Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on February 07, 2012, 08:01:08 am
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Got a Bassman RI feeling ill. No Standby light. Rectifier tube working, but I have no heater voltage. A quick look inside lets me know this is not really like an original Bassman. Just checking to see if anyone has seen this and it is a common problem with PCB Bassmans before I begin taking out the board.
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Have you looked at the schematic? Look in the area of CP18/19 for a loose connection or poor solder joint, cracked trace, etc. I wouldn't pull the board just yet.
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Haven't looked at anything yet. Usually I dread a bad winding when this happens. I will check for common problems first as you suggest. I did print the schematic and found the c18/19. These looked to be soldered to the bottom of the board, but I am not sure. I was throwing this out to see if it is common problem. Tonight I will pull the tubes and check the PT winding to see if I get 6.3+ volts. Here is hoping.
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My experience with the many that I've seen and the few I've owned....
Once they start messing up, I strip them and slap a Hoffman board inside... Everybody is sooo much happier. :-)
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This is a friends amp and I think I have some bad news for him. Since it is a stock Bassman PT. I can see where one the heater leads have rubbed through. Since he did not give me the cabinet I assume it has shield tape or something similar on the back cover what ever you call that. Anyway, one lead gives me a solid 7v whereas the other is running from 30v to 15v according to different tests with the on off switch. I would think they should be very close to each other and with the tubes not in somewhere around 8v or less.
I am thinking it is a burnt winding. The other leads test perfect. My question now is good idea or bad to install a small transformer for the heaters only? I have done this for bias taps, but never for heaters. Best idea to replace the PT and be done with it?
I do, however think the board replacement is a good idea, but my friend has shallow pockets.
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Measure filament voltage *** DIRECTLY ACROSS *** the two PT filament leads.
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Thanks for the DIRECTION. Check out fine. Now onto the board.
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I have located the problem. It is a cracked trace where CP18 plugs in. What a nightmare. It is necessary to remove everything to get to the bottom side of the board. I have a question. I have never soldered to this type of PCB. I have cleaned the area removing the coating and have a shiny trace. Is there any way to test heat BEFORE attempting to solder the crack. I want to make sure I do not burn through. I did test the iron in a area where there is no trace and it seems like it will take the heat. Much more than a PCB inside a pedal. I am assuming these are fairly tough, but just want to ask to make sure. Secondly, should a coating be reapplied after the repair? This coating seems different than on regular old green PCB's, but I am fairly sure the spray I have should work fine.
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That's a high current trace so don't just flow solder across the crack. Clean the trace until you see bare copper on each side of the crack. Then smear some paste or liquid flux on the repair area. Now lay a bare tinned 18AWG jumper wire across the crack to make a bridge. Next carefully solder the jumper to the trace. Get your iron outta there as soon as the solder flows. The flux and tinned wire should allow you to do this quickly. Once the jumper is in place, take a small common screwdriver (jeweler type is fine) and apply pressure to the wire on one side of the bridge. Touch your iron once more to this side of the bridge to reflow the solder. The pressure from the screwdriver will ensure a good surface bond. Repeat for the other end of the jumper wire. Finally, clean up the solder flux with mineral spirits and/or denatured alcohol.
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Understood, thanks Steve. Man I absolutely hate working on boards and since I began drilling turrets and I'm getting that way about eyelets.
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Completed. Working like a Champ. Thanks or the help.