Sansui G-9700

[kasbah512]

The vacuum tube display and tuner are not giving any output/display when on FM, but i really don't even know were to start with this. There is ALOT going on with the circuitry in this unit. I marked the voltages at all the jumpers and wires going in. I'm hoping the MSM5540 isnt the culprit, but any help would be appreciated. If there is nothing in the sheet, that means it either isnt there, or has not been found yet. I havent started a recap or detail cleaning, because i don't even know if it is salvageable, given the lack of MSM5540 available. Should i report back after cleaning?? (I think i know the answer, but i figured ide ask...)

 

[Sansuiman]

If the VFD is completely dark, you first need to verify all voltages required for the display are present and correct. There will be a low voltage (probably around 6 volts) unrectified AC supply for the the filaments. I'm not familiar with the 9700, but this may be a direct fed secondary on the power transformer. A higher, probably negative DC supply (30-50 VDC) for the grids is also required. If either or both are missing, the display will not fluoresce, even if the driver voltages to the individual segments are present coming off the driver IC.

You mentioned it had no output in FM. What about AM, any activity there?

 

[kasbah512]

I'm not catching anything with AM either. Ill give the switches a clean and see how far that gets me since even on aux this thing doesn't play to well on one channel, and wiggling the switch does help a bit with the sound quality.

Here's the insides. It came in pretty rough shape.

 

[Hyperion]

I suggest you try to get the AUX input working first - with the channels balanced, then move on to getting the Tuner livened up - possibly a Tuner PSU fault? - there is a separate 12v regulator for the tuner section - F-3071 = RF Power Supply.

Whatever you do take care measuring voltages on or near the MSM5540 IC - IIRC the fault you describe with the display is not usually attributable to the MSM chip, as you know they are very nearly unobtanium - so damaging the one you have with a 'probe slip' would be a tragedy. One trick to help prevent this is to wrap insulating tape round your probe tips so that only the absolute tip is exposed.

Good luck!

 

[kasbah512]

I suggest you try to get the AUX input working first - with the channels balanced, then move on to getting the Tuner livened up - possibly a Tuner PSU fault? - there is a separate 12v regulator for the tuner section - F-3071 = RF Power Supply.

I got the aux and input switches cleaned up and everything sounds as clean as it can for the time being. I'm definitely not seeing 12V anywhere on F-3071. I do see 100 mV where i should be getting 12v on F-3071, and I'm guessing that's where I'm also catching 100.1 mV on the display board. Ill see if the resistors are in spec and if there is a faulty transistor here.

Whatever you do take care measuring voltages on or near the MSM5540 IC - IIRC the fault you describe with the display is not usually attributable to the MSM chip, as you know they are very nearly unobtanium - so damaging the one you have with a 'probe slip' would be a tragedy. One trick to help prevent this is to wrap insulating tape round your probe tips so that only the absolute tip is exposed.

I appreciate the warning. I was trying to find a datasheet in order to manufacture 50-250 PCB versions of the MSM5540 to save anyone who is out of luck, but i cant seem to find one and my reverse engineering abilities are limited, especially with electrical. If i had a schematic i could probably put something together, but it seems there is no information out there. Realistically it would be dirt cheap (relatively speaking) to do something like that if the information is out there.

 

[Sansuiman]

I tried to turn up a data sheet for the MSM5540RS a couple of years ago, with no luck. Oki sold their semiconductor division to Rohm some years ago. I figure one might try to inquiring with Rohm to see if they have a paper copy of the data sheet archived, but no digital copy has ever surfaced online, to my knowledge.

I picked up some spares when Consolidated still had stock of them, so haven't needed to pursue doing a discrete reproduction yet. Oddly enough, none of my gear that uses the MSM5540 has had a failure of that IC...yet.

 

[Hyperion]

A couple of times now, people with the know-how on here have said that they can design a semi-discrete replacement/equivalent for the MSM5540 IF someone can give them an internal block diagram and schematic with enough detail.

The IC itself seems quite complex in that it performs several functions so it is difficult trying to deduce exactly what it does from just its external connections and the labelling of same (which is all we have at present). I can't see why Rohm or anyone else wouldn't release the IC's details IF they still have them (!?), and if we could communicate with the right person/department.

 

[BeerLegs]

Those do pop up on eBay every so often. I bought one for a spare about a year ago on eBay from Consolidated Electronics in Dayton Ohio. Might be worth a try to inquire if they have more.

 

[BeerLegs]

I think the data sheet for it is here, you need to be able to log in to view it. I don't know if they have any for sale.

 

[Sansuiman]

Unfortunately, no dice on the datasheet from that source. Once logged in, it calls to a dead link at okisemi.com, which no longer exists since they sold that division to Rohm. A fresh search of Rohm's support and data website renders no results either, nor does the internet archive. I think someone will have to directly contact Rohm with an inquiry and hope they have a hard copy archived somewhere.

 

[kasbah512]

I suggest you try to get the AUX input working first - with the channels balanced, then move on to getting the Tuner livened up - possibly a Tuner PSU fault? - there is a separate 12v regulator for the tuner section - F-3071 = RF Power Supply.

Whatever you do take care measuring voltages on or near the MSM5540 IC - IIRC the fault you describe with the display is not usually attributable to the MSM chip, as you know they are very nearly unobtanium - so damaging the one you have with a 'probe slip' would be a tragedy. One trick to help prevent this is to wrap insulating tape round your probe tips so that only the absolute tip is exposed.

Good luck!

I just got back to work on this guy, and there is definitely an issue with the power supply. I swapped out 2SD313 with a MJE15034G because the original had a gain of 23 hFe. Heres the voltages im seeing now with the power supply outputs disconnected.

pin 3: -481 mv DC
pin 7: 5.34 vDC (No ac voltage here)
pin 3: -1.046 vDC

 

[kasbah512]

If the VFD is completely dark, you first need to verify all voltages required for the display are present and correct. There will be a low voltage (probably around 6 volts) unrectified AC supply for the the filaments. I'm not familiar with the 9700, but this may be a direct fed secondary on the power transformer. A higher, probably negative DC supply (30-50 VDC) for the grids is also required. If either or both are missing, the display will not fluoresce, even if the driver voltages to the individual segments are present coming off the driver IC.

You mentioned it had no output in FM. What about AM, any activity there?

I went through the trouble of modeling the circuit on LTspice, and I'm basically getting the same readings as I did with the actual device in front of me. Here's what i have so far. I'm not sure how i should proceed at this point.

 

[kasbah512]

Looks like AK compressed the images. Maybe they will be easier to see this way.

Google Drive: Sign-in

Access Google Drive with a Google account (for personal use) or Google Workspace account (for business use).

drive.google.com

 

[czar]

Anyone know where the MSM5540rs chip can be obtained ? Does anyone here have one they could sell me ? Thanks

 

[BeerLegs]

They do show up on eBay from time to time. I don't see any right now. I bought one a couple years ago to keep as a spare. Do a search for MSM5540 on eBay, then right below the search field, click on Save this search. eBay will email you when one comes up.