Howdy, I have the mains light on and the pre-amp valves light up but the power valves wont light up. I've replaced the 10amp ceramic fuse and can hear hum but still no light on the EL84's. The amp was running sweetly and reliably up until yesterday. thankfully it failed at a rehearsal and i have a JTM-45 to work with, but, this is somewhat unusual and I do like the amp for certain gigs. Any assistance in this matter will be gratefully appreciated. Adios Tex

Hi,

Looks like the power module went out, could be either LM3886T Power IC or Mosfet Module But certainly is in the output stage where your problem is, hope this helps to troubleshoot the issue at least...

1- check the outlet for power by plugging in a lamp or something
2- check the amps fuse. Most power amps have user replaceable glass fuses.

Likely the filter capacitors have a problem. Best to take this in for repair... it is ODD that there is noise with the standby switch is in standby... That makes no sense... In standby it appears that power is removed from the power amp high voltage so nothing should come through the speaker... Unles maybe one off the power tubes had a total meltdown which should have blown the fuse.

If it has 2 fuses, the second one might be blown. Try replacing the fuses, if its not that then there is a problem with the amp and you will probably need to take it for repairs.

UNPLUG THE AMP
First you need to check any visible fuses either in the plug or on the back of the amp.
If you have an electrical testing screwdriver or meter you can test the fuse. If this sounds like gobbledegook (daft talk) then stop and take it to the dealer where you bought it.
If you can remove the fuse or fuses then replace them if it is obvious they have blown - dark and kinda burnt looking.
If replacing the fuses doesn't fix it - STOP.
Valve amps are serious bits of electrical hardware - they are not for home repairs - take it to a GOOD dealer / repair shop that deals with valve amp - nowhere else should play with it. When they have it sorted you can have the valves re-biased/changed to adjust the amp for the sound you want - yes the valves can change it.
Hope this helps - don't mess with it - even when turned OFF valve amps can give you a shock!!
Keep on Rockin'

Chances are that one of your tubes (valves) has failed.
If your amp is still in manufacturers warranty take it back to the place where you bought it.
Otherwise you need to identify which tube has gone and replace it. This will entail gaining access to the inside of the amplifier case. BEWARE this can be VERY DANGEROUS if you don't know what you are doing.
IF IN DOUBT TAKE IT TO A REPAIR SHOP
Always disconnect from the mains electricity supply before dismantling.
Once you have got the case off you will need to plug the amp in again and turn it on - you don't need to plug a guitar in just the mains supply. TAKE GREAT CARE not to touch any metal parts.
Look at each of the valves (there are some on the pre-amp stage behind the front panel as well as on the main amplifier chassis) they should all be glowing a nice bright red colour in the middle if they are healthy.
If any valve is not 'lit up' that will be the faulty one. DO NOT TOUCH IT - apart from the high voltage electricity associated with them valves get very HOT.
Turn the amp off, unplug the mains supply and WAIT at least 10 minutes for it to cool down again. Carefully slip the metal retaining spring off the top of the valve and, gripping it with a tissue or soft cloth, gently pull it out with a slight rocking motion. Carefully note down all the numbers and letters on the valve to identify the correct replacement. There are literally hundreds of different valves - putting the wrong one in may cause further damage to the amp.

You did a lot of damage, and replacing the fuse and trying it has caused much more.

There will be about seven blown parts on the amp and a dozen on the power supply. To give you an idea of the extent, the WHOLESALE parts cost is roughly $50, including the main power supply switchers IGBT's which cost nearly $20 for the pair by themselves.

I have repaired several of these and the larger cousin the PMP5000. It is a lot of skilled work to repair these.

The cause is a marginal insulation between layers of the multi-layer amplifier board. Usually the rearmost amp arcs between the layers. This fries three big switching transistors followed sometimes by a metering resistor and two small surface mounted transistors. A zener diode often goes. The cancer and burned area has to be ground out with a dental drill and new wire replaces the blown circuit traces.

If you are under warranty, take the unit for replacement.

