Troublesome B15N electronics ( please help )

[Rover Eric]

Hey, all. Need some help with a B15N i picked up on craigslist.

1) It's a mid-late 70's B15N ( 4-input, white switches ). It has a full compliment of magnavox tubes, all with yellow silkscreened printing - perhaps original to the amp.

I got the amp on craigslist, advertised not working. Opened it up, and noted that the fuse on the chassis was disconnected in such a way that it appeared someone had been working on it. I re-soldered the standard 3-amp chassis fuse back in place per the schematic and fired it up with no tubes in place. The power transformer hummed and clicked and smelled funny. I put the rectifier tube in, and it began to spark and such. ( this was just on the heater filament - still on standby ) I put in a different rectifier tube of the same type, and got the same results. At this point i assumed old / bad filter capacitors.

2) I ordered the proper cap replacement kit from fliptops.net. I soldered them all into place yesterday ( original were all Sprague that had a "78" on them, so i assume that to be the era of the amp ). Popped the rectifier tube back in - same thing. Sparks, clicks, funny smell, etc.

With just the rectifier tube in place, flip OFF the standby switch and within seconds it pops the fuse.

With the rectifier tube OUT, i can sit and flip the standby switch off and on every few seconds and not pop the fuse ....but there appears to be a spark arcing behind the switch itself - it's visible through the white plastic.

3) I'm wondering if it's a bad power transformer at this point...all signs point to that, since i've tried different tubes, replaced the caps, and still the same thing ....and since all humming, ticking and popping sounds are coming from that item. I just wanted to know if anyone else had any opinions before i drop $200 on one only to find out it was a bad hum balance pot or a cold solder joint somewhere else.

Anyone have a similar experience ever?

thanks in advance.

(also, i posted a question a few weeks back regarding the fuse on this thing, and it's got some pics of the schematic and such in it : http://www.talkbass.com/forum/showthread.php?t=670409 )

 

[JimmyM]

bump for the late night crowd of amp techs.

 

[T-Bird]

Hi.

The PT is easy enough to diagnose with a multimeter, and way, way safer than just messing around with changing random components.

No offense intended, but it sounds like You're way over your head with this one.

Tech time.

Regards
Sam

 

[Rover Eric]

Hi.

The PT is easy enough to diagnose with a multimeter, and way, way safer than just messing around with changing random components.

No offense intended, but it sounds like You're way over your head with this one.

Ideally, i'd like to fix this myself... not because i'm cheap, but purely out of the satisfaction i get from doing it, and the learning experience. I restored a Plush 1060C last weekend, recapping and retubing it as well as converting it over to the safer ground cable / fuse wiring. I've got an armload of tube amps and the desire to learn to work on them.

The way i see this - i'm pretty sure it's the power transformer at this point. I could pay an amp tech however much to diagnose that himself.... or i could just order up the power transformer and swap it myself and see if that fixes things. If i should do something like unsolder the current PT leads and check them for voltage first, then that's a fine troubleshooting step i'm happy to take away and do on my own.

I do appreciate your input, however. Maybe i'll find an amp repair guy nearby just to shadow for this operation so i know how to do it in the future.

 

[T-Bird]

Hi.

Like I said, I mean no offense when I direct people to techs when they ask for tube amp related repair advice. It's just that the voltages inside tube amps are lethal or worse (for me anyway), those voltages can cripple You for life. I don't want to be the one to give a lethal advice.

That said, a simple continuity test reveals whether the PT is shorted or not. I always start with the primary, and then proceed to the secondaries. Unless the laminations have moved, the fuse(s) have been replaced with a nail or the transformer is a really crappy one, the short is somewhere else 80% of the time.

Regards
Sam

 

[beans-on-toast]

Since someone has been working on your amp, the first thing to look for is any differences between the schematic and how your amp is wired. Ignore the 6A fuse in the schematic.

An abnormal hum and clicking from the transformer is an indication of internal arcing and a bad transformer. It is reasonable to look here next.

As suggested, testing the resistance of the input and output windings is the first thing to try. The 5 and 6.3 volt heater winding should read around 2-3 ohms. The high voltage windings should read 50 ohms or higher. It depends on the transformer.


This next step is very dangerous and should be done with the wires connected to something like a terminal block for safety.
With the transformer removed, you can power it up and test the AC voltages under no load. Even if the voltages appear to be correct at this point, the transformer can still have a problem. This is because internal arcing (the clicks and smell) could only happen when a load is present and current is flowing. So the next step is to test the transformer under a load.

 

[Rover Eric]

As suggested, testing the resistance of the input and output windings is the first thing to try. The only winding you can test without desoldering the transformer leads is the 5 volt heater winding (with the tubes unplugged). The 5 and 6.3 volt heater winding should read around 2-3 ohms. The high voltage windings should read 50 ohms or higher. It depends on the transformer.

This is good info. Can i just put the two leads of my multimeter into the heater pins of the 5A4 socket ( with all tubes removed, of course ), set it to ohms, and turn it on ?

 

[beans-on-toast]

Can i just put the two leads of my multimeter into the heater pins of the 5A4 socket ( with all tubes removed, of course ), set it to ohms, and turn it on ?

