MSF 5000 "CLB" (EPROM Version) voltage regulator?

[d119]

I have an MSF 5000 that the capacitors went south on and the regulator/transistor on the SCB, Q706, burned up on the board.

Is there a common replacement for this? I've replaced the capacitors and fuse on the board but with the component removed I just get a dim TEST led, where before it would just blow fuses. Anyone know what this part is and what a common replacement for it is???

[Andy Brinkley]

Q706 is NLA at Motorola (P/N 4883875D05). The description comes up as a M7505 SCR.

Q706 and VR702 form a crowbar circuit and will fire if the +5V supply voltage goes above ~ 6.3 volts. When it does fire it will take out fuse F701. (as you have found out)

Pull Jumper JU8 out and check the voltage on Pin1. This disconnects everything down stream and lets you check the power supply portion. Voltage should be a little over 5 volts.

Also check your +9.6 v power rail, the MSF needs both +5v and +9.6 to make the LED light up.

[d119]

Andy,

OK. Well my Q706 is shot - is there another replacement part, or does this mean the SCB is garbage?

JU8 PIN1 has 16v (!) DC on it. I found a clearly failed capacitor on the interconnect board on the bottom of the station.

[kcbooboo]

If in fact you've got 16V on the 5V line with JU8 installed, chances are it fried a few more things down the line, like the microprocessor. Sounds like the switching regulator is shot or shorted, although I wouldn't expect 16V to be coming in from the A+ supply (maybe 15V). This excess voltage probably caused the SCR to fire, but then it ended up opening, or a foil going to it burnt up, hence the high voltage you're now seeing.

It doesn't look good. Start boning up on Dr. McCoy's favorite line that he always said to Captain Kirk... It's dead, Jim!

Bob M.

[d119]

Well one never knows... Question is, what can replace this mystery M7505 SCR?

Did the failed 1000uF 12V cap on the interconnect board cause 16V to go rushing into the SCB?

I'd really like to at least make an attempt to repair the station.

[kcbooboo]

I'm sure it's not a critical part. Just an ordinary SCR good for 5-10 amps, so it'll blow the A+ fuse. The zener is probably shot too. Get something that's the same physical size and good for 50-100V.

If it were me, I'd pull that jumper and feed in +5V from an external supply to all the downstream circuitry, just to see if the rest of the SCB is working. Yes, it will also need +9.6V, and I think the 5V regulator depends on that. The 9.6V regulator is on the IPA board in the RF Tray. It should provide that voltage without the control tray attached but you should check the schematic just to be sure.

The failed 1000uF 12V cap probably went to ground. It might have failed when the voltage across it went too high. Was that perhaps on the 9.6V line or on the 5V line that came back from the SCB to power the synthesizers? If so, the excess voltage in the SCB may have toasted stuff in the RF Tray as well.

SCRs are great - until they go open and don't do their job. Then all sorts of nasty stuff gets destroyed.

Bob M.

[d119]

OK. Any recommendations on a commonly (hopefully locally) available part number to replace the SCR with?

[kcbooboo]

You don't need the SCR if the PS is working properly. It's only there for overvoltage protection. Sort of like a reverse-polarity protection diode across the DC input of a mobile radio. Unfortunately in your unit it seems the SCR was a strong believer in Murphy's Laws: the SCR opened (taking a lot more with it) to protect the fuse.

I've never seen an SCB so I don't know what Q706 looks like. My guess is either a TO-220 or TO-92. I'd just look in the Mouser catalog and choose one that is good for 50-100V, 5-10A, the proper case style, and the right lead configuration, and buy it. I don't know what electronic supply houses you have locally. There are none near me so I don't have much of a choice of where to buy parts. Chances are, there's other stuff blown up in the station so you'll need more parts anyway.

You might find a cross-reference for an NTE replacement for the Motorola part number which will give you a better idea of the case style and device ratings. They used to have those numbers a long time ago. Of course the SCB manual is no big help: Q706: SCR, type M7505.

Bob M.

[Doug]

d119,
I've got a couple extra scb's laying around....Your welcome to one of them should you want to salvage a couple parts from. No promises that they are any good. Its been years since I've tested them. If I get clb station I automatically switch them over to cxb's, it makes life easier for me
Doug

[kcbooboo]

I did some research. Q706: 4883875D05 has been discontinued and has no replacement. However, for about $4.50, you can buy one of two brand new ones at: http://www.talonix.com/shop/item.aspx?itemid=11209

The leads are oriented KAG and the case is supposedly TO-126. As there's a 3A fuse in the SCB, the SCR needs to be rated for at least that. You can buy lots of 6A 600V SCRs in TO-220 case with the same KAG lead orientation from Mouser for between $0.50 and $1.50, Littelfuse brand. I don't think it's critical as long as it blows the fuse before everything else gets destroyed.

In the meantime I'd take Doug up on his offer as you'll probably spend more time replacing the rest of the blown parts on your existing board.

Bob M.

[kcbooboo]

Any progress? What else was blown up on that SCB? Inquiring minds want to know!

Bob M.

[d119]

Hi booboo,

I haven't had a chance to do anything with it thus far. A stand up gent is sending me a board to play with so hopefully between the two boards I'll have something running. I hate trashing a CLB station, as I have the R1801 to program them and it's not too much of a hassle. Plus the 6W station is pretty neat - would hate to junk it (and it's in the mini cabinet too which is cool!)

I'll hammer on it some more tonight and see what comes up.

Apparently the voltage regulation is done on the SCB in the CLB stations and elsewhere on the CXB.

[kcbooboo]

The SCB provides 5V for itself and I believe that is also fed back down to the interconnect board under the RF Tray for use by the synthesizers. The newer stations with SSCBs have their own 3-terminal 5V regulator on the interconnect board.

9.6V is generated in the RF Tray and that feeds the control tray. I think this is also used by the SCB's 5V regulator. The SSCB only needs A+ and ground to power up, however it needs +9.6V for all the analog circuitry. Without it, you'll get dozens of error codes.

Bob M.