The thing that can arc the amp is if speaker leads are marginal or get pulled out while under power. Excessive volume, especially pulses at high level can break down the insulaton causing the arc and failure. The black smoke is likely from the metering resistor.

DO NOT REPLACE THE FUSE !!! You will do more damage.

I just repaired one of these. A small ceramic surface mounted cap very near where the +/- 85 volt connectors plug onto the board blew up.

This left a sputtered mess on the insulating material at the right rear corner of the power supply.

The net parts list of parts I had to replace:

1. Main IGBT transistors on heat sinks. You have to remove the heat sinks to get at the mounting screws. 1MBK50D or equivalent. I used DigKey part APT28GA60BD15

2. One 1N4148 surface mount diode that is for gate drive speedup. D24 and D25. (don't remember which one was shorted)

3. All four driver transistors for the gate of the IGBT's. @ each 2SA1020 (PNP) and 2SC2655 (NPN). These are tall TO92-like transistors and I believe that at least one of these I had to get from Mouser.

4. Transistor T9, suface mount BC856B (PNP)

5. Replaced all four surface mount caps, one had exploded.
C50, C51,C54,C56. Replace with 0.01mfd/200 Volt 1206 style surface mount caps Digikey 478-1511-6

6.Replaced ceramic wire wound 10 Watt stand up resistors that were blown open. Replaced with 20 ohm design change. These slowly charge the main input filter caps and when supply is stable, a relay shorts them out. If you have replaced the fuse, you LIKELY blew them out. Original value was 10 ohms on some units. This was a Mouser part. 280-PRM10-20-RC

7. The 2153 switching reg chip IC1. This chip shorts. It is made by International Rectifier and I believe the full number is something like IRF2153. I had these in stock and not sure of which source I had procured them from... It was from one or the other.

Lots of destruction and one really needs a desoldering station to work this. Removal of the blown IGBT's is especially tough. All solder must be removed from the transistor leads AND the thermistor on one of the heat sinks. Once you have verified the leads are loose, one has to rock the heat sinks to get the tabs free. The tabs are NOT soldered! Be careful when restoring to get the thermistor leads started in their holes.

Note: ALL these repair parts were procured from DigiKey.com and Mouser.com. Neither of these places had all, I had to order from both. Mouser had the ceramic resistors.

When restarting this, leave outputs disconnected until it is running again. USE a series 75 watt light bulb in the 120 volt input. ONLY work on the unit using an isolation transformer as a large portion of this is connected right to the power line.

There is a small 15 volt switching supply that comes on first that runs the relay to short out the inrush limiting resistors. With the 2153 shorted as usual, this auxilary supply can't reach voltage enough to turn on the relay. The power for this is supplied through the ceramic resistors.

The schematics I found show SEVERAL fuses on the secondary side of the power transformer. I suspect one or more of those are blown. The fuses include the filament power.

As an electrical engineer I will have to caution you on leaving the amp in standby for extended periods. The high voltage rectifier circuit has NO bleeders in this unit and IF a power line transient occurs while in standby, the filter caps CAN be pumped up over their ratings causing them to fail.

Since you had no heaters lit in the tubes, you better pray that the heater fuses have opened as the only other thing is a failure of the power transformer since the neon pilot light is right across the 120 volt primary connections.

BIG problem! You should NOT have replaced the fuse as additional damage has been done. I repaired a PMP5000 and I can tell you likely what you will find: The smoke and smell came from two power resistors that are part of the soft start circuit that now are toast. You will likely find several components in the main portion of the power supply that are toast. These will include transistors, resistors, diodes, and probably the small 8 pin switching regulator chip. There is a small power supply that comes on first which enables a relay that shorts out those soft start resistors AFTER the power amplifier BALANCES and normalizes.
The unit I repaired had severe damage due to arc-over at one power amp section. Three power transistors were destroyed there and board traces and charring had to be handled in that section.
The repaired unit had to be brought up with a light bulb in series with the line power cord to prevent further damage until all bad parts were discovered and repaired. You will need to take the unit to a shop that is expperienced with the product. The repair cost MAY be pretty high.