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January 29, 2022, 12:02:50 AM

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91
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on December 29, 2021, 11:25:35 PM »
Well as the output is stable then read the DC voltages that supply the preamp.
Those 2 rails are regulated via 2 Zener diodes and filtered via 2 ElCaps.

Using the first picture posted they are top left corner next to the mounting post at the edge of the PCB. The 2 small Blue Elcaps are just near the Silver Zeners.

Use your meter to check you have both rail voltages +12v & -12v. they will likely be 12 Volt but could be 15V rails, both rails should be very close to the same.
likely easy to check voltages at the pig tails on the Blue ElCaps rather than the Zeners.

If a Zener is blown then the voltage will be almost equal to the main voltage which can be read on the bottom side of those 470R resistors below the zeners.
Also the Capacitors could be dying causing hum.
Phil.
92
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on December 29, 2021, 09:16:36 PM »
Is the Buzz still present with guitar plugged in and the volume knob on guitar set to Zero?
Also does the buzz increase as you turn up the gain knob on the amp?

Just be aware that a lot of amps will have some residual noise and in a QUITE room near computers, Fluro lights and many other sources of RFI/EMI it will be noticed.
If the buzz/hum is louder than your guitar playing then yes there is a major problem, other wise you may have to live with it.

The buzz is present when a guitar is plugged-in and the volume set to zero.
The buzz does not increase with the gain.

It is way louder than a residual noise, it's quite loud, louder than the guitar.

Thanks!
93
Amplifier Discussion / Re: newbie question about cranked amp overdrive
« Last post by joecool85 on December 28, 2021, 08:00:35 PM »
I'm not trying to be difficult, but unless I am blind, there is no model # in either of those posts.
Schematics are found by model number.  If we had the number, perhaps we would already be looking at the schematic, which would make for a much more fertile discussion.
Apologies if there is no model # to be found on the unit, but if so the OP should just say that.

You aren't being difficult.  The model name is "Guitar/Bass/Organ amp" by Montgomery Ward.  There should be a model number as well though which no one has shared.  This video shows one of the same name, but very different configuration (head/cab vs combo anyway) vs what was linked to by Phatt.  https://www.youtube.com/watch?v=gWmjj-eag3g
94
The Newcomer's Forum / Re: Am I reading this correctly and does this look suspicious?
« Last post by Enzo on December 28, 2021, 07:00:48 PM »
Perhaps if you told us the make and model of what these parts are in, we could look on the schematic?

You have a good board?  Stick your ohm meter probes on the good one and see what it measures.

Resistors don't burn up on their own, something caused the excess current.   Very often a semiconductor.

CAp, hard to say.  Replacement cost well under a dollar.  WHy take chances?
95
The Newcomer's Forum / Re: Am I reading this correctly and does this look suspicious?
« Last post by Loudthud on December 28, 2021, 06:57:03 PM »
That resistor looks like 1/8W. At 180K it would take 150V to reach 1/8W. If it is 18K ( multiplier looks like orange to me ) it would take 47V. E=Square root (P*R)

Is there that much Voltage in that circuit ? If it were me, I'd unsolder one end of the resistor on the good board and measure it.
96
The Newcomer's Forum / Am I reading this correctly and does this look suspicious?
« Last post by Den. on December 28, 2021, 04:26:15 PM »
Although I have an identical working board I'm going to try to repair this burned circuit board. This resistor (R55) is small and my eyes are getting old. I see brown, grey, yellow, gold 180K 5%, or maybe red, grey, yellow, gold 280K 5%. And, does this cap look suspicious? The cap on the good board looks much "cleaner".
97
Amplifier Discussion / Re: newbie question about cranked amp overdrive
« Last post by g1 on December 28, 2021, 02:52:44 PM »
Any source for the schematic?
I could be wrong, but it seems we have not even found out the model # yet?
Perhaps that would be a little bit helpful?  ;)
Mentioned on the first post, Also I linked to a few pictures I found on Reverb site in (post 15) ;)
Phil.
I'm not trying to be difficult, but unless I am blind, there is no model # in either of those posts.
Schematics are found by model number.  If we had the number, perhaps we would already be looking at the schematic, which would make for a much more fertile discussion.
Apologies if there is no model # to be found on the unit, but if so the OP should just say that.
98
Amplifier Discussion / Re: newbie question about cranked amp overdrive
« Last post by phatt on December 27, 2021, 09:08:13 PM »
Mentioned on the first post, Also I linked to a few pictures I found on Reverb site in (post 15) ;)
Phil.
https://reverb.com/item/2706505-vintage-montgomery-ward-3-channel-guitar-bass-organ-amplifier-15-speaker-usa
99
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on December 27, 2021, 06:17:05 PM »
Hi Cin,
Yes many factors can cause the issue.
If the Ground/Common has many return paths it can cause what is called a ground loop,
often picks up the 50Hz hum.
If it's more of a buzz it might be a shielding issue.

Is the Buzz still present with guitar plugged in and the volume knob on guitar set to Zero?
Also does the buzz increase as you turn up the gain knob on the amp?

Just be aware that a lot of amps will have some residual noise and in a QUITE room near computers, Fluro lights and many other sources of RFI/EMI it will be noticed.
If the buzz/hum is louder than your guitar playing then yes there is a major problem, other wise you may have to live with it.
These are the low budget models and unlikely they do much R&D 8|
Phil.
100
Amplifier Discussion / Re: newbie question about cranked amp overdrive
« Last post by g1 on December 27, 2021, 02:28:37 PM »
Any source for the schematic?
I could be wrong, but it seems we have not even found out the model # yet?
Perhaps that would be a little bit helpful?  ;)
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