Laney nexus 400 watt tube bass amp

[rockett]

Query: Hi I have new but old stock demo Nexus 400 watt Tube bass amp which
cuts out after 15min. of playing? Straight out of the box. All lights on
front panel go out valves heaters stay alight switch off for 10 min. And
amp
will go again
There is know warranty with this amp and I think it was a common fault
posably a poly switch I have been a electronic tech so can you help with
some info thx. Rod

[Enzo]

So go inside and find the low voltage power supplies. Are they working and healthy? At first? and after it quits?

[rockett]

Thx Enzo I will when I get off work next 2000kl away from amp just getting some ideas for my return.

[g1]

They did have a bad batch of re-settable polyfuse thermal fuses but I'm only aware of that problem in the Cub head. If you read about that for this amp, they may use them in this model as well.
Here is a link to the service manual, it is quite large, perhaps someone could extract the schematic and post it:
LANEY NEXUS TUBE NXT SCH Service Manual free download, schematics, eeprom, repair info for electronics

[mozwell]

tube power amp & mosfet power amp versions

[g1]

See schematic page "NXT PSU Board". There are 2 polyfuse F4 and F4 which supply AC to all the low voltage supplies, +/-24, +/-15, and +5V (which runs off +15V).
If one of those polyfuses were the problem, you would still have heaters so you do have a plausible scenario.

[rockett]

Thx g-0ne and mozwell
The poly switch could be to close to another hot component in the power supply and if you move it away it could fix the problem so when I get back home I will investergate also thay have a life of 20 odd switches someone said apparently ?

[mozwell]

Hi Rockett
From the data sheet, these have 100 "tripping" cycles. Other manufacturers do not quote a figure.
If these are your problem, then you could jumper them temporarily or replace them with a 2A fuse temporarily, they just protect the 24V & 15V supplies from a short. Make sure the bridge rectifier BR1 is ok, and there are no shorts on the +24, -24, +15 or -15V rails.
The poly switch seems to be a standard item from Mouser (their p/n 652-MFR160-LF)
Many amps would not bother with a fuse or a poly switch in this position.

Measure the low voltage power supplies, see if these hold up, or if any drop when you have your problem. Then you can measure the incoming AC to see where the problem is.

[rockett]

Mozwell thx mate i think that is it as i said it could be very close to a WW resistor and getting hot so if that is the case presto fixed it will have to be when i get back to the sunshine coast in a few months reg.

[mozwell]

Rockett
You are in banana bending country, so Jaycar RXE160 RN3466 will be ok, also a great place in AU for spares is Wagner Electronics in Sydney. I have used them & they are great.
Wagner also have RXE160 Ptc Polyswitch Protectors, Polyswitch | Wagner Electronics, wagneronline.com.au

[mozwell]

Rockett
i have a some RXE185 at work, i am happy to post them to you. PM me with your address & ask me for 2x RXE185 & i will send them off to you