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There is so much to get to in this one, it's hard to know where to begin...
So I'm going to do this write up in case someone happens to get one of these in their shop for repair, and learns that there is no schematic available as of yet. I guess we can start at the point of the story where it became my responsibility to get it working right and get it out of the shop. Here's basically what I get for the job notes "blows fuses(or maybe just told "not working", I can't remember); was at another repair shop, tech hit a roadblock and couldn't make sense of the failure and couldn't fix it; was returned to customer "as is" and he's moving to the west coast and wants to take it with him.
This is a porta-flex type guitar amp, with a very wide chassis and a lot of tubes. I notice in the chassis that the filter caps were new replacement type, but they looked like they had been lifted from the circuit. One end of the terminal board was brittle and had been broken, with an attempted repair. So, the entire power supply was in shambles, and absolutely no trace of this guys particular amp that I could find anywhere on the web. It was a weird one, yet printed on the chassis as it left the factory was model "B-12X".
Was not this:
B-12X
The amp in our shop had 6V6 single ended speaker driver for the reverb amp using parts of 2 compactrons. So I managed to come across schematic for a B-12XY. Still not the same amp, but this version managed to include some drawings for compactrons.
B-12XY
Basically, I was able to reference 3 different ampeg schematics and rebuild the power supply. At least, the point in the power supply up to the driver and output stages to power it up and see if it would pass signal. I took the oportunity to make some improvements in layout and wiring during the process, and I was ready to power up and run some tests. Here's where it gets F*ck'd. - With a new good rectifier tube installed, and good bench tubes in the output stage and driver–I bring the mains up slowly with the amp in standby mode, and everything starts out nicely and looks good on the current meter. The amp warms up for a bit; Put the variac dial to zero, switch it out of standby and bring the voltage up again while I monitor the current meter pin. It goes easy, easy, until I get to about 80-85V on the primary, then, SLAM! The Meter dimes out, and I have to throw the switch off.
Well, there's the issue at least. But interestingly, I connect an incandescent limiter to continue testing, but now the amp fails while just the power switch engaged and the mains voltage in the 80's. So, we start eliminating things– use solid state diodes in place of the tube; start lifting caps and supply nodes; and it hits me: "We're going down the same path as the previous tech working on this" It made sense now. This is why even with new components installed, the leads and connections were all lifted when we recieved it!
I was like, "dude, I don't want to end up redoing all the trouble shooting the last guy did without finding the problem and end up scribblig on notebook paper 'All work and no play, make's Jack a dull boy' over and over again." "Let's pull all the secondaries, I think there is a fault in the mains transformer, where the insulation or something is failing under load and when the voltage rises to a certain point." So we pulled all the secondaries and powered it on with no load.
The transformer took the full mains voltage across the primary with no indication of any problem. Okay, but now we're gonna get to the spooky stuff. With no other secondary connetions, the moment we connected the HT windings to the 2-phase diode rectifier(connected to one of the legs of the 5V winding), the transformer fails, drawing fault current. BUT!! only when both legs of the HT windings are connected to the 5V winding through their diode. A tested. and retested, good diode. With only one leg of the HT connected, it wouldn't fail.
When we removed the connection of the rectifier to the 5V winding, the transformer seemed okay. What the actual hell is going on with this thing? Never seen anything like it before.
I mean, we're like, most likely some kind of failure in the mains transformer, so we disconnect it to be sure and tack in another transformer just to power up the circuit and see if it even works. With the substitute test transformer, the amp powers up fine and we drive the output by feeding signal to the phase inverter. So, we do what anyone would do after all that. Tell the customer he needs a new mains transformer and we call up Bruce and fliptops and order one up.
This case had both my boss and I scratching our heads trying to suss this one out. I pull the original mains transformer from the amp and set it aside with the chassis and hardware while we wait for the new transformer to show up.
Meanwhile, a sometime later on. I see my boss with the pulled transformer at his bench and clip leads all about, and I'm thinking "dude, why are you doing this to us." I don't even want to know, but there it is: Both HT windings connected to a rectifier with the 5V leg of the heater supply on the load side of the diodes with a reservoir capacitor grounded to the center tap running at the full HT voltages. I'm looking at the connections trying to find out something, someway he didn't make the connections correctly. But it was working.
I'm thinking "what the eff is that", but the new transformer was already on it's way. Regardless, I still thought we had enough evidence to suspect PT failure.
I will say this..... turns out, it was a fault in the power transformer, but it wasn't until the new one arrived that I figured out why it had faulted. This one was destined to fail, and it all started out during the assembly back in the ol' Ampeg factory.
I'll tell you, I didn't see this one. Obviously, neither did Ampeg. There was a chance it could have snuck by and ended up being shipped out to Seatle only to ruin this guys life good feelings, and savings account again. But this is why they bring all the tough ones to me, boys
Okay, I'll wrap and finish the story next time.
