RSD Studiomaster 400C schematic and repair help

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   Picked up this little beauty. One channel working, other has + rail voltage on the output. The problem is in the driver card, as the problem moves from one channel to the other when cards are swapped. Any common faults with these? Anyone have the schematics for them? Quick continuity test of all the transistors revealed no shorts.

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JJax Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 10 April 2015 Location: Estonia Status: Offline Points: 17	Post Options Post Reply Quote JJax Report Post Thanks(0) Quote Reply Topic: RSD Studiomaster 400C schematic and repair help Posted: 05 March 2019 at 9:59pm
	Picked up this little beauty. One channel working, other has + rail voltage on the output. The problem is in the driver card, as the problem moves from one channel to the other when cards are swapped. Any common faults with these? Anyone have the schematics for them? Quick continuity test of all the transistors revealed no shorts.

jbl_man Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155	Post Options Post Reply Quote jbl_man Report Post Thanks(0) Quote Reply Posted: 06 March 2019 at 8:57am
	The common fault on these are the two little siamesed signal transistors, (BC184 from memory) glued back to back on the driver board. I have the schematic,will post it up.
	Be seeing you.

jbl_man Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155	Post Options Post Reply Quote jbl_man Report Post Thanks(0) Quote Reply Posted: 06 March 2019 at 9:00am

	Be seeing you.

jbl_man Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155	Post Options Post Reply Quote jbl_man Report Post Thanks(0) Quote Reply Posted: 06 March 2019 at 9:01am
	^ This is the 800B/800C, is the same driver board as 400, just with more output devices. (Yours will probably have RCA mj15003 outputs, not the 2N 3773 shown above.) See photo below..check the siamesed transistors bottom left corners. Also check the two silver 2n4515 (with the heatsinks) either side of the board at the top. Edited by jbl_man - 06 March 2019 at 1:51pm
	Be seeing you.

JJax Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 10 April 2015 Location: Estonia Status: Offline Points: 17	Post Options Post Reply Quote JJax Report Post Thanks(0) Quote Reply Posted: 11 March 2019 at 4:10pm
	Thanks for the info, jbl_man :) Checked the drivers and both channels measure the same. Also changed the two glued together transistors, now sometimes the amp goes to rail voltage, other times lower DC voltage, forgot to measure but the needle stays at about 1/3 or 1/4 way up the VU scale. Checked for open transistors also but can't find anything.

jbl_man Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155	Post Options Post Reply Quote jbl_man Report Post Thanks(0) Quote Reply Posted: 11 March 2019 at 8:19pm
	Check every transistor and diode on that board...then every resistor ( compare the values with the working board) if all measure ok then, also check for continuity on all the pcb tracks, old boards can have tiny hairline cracks or dry joints in the silvered tracks that might not be immediately visible.
	Be seeing you.

JJax Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 10 April 2015 Location: Estonia Status: Offline Points: 17	Post Options Post Reply Quote JJax Report Post Thanks(0) Quote Reply Posted: 15 March 2019 at 8:00am
	Well looks like it's working now, thanks jbl_man :) The problem was the resistor that's in the lower right corner on your pics, the one right on the corner. On my amp it's an old red carbon type, supposed to be a 2k7 but measured at over 100k and went completely open when desoldered. Replaced it now both channels click right in with no DC at all! Didn't get a chance to test it yet as I only had a 1/4W 2k7, enough to try it out but not gonna leave it in there, gets pretty toasty, got some 1W 2k7 on order so gotta wait to use it. Are the power switch and VU meters supposed to light up? Mine don't, amp turns on fine but no lights. Are there supposed to be lights in there and are the bulbs replaceable?
	When life gives you lemons, sell them and use the money to buy more audio gear.

jbl_man Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155	Post Options Post Reply Quote jbl_man Report Post Thanks(0) Quote Reply Posted: 15 March 2019 at 10:36am
	Will be a tiny little filament bulb in the VU meters...you will have to take each meter out,carefully undo the sticky tape holding the front lense on, the bulb should then be visible. Best to fit led's instead,as the old filament bulbs dont last long, (and thats if you can even find them). Find the 12v dc feed to the lamps, fit two leds and a 240ohm resistor in series in each meter. Is a bit fiddley,but worth the effort. Here's an old Pioneer hifi amp i did a led conversion on, you get the idea.
	Be seeing you.

kedwardsleisure Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4947	Post Options Post Reply Quote kedwardsleisure Report Post Thanks(0) Quote Reply Posted: 15 March 2019 at 12:22pm
	Alot of those old studiomasters used series-connected lamps plus a resistor or two to drive them straight off the DC bus, including the one in the mains switch where fitted. So you might need to be creative!
	Kevin North Staffordshire

Tom kruse Members Profile Send Private Message Find Members Posts Add to Buddy List New Member Joined: 26 September 2022 Location: Denmark Status: Offline Points: 1	Post Options Post Reply Quote Tom kruse Report Post Thanks(0) Quote Reply Posted: 26 September 2022 at 10:11pm
	Hi there ! Got a SM400c repairer it , and find out all lamp are in serie and power switch bulb was burned.. VU bulbs are ok Sounds btw great