Fender Bassman 100 Head Silverface 1974 - Blows Fuse

[misterc57]

Good Morning Forumers!

Looking for advice on my Bassman 100 head that I bought this week. The seller said it is blowing the 3 amp fuse.

Everything looks original, maybe even the RCA tubes. All tubes are out and tested good on my tube tester.

I plan to order a capacitor kit, but I was hoping to isolate the issue before ordering any parts.

With tubes out, chassis plugged into a bulb limiter, bulb limiter plugged into a variac, I brought it up to full voltage and had a very bright bulb.

I suspect the filter caps, but also wonder about the rectifier diodes, and the PT.

The filter caps look okay and also look to be from 1974 if I am reading the date code correctly. I measured continuity to ground from each filter cap. Everything seemed okay except the "+" side of the two 100uf wired in series, that "+" side measured about 6 ohms to ground. That seems wrong to me, but I am not sure.

I could start clipping out one end of each cap and test each, same with the rectifier diodes.

Any suggestions on isolating the issue? I am attaching the schematic and a few pics.

As always much thanks and gratitude!

[Leo_Gnardo]

I suspect the filter caps, but also wonder about the rectifier diodes, and the PT.

The filter caps look okay and also look to be from 1974 if I am reading the date code correctly. I measured continuity to ground from each filter cap. Everything seemed okay except the "+" side of the two 100uf wired in series, that "+" side measured about 6 ohms to ground. That seems wrong to me, but I am not sure.

I could start clipping out one end of each cap and test each, same with the rectifier diodes.

That 6 ohms to ground reading looks ominous. You could also suspect a short from hi voltage to ground thru the output transformer or filter choke. Rare, but they happen occasionally.

Filter caps of this age are highly suspect. I'd replace them anyway, plus filter cap(s) in the bias supply.

Before clipping out parts, you could warm up the soldering iron, pull the + end of that series cap off its board, then check resistance again, both cap and the eyelet you removed it from. That will tell you real fast whether the cap - and its series companion - have an internal short, or whether you need to continue the short hunt elsewhere.

[misterc57]

I lifted the + side of the filter cap. Same reading, about 5.7 ohms to ground at the eyelet. The + side of the cap now had high resistance to ground.

So that 5.7 ohms to ground is all over that path, including before and after the bridge rectifiers, both red wires in the PT secondary included. With my limited experience, that does not seem right. If I read reistance from the beginning and end of each 3 series diodes, I have continuity on end to end arround 0.25 ohms.

Bad diodes?

I tested each of the 5 filter caps with the + side lifted out. All test good. I will still be replacing them all.

Thank you.

[nevetslab]

Check to see if your DMM has a Diode/Semiconductor setting on it, which will provide a 2V 1mA current source, so the reading will be showing you the typical 0.5 to 0.7V junction readings. I'd also unsolder the red wire that leaves the summing connection of the two diode strings and feeds the power supply buss caps at the other end. Then, see if you still are reading this very low ohms reading to chassis. If that's still there, unscrew the rectifier board and lift it up off of the insulator board beneath it, and check again.

If you're still seeing this low resistance reading, you may have the infamous leakage on that fiberboard material. Remember, you still have a ground connection to the chassis where that short braided wire is attached to the cap terminal of the bias supply.

[misterc57]

Hi Folks

I did not see post 4 until this morning. I have since removed all the rectifier diodes and all 6 are shorted.

I wonder what could have taken them all out and also wonder about the health of the PT.

Also need to figure out what diodes to order.

Thanks!

[misterc57]

Check to see if your DMM has a Diode/Semiconductor setting on it, which will provide a 2V 1mA current source, so the reading will be showing you the typical 0.5 to 0.7V junction readings. I'd also unsolder the red wire that leaves the summing connection of the two diode strings and feeds the power supply buss caps at the other end. Then, see if you still are reading this very low ohms reading to chassis. If that's still there, unscrew the rectifier board and lift it up off of the insulator board beneath it, and check again.

If you're still seeing this low resistance reading, you may have the infamous leakage on that fiberboard material. Remember, you still have a ground connection to the chassis where that short braided wire is attached to the cap terminal of the bias supply.

