Fender Custom Vibrolux Reverb won't power on

[Yreva]

Hi,
I have a Fender Custom Vibrolux Reverb that fails to power on intermittently. Fuse intact, new tubes, although the power switch tested okay I replaced anyway. Still failing to power on consistently. Any advice what to check next would be appreciated.
Mark

[eleventeen]

Get your ohmmeter and clamp the leads on to the two prongs on the power cord and turn the power switch to the amp on. Normally, this should show a very low resistance connection, 2-3-5 ohms. Wiggle the cord. If you don't have alligator clips you can stick the probes through the holes in the blades on the plug and hold them by hand. That will be a 100K++ connection that will be swamped by low resistance of the AC primary (if it's working right) If as you wiggle the cord you see the resistance change, a conductor is broken and the cord requires replacement. Sounds like an intermittent power cord to me. It's most likely to occur within the molded AC plug or right where the cord enters the chassis. The other alternative is you have an intermittent primary on the power transformer, extremely unlikely but not impossible.

[Yreva]

Thanks for the reply. No change in resistance  when wiggling the power cord. I'm using this amp as learning experience, what's the next step to address an intermittent primary on the power transformer? Thanks again for any suggestions!
Mark

[sluckey]

Primary schematic for that amp is attached. Not much to go wrong. Reseat those four connectors (CP1, 2, 7, 8). Check the connections on the fuse holder. Put a jumper across TH1.

You say it fails to power on consistently. Does that mean even the indicator lamp does not come on?

[Yreva]

Sluckey,
Thanks for the reply. It fails to power on intermittently and the indicator lamp does not come on when this happens. I'm not sure what you mean by reseating the four connectors you mentioned and putting a jumper across TH1. I can check the connections on the fuse holder. Thanks for your help and most of all for your patience. Obviously I'm new to troubleshooting amps but trying to learn. And I really want to get this one up and running reliably.
Thanks,
Mark

[Yreva]

TH1 is a thermistor correct? There does seem to be a correlation, albeit inconsistent, that failure to power on occurs after the amp has been played long enough to get hot and then cooled back down for an hour or so. Do thermistors typically fail and are they affected by temperature change?
Thanks,
Mark

[sluckey]

Thermistors are subject to failure just like any other electronic component. Put a jumper across TH1 means solder a short wire to each terminal. This takes it out of the circuit. If this jumper fixes the intermittent power on problem then replace it with another thermistor, or take it out permanently.

I'm assuming those connectors (CP1, etc) simply plug in. Reseating the connectors means pull it off the component then put it back on. Do this a couple times for each of the 4 connectors I mentioned.

BTW, in case you don't have it,  the service manual for this amp is in the schematics library. Follow the link at the bottom of this page.

[Yreva]

Thanks for the help with this. I reseated and used a little Deoxit on the connectors and checked continuity on the thermistor (not sure if that is meaningful when it's cold) , but so far so good.
Thanks again,
Mark