Turn on your volt meter, wait a couple of minutes before taking your readings. Taking the resistance readings with the transformer removed is the best way to check it out as you can test all the windings both for resistance, and with the power applied to the primary windings, for the unloaded secondary voltages. It is best to do the job properly.

You can test the heater windings in place but the readings won't be the same as if the transformer were removed from the circuit.

 

[gerryjazzman]

This is good info. Can i just put the two leads of my multimeter into the heater pins of the 5A4 socket ( with all tubes removed, of course ), set it to ohms, and turn it on ?

I wouldn't bother with the heater pins as most likely the problem is in the high voltage winding. You could check the heater but you'll probably get a reading less than an ohm (which would look like a short to you but would be normal). i would check the high voltage winding which would be across pins 4 and 6 of the rectifier tube (same for 5U4 or 5AR4, not sure which one you have). But all tubes should be out, and make sure that the amp is unplugged. First try with the standby switch off (open) and check the resistance. That is across the entire HV secondary. Then you can try with the standby switch on (closed), so the center tap is grounded. Then try measuring from 4 to ground and then 6 to ground. The readings should be idential. If not, then there is most likely a short on one side of the HV secondary.

Also, with everything disconnected from the transformer (all tubes out, standby switch off just to make sure, if you connect a light bulb (say 60 watt) in series with the AC cord to the outlet, plug in and turn the amp on, the bulb should not light or show only a VERY dim glow from a small current due to leakage reactance of the transformer. If it glows noticeably brightly, then the transformer is definitely shorted somewhere. If you try this be VERY careful with wires so you don't electrocute yourself or start a fire if wires accidentally short. This is a nice thing to assemble on a board with a proper bulb fixture, outlet and switch in electrical junction boxes for general amp testing and is much safer to jury rigging clip leads around AC line circuits.

 

[Rover Eric]

I wouldn't bother with the heater pins as most likely the problem is in the high voltage winding.

Just terrific, terrific info, Gerry...and i really appreciate your time. But i'm confused about one thing:

That transformer is sparking and ticking and stinking of burning before i ever flip on the high-voltage windings. Correct me if i'm wrong, but with the standby switch "on" isn't the only winding that's distributing power the heater one ?

If the rectifier tube is in place, and just the power is flipped on ( standby engaged ), the rectifier will begin to spark around the plates.

Sorry if this is a stupid question - i'm trying to learn. I think i know enough not to kill myself ( at least, to know enough NOT to attempt the dangerous stuff ) ...but i thank everyone for their help in guiding me through.

-Eric

 

[BassmanPaul]

With no tubes installed the only circuit getting power from The PT is the bias supply. Check that out for a shorted cap or a bad silicon rectifier. You should also check for actual shorts in the wiring from the PT. If everything seems OK then the PT is probably bad. Mercury Magnetics will rewind the transformer for a reasonable price.

Trying to repair a tube amp is not something that should be entered into lightly. Verifying that a transformer is bad is basic 101 stuff and you don't seem to be too sure of how to use a multimeter. I'm going to endorse Sam's post and suggest you take the amp to a tech.

Paul

 

[Rover Eric]

Fair enough...

my expertise is in vintage scooters ( i collect and restore old vespas and lambrettas ) , not in vintage amps But i do know how to use a multimeter well enough to troubleshoot all my electrical wiring / harness / shorts and such when i restore bikes.

My point is - i'm a smart guy. I have a decent grasp of electronics. I've built some tube power and preamp kits on the hi-fi side of things. I've repaired other amps before. I feel like i know enough of the essentials to not get myself killed...but i need some guidance in these early projects, particularly when i'm running into something i've not done before.

I've called around to some amp repair contacts today locally to see if they can help me out, ( heeding everyone's advice ) ...but i'd still like to be able to use this opportunity / project to advance my own repair skillset.

*edit* better yet - i just talked to Mercury Magnetics. I can remove my current PT, ship it to them for testing ( which is free ) and if it's not the PT i know i have a short in my system. If not, it's a $200 new PT.

Thanks all,

-Eric

 

[johnk_10]

everything that you have stated leads me to believe that your power tranny is toast.

 

[BassmanPaul]

If you could get an experienced guy to mentor you you could learn that way. Some repair guys will let you watch him work and describe what he is doing. This is something I would do but not everyone would take the time.

The best way for you to proceed is to identify the PT wiring. disconnect the wire feeding the bias supply and insulate it. THe CT of the HV secondary going to the standby switch can be left but put the switch in the open position. Disconnect and insulate the HV wiring from pins 4 and 6 of the rectifier socket. All this done with the power disconnected from the amp.

Gerry's post about the light box is something that you should build. It limits the current flow into an amplifier. Simply by screwing a different wattage bulb into the socket you change the current that's allowed to flow.

When you power the amp on there should be a flash from the bulb from the inrush current then it should fade out. It the lamp stays lit and you have disconnected the wiring that I indicated your PT is toast.

Paul

 

[Rover Eric]

I will build one of these boxes. Thank you for the suggestion.

I already pulled my PT to send off to mercury, though, for good measure.

I like my Power Transformers like i like my women; potted, and from 1978