So I'm going to do this write up in case someone happens to get one of these in their shop for repair, and learns that there is no schematic available as of yet. I guess we can start at the point of the story where it became my responsibility to get it working right and get it out of the shop. Here's basically what I get for the job notes "blows fuses(or maybe just told "not working", I can't remember); was at another repair shop, tech hit a roadblock and couldn't make sense of the failure and couldn't fix it; was returned to customer "as is" and he's moving to the west coast and wants to take it with him.
This is a porta-flex type guitar amp, with a very wide chassis and a lot of tubes. I notice in the chassis that the filter caps were new replacement type, but they looked like they had been lifted from the circuit. One end of the terminal board was brittle and had been broken, with an attempted repair. So, the entire power supply was in shambles, and absolutely no trace of this guys particular amp that I could find anywhere on the web. It was a weird one, yet printed on the chassis as it left the factory was model "B-12X".
Was not this:
B-12X
The amp in our shop had 6V6 single ended speaker driver for the reverb amp using parts of 2 compactrons. So I managed to come across schematic for a B-12XY. Still not the same amp, but this version managed to include some drawings for compactrons.
B-12XY
Basically, I was able to reference 3 different ampeg schematics and rebuild the power supply. At least, the point in the power supply up to the driver and output stages to power it up and see if it would pass signal. I took the oportunity to make some improvements in layout and wiring during the process, and I was ready to power up and run some tests. Here's where it gets F*ck'd. - With a new good rectifier tube installed, and good bench tubes in the output stage and driver–I bring the mains up slowly with the amp in standby mode, and everything starts out nicely and looks good on the current meter. The amp warms up for a bit; Put the variac dial to zero, switch it out of standby and bring the voltage up again while I monitor the current meter pin. It goes easy, easy, until I get to about 80-85V on the primary, then, SLAM! The Meter dimes out, and I have to throw the switch off.
Well, there's the issue at least. But interestingly, I connect an incandescent limiter to continue testing, but now the amp fails while just the power switch engaged and the mains voltage in the 80's. So, we start eliminating things– use solid state diodes in place of the tube; start lifting caps and supply nodes; and it hits me: "We're going down the same path as the previous tech working on this" It made sense now. This is why even with new components installed, the leads and connections were all lifted when we recieved it!
I was like, "dude, I don't want to end up redoing all the trouble shooting the last guy did without finding the problem and end up scribblig on notebook paper 'All work and no play, make's Jack a dull boy' over and over again." "Let's pull all the secondaries, I think there is a fault in the mains transformer, where the insulation or something is failing under load and when the voltage rises to a certain point." So we pulled all the secondaries and powered it on with no load.
The transformer took the full mains voltage across the primary with no indication of any problem. Okay, but now we're gonna get to the spooky stuff. With no other secondary connetions, the moment we connected the HT windings to the 2-phase diode rectifier(connected to one of the legs of the 5V winding), the transformer fails, drawing fault current. BUT!! only when both legs of the HT windings are connected to the 5V winding through their diode. A tested. and retested, good diode. With only one leg of the HT connected, it wouldn't fail.
When we removed the connection of the rectifier to the 5V winding, the transformer seemed okay. What the actual hell is going on with this thing? Never seen anything like it before.
I mean, we're like, most likely some kind of failure in the mains transformer, so we disconnect it to be sure and tack in another transformer just to power up the circuit and see if it even works. With the substitute test transformer, the amp powers up fine and we drive the output by feeding signal to the phase inverter. So, we do what anyone would do after all that. Tell the customer he needs a new mains transformer and we call up Bruce and fliptops and order one up.
This case had both my boss and I scratching our heads trying to suss this one out. I pull the original mains transformer from the amp and set it aside with the chassis and hardware while we wait for the new transformer to show up.
Meanwhile, a sometime later on. I see my boss with the pulled transformer at his bench and clip leads all about, and I'm thinking "dude, why are you doing this to us." I don't even want to know, but there it is: Both HT windings connected to a rectifier with the 5V leg of the heater supply on the load side of the diodes with a reservoir capacitor grounded to the center tap running at the full HT voltages. I'm looking at the connections trying to find out something, someway he didn't make the connections correctly. But it was working.
I'm thinking "what the eff is that", but the new transformer was already on it's way. Regardless, I still thought we had enough evidence to suspect PT failure.
I will say this..... turns out, it was a fault in the power transformer, but it wasn't until the new one arrived that I figured out why it had faulted. This one was destined to fail, and it all started out during the assembly back in the ol' Ampeg factory.
I'll tell you, I didn't see this one. Obviously, neither did Ampeg. There was a chance it could have snuck by and ended up being shipped out to Seatle only to ruin this guys life good feelings, and savings account again. But this is why they bring all the tough ones to me, boys
Okay, I'll wrap and finish the story next time.