Ampeg V4 dead

[bigsbybender]

Hey guys,

I just got a frantic call from a regular customer that his "new" (but really 1973) Ampeg V4 blew a fuse. He told me that he had accidentally set the impedance selector to 2 ohm when using his 4 ohm cab. I figured that a beefy Ampeg would handle this so I discounted this...at first.
 I've just got the thing on the bench. It appeared that he messed with the fuse holder, but then I looked further... that fuse blew so violently that the holder was damaged. I replaced it... put it on the current limiter and it's glowing bright. Then I pulled the 7027s and I'm getting the same glow....
One odd thing is that I'm getting an audible hum through the speaker (even with the valves out). I'm really worrying about the OT at this point....
I worry about prodding this era of Ampeg as they have notoriously high voltages inside and on a PCB.

What do you guys think?


As always, any help or insight is greatly appreciated.

j.

[navdave]

Well you could sell it to me.....
LOL prob needs a cap job.
38 years is a long time.

[LooseChange]

Just walk away. :-)

Many things can go wrong with these old beasts. Check the diodes and of course if it has original caps, probably should replace them too. These violent blow outs can burn the traces off the board. Check them all.  I had one where the PT went too.

Heavy amp. Lift with your legs!

[dpm309]

Just finished working on one of these that fortunately only needed 3 new tubes.  I told the owner he needed new caps but he did not want to spend any more on it than he had to.  Amp seemed to work fine after I changed out the tubes.  Yes, these amps are a beast and hard to work on.  A couple of the original multi-cap cans are not available anymore so you would have to install some single caps and do some other mods.  Don't know if you have been to the Ampeg V4 forum but there is a lot of help and advice over there for these amps.  Also, I have come into possession of several NOS 6K11 tubes that are used in these amps so drop me a line if you need one of these.

And yes, lift with you legs!

[bigsbybender]

Thanks for the help so far guys....   I've since opened everything up, and can't find burn marks anywhere. A visual inspection of the cap cans looks good....however that doesn't  mean anything. This guy will resist me changing anything unless it looks absolutely fried. I haven't had too many Ampegs of this era come across my bench...I'm impressed with the build and layout, especially versus the SVT. My next step is to disconnect the OT and check for shorts. Though I'm with you guys on the cap job....  I just want to make sure nothing else went.

j.

[The_Gaz]

Well, really hard to say without any other info. Check your voltages. As other people have suggested, the amp is old, and a cap may have shorted to ground. In fact, I fixed a V4 once that burned up a big resistor in the reverb driver circuit. Turns out a .022uf cap to ground had shorted.

I hate working on V4s, but they have a bass response to die for, so I gotta have respect :) You're right about high voltages, and just be super careful when probing. You especially don't want slip with the probe and fry any part of the PCB - they are fragile and hard to repair.

Good luck, and take your time!

[bigsbybender]

Yeah, this seems to be the ONLY amp this guy is satisfied with... I've worked on 100w JMP's, Twins, Bassmen etc for the guy... this is the first amp he's liked for more than a month.

I'm calculating the amount to build a little cap farm inside, then I'll leave the unused cans for visual originality sake. This thing is in gorgeous cosmetic condition! (Looks like it spends much of it's time in a Flight Case).

j.

[PRR]

Disconnect all the PT scondary leads.

If still FULL-bright on the lamp limiter, and no shorts in the primary wiring, then you want a new PT.

If the PT is fine, re-connect the heaters. Try that, without and then with tubes.

If PT is fine and heats the tubes, take the tubes out again.

Next, connect the rectifier BUT dis-connect it from the main filter cap(s). If this is sick, replace rectifiers.

With good through the rectifier, connect main cap BUT dis-connect the OT CT.

You can probably ignore the B+ to the small tubes. The B+ dropper resistors mean this chain can't pull enough power to ex-plode a fuse.

There are several versions of the big 1970s Ampegs. The Screen supply may be dropped from plate supply or may be on a separate winding and rectifier. Disconnect that while proofing the plate supply, then connect a part at a time (rect, cap, screens).

[bigsbybender]

This thing is nothing but headaches...

I've recapped and the bulb still glows.  I disconnected the HT leads from the PT, glow is low (presumably just heater current going through). I reconnected that and disconnected after the bridge rectifier, the glow is good. Since the first 2 stages of caps are clustered and paralleled I then left those connected and disconnected the power rail after the first set of caps....  the limiter lit brightly (Even with new caps). This leaves perhaps the OT left?  I was going to blame the bias circuit, but it would have shown up with the when I tested the first set of caps disconnected, if I am reading the schematic correctly.

Schematic.

http://www.el34world.com/charts/Schematics/files/ampeg/Ampegv4poweramp.pdf

[bigsbybender]

Well I unplugged B+ from the OT and the limiter barely glows..... OUCH, I've got bad news for this guy...  I think I'm going to replace the impedance selector too, I have a suspicion that the switch may be the original culprit.

