91
« Last post by g1 on March 09, 2019, 03:39:06 PM »
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.
92
« 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.  Always wise to have some head room as well so +/-21V rails leaves a few volts safety margin for the filter caps.  Phil.
93
« Last post by g1 on March 08, 2019, 01:34:19 PM »
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.
94
« 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
95
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.
96
« 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
97
« Last post by lakesidepark on March 07, 2019, 02:33:41 PM »
Hello guys maybe anyone can help me. I bought an Vintage Marshall 5203, but the mains transformer is burned, does anybody have this amplifier model can send me the output voltage.
By the data-sheet of diode bridge I discover it is for 4A -5A, but I still need the secondary AC voltage. All Internet schematics don't have the transformer voltage.
Thanks
98
« Last post by smackoj on March 07, 2019, 02:19:06 PM »
Hi Phatt; the wiring is correct but I found out where the faceplate meets the aluminum reverb pan is the noise problem. I put some non-conductive strips between the 2 pieces and it is all quiet.
thanks a million,
Jack D
99
« Last post by phatt on March 07, 2019, 03:56:03 AM »
Check the reverb wiring, you may have re-connected the tank backwards which would induce the hum symptoms you describe.
Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)
If the tank connections use RCA plugs then swap them over and see if the hum improves.
Input and Output are often marked on the tank case but some folks get them mixed up.
Phil.
100
« Last post by smackoj on March 06, 2019, 06:02:00 PM »
I have not been testing with the aluminum plate and reverb tank sitting in posistion. I put it together outside the box just like it would be inside. Lots of hum. I disconnected the reverb leads from the preamp board. Still humming. I moved the alum. plate so it was not touching the power amp chassis. still humming. I removed the plate/reverb pan completely so not touching the preamp faceplate/board or the power amp .... dead quiet. The last piece to be moved from touching the reverb plate was the preamp faceplate. That is when it stopped humming
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