Crate battery amp V-reg problem

[Enzo]

Alright, I don;t know what I am missing, but I can;t see the problem to save my soul.

I have a Crate TVJR here, and the voltage regulator and charging circuit doesn't work. I have included the schematic for the older TX15. The amps are not the same, but the power supply board is the same.

Powr adaptor is fine. AMp circuits work on my bench supply. This amp will run on battery or on the adaptor. Tha battery will charge when the adaptor is plugged in, and even when the power switch is off. But not for me.

It is always some dumb little thing that hangs me up.

The adaptor is 15v, 1250ma, and runs about 20v unloaded. The regulator will spit out about 18v unloaded. Any load will drag that voltage way down. A 220 ohm resistor for example will draw about 50ma from this, but that is sufficient to knock the output down to like 10v. Trying to charge the 12v battery is way over its head. I could assume the battery had failed, except the amp won;t run off this supply either. Even at idle, the load shuts down the regulator. The input voltage from the adaptor is stable other than a tiny drop under load.

After testing stuff, I replaced all the transistors, replaced the 10uf cap. Verified all the resistors. In operation the voltages across the zeners are right. The 10v stays there, the two 6.8vs are OK until the output drops below their combined voltages. The output diodes are OK, and it doesn;t matter which side of them I put the load on. I can ground collector Q3 and it comes full on, and removing Q4 makes it come up fill, but it won;t regulate. part to part continuity as far as I can tell on the tracery.

Just for grins I found the voltage drop across the 180 ohm shunt resistor to be exactly what the current drop would be, and removing it showed the transistor circuit was not turning on at all when the circuit was complete.

Grounding base Q4 makes it come up, so those 6.8v zeners are shutting me down, but they are at the proper voltages.

I DID sleep on it, and my head keeps running the same path over and over. I got some mental block over something simple.

[Steve L]

I could probably help out a lot more if the amp was in front of me, however I would try changing R-14.

More than once I have seen those power resistors measure fine with an ohm meter, however when power is applied, they either open up or change value, sometimes quite a bit.
Slo-blo fuses sometimes do the same thing. I know it is a shot in the dark, however given the symptoms you describe it would make sense.

Other than that, you have covered all of the usual suspects. I hope the best for you.

Steve

[HDSarge]

I remember repairing a TX15 with similar frustration. What I found was that the design, not mine, was intended to run on the battery. That's it, the adapter is just to (Trickle) charge the battery, and will not run the the amp on its own. If the battery get too discharged, the adapter can not coup with that either....maybe it would if given "lots" of time, but fires can start that way too.

I charged the battery with my bench supply and all worked well. I sold the owner a new battery (I hate call backs), and advised that he leave the amp plugged in when not in use. He said he had never done that, but would give it a try. Last report, still working a year later.

Hope that helps.

[rf7]

Yeah, those things only run off the battery. A new battery should do it.