BV120H Two power tubes not working but check out ok

[JBeast]

Hi,
I'll save you the typing and say right off that I know this amp is a piece of trash. I've had problems with it 1-2x a year for the past three years. I completely re-capped it and put silicone washers on the board mounts, etc... It has been working fine for quite a while, until last week.

Anyway, I need help and my problem is this:

Two of the power tubes aren't heating up. I switched tubes around so it's not the tubes. I checked continuity on the fuses and they all check out good. I traced all connections around every pin to their sources and made sure the resistor values were all good. As far as I can tell it should be working fine... but it isn't. I'm no troubleshooting expert. Can anyone tell me where to start diagnosing?

~Justin

[Enzo]

If a tube doesn;t light in a socket, but does elsewhere, then the tube heater is OK, and hte socket is not able to put heater current through the tube filament.

Just a hunch, are you measuring heater voltages to ground? And your resistance tests, dod you take them with the tubes still in their sockets?


Heater windings on power transformers have VERY low resistances. A bunch of tubes wired in parallel in their sockets ALSO have a VERY low resistance. I see a 6A fuse F6 for the heaters. Power off, leave the tubes all in, and measure resistance across that fuse. I bet you get continuity and the fuse is OK, right? Now remove the fuse and measure resistance across the empty fuse clip. You might expect an open reading, but I bet you get a low resistance. With the fuse in the clip, your meter sees the fuse as continuity. WIth no fuse your meter is reading the resistance through teh 6v transformer winding, through the tube heaters and back to the other end of the fuse clip. A false parallel path. That is why we always remove fuses from the holders to check them.

But your fuse is OK. That was just an experiment. The fuse can;t selectively refuse to power SOME of the tubes, it is all of them or nothing.

Look at your schematic, the power tube page. I see the power tubes wired as two pairs, two 13-pin connectors to the board. The tube heaters are wired in pairs, one pair to one connector J205, and the other pair to the other connector J206. That screams to me that you have a failed connector on one of those 13-pin ribbons. I see four light bulbs by the tubes. I wager the two bulbs behind the dark tubes are also not lighting up. Yes? No?

What to do here is pull the power tubes and measure for AC volts between pins 2 and 7 of each socket. I bet the dark-tube sockets don;t have 6v across those pins. You need to track this down with a meter rather than lit tubes - remove all tubes during this work. That will eliminate most false paths for current.

The weak link in this will be the ribbon connector. Note the two 13-pin connectors are similar but NOT wired identical. Of course you could have a break elsewhere, but I;d look there first. The male header pins break their solder, the female pins in the plastic housings overheat and even burn up. If nothing else, you can hard wire around the connector and eliminate it.


I won't disrespect your amp, I leave that to people with bad attitudes.

[JBeast]

Thank you Enzo. I appreciate your unbiased assistance.

These amps really are made horribly. The boards are super thin and prone to cracking and hardening thus sympathetic vibrations in addition to super thin traces that lift easily, cold solder joints, poor quality components etc. I keep it around for three reasons 1. Because it's my backup amp in a stereo recording rig 2. It's my project/learning amp to learn repair, maintenance and troubleshooting 3. I have put tons of work and modifications into it and it sounds really good right now, hahaha.

are you measuring heater voltages to ground? And your resistance tests, dod you take them with the tubes still in their sockets?

Yes, I measured heater voltages to ground and Yes I did resistance tests with tubes in but I also did them with tubes out

I see a 6A fuse F6 for the heaters. Power off, leave the tubes all in, and measure resistance across that fuse. I bet you get continuity and the fuse is OK, right?

Yes, I get continuity and the fuse tests as good

Now remove the fuse and measure resistance across the empty fuse clip. You might expect an open reading, but I bet you get a low resistance.

Yes, I get positive continuity. That is certainly unexpected.

Look at your schematic, the power tube page. I see the power tubes wired as two pairs, two 13-pin connectors to the board. The tube heaters are wired in pairs, one pair to one connector J205, and the other pair to the other connector J206. That screams to me that you have a failed connector on one of those 13-pin ribbons. I see four light bulbs by the tubes. I wager the two bulbs behind the dark tubes are also not lighting up. Yes? No?

Yes, both lights behind the dim bulbs are out as well and the lights behind the lit valves are lit.
The 13 pin connectors are soldered directly into both the main board (female) and the power tube component board (male). There is no ribbon cable.

What to do here is pull the power tubes and measure for AC volts between pins 2 and 7 of each socket. I bet the dark-tube sockets don;t have 6v across those pins. You need to track this down with a meter rather than lit tubes - remove all tubes during this work. That will eliminate most false paths for current.

You are correct, there is no voltage across the pins 2&7 of the dark tube sockets...

The weak link in this will be the ribbon connector. Note the two 13-pin connectors are similar but NOT wired identical. Of course you could have a break elsewhere, but I;d look there first. The male header pins break their solder, the female pins in the plastic housings overheat and even burn up. If nothing else, you can hard wire around the connector and eliminate it.

Ok, so you are saying that you think the heater connections are bad somewhere along the line, possibly at the 13-pin connectors? I will look at those and follow up.

I won't disrespect your amp, I leave that to people with bad attitudes.

Thanks again!

[Enzo]

There may not be an actual "ribbon" wire, but I tend to call any cable of 13 wire width a ribbon. The point being the clue here is that all those unlit things have that interboard connection in common. A ribbon connector will have a pin header soldered onto the board and a female on teh wire, but a hardwired cable can also have failed joints. SO a bad solder connetion, one of the wires wroke off, etc etc. Or even a cracked trace between teh ribbon landing point and its first destination at some socket.


The reasona I brought up measuring heater voltage to ground was that it doesn't matter what it reads to ground, it only matters that it be between the two ends of each heater. So in many amps we get 3 to ground from pin 2 and from pin 7, what really matters is 6 from 2 to 7.

[JBeast]

There may not be an actual "ribbon" wire, but I tend to call any cable of 13 wire width a ribbon. The point being the clue here is that all those unlit things have that interboard connection in common. A ribbon connector will have a pin header soldered onto the board and a female on teh wire, but a hardwired cable can also have failed joints. SO a bad solder connetion, one of the wires wroke off, etc etc. Or even a cracked trace between teh ribbon landing point and its first destination at some socket.

The reasona I brought up measuring heater voltage to ground was that it doesn't matter what it reads to ground, it only matters that it be between the two ends of each heater. So in many amps we get 3 to ground from pin 2 and from pin 7, what really matters is 6 from 2 to 7.

That's good information with regards to measuring heater voltage. I'm going to try to get to it tonight but it will more than likely be over the weekend. I have my work shop bench covered with wood working projects, I need to clean it off to put my amp down under some good light. I'll definitely post my findings here once I get it figured out.

[JBeast]

Ok,
Got it fixed today. Turns out the pins or the socket were corroded or not making contact for some reason. I noticed a little dark mark at the base of one of the filament pins on the bad side. I traced all of the connections and everything checked out fine - nothing broken, no bad components or bad solder - so I cleaned the sockets and pins with deoxit and deoxit gold after lightly sanding the pins with 600 grit emory cloth. I bent the pins forward a very tiny bit to make sure there was contact in the socket. Put it all back together and it sparked up like nothing was wrong.

Thanks for your advice Enzo, I greatly appreciate it!