[Willabe]

I think I'm going to replace the impedance selector too, I have a suspicion that the switch may be the original culprit.

That's done in many an amp, which is/has been well documented here and on other sites as well.    

Our good friend and fellow member Ritchie200, Jim, has posted a number of times about this being a known and common problem for taking out the iron in many a Marshall through the years, along with others.

Could have been way be worse, could have taken out both the PT and OT and any # of filter caps, tubes! Even the PCB, burnt traces.    

[bigsbybender]

Well I replaced the OT and all the filter caps but it's still showing up with a short on the limiter....   This is with the tubes out..  at this point I don't know what it could be. A brand new OT shouldn't show up with a short. I can disconnect the OT and the limiter calms right back down. The only thing I can think of is arced PCB or Tube socket... I can't think of anything else that could be shorted....  The OT center tap would feed each plate and that's it and without tubes in, what else could it be? 

So far I still have the PCB in and the traces are on the underside, but with a dental mirror all looks well. The sockets look excellent too. Am I missing something on this?

j. 

[sluckey]

With the OT disconnected, check resistance from all pin 3s to chassis. Bad socket(s) could be the problem.

[bigsbybender]

Thanks Steve, you put me on the right track. I think I found it....   It wasn't the sockets but as I was probing the plate leads I got a funny reading on one so I traced back and found an old mod under the board.  Someone had rigged in some "flyback diodes"  and of course, being out of sight and not on the schematic, I wasn't looking for them. One leg failed open causing a short.

Now my question is, should I replace the flyback diodes or go without? The schematic doesn't have them, but whoever put them in looked like they knew what they were doing (like a recommended mod). 


j.

[sluckey]

A lot of big amps have them and they serve a good purpose. But, when one fails shorted, it puts your OT and possibly PT at risk. I'd probably remove them.

[bigsbybender]

The output is working! There is a new problem though.    This thing works perfectly when connected to the current limiter (albeit with the lower output and fuzzier tone of lower voltage).  The bulb glows dimly as it should, however when I plug the thing in to full mains without the limiter, I get a huge 60hz hum that is nearly full the full 100w volume and a bunch of crackling sounds. I believe the crackling were sparks as I could see a blue light in there but I was unable to pinpoint where they came from.
 I've done this twice and have managed to catch it before anything serious happened.... I'll plug it back into the limiter and all works well.

I'm not even sure where to start... I'd like to do voltage readings (as there is a voltage chart in the owner's manual) but I can't get accurate readings while it is plugged into the limiter.

j.

[John]

Only a guess, but I'd say the limiter doesn't allow the arc to jump, while full voltage does. I mean, it's not a "dead short",but just that little extra juice lets whatever it is jump the gap. 

[bigsbybender]

That's kind of the conclusion that I've come to, but I can't figure out where....

I was also wondering about the bias circuit since there was a hum and I've replaced all the big filter caps, but I'd think if the bias circuit caps had failed, I'd still notice a hum even with the limiter on...this amp operates quietly.


j.

[bigsbybender]

I got rid of the crackling....  I actually found an old nail among the wires near the power switch....  It must have been in there for years since it didn't come from me! The nail had one end touching a ground point and the other end very near a terminal point..... What a messed up deal.

Now the amp still hums but audio comes through..  I tried the hum balance, but it's turned out not to be heater hum. Should I attack the bias circuitry next?  Sadly the bias caps are under the board which means I'd have to unsolder dozens of wires to get at....

[John]

I actually found an old nail among the wires near the power switch.

Well obviously, a farmer got ahold of that somewhere along the line! 

[bigsbybender]

I actually found an old nail among the wires near the power switch.

Well obviously, a farmer got ahold of that somewhere along the line! 

I am in Iowa.... 


I should add that the PT hums at the same frequency as the noise through the speakers... The hum is nearly non-existent when I operate through the current limiter.

[bigsbybender]

The hum is also independent of the volume controls...

j.

[John]

I am probably WAY off base on this, but it seems like I read somewhere about PT's naturally humming because of the EM field (normally not heard), and if they get over heated the insulation gets damaged, allowing for more of that hum. Maybe that nail was not quite shorting out for quite a while, but making the PT work harder, thereby damaging some insulation? I dunno, but I'm betting one of the guys that actually knows will be along shortly.

[bigsbybender]

Just walk away. :-)
[/quote

I should have.

[bigsbybender]

Patience persevered! I found a bad ground to a filter cap. Nice and clean now.
The PT still hums but it is hardly audible through the speakers. Too bad I it would be impossible to charge for all the time I put into this.


Very many thanks to all of you for your help and guidance.

j.

[birt]

Too bad I it would be impossible to charge for all the time I put into this.

true. but you have learned a lot that will help you with future repairs. congrats on this one!

