Peavey TNT 150 - Technical Help Required

[John N Scott]

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

[J M Fahey]

Hi John.
Watch carefully the speaker cone: if when you turn the amp on it thumps, hums loud and jumps forward or backward and stays there, you have DC on the speaker (because of an amp fault) and you will kill it in no time.
After (briefly) checking this, pull one of the speaker wires (you'll have to unscrew and pull the speaker to have access to its back side) and measure across the wires on the 200V DC scale.
You might have up to 50VDC there.
Post your results.

[John N Scott]

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

[Enzo]

You can pull a speaker, but since you will have to pull the chassis to do anything, I wouldn;t.  Top screws come out, and a few screws along the bottom rear of the chassis, and it slides out the rear.  There will be a blue and yellow pair of wires feeding into the speaker enclosure.  They connect to the circuit board with a white Molex plug.  There may be a cable tie around the wires - clip it if so.  With the speaker disconnected remove the chassis entirely.

Now you can fire it up on the bench and check for DC at the Molex pins.

Unlike your tube amps, solid state amps do not require a load.

Don;t sweat the circuit board.  From that era, most Peavey amp socketed their ICs.  If you have to change out a transistor or resistor, then yes, you will have to solder on a board.

Isolate the problem.


There is a power amp in jack.  Plug a cord into that and apply a test signal.  CLip a speaker to the molex posts if you need to hear it.

Does the symptom remain?   Likewise, you can run a cord from PREAMP OUT to some other amp for a listen.  Is that signal affected?

Assuming the power amp is the problem, the preamp and power amp are on separate boards.  The power amp will run standing alone, you can disconnect the unbilical to the preamp board and clip a test signal to the power amp at the exposed pins.

There are three ICs on that board, two 4558 dual op amps, and a "87478" used as a compressor.

You can remove the 87478 from its socket during test.  Note its orientation.  ANy change?

U1 is the signal; IC, is there undue DC offset at either output pin?

Muting JFET Q1 could be faulty, remove it and see if the problem goes away.

Note that U1 has its own zener derived +/-15v rails.   There are also 7815/7915 dervied rails for the preamp.  make sure U1 has +/-15v.

Due to file size limits, I can't post the schematic here, but customer service at Peavey will send it to you.













xxx

[John N Scott]

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

[Enzo]

OK, no speaker for now.

Check the power supplies.  The amp runs on +/-45vDC.  A convenient place to check is the cases of the four power transistors.  With respect to ground, the cases of two on one end are -45v and the cases of the two on the other end are at +45v.

Now flip your meter to AC volts and measure at the same points.  You should see less than a volt of AC on those transistors.

If one of them shows substantial AC voltage, and most likely one of th DC voltages is considerably lower, then a main filter cap is suspect.  The filters are the two cans next to the fuses.   Any cracks in the solder under those cans?

[John N Scott]

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

[Enzo]

The voltage is approximate 50v is just as good as 45v.

That 32vdc with 16vac on it is the dead giveaway that you have a missing filter - caused by that cracked solder.

[J M Fahey]

If one of them shows substantial AC voltage, and most likely one of th DC voltages is considerably lower, then a main filter cap is suspect.  The filters are the two cans next to the fuses.   Any cracks in the solder under those cans?

There you have it, Enzo's loooooong bench years at its best.
Congrats.