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June 24, 2024, 01:32:02 PM

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#1
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by phatt - June 22, 2024, 06:13:19 PM
Great to hear you fixed it. Good work G1  8)
Phil.
#2
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by Timko - June 20, 2024, 01:50:44 PM
I ended up running to my electronics store (yay big city living), picking up a pack of 2.7k 1/2 watt resistors, and getting them into the amp.  It works like a champ now!  Thank you so much for the help and direction.
#3
Preamps and Effects / Re: Joe Davisson's Vulcan prea...
Last post by Miyagi_83 - June 20, 2024, 11:20:12 AM
No problem. I breadboarded it to see how it sounds and shared my findings for others to have a point of reference because I had trouble finding any. Maybe my recordings will not get lost over time.
NB: I plugged the output of the preamp directly into the effects loop return of my DIY tube amp for this recording. Later I ran it through a buffer (a bypassed Boss DS-1) and it sounds different than in the sound sample.
EDIT: Of course, I can post the schematic if anyone's interested.
#4
Schematics and Layouts / Re: Ibanez TSA30 Schematic and...
Last post by Frank75 - June 19, 2024, 05:59:56 PM
I was on the road for a while, so today was my first opportunity to open up the amp and take a peak. What a mess. Some of the soldering looks like it's been done by a 5yr old. The tube sockets are mounted on stilts.
Anyway, visual inspection didn't really bring any results. I took a few measurements and these put the biasing VR on the suspect list. I can't get a steady reading from it. There's also a crack in the PCB near the VR, although I don't see an immediate correlation there.
All 9-pin sockets feel a bit loose, but I suppose that's just wiggle room. ::)
Respect for those in this thread who opened this soab and lived to tell the tale.
#5
Preamps and Effects / Re: Joe Davisson's Vulcan prea...
Last post by joecool85 - June 19, 2024, 11:14:13 AM
Quote from: Miyagi_83 on June 12, 2024, 04:57:39 AMI added another gain stage and tweaked it again. Here's how it sounds now

https://m.soundcloud.com/user-996907905/joe_davisson_preamp_modded_bc3

EDIT: Again, I'd like to apologize for the sloppy playing.

Sounds great!  Thanks for sharing!
#6
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by g1 - June 17, 2024, 07:44:33 PM
Yes, they would likely be 1/2 watt resistors.
It's a bit odd they are using 5 band color code (usually precision) resistors.  With a green band in there it would be 2570 ohms.  I would have expected regular 4 band color code with red-violet-red and gold for tolerance.
#7
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by Timko - June 17, 2024, 01:27:50 PM
Thank you for the testing tip!

Also, I think this may be the culprit.  I tried the resistor against 2 different multimeters after removing it from the circuit.  Both registered 140k.  The resistor color (red, green, purple, brown, gold) aligns to the 2.7k that I'm expecting.  So I'll look to replace that. 

I generally use 1/4 watt resistors in all of my guitar pedal builds. I don't see anything on the schematic that denotes the wattage on these resistors.  Is there a general standard I should be adhering to?  The resistor I remove looks about 25% bigger.  Would that be a 1/2 watt?
#8
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by g1 - June 17, 2024, 10:40:08 AM
As far as the additional testing, you should never try to do resistance measurements when unit is powered up.  You could damage the meter, and the readings are meaningless anyway.  Also applies to batteries or capacitors that are charged.  Or anywhere voltage is present.  Resistance measurements are only done when power turned off, and power supplies given time to discharge.

The voltage readings on the bias string indicate something is wrong with R47 or it's connections.  36V across it (points F to G) would mean 13mA through that string, That in turn would cause approx. 20V across R46, which you do not have.  What does R47 resistance measure?
#9
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by Timko - June 16, 2024, 09:30:23 PM
A   1.141
B   1.10
C   0.838
D   0.564
E   0.558
F   0.364
G -37.63

I've done some additional testing around the amp to see what I can find.  I noticed that when the amp is powered on, both of the terminals on C52 (one of the two 3000uF capacitors in the rectifier section) both connect to ground per my continuity tester.  When the amp is off and the caps are discharged, there is no connection.  But the moment the amp powers on, the resistance reads 0 and they have continuity connection.  So such thing happens on C53.  When I remove C52 from the circuit, it reads as expected.

C and D for the power transformer function how I think they should.  When the amp is off, the two terminals have continuity.  When the amp is on, the have a resistance of around 128E. I'm not sure if that tells me anything but it was something I observed. 
#10
Amplifier Discussion / Re: Repairing a Gibson G-105 G...
Last post by g1 - June 13, 2024, 02:02:59 PM
Measure DC voltages (no signal) at points A through G.

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