Mk5 1N4007 replacement.

[steve Rhenius01]

:my mk5 90 12v supply failed when switched on after a long rest during covid pandemic.1N4007 diodes and 12V regulator needed replacing. After that heater supply returned but no drive t v7 grids. Tried replacingJ175 M1 and M4 without improvement. Boogie suggests using 1N5392 diodes instead of 1N4007 but how would that improve relay operation ? Any advice / help , please.

 

[bandit2013]

Interesting. The 1N4007 has a 1KV reverse breakdown voltage but the 1N5392 is only 100V. A 1N5399 would have the same reverse breakdown voltage as the 1N4007. The 1N539x diode does have a higher voltage drop though at 1.4V for 1.5A. Both are standard recovery types. RL255G is rated for 600V with a 1.1V drop and 2.5A capacity. I would ask Mesa if the reverse breakdown voltage of the 1N5392 is sufficient.

Since the 1N5392 can support up to 1.5A, the 12V supply will not be choked off as much due to the limitations of the 1A diode. Not much else will make a difference. The voltage regulator also has limits on its ability to maintain regulation at a specified current rating. Once the parts get taxed with too much load, they tend to fault or fail. Most of this is due to thermal degradation.

From what I saw on the schematic, V1, V2 and V5 have the heaters supplied by the 12V power supply.
The rest are supplied by 6.2VAC: V3,V4,V6, V7 and all 4 power tubes. It is the green wires from the PT. the Green/Yellow is a center tap on the winding connected to chassis ground. It would not be difficult to see if the heaters are active on V7, you will need to remove the tube shield to see if the cathodes or the heater element is glowing. Also, if there was no heater voltage to that tube, it will also remain cold to the touch. It that is not the issue, and you have no signal at V7 control grids, the issue could be part of the strobe mute circuit. The only schematic I have seen is missing the switching circuits and strobe mute circuit that controls M1 and M4.

 

[steve Rhenius01]

Interesting. The 1N4007 has a 1KV reverse breakdown voltage but the 1N5392 is only 100V. A 1N5399 would have the same reverse breakdown voltage as the 1N4007. The 1N539x diode does have a higher voltage drop though at 1.4V for 1.5A. Both are standard recovery types. RL255G is rated for 600V with a 1.1V drop and 2.5A capacity. I would ask Mesa if the reverse breakdown voltage of the 1N5392 is sufficient.

Since the 1N5392 can support up to 1.5A, the 12V supply will not be choked off as much due to the limitations of the 1A diode. Not much else will make a difference. The voltage regulator also has limits on its ability to maintain regulation at a specified current rating. Once the parts get taxed with too much load, they tend to fault or fail. Most of this is due to thermal degradation.

From what I saw on the schematic, V1, V2 and V5 have the heaters supplied by the 12V power supply.
The rest are supplied by 6.2VAC: V3,V4,V6, V7 and all 4 power tubes. It is the green wires from the PT. the Green/Yellow is a center tap on the winding connected to chassis ground. It would not be difficult to see if the heaters are active on V7, you will need to remove the tube shield to see if the cathodes or the heater element is glowing. Also, if there was no heater voltage to that tube, it will also remain cold to the touch. It that is not the issue, and you have no signal at V7 control grids, the issue could be part of the strobe mute circuit. The only schematic I have seen is missing the switching circuits and strobe mute circuit that controls M1 and M4.

 

[steve Rhenius01]

I've found an equivalent to the IN5292 and am in the process of installing the replacements. Will report back on the result ! Thanks for the comment. Steve

 

[steve Rhenius01]

Success !!!! I sourced alternative to the suggested replacement for the IN4007 (IN 5392 which are said to be obsolete). Installed these yesterday and to my delight I now have signal from input to anodes of V7 so assume this will now drive the power valves - final tests this weekend but expect to be able to box the amp up and return it to user next week. Thanks for all the comments and advice. Steve

 

[steve Rhenius01]

Latest - amp worked fine after installing IN5392’s and replacing 12V regulater. Then the valve rectifier failed and was replaced after which one of the 6L6 power tubes failed short circuit. I replaced all 4 with matched set but now no sound, again! 12V supply is the and fan is running but suspect either J175 M1 or M4 in mute circuit. There are also npn 6294 transistors - when I look for datasheet they com up as Darlington pair - is that correct? If not, what is the correct replacement ? Help !

 

[bandit2013]

If you have the complete schematic, the 2N6426 are Darlington pairs. I could not find any old resources to state otherwise. I would assume if is one of the JFETS more so than the NPN transistors.
The 2N6426 is used in the Roadster as well as the Road king. The symbol is drawn as an NPN but does not mean it is not a Darlington pair. One is used for the strobe mute, the others are used to control relays.

If you had to remove the cables and such to replace the diodes, Check the ribbon cables to confirm they are all connected to the pins properly. If one cable was unconnected, it could result in a JFET fault or damage.

Way to find out if it is the one in front of the phase inverter, turn on the FX loop, set to all on the back panel, insert a guitar cable into the FX return, If the J175 that mutes in front of the phase inverter is still good and the circuit is working properly you should get sound through the amp from the FX return trick.

The strobe mute circuit will have a pull-up resistor to drive the JFET gates to +12V when the NPN transistor or 2N6426 is released. It will become an RC time-based charging circuit based on the capacitors used in that circuit. If for any reason the voltage does not change and remains at close to ground, the 2N6426 is either latched on which may be related to the switching circuits. If the gates get to +12V on the J175 they should be turned off and not shunting the signal. I believe the Mark V has different timing for the reverb mute circuit than the other mutes.

I do not have access to the full schematic so what other components could keep the strobe mute latched on is hard to say. A pulse input is what triggers the strobe mute and that is caused by change in the relay network used to define channels. If the base on the 2N6426 is above 1.4V (assuming it is a Darlington) it will be latched on and no signal. Some of the components are on the rear board, but not all of them. Would be nice to have the full schematic as there are many signals and control lines that are associated with the ribbon cables.