[bigsbybender]

That is true birt... I have learned a lot about Ampeg layout from the 1970's from this amp. I rarely see Ampeg from this era. Mostly a few older and many later models.  Around where I live I see a majority of Fender for Vintage, and mostly Peavey overall. (I take in Solid State, albeit reluctantly)


j.

[bigsbybender]

Well it's back 


It blew a fuse 20 minutes into the first session the customer used it.  He said it made "Electrical Crackle Noises" (his words) for a minute then the fuse blew. I've replaced the fuse, but it on the current limiter and can't find anything wrong. It works great connected to the limiter and the fuse doesn't blow. My problem is how to diagnose this thing without going through a case of fuses.

If I start lifting leads like the heaters or the secondaries then powering up full, I risk losing a fuse on each test and potentially damaging the amp further....but I can't replicate the problem while on the current limiter.... It feels like a lose-lose situation.

I can't wait to get back to Blackface Fender restorations...  Any guidance is greatly appreciated!

j.

[sluckey]

Buy some circuit breakers. Use gator clips to connect across fuseholder terminals. Pull fuse.

https://taweber.powweb.com/store/fuseord.htm

[Dave]

Power tubes may have an intermittent short after they warm up

Dave

[bigsbybender]

Buy some circuit breakers. Use gator clips to connect across fuseholder terminals. Pull fuse.

Thanks Steve!  I should have some of these anyway....but I've gotten away without them.

Power tubes may have an intermittent short after they warm up

I'm wondering that... Unfortunately I don't have any spare 7027's around to try out.

[LooseChange]

>> I'm wondering that... Unfortunately I don't have any spare 7027's around to try out.

Use JJ 6L6's. They are exactly the same without the extra pin connections as the 7027.

[bigsbybender]

Use JJ 6L6's. They are exactly the same without the extra pin connections as the 7027.

So they'll just stab in?    No re-wire?

Edit

http://www.triodeel.com/7027.htm    This appears to answer my question. Now I'll have to drive to my studio after work and steal the JJ 6L6's out of my Twin.


j.

[bigsbybender]

Ok. I fired it up with 6L6s after work... Still on the current limiter. After a few minutes I still heard pops like arcing....this was not through the speaker but from the Power section and rectifier end of the PCB. I cannot see any carbon traces or burns with the naked eye....is there a better way to spot these?


j.

[Dave]

With the lights off.

Dave

[bigsbybender]

Well I'm back again..   I hooked up a circuit breaker (bypassed fuse) and ran full voltage...  I ran the thing for about 5 minutes and finally saw the arc. It arced across the HT secondary leads off the PT.  I inspected the insulation and it seemed pretty good, no holes or burns.   What should I check next? Bad PT? Bad Recto?

I'm also thinking I should hook up new Flyback diodes, wondering if this has any effect on the amp.

j.

[sluckey]

It arced across the HT secondary leads off the PT.

Put some electrical tape on both HT leads at the point of arc. If this fixes it you may leave it or you may want to replace the tape with some heat shrink tubing for a better look.

[bigsbybender]

I put heat shrink over the leads and re-routed them in a looser braid, this has appeared to stop the arcing.  The next challenge is output. The amp doesn't put out sound, almost like there are no tubes in or on standby. I'd like to use my listening amp but these PCB mount sockets give me nothing for my alligator clip to grab on to.   
Slowly getting there..... I hope.

[J Rindt]

I am working on my own V4-B nightmare.
This is the most frustrating chassis I have ever worked on.
I got it for free, and never heard it run. Needed a cap job, bias resistors, and a few other things. I was also considering removing the "fly-back" diodes.
Guess I will have to start my own post with pics as well.
Good Luck with your Television Amp

[bigsbybender]

A new Twist!

I cannot get any sound from the speakers...like the amp is on standby. I'm using an iPod for sound source. I usually keep the signal low on the iPod as guitar signals are not nearly as hot as a digital music player.  Here's where it gets strange.... I played the music but heard nothing from the speaker, I turned it up a little and heard something... so I put the iPod on full volume and sure enough I heard the music....but it wasn't through the speaker... it was harsh and tinny and coming from the chassis.  I used a dowel to my ear (Like the old broomstick test on a car looking for bad bearings and lifters.) I found the sound was coming from the OT.  Would this be a bad OT or do you suppose a secondary could be grounded causing this issue?


As Always, Thanks!

j.

[PRR]

> a bad OT or do you suppose a secondary

You really want to find any bad-wiring before you pay shipping on a new OT. So I'd start with intense eyeballing and metering.

[sluckey]

I would suspect the OT is not properly connected to the speaker.

[bigsbybender]

Thanks, Paul and Steve!   I was testing with an 8 Ohm speaker, and I found the 8 ohm setting on the switch was bad.  We're in business....I hope.

I hope this is it... I have a backlog of amps that I know much much better just waiting.

Thanks, and fingers crossed!

j.