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Having just restored a Fender TB-1200 Power Amp section of their Bass Guitar amp, I thought I’d post some details on it. It was from our Guitar & Bass Equipment Rental Dept at CenterStaging, LLC, and after pulling the top cover to survey the damage, it looked like the entire output stage failed. 5W & 3W emitter resistors burnt open, base & collector resistors around the bias transistor burnt open & scorched the board, some traces that overheated lifted off the board before burning open. On the bottom side, the V++ high current buss burnt open and the foil arc’d to the chassis floor.
So having removed the main PCB assy, I checked with Fender on the cost of a complete replacement board assembly, as it would have been cheaper to replace it. I learned they don’t support the product, and could only offer another complete amp.
It’s a two-tiered output stage, 4 stacked pairs of NPN/PNP transistors, running on roughly +/- 90V & +/- 45V supplies. Output xstr’s were TO-247 Sanken 2SA1294 & 2SC3263’s (130W 230V/15A parts, with SOA 2nd breakdown at 80V/1.7A). I was going to change those to the stronger (and slower) MJW21193 & 21194 On Semi parts (200W 250V/16A with SOA 2nd breakdown at 80V/2.25A)…the smaller cousin to their TO3P parts. The heat sink for this 1200W/2 ohm rated amp was a 3/16” folded aluminum part, with segmented vertical fins punched and spread to yield two rows of fingers on each side of the center array of power xstr’s. A variable speed fan blows into the center of that from the rear panel. Still, woefully inadequate cooling IMO. Bergquist K6 TO-247 insulators were used, which I would change to greased mica, just to get as much heat out of the xstr’s as possible on the rebuild. Also, very odd, I thought…the bias transistor was NOT mounted to the heat sink, though positioned between two of the outside H/S fingers. In the thermal air flow path, as long as the fan works, but........
I tore it down, not knowing if there was further foil failure underneath the heat sink on the top side of the PCB. There wasn’t. Lead-free solder in use, so de-soldering was far more tedious (hate that stuff!). Top side PCB was ok. During the rebuild, using 2SA1837 & 2SC4793 TO-221 pre-drivers, and the faster Sanken output xstr’s they used as drivers, I discovered the PCB solder holes for the power xstr’s were only drilled to 0.055” ID (now smaller from use), and the MJW21193/21194 leads wouldn’t fit the holes! The Sanken lead width was only 0.037” typ, whereas the new parts are 0.045” wide, needing larger holes. I hadn’t planned for that! I’ve always provided 0.062” ID in my boards so anyone’s transistors would fit.
Double-sided plated-thru holes, so drilling them out was out of the question. I had to set up my Foredom hand-piece, chucked in a fine-flute carbide burr and lead by lead, dragged the tin-plated copper leads over the hi-speed burr to taper them down to fit the holes. PITA operation, but worth the effort to get a much stronger output stage into place.
On the bottom side of the PCB, I peeled up the damaged V++ power trace to where it became solid and cut them. Scrapped away the solder mask. Then, I laid in new copper foil from a roll of 3M 0.5” W x 2mil thk, adhesive backed material and routed the replacement trace in segments, cut to fit. I could have done it with a piece of wire, but liked the copper foil method. Covered the new foil with Kapton tape to complete that task. The thin traces that failed I did use 26AWG Teflon buss wire to restore order.
After all the failed parts were replaced and double-checked, I put it back into the chassis….a real shoe-horn operation that offered very little fit tolerance….and cautiously powered it up. All came up fine from a DC standpoint, though under drive, now using slower output xstr’s, there was oscillation birdies in the transition between upper and lower tiers as you got close to clipping. Adding additional 430pF mica across the existing 100pF miller caps on the upper pre-driver stage cured that.
I had another fully functional TB-1200 to compare against, and the two looked the same under full power drive at low impedance (2 & 4 ohm).
