

Test 5
Guitar connected to Input, Speaker connected to molex posts.
87478 removed.
Switch on –
  'COMP' LED illuminated.
  Constant hum when guitar not played.
  Undistorted guitar and pulsating hum when guitar played.
  Lower output than test 3.
Switch off –
  Load bang.

Test 6
Guitar connected to 'PWR IN', Speaker connected to molex posts.
87478 removed.
Switch on –
  'COMP' LED illuminated.
  Constant hum when guitar not played.
  Undistorted guitar and pulsating hum when guitar played from speaker.
  Lower output than test 5.
Switch off –
  Load bang.

I will wait for the circuit before attempting to remove JFET Q1 (I need to identify it).

I appreciate that the symptoms differ from those stated in the 1st post, I am now posting what I find not what I was told.

The speaker connected to the molex posts is an old 10W 8R 2 way unit and it can take full output without any problem.  This is not 150 W.

Does any of this help.

Regards
John

Hi Juan

Thanks for the information.
I will collect the amp and carry out the tests.

It will take a day or two to post the results.

Regards
John

Hi

I'm new to this site, normally over at DiyAudio.com

I have built a number of simple point to point SE Tube amplifiers over the years and acquired a little knowledge as I went along.

My son asked me if I could look at a guitar amplifier for his girlfriend's father.
I grudgingly agreed (I work on a principle of Never Volunteer) and asked him to find out the make, model and the fault.
I don't have the amp in my possession at this time or a circuit diagram.

The amp is a Peavey TNT 150 and the first thing I find its solid state not tube.
This presumably means circuit boards!!! this is outside my comfort zone and its not my amp.

I'm committed to doing something and therefore asking for your help, if the probable fault can be determined and it's a simple repair I may go ahead, if not I will have done my bit and everyone will be happy.

The symptoms.
When switched on the 'Compression LED' illuminates and the amp hums.
The hum then starts to pulse.
No bass output normally but intermittent distorted bass when moving the 'Gain Control'.
Loud thump when turning off.


From Peavey TNT 150 User Manual.
'The compression circuits eliminates audible power amp clipping, reduces distortion and protects the loudspeaker.
In use, the compression LED may occasionally illuminate, indicating that compressor is engaging to prevent distortion.  If the LED is continuously illuminated, Gain control or Low EQ settings should be reduced to prevent possible loudspeaker damage.'

Thanks in advance.
John