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Topics - Peter Blair

Hello all,
Well, last time I was here I got some great advice. I'm hoping for 2 for 2.
So, I have a XR500C Peavey powered mixer that I've been using for several years in an acoustic duo. The sound has always been excellent, in spite of the age of the unit.
My problem: Everything sounds great at sound-check. One and-a-half, to two hours into the gig, I start getting distortion. Sometimes it's worse than others. Had a local guy look at it. He found nothing after running it for several hours. I suspect it was not being run at the sound level that we perform at. He said he just listened to music from his I-pad in it. In my mind, it won't start distorting unless you're really pushing some air. The distortion happens across all channels, suggesting (to me) it's not the pre-amp section. Upon the distortion starting, I have tried omitting everything in the input signal chain except for a single guitar. Same distortion. Try another guitar, same results. Just to be sure, change out cord, same results still. Upon inspection nothing visual is jumping out at me. Everything looks clean. Nothing looks like it's experienced any overheating. No apparent physical damage. I'm getting quite frustrated. Every gig I go to, I just hope it's not going to happen again. I'd just go out and buy new, but to be honest, I have a hard time finding anything that really measures up to this relic Peavey.
Anybody care to weigh in?
Thanks and appreciation to any suggestion,
All I have for diagnostic equipment is a Borbede (BD-168A) multimeter. In light of this I would be willing to try replacing a few components if you think they might be the likely suspects.
Hello, and thank you in advance for reading my post, and/or trying to help out a newbe.
I have a Peavey, KB100 keyboard amp that I'm having a problem with. When I got the amp, it had a burned out transistor, which I have replaced, along with a few obviously overheated resistors. I replaced the filter capacitors for good measure as well. Well, overall the outcome was quite successful except for . . .
As the input signal dies out and becomes very quiet, (in my case, this would be a guitar and/or a keyboard) the signal seem to break up and sound distorted. You wouldn't notice it at normal listening levels, but if you let the note sustain, as the volume decreases the distortion sets in. Maybe better described as "breaking up", more than to call it distortion. I don't know if that's enough information to go on, but can anyone tell me what I might be looking for to remedy the situation? What might be causing it?
Thank you,

(link to schematic, or see attachment that I tried to include)
Hi everyone,
Before I start, I just want to say, to anyone who remembers, I am still working on my Masco amp. I just haven't had enough time to get back into in yet. I will report back when I can talk about results.
Now to the issue at hand;

I recently came into possession of an almost free Fender Stage 112 amp. Previous owner could tell me nothing but, "It doesn't work". Well, yes and no. Everything on the normal channel seems to be working fine. On the overdrive channel, the treble and bass controls seem to have almost no affect on the output. It WAS almost free, so I COULD live with that. But I'd love to resolve the issue.

So here's what i found. With some of these resistors, I had to lift one leg to get a proper reading. I will note location mapping on the schematic. I'm not sure that I'm doing that in the correct format, but I'm sure it will help even if I'm not.

R33, R37             
Location approx. 4-D   
I just did these because they were in the neighborhood.
Both of these checked out fine. Very close or right on original values.

R39, R41, R44, R45, VR46, R47, R48
Location approx. 2 to 3-D     
All in the tone stack that seems so unresponsive. I took the bass pot out to test, just because that one seemed the worst, and in circuit, the reading was peaking and then dropping, and non of the readings were close to value. Once isolated, it check out fine.
All of these check out fine also. Again, very close or right on original values.

I do have some suspicions about C20, (location 5-D). It looks like it might have taken a little heat at some point. Not to any great measure, but the shrink wrap at on end is sagging away from the lead a little bit?!

The other obvious thing I found, and I don't think it's related, but if I knew I wouldn't be asking;
R75, and R76 (location 5.5-A) are TOAST!! They Put a pretty good half inch diameter brown dot on the board.
I don't see any obvious signs of trouble with C49. (same location)

I've already ordered replacement components for C49 and the previously mentioned C20, just because I have some doubts about both of those. R75 and R76 are also on the way.
I guess my question is this;

Does anyone see what might be causing the more-or-less absence of response from the treble and bass control?   And;

Would anyone care to speculate about why R75 and R76 went up in flames? What else should I be checking?
Or do you think I should just replace what's obvious, and items of suspicion, hope for the best and see what happens?
(All I have for test equipment is a multimeter.)

Thank you all for indulging me.
The Newcomer's Forum / Masco tube amp
January 22, 2015, 12:26:02 AM
OK I know this is a SS amp forum, but I'm hoping someone will have mercy on me.
Knowing almost nothing about electronics, I'm faced with the following problem. Working on a 1954 Masco model MU-17. (I've attached a schematic, but it is quite small)

The schematic says i should have 330v at the plates of the two 6L6GT power tubes. I have 468v.
I'm supposed to have 340v off the 5Y3GT rectifier. I have the same 468v.
Checked that with a new tube also. Same reading.

If I plug in either of the two old power tubes (one of which I KNOW is bad), into either of the two sockets, I get some kind of short as soon as the tube warms up, and my in-series 300w safety light starts to glow. Same thing with two new replacement tubes, again in either socket. I see no conspicuous faults in the circuitry or components. Everything is clean and appears to be in outstanding condition. No mess in the tube sockets or anything like that. If someone could have a look at the schematic and maybe tell me what to check. I've checked all the voltages from the power and the output transformers. Can't find anything that isn't just a few volts higher than what they should be. I sure would appreciate any suggestions. Note; All I have for test equipment is a multimeter.
Thank you in advance,
Hi anyone reading this. i really appreciate you trying to help me. Not only am i a complete NOOB, but i know almost nothing about electronics. that said, i have actually managed to make some simple repairs, and save a few junk-heap destined amps.

so i have this pa head. everything works EXCEPT, if i turn the volume, the tone, or the bass pot for any particular channel to full output, the "protection" light comes on and the amp makes a loud buzz. mind you, the volume might only  be at 50%. but turn the tone or bass up full, and the buzzing starts. the main output volume might only be half way up (or less) also.

Any suggestions what to look for? i do have a small multi-tester, but that's about it for test equipment. i'm willing to replace parts as an experiment. any ideas? attached is a pdf with a schematic for the individual channels, and one for the power amp. nothing there for the EQ section (in the pre-amp?)
Thanks in advance,