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21
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by phatt on March 09, 2019, 11:09:20 PM »
Another bad idea :duh

**IF** the circuit is not be designed to run at higher voltage then expect tears when the smoke escapes from the power amp :'( :'( :'(
Phil.
22
Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« Last post by phatt on March 09, 2019, 11:06:26 PM »
Answer; You just removed the ground loop. 8|
If you ground the case there are two ground paths which will cause hum. >:(
When looking at schematics the ground paths are hardly ever shown as they are all assumed.

Translating that drawing onto the actual circuit can be a challenge especially if you don't have a good understanding of ground paths.

I fixed a circuit years back and forgot one small fiber washer and hum was a problem.
I had created a ground loop as the fiber washer isolated the PCB from chassis.

i've seen reverb tanks where the rubber grommets had rotted away and gave intermittent hum problems as the tank would make contact with chassis as it vibrated.
Stuff like that can drive you insane if you don't know about grounding techniques.
Phil.
23
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by lakesidepark on March 09, 2019, 10:52:24 PM »
I replaced all capacitors to 35V.
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Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« Last post by smackoj on March 09, 2019, 10:11:50 PM »
I am not sure about what changed with the grounding but it is playing nice and the reverb sounds real nice too.
Thanks for all the help amigos.

Jack D
25
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by g1 on March 09, 2019, 03:39:06 PM »
Quote from: phatt on March 09, 2019, 01:25:25 AM
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.
That sounds better.  Solving backwards from 30W into 4ohms, rails would be approx.  +/-15.5V fully loaded.
26
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by phatt on March 09, 2019, 01:25:25 AM »
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.

Never run Caps above the rated voltage otherwise they will fail. 8|
Always wise to have some head room as well so +/-21V rails leaves a few volts safety margin for the filter caps. ;)
Phil.
27
Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« Last post by g1 on March 08, 2019, 01:34:19 PM »
Quote from: phatt on March 07, 2019, 03:56:03 AM
Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)
I think you are right that something has changed with regard to the tank.
Other possibilities are that the tank was replaced with same type but different jack grounding scheme (5th character of accutronics tank code), or there may have been rubber mounts insulating the reverb tank from the aluminum plate it is mounted on.

Anyway, good to hear smackoj found a workaround and it's all working fine now.
28
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by lakesidepark on March 07, 2019, 08:34:44 PM »
Oh Thanks, I saw other schematics from same era, by Marshall 5205 (Lead Reverb) the transformer is for 21+21V. And the power output stage is very near of 5203.

Maybe AC 19+19 Volts will be an good shot. By looking the first stage after bridge have 2200uF/25V, with a simple calc -> 19 *sqrt(2) = (+/-) 26,9 V with no load, and may decrease to 24.5 V with full load in power amplifier


Thanks
29
Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« Last post by Dino Boreanaz on March 07, 2019, 05:51:23 PM »
I don't own a 5203 and I'm not an electronics expert, but I have several small, solid-state Marshalls from the same time period.  Can you read the model number from the top of the transformer?  What input voltage are you using?  The following may be useful for comparison, but keep in mind that mine are 120V input.

My 3005 (Lead 12) uses a 4942 transformer and its schematic specifies +/-14.3V output with 120V input.  I've measured +/-14.8V (29.6V between terminals) output with 122V input.

My 3505 (Micro Bass) uses a 4937 transformer.  I've measured +/-17.3V (34.6V between terminals) output with 122V input.

The only photo I could find of the 5203 showed a 5010 transformer which is the SEMKO model number shown on the schematic of the 5002 (Lead 20) which uses the 4937 in USA for 120V.  So based on this, I wonder if the required secondary voltage of your 5203 is the same as that of my 3505?

Hopefully someone with more direct experience can offer some feedback.
30
Amplifier Discussion / Re: Peavey Musician 400 (1970's) Parallel Inputs
« Last post by lakesidepark on March 07, 2019, 02:34:31 PM »
As seen in schematic the preamps gone to mixer amplifier before the power amplifier
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