Bugera V55 problem

[mresistor]

OK  got the schematic courtesy of John, thank you. This amp has no output.  Have checked the tubes and all are functional (good).  Pulled up board and did visual with nothing out of whack noted, no burns no scorched traces no burned/broken components. Checked all 4 fuses, good. Pwr input fuse good.  Continuity checked the OT and it is OK. Wondering about the input relay. I've read that these relays use 15v as a latch voltage. It's a 24v relay though, a BESTAR BT-24S. And I'm not sure +15v has anything to do with it. I can hear the relay switch. Guess next is to check the regulator for full 15v. I've also read that the regulator needs to have a better heat sink setup, by either adding a sink or remoting the regulator to the chassis as a sink.

Bias voltage is -48.2v

Any one want to help or has worked on one of these? It's a head not a combo fyi.

[mresistor]

Looks like I don't have any plate voltage on V1 or V2. why? Looks like the yellow wire input power from TR1 has a bad molex connector. Going to hardwire to the board.

[eleventeen]

A 24 volt relay will work fine on 15 volts. It makes sense to underpower such a relay because it might be used in the energized position for long periods at a time. Hence, "keep it cool". A 7815 (or other 3-terminal) regulator is probably one of the most reliable electronic components you can buy, they are almost indestructible.


If you *are* getting good volts on the higher power supply nodes (A, B, C) but are *not* getting good volts on "D" then the immediate suspect would be the dropping resistor R23, or possibly a shorted or leaky cap C10. To check this out, you'll somehow have to break the connection from node "D" to the four places it supplies. Before you carve up PC traces or unsolder parts, I would remove V1, V2, one, the other, both, and see if your volts come back at node "D", on the lowish chance that something in the preamp stages is dragging down the volts you expect at "D".
 
But if you really have ZERO plate volts (and not 12, or 20 or 40 volts) on V1, V2, it almost has to be an open R23. Nothing at all unusual about an open power resistor. You could measure R23 without unsoldering one end if you turn the amp off and let it sit for a while with power off. Have the stby switch flipped to the "on" position as the amp sits for ~~ 5 minutes with power off so those 220K resistors (R29, R30) discharge all your caps.


I would also say that is hard to believe that you are getting 0 volts at point "D" and NOT having your REAL problem at node "C". Why? Because the difference between "C" and "D" is 20 volts, 340-320 per the schematic. Across R23, a 10K resistor, 20 volts is a mere 2 mils....thus R23 dissipates .002 * .002 * 10K a mere .04 watt. For that tiny amount of power to blow a 5 watt resistor is hard to believe. Possible, but unlikely. 


So, on the chance you found 0 volts at "D" and are saying "aha! that's it!" Check also node "C", in fact all the other nodes, R50, R20, R23.

[eleventeen]

A 24 volt relay will work fine on 15 volts. It makes sense to underpower such a relay because it might be used in the energized position for long periods at a time. Hence, "keep it cool". A 7815 (or other 3-terminal) regulator is probably one of the most reliable electronic components you can buy, they are almost indestructible.


If you *are* getting good volts on the higher power supply nodes (A, B, C) but are *not* getting good volts on "D" then the immediate suspect would be the dropping resistor R23, or possibly a shorted or leaky cap C10. To check this out, you'll somehow have to break the connection from node "D" to the four places it supplies. Before you carve up PC traces or unsolder parts, I would remove V1, V2, one, the other, both, and see if your volts come back at node "D", on the lowish chance that something in the preamp stages is dragging down the volts you expect at "D".
 
But if you really have ZERO plate volts (and not 12, or 20 or 40 volts) on V1, V2, it almost has to be an open R23. Nothing at all unusual about an open power resistor. You could measure R23 without unsoldering one end if you turn the amp off and let it sit for a while with power off. Have the stby switch flipped to the "on" position as the amp sits for ~~ 5 minutes with power off so those 220K resistors (R29, R30) discharge all your caps.


