Peavey TNT150 with loud hum - help required!

[dorsetfender]

Hi all,
Although I have been carefully reading earlier advice on this site relating to this amp, my problem seems to be slightly different, so a post is in order. I urgently require some techincal advice on a faulty Peavey TNT150 combo, which hums like a boxfull of bees.  I've been gradually isolating the problem with the help of schematics, but have not yet cracked it.  It seems to be in the 25V/16V circuitry on the amp board.

The -16V rail is right where it should be, with no ripple, so I'm happy with that; however, the +16V rail is showing 21.7V with 233mV ripple, exactly the same ripple that's on the +25V rail.  Furthermore, the middle pin of the 7815 regulator is showing 23.3V with the same ripple.

The 1000uF smoothing caps and the 7815 are all new, so I could really use a clue as to where to check next...

thanks in advance

[J M Fahey]

Clearly your positive regulator got disconnected from ground.
Resolder the center leg and follow the track from its pad to ground, it's cracked somewhere.
To reinforce it, solder a short piece of wire from that pad to some suitable ground, as close as possible (or rather, where it was going originally).
Post results.

EDIT: as Enzo always says, problems are not only "bad parts" by a long shot; bad connections too.
You may throw a truckload of new parts at a dead amp, change *everything*, and it still may malfunction.

[Roly]

 :tu:

A clear majority of the repairs I do originate from poor connections, sometimes dirt or other contamination, sometime poor contact pressure as in the valve/tube amp currently on the bench where one of the output valves wasn't working simply because it wasn't actually making connection.

Printed circuit fractures are very common with the modern practice of mounting external connectors directly on the printed circuit, often with no other support at all.

With computers the proportion of "bad contact" faults is even higher, I estimate around eight out of ten of the machines I deal with.

It's actually a pleasant change when I have to fire up the multimeter or CRO and actually "technish".  :cheesy:

[Enzo]

Indeed.

I constantly harp on people about it.   All the time I see repair threads open with "what parts do I change, what parts do I change, what parts do I change."  And quite often there are no bad parts.  A broken lead, a cracked trace, a failed solder joint will stop a circuit from working just as much as a "bad" part.

[dorsetfender]

Thanks so much for your help folks, I wasn't expecting a response so quickly - and you're preaching to the choir when it comes to checking for cracked tracks and suchlike!  As you rightly say - that's the first thing I always check, and there were no problems there, but what J M Fahey said about the regulator losing touch with the ground was spot on   there's a small diode between the middle of the regulator and the ground track - I tested it off the board and found it was stone dead.  put a new one in and the voltages are now fine - many thanks!

That's the preamp up and running again - but there's a more fundamental problem on the power amp side, which causes the primary fuse to blow as soon as it's switched on.  As Enzo often advises "Isolate the problem", I'll see how I get on...  I know the transformer's working fine and its voltages are good.

[Roly]

The diode in the ground leg is often simply to lift the output voltage a little bit (0.6-0.7V), just be sure it isn't a zener to lift it more than a bit.

To diagnose the shorting output stage you will need a light bulb limiter so you can get some power for measurements without blowing fuses or doing damage;

http://www.ssguitar.com/index.php?topic=2093.0

[dorsetfender]

Thanks,
it's shown as just a plain diode on the schematic, although there are zeners in a couple of places elsewhere on the board.

Bulb limiter duly constructed - will report on progress in due course.

[J M Fahey]

If the cathode points to ground, it's a regular diode; if away, it's a Zener.
What does the schematic show?

[Enzo]

They are just common diodes, in the ground leg of a three terminal regulator.  It is there to lift the +15 to +16v, more or less.  Very very very common in Peavey power supplies.

[dorsetfender]

...Further to my earlier posts, I got a very bright 60W bulb when I switched on the amp through the limiter - so having viewed advice given for other amps I tested all the main transistors off the board and found two that were shorted (Q4 and Q9 on the schematic).  These are SJ8226 (NPN) and SJ8227 (PNP).  The NPN is shorted in all directions, and the PNP is shorted between collector and emitter.

I've read that NTE388 could be a suitable alternative for SJ8226 and NTE68 for SJ8227.  Is that correct?  are there any other alternatives I should investigate?

[Enzo]

Investigate anything but NTE unless you just can't buy anything else.  NTE stuff will cost at LEAST three times what real parts will cost, and as much as ten times.

I don't recall we posted it here or not, but Peavey has a semiconductor cross reference guide.  Customer service there will send it to you if you ask.   I know it is a sticky in the schematic request section of Music-Electronics Forum, you can get it there too.

You can buy the exact 8226 and 8227 from Peavey directly.  Prices will be reasonable.  The generic number for those house numbers is:
SJ8226 = MJ15052, current part number 70484140
SJ8227 = MJ15053, current part number 70474140

But the more common MJ15003 (NPN) and MJ15004 (PNP) will work fine.

MOuser sells the MJ15003G for $3.78 each and has them in stock.  They sell the MJ15004 for $3.20 eahc and has them in stock.    Sitting down?   Mouser sells the NTE388 for $14.04 each and the NTE68 for a mere $10.33 each.

If you have a couple outputs dead, some things come to mind.  First check all four of those 10 watt 0.33 ohm ballast resistors for opens.  Next, the amp will function with one pair of outputs removed, for testing purpose only, you can see if the rest of the amp is OK.   But I highly suggest, if outputs died, that you also replace the driver transistors  Q3 and Q7.  I recommend getting them from Peavey rather than subbing.  Also check small resistors R14, R15 associated with the drivers, as well as the R23 between the bases of the outputs.

[dorsetfender]

Sorry for the delay in replying - have been busy!

so here we go...
The four 0.33 ohm ballast resistors were all ok, as were the R14, R15 and R23.  Peavey UK ('cause that's where I am) were very helpful and prompt in delivering the required parts (and their handling charge was very reasonable), so I 've replaced the bust pair of power transistors Q4 (8226) and Q9 (8227), plus the driver transistors Q3 (5331) and Q7 (7036) as advised.  I must just add that it was very useful to be able to run the amp with the Q4 and Q9 missing.  So all now running well again 

With everything else fixed there was an annoying little bit of hum (<20mV p-p) left over, which was due to poor grounding through the riveted board mounts.  Therefore I've added a grounding lead, soldering it to the earth track at a convenient place and with a tag so that it can be attached to the chassis at the main earth point.  With that done, it all now sounds very clean.

Very many thanks to you all for your insight and assistance - there were moments when I thought I'd bitten off more than I could chew.