Best of all, you don't have to disconnect the caps to use it. It's especially handy for electrolytic caps. It's a good tool for quickly troubleshooting a bunch of capacitors. But if you find two similar caps and one gives a dramatically different reading from the other, then you know something is wrong. The ESR can be a bit difficult to read, as you don't always know what a capacitor should read. Here's what the ugliness looks like on a scope, connected to the speaker load (sorry for the fuzzy focus).Ĭlick to expand.We have the same scope! I just didn't put it together that it also had the POS 022848 transformer until years later when I was doing a bunch of BMR to Vibroverb conversions. That amp worked fine with the new OT as well. He got back to me after a while and said that the amp was fine, but that the OT had a strange problem, and that he had replaced it. I couldn't get the amp to run right, and actually sent it to John to have him try to chase the gremlin out of it. For others, that doesn't quite get it, and a replacement OT is the best path.Īctually I had encountered the problem with the 022848 even earlier, back in the early 90's, when I build a MacIntyre Bluesmaker using a Bandmaster (with an 022848 transformer) as the donor. For some amps using this transformer, it is enough to install a 120pF cap between the PI plates. If you run into this, the litmus test is to disconnect the negative feedback, the problem goes away. The amp is bursting into oscillation at a certain point as the volume is increased. You may even, as I did about 15 years ago when I first encountered it, order a new speaker. Because it will sound like a very horrible speaker rub. Oh crap, you exclaim, I blew the speaker. Then in a week or so, you wind the amp up a bit. The symptoms go like this: you blackface the phase inverter, power it up and test it at low volume. Those transformers exhibit weird behavior if you blackface the PI, to the point where the negative feedback is not always negative. Your amp probably has the 022848 output transformer. If you are thinking of 'blackfacing' the phase inverter, you may run into a strange phenomemon. I should throw one more item your way, regarding your BMR. Unclip one lead? I can just replace them with orange drops too but not sure if that's too brute force. I've tested caps fresh from mouser out of circuit, but not sure how to do it here. Not sure if that's just how they are, or there's an issue. Other than that, many of these blue coupling/tone stack caps on the eyelet board seem to have "bulges" in them. I'm not sure if someone did this in a later service but wonder what effect the slightly higher capacitance would do here.Ĥ) I've measured all the resistors and found several out of spec that I'll replace. There shouldn't be any issue with these being a tad higher in the filter section right?ģ) Schematic calls for 50uF 70v bias cap(s) but this amp actually has 80uf 75v. They are the original cardboard mallory's so I'll be replacing them. Any recommendations on if/how to clean the power transformer?Ģ) The original schematic calls for 20/20/20/70/70 filter caps. Pictures at the end of the post.ġ) The power transformer is filthy, and there seems to be some sort of "coating" on some parts of it (you can see a couple drops of that coating in the picture). Sounds good when I played it but after inspecting it in more detail at home, I've definitely got some work to do.Ī couple questions for some feedback. I've confirmed it's a AA270 circuit after comparing to schematics. Picked up a 76 silverface Fender Bandmaster Reverb that I'm in the process of renovating.
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