I would also say that is hard to believe that you are getting 0 volts at point "D" and NOT having your REAL problem at node "C". Why? Because the difference between "C" and "D" is 20 volts, 340-320 per the schematic. Across R23, a 10K resistor, 20 volts is a mere 2 mils....thus R23 dissipates .002 * .002 * 10K a mere .04 watt. For that tiny amount of power to blow a 5 watt resistor is hard to believe. Possible, but unlikely. 


So, on the chance you found 0 volts at "D" and are saying "aha! that's it!" Check also node "C", in fact all the other nodes, R50, R20, R23.

And obviously, if there are no bad resistors but there *is* a bad mechanical connector item, that could do it too.


Edit; Untangled quote. Brad

[mresistor]

Hey eleventeen,   good to see ya, and thanks.   I am getting 448v on node B but not getting the rest of the nodes, so it looks like I need to check the 22K  ohm R20 resistor and C12.

And I added a heat sink to the rectifier and wired the yellow wires straight to the PCB as the molex pins were showing signs of excessive heat.

[mresistor]

OK   I test pin 4 of the 6L6 socket, in triode mode I have 448 and in pentode mode nothing. Is this correct? 6L6 pin 3 is correct  457 volts on it in either triode or pentode so I have nodes A and B. Good.   Checking..  R20 reads 21.37K ohms. 

this is weird...

(being an old guy, I gave up for the night and succumbed to a brew and the living room)

[eleventeen]

That *is* weird. Unless I am reading the schematic wrong you should have big volts ~~450 on 6L6 pin 4's in either triode or pentode mode. Suggest you recheck. The only difference (voltage wise) should be small, less than 10 volts.


What about the triode/pentode switch itself? I have never been inside one of those amps, but it's not every cheesy little (toggle?) switch that can handle 450 volts. Is there an instruction with that amp that states that the triode/pentode switch is not to be switched under power? Maybe I am exaggerating, but it's possible that the insides of that switch have been arc-welded into nothingness. I would not want to switch that under power. Just wouldn't. Yet, in someone elses' hands, it might be just the thing they go for first, to really "A/B compare" how the amp sounds in triode vs pentode mode.



I would also like to see you grab the "send" output and feed a scope or a listening amp or another amp with the "send" output just as an overall quick check of the preamp section. That would be actually be a nice test, because if you DID get "send" output, it would show that your preamp is working, and for that to happen, the node-separating resistors would have to be OK, and the loss of big volts you are seeing would be more likely mechanical (bad connector pin) or bad tri/pen switch


From your description so far, it appears that you've lost a node or two from the power supply. The question is, is that because one of the series resistors R56, R20, R23 is open? Is it because you have a hinky wire on a molex or other multi-pin connector? Or are the insides of the 3/5 switch vaporized?


It's also possible that a trace on a PC board acted like a fuse and blasted itself open. If this happened fast enough, you might not notice any gross discoloration and you might have not any burnt smell.




Once you get the voltmeter involved, measure (and write down) ALL nodes A-B-C-D; at the power supply, AND at the various destinations  in the amp.


Seems to me like the switch could be the issue. 

[mresistor]

I forgot to originally add, I have already taken the effects send output and plugged it into a Champ amp. Nothing at all. But. when I turn off the Bugera the Champ made a real nasty shrill squeal noise, which I attribute to that BC546B transistor. The owners manual doesn't offer any warning at all about the half power switch, but later in the manual it says to go to standby first then switch it. The switch appears just fine. Feels good in operation. Looks brand new. This whole amp for the most part looks in fine condition. I've inspected the board with magnifying glass and under light and don't see any burned runs. They are very dark though and difficult to examine. I'll update on the voltages later. When testing you have to keep taking the tubes in and out and screwing the board down and unscrewing it, its a royal pita.   

[eleventeen]

Buzz out that switch. With power off, see if it works as a switch.

[mresistor]

OK  found it    the 390 ohm 5 watt R56 was open .    Must of been tired yesterday . thanks eleventeen .