Need help with a Silverface Vibrosonic short

[sonicmojo]

I am trying to fix a '73 Vibrosonic that was blowing fuses after a little bit of playing time. It is not a straight out short but something is sucking major current. I'm using a lightbulb limiter to debug and it stays brightly lit only when power tubes are in. First I tried swapping tubes to no avail and have checked all the socket resistors and everything looks okay there with no obvious shorts. Filter caps are newer and look okay, no obvious shorts, and have good voltages. The rectifier diodes appear to test okay. The PT primaries seem okay. PT Heater voltages look good. PT secondaries have good voltage. Here is what doesn't seem right - I am getting 22 Ohms resistance across the PT secondary leads (red wires) and only 11 Ohms to chassis for each lead. I disconnected the leads from the rectifier board just in case and get the same measurements. That seems like a very low resistance to ground to me but I don't have much experience with Power Transformers and wanted to ask here and get an opinion. What kind of resistance is typical for high voltage secondaries? Am I right to think that something is possibly shorted within the PT? If not, what else should I check?

Thanks!

[Leo_Gnardo]

Those HV secondary resistances seem awfully low BUT I repaired a 100W Rivera the other day and it had comparable resistance measurements, works just fine. How to tell on your Fender? leave the HV connections disconnected, you could pull all the tubes too so no filament current is used , then you should see a very low current on the AC line supplying the amp. If the transformer draws significant current say over 100 mA in this condition, and heats up besides, then it's suspect.

How's the bias supply? I rarely see bias filters gone bad in 70's Fenders but it does happen. When swapping out hi voltage filters I ordinarily change the bias filters too, heck they're 40+ years old too. Although original parts are typically some unobtainable value like 70 uF, I use 47 uF 100V parts, plenty enough for the task especially in those amps that have a two stage bias filter. Sometimes the connections on the bias board are cracked too. Inspect carefully & freshen' em up if they're not looking up to snuff.

Back to hi voltage, something to check once you've cleared the PT of suspicion - although the hi voltage filters are "newer" that's no guarantee one or more hasn't failed. I've seen 'em go in as few as 5 years (new-ish Egnater) to 18 years (Mesa with filters roasting atop the PT), so don't discount the possibility. For a quick field test, a leaky filter cap will heat up enough for you to feel it with your finger. To be safe while poking around in there, power down and discharge the hi voltage, then see if you feel a warm/hot cap. The primary filter caps have balancing resistors and those heat up just a little so don't be fooled by that.

[Stevem]

Due not assume that just because the filter cap look new test they are good!
I would disconnect each filter node starting in the preamp section one by one until the fuse holds!

[sonicmojo]

How's the bias supply?

I'm checking into that. The cap has been replaced at some point with a 47uF 100v but there are also a couple of extra resistors spliced onto the lugs of the bias pot and I'm trying to determine what else may not be stock in that area. I'll test that cap and post a picture of the area while I am at it. I can't find a layout for this particular amp but I think I found the right schematic. One other thing that I'm noticing, why would there be two carbon comp resistors to ground on the pilot light leads? That detail is not on the schematic that I have.

If that is not the problem, I will also check out the filter caps better. I'm not seeing any low resistances when measuring them as attached but whoever installed them left the leads too long and made bent spaghetti out them. My first thought was that the short may be caused by those possibly touching the aluminum cover but the problem persists with the cover removed. I was planning to clean that mess up anyway. Perhaps one of them is damaged.....

Thanks for all the replies. I'm now leaning away from the PT at the moment.

[sonicmojo]

Here is a pic of the bias area. I think it was changed from bias balance to bias adjust but I am not certain until I research a little more. Also attached is the schematic that I think goes with this amp. Thanks for the eyes!

[martin manning]

The two resistors going to ground on the pilot light are an artificial center tap for the filament circuit to reduce hum. Since that winding also powers the pilot lamp it's a convenient place to put them.

Check the bias voltage on pin 5 of the power tube sockets (on full line voltage with the tubes removed) to see if it looks reasonable. While you're at it get the voltages on pins 3, and 4 too. That should give you some idea of what state the power supply is in.

The connection between the two 4700 ohm resistors on the bias pot looks sketchy, like the leads are just twisted together; check that carefully.

[sonicmojo]

The two resistors going to ground on the pilot light are an artificial center tap for the filament circuit to reduce hum. Since that winding also powers the pilot lamp it's a convenient place to put them.

Check the bias voltage on pin 5 of the power tube sockets (on full line voltage with the tubes removed) to see if it looks reasonable. While you're at it get the voltages on pins 3, and 4 too. That should give you some idea of what state the power supply is in.

The connection between the two 4700 ohm resistors on the bias pot looks sketchy, like the leads are just twisted together; check that carefully.

Pin3 448V
Pin4 447V
Pin5 -8V

[martin manning]

The -8V on pin 5 is much too high; it should be ~50V lower. Start checking voltages at the junction of the bias rectifier and the electrolytic cap on the rectifier board and move towards the bias balance pot tap (where the 3k3 resistor connects) and then check the other three lugs to see where the failure is.

[sonicmojo]

Yep. I'll get back in there later today and check some more things but it was only -48 (should be -63 per schematic) at the junction and about -8 after the 3k3. I'll pull the bias cap to check it and also check all things around the bias pot. That spliced resistor is soldered but I'll double-check.

[sonicmojo]

Progress! The vintage electrolytic (80uF/75V) on the rear wiper of the bias pot was bad. I pulled it and it was only offering 600 Ohm resistance to ground. I replaced with a 100/100 and am now getting -63V at the diode junction and average -46V on all the Pin5s. Do you think this faulty cap alone would account for the blowing fuses?

[pops]

The two red leads from the power transformer to the diode rectifier don't look like they are soldered. Maybe it's my old eyes, but looks suspicious to me.

[sonicmojo]

The two red leads from the power transformer to the diode rectifier don't look like they are soldered. Maybe it's my old eyes, but looks suspicious to me.

You're right. I had pulled them out to measure. They are soldered back in now.

EDIT: I answered my own last question. I replaced one last old, white electrolytic in the preamp section just in case and fired it back up on the limiter. All looked good so I put in a new fuse and it holds. Transformers are still cool after 10 minutes. I plan to play it for a while before I give it back to its owner but I think it is solved. Thanks for the help!

[martin manning]

Great! Do you know what the power tube idle current is now?

[Leo_Gnardo]

Progress! The vintage electrolytic (80uF/75V) on the rear wiper of the bias pot was bad. I pulled it and it was only offering 600 Ohm resistance to ground. I replaced with a 100/100 and am now getting -63V at the diode junction and average -46V on all the Pin5s. Do you think this faulty cap alone would account for the blowing fuses?

That sure would do it. -8V on grids, that was definitely a sign of big trouble. Glad you got it sorted.

Too many times techs swap out the hi voltage filters, maybe that's needed, but neglect bias filters. Half the time I look in an old Fender with fuse blowing problems, the bias filter is shot. Cheap insurance to change out, even if it's not misbehaving at the moment.

[sonicmojo]

Great! Do you know what the power tube idle current is now?

My bias probe is on the fritz but assuming I am doing this alternate method correctly (center tap OT primary to Pin3), I am reading average of about 25mA which seems cold. I'm reading about -45V on Pin5 so I'm thinking the funky resistor cluster on the bias pot is perhaps bleeding too much to ground. What target values do you think I need to get to for the bias?