I lifted the red wire that leaves the summing connection. All diodes are also out. There is no path to ground at any of those eyelets now. However I do have 10 ohms to ground at both the red wires at the start of the diode strings, those two red wires are still soldered to the eyelets.

edit: I lifted the board and I still have 10 ohms to ground on both red wires.

[Leo_Gnardo]

I lifted the red wire that leaves the summing connection. All diodes are also out. There is no path to ground at any of those eyelets now. However I do have 10 ohms to ground at both the red wires at the start of the diode strings, those two red wires are still soldered to the eyelets.

edit: I lifted the board and I still have 10 ohms to ground on both red wires.

Center tap of the high voltage winding is bolted to the chassis. You're measuring DC resistance of each leg of the high voltage winding.

[misterc57]

Center tap of the high voltage winding is bolted to the chassis. You're measuring DC resistance of each leg of the high voltage winding.

I see. I think I should power up again and see if my bulb limiter gets bright.

[misterc57]

I powered up on variac. Bulb in limiter did not come on.

I measured AC from each red wire to ground. One red wire reaches 296 VAC, the other red wire reads nothing. Shouldn't they both read 296 VAC?

[Helmholtz]

One red wire reaches 296 VAC, the other red wire reads nothing.

This is strange, as both windings showed about correct DCR to ground (10 to 13 Ohms). Yes, both red PT wires should have equal AC voltages to ground (around 335V at 120V mains input).
Were the wires disconnected from the board? What's the voltage between both red wires (should be around 600VAC)?

[misterc57]

This is strange, as both windings showed about correct DCR to ground (10 to 13 Ohms). Yes, both red PT wires should have equal AC voltages to ground (around 335V at 120V mains input).
Were the wires disconnected from the board? What's the voltage between both red wires (should be around 600VAC)?

At 120 vac input I have 612 vac across both red wires. I tried each leg again and now both are reading around 310 vac. I must not have had a good connection on my earlier readings.

And I have 6.4 vac at my jewel light!

So, looks like the PT is okay!

All the rectifier diodes were bad. What could have caused that?

Any suggestoin on rectifier diodes? Should I stay at 3 amps or go higher?

Thank you

[Helmholtz]

All the rectifier diodes were bad. What could have caused that?

Assuming that all 6 diodes are shorted, my theory would be the following: Diodes may fail from overvoltage, overcurrent and sometimes in rare cases one fails without a special reason (all electronic components have a non-zero probability of spontaneous failure, expressed by the FIT (failure-in-time) number).
Diodes typically show a short when defective.
Imagine one of the diodes shorted spontaneously. This would cause the other 2 in the string to see too high blocking voltage, causing them to fail as well. If the whole string is shorted, the other string would fail from overcurrent as it shorts the whole winding.
Another possibility for the initial failure could have been a voltage spike on the mains caused by a lightning discharge leading to the same chain reaction.

[misterc57]

Assuming that all 6 diodes are shorted, my theory would be the following: Diodes may fail from overvoltage, overcurrent and sometimes in rare cases one fails without a special reason (all electronic components have a non-zero probability of spontaneous failure, expressed by the FIT (failure-in-time) number).
Diodes typically show a short when defective.
Imagine one of the diodes shorted spontaneously. This would cause the other 2 in the string to see too high blocking voltage, causing them to fail as well. If the whole string is shorted, the other string would fail from overcurrent as it shorts the whole winding.
Another possibility for the initial failure could have been a voltage spike on the mains caused by a lightning discharge leading to the same chain reaction.

Yes all 6 are shorted. Thank you

[misterc57]

Thank you to all for the help. I will order diodes, capacitor kit, some resistors, etc for an overhaul. Will report back when all is installed.

[nevetslab]

Regarding the diode type, I looked in the Twin Amp service manual to see what they call out. In the parts list, they called out 1N4006 1A/800V rectifier diodes, though in the schematic, it shows 1N5062TR diodes, also 800V but 2A rating. I'd go with those....Mouser has stock. The TR suffix is Tape & Reel (instead of loose).