Hot rod CR4 / CR5 equiv

Danny1969

@ICBM ;

Do you know a common replacement PN for these with a similar rating at 3KV 250mA ? 

Got a hotrod in with burn out screen resistor, diode and big patch of carbonated PCB 

ICBM

Ouch... it must be the season for it, one of my jobs this week is a Blues Junior with the usual valve PCB fire .

https://uk.rs-online.com/web/p/rectifier-diodes-schottky-diodes/6298679/

Danny1969

Cheers mate, I did look but couldn't see that, ordered so should be here tomorrow. 

Danny1969

Well it gets worse, fixed the PCB, new valves,  turned on ... 240V on the primary but bugger all on any secondary. Are these transformers thermal fused internally ? There's no evidence the transformer even got hot, no smell, no deformed insulation of the wires 
Best place to get another PT ? 

jpfamps

Danny1969 said:

Well it gets worse, fixed the PCB, new valves,  turned on ... 240V on the primary but bugger all on any secondary. Are these transformers thermal fused internally ? There's no evidence the transformer even got hot, no smell, no deformed insulation of the wires 
Best place to get another PT ? 

There is a thermal fuse between the violet wire and black/red wire on the transformer primary.

You can buzz this out to check if there is continuity.

You can bypass the thermal fuse, but of course I am not recommending you do that....

Getting replacement parts from Fender has been a pain recently, but if you can't get on from them Hammond make a good repro unit, part no 290UEX, which you can get from Tube Town, or Mouser.

ICBM

jpfamps said:

You can bypass the thermal fuse, but of course I am not recommending you do that....

Nor am I. Cough... cough...



Also, if you haven't done it already then do the 230/240V swap so the amp runs cooler as well.

Danny1969

Do you mean the MOV on the PCB in series with the primary ? That's good and that feeds the purple wire into the primary, neutral is white and black with 240V across them so it's like the primary is open circuit in the actual transformer ...

But I could be being a bit dense, I probably repair less than 5 valve amps a year so appreciate your help @jpfamps and @ICBM ;

 

ICBM

Danny1969 said:

Do you mean the MOV on the PCB in series with the primary ? That's good and that feeds the purple wire into the primary, neutral is white and black with 240V across them so it's like the primary is open circuit in the actual transformer ...

No, there's a non-resettable thermal fuse in the transformer winding. It's connected between the purple wire (P3, incoming supply connection) and the black/red one (P5, spare terminal) - check the continuity between these, and if it's open circuit then you can either replace the transformer, or simply swap the two wires... which will give you the black/red connected to the incoming supply.

I don't regard this as dangerous since very few amps actually have them anyway, and transformer fires are very rare - at least as long as you have the correct fuse in the amp.

And if not already done, swap the black and the white/black wires so black is on P4 and white/black is on P10, this sets the amp to 240V not 230V which reduces the stress on everything - including the risk of board arcs and fires.

Danny1969

Ahh, right done that and amp now running nice. I've also done the 220 to 240V swap as advised, biased and all seems good now. 
Thanks for your help guys, much appreciated