The driver stage on the upper rails of the TB-1200 isn’t conventional. http://support.fender.com/schematics...c_%20Rev-B.pdf
When I first found instability under drive as I had the upper stages driving with the lower stages, I looked at their circuit complexity and took a deep breath, and a sigh of relief when just raising the miller value at the pre-driver cured it.
So having removed the main PCB assy, I checked with Fender on the cost of a complete replacement board assembly, as it would have been cheaper to replace it. I learned they don’t support the product, and could only offer another complete amp.
It’s a two-tiered output stage, 4 stacked pairs of NPN/PNP transistors, running on roughly +/- 90V & +/- 45V supplies. Output xstr’s were TO-247 Sanken 2SA1294 & 2SC3263’s (130W 230V/15A parts, with SOA 2nd breakdown at 80V/1.7A). I was going to change those to the stronger (and slower) MJW21193 & 21194 On Semi parts (200W 250V/16A with SOA 2nd breakdown at 80V/2.25A)…the smaller cousin to their TO3P parts. The heat sink for this 1200W/2 ohm rated amp was a 3/16” folded aluminum part, with segmented vertical fins punched and spread to yield two rows of fingers on each side of the center array of power xstr’s. A variable speed fan blows into the center of that from the rear panel. Still, woefully inadequate cooling IMO. Bergquist K6 TO-247 insulators were used, which I would change to greased mica, just to get as much heat out of the xstr’s as possible on the rebuild. Also, very odd, I thought…the bias transistor was NOT mounted to the heat sink, though positioned between two of the outside H/S fingers. In the thermal air flow path, as long as the fan works, but........
I tore it down, not knowing if there was further foil failure underneath the heat sink on the top side of the PCB. There wasn’t. Lead-free solder in use, so de-soldering was far more tedious (hate that stuff!). Top side PCB was ok. During the rebuild, using 2SA1837 & 2SC4793 TO-221 pre-drivers, and the faster Sanken output xstr’s they used as drivers, I discovered the PCB solder holes for the power xstr’s were only drilled to 0.055” ID (now smaller from use), and the MJW21193/21194 leads wouldn’t fit the holes! The Sanken lead width was only 0.037” typ, whereas the new parts are 0.045” wide, needing larger holes. I hadn’t planned for that! I’ve always provided 0.062” ID in my boards so anyone’s transistors would fit.
Double-sided plated-thru holes, so drilling them out was out of the question. I had to set up my Foredom hand-piece, chucked in a fine-flute carbide burr and lead by lead, dragged the tin-plated copper leads over the hi-speed burr to taper them down to fit the holes. PITA operation, but worth the effort to get a much stronger output stage into place.
On the bottom side of the PCB, I peeled up the damaged V++ power trace to where it became solid and cut them. Scrapped away the solder mask. Then, I laid in new copper foil from a roll of 3M 0.5” W x 2mil thk, adhesive backed material and routed the replacement trace in segments, cut to fit. I could have done it with a piece of wire, but liked the copper foil method. Covered the new foil with Kapton tape to complete that task. The thin traces that failed I did use 26AWG Teflon buss wire to restore order.
After all the failed parts were replaced and double-checked, I put it back into the chassis….a real shoe-horn operation that offered very little fit tolerance….and cautiously powered it up. All came up fine from a DC standpoint, though under drive, now using slower output xstr’s, there was oscillation birdies in the transition between upper and lower tiers as you got close to clipping. Adding additional 430pF mica across the existing 100pF miller caps on the upper pre-driver stage cured that.
I had another fully functional TB-1200 to compare against, and the two looked the same under full power drive at low impedance (2 & 4 ohm).
The driver stage on the upper rails of the TB-1200 isn’t conventional. http://support.fender.com/schematics...c_%20Rev-B.pdf
When I first found instability under drive as I had the upper stages driving with the lower stages, I looked at their circuit complexity and took a deep breath, and a sigh of relief when just raising the miller value at the pre-driver cured it.