Filmosound 385 no output.

[BrainDeadAmps]

Hey guys,

Backstory:

 6-8 months ago I purchased a converted and working filmosound 385 amp. Great, love it. 2 weeks ago i get the bright idea after watching some uncle Doug videos to open her up and see if I could understand the modifications done to the amp. To my surprise, all I could see was that the filter cap network had been replaced already and with a 3 prong grounded power cord. A few capacitors in the preamp section were also replaced but appear to be the same values as on the orig. The Pec was also disabled and epoxied.

Anyways, before I stuck my nose in the amp worked fine, yes the sound was compressed and perhaps a little dark. Very low noise floor.

The issue:

First I removed the oscillator coil circuit, great, no problems yet. I install a 250k pot as a variable Nfb control...Still working, and then the neon bulb circuitry.

I remove the entire neon bulb circuit. This time, I start her back up, foolishly through just a surge protector. No sound coming through speaker after letting it run for just under a minute.

Open her back up, In the process of my mods I managed to disconnect the primary center tap from the output transformer from the wire that carries over the r40. Also in order to work in here I had to remove the large wire wound 225ohm 5watt resistor to free up some room. Well when I put it back I didn’t realize that when I removed it in the beginning their must have been a bare wire attached to the connection point on the little terminal board near the tone stack that joined all 3 eyelets together and carry on to ground.

Once this happen I freaked, immediately shut it down, and began searching for solutions on the web.

I now have built myself a light bulb limiter and spent last night attempting my hand isolating the issue following the rob Robinette amp troubleshooting guidelines.

I will post the reading I measured last night when I get home to my notebook.

Again, when I turn the amp on the light bulb limiter turns on just enough to turn the filament red but not really producing light. Absolutely no sound is coming through the speaker. All tube filaments are glowing when I power on the amp.


I hope what I described here wasn’t a death sentence for my amp. Any help or suggestions are greatly appreciated.

I attached the schematic and I highlighted in the red, the known problems I described above, and in green the components removed from circuit.

[BrainDeadAmps]

I opened the chassis back last night, took some readings with my multimeter.


Power transformer Secondary green > green = 5vAC
Heater V4 pin 7>2 = 5.3vAC
Plate V4 pin 3 = 420 at start, dropped quickly to 327vDC
Cathode V4 pin 8 = 15vDC


All tests were done with tubes installed and amp plugged into light bulb limiter.

[sluckey]

Do you have any electronic training and experience?

You don't need the light bulb limiter. Set it aside and plug straight into the wall. Now voltage readings will be meaningful.

You have disconnected something that should be connected. Could be a part or wire you intended to remove or it could be something that was accidentally disconnected. Either way, look over the work you have done. You'll likely have to compare the as is wiring to the schematic to find the error(s). Those things are packed so tight that it will be very difficult to diagnose over the net.

Measure voltages for all tube pins and power supply caps. Many times there will be clues in the voltage readings.

[BrainDeadAmps]

Do you have any electronic training and experience?

No.

Of course since the incident, I’ve been reading up on amp circuits to try and understand the design and troubleshooting.

Thanks for Replying, nothing like breaking your only amp to inspire you to learn how the damn things works!

I will take more readings tonight.

[bmccowan]

Make sure you read up on safety too!
As Steve said, those damn Filmosounds are cramped. Probably the hardest amp to work on. So if you get the amp bug, and having only one amp, you are vulnerable, you may want to build or rebuild something simpler with more room to work to learn the ropes. In the meantime, Steve is the right guy to help you get it working again.

[BrainDeadAmps]

Update

I opened her back up, and beat my brains comparing the wiring to the circuit for another hour or so when i finally spotted a burn mark on a 1.5k resister nestled in a wrapped bundle along with the power filter caps.

 I decided to cut open the rubber insulation bundling the filter caps together. Alas I found that a 22uf 500v cap had burst along with the 1.5k 5watt resister which were wired to pins 3 & 4 on V6 which is the 6v6 tube for the oscillator circuit.


The other 2 filter caps housed in the same bundle appear fine (physically) along with the resisters which i measured (in spec).

I removed the Capacitor and resistor along with the wire to pins 3 & 4 of V6 and took the measurements I attached. These were the pins I have access to without removing components.
 

[BrainDeadAmps]

Sorry about the image, I tried rotating it and uploading it in different orrientations and they always end up sideways.

[PRR]

.

[BrainDeadAmps]

I’m going to do the best I can tracing the circuit backwards from the speaker out, checking for continuity.

Any other reading that might be helpful in diagnosing the issue?

[BrainDeadAmps]

Removed all electrolytic caps and waiting on them in the mail.

I found a modification done to the amp by the previous owner.

25uf/25v cap on pin 3 (cathode of the V2 (12ax7) along with another one to pin 8 (cathode) of same tube, both are in parallel with the original resistors from the schematic.

 I believe it is to increase gain?

No harm no foul if I remove them?  to move back towards original spec.

Thanks!

[TitaniumValhalla]

Removed all electrolytic caps and waiting on them in the mail.

I found a modification done to the amp by the previous owner.

25uf/25v cap on pin 3 (cathode of the V2 (12ax7) along with another one to pin 8 (cathode) of same tube, both are in parallel with the original resistors from the schematic.

 I believe it is to increase gain?

No harm no foul if I remove them?  to move back towards original spec.

Thanks!

Cathode bypass caps added, as you say, to increase gain. A value of 25uf is considered fully bypassed, which should boost all frequencies, so essentially it's an attempt just to get gain without affecting frequency response too much. You can go smaller with this cap just to get a treble boost if needed. Removing both caps entirely will probably make a pretty noticeable decrease in gain but shouldn't change frequency response much due to cap's value.