Sansui G-7500 Fried Resistors F2980 Board

Young46

AK Subscriber
Subscriber
Hello, I am an AK member for approximately a year now. I have received help with repairs to Pioneer Receivers from AK members and this is my first plea for help with a Sansui. I have this beautiful G-7500 but it does not have any sound on any channel. After receiving it I powered it up on my DBT and the bulb (100W Halogen) glowed bright and then quickly dimmed. I opened it up and discovered and area of fried resistors on the F2980 Board. I am really seeking any help that I can get to get this fine piece of equipment playing again. I have several photographs and would like to start a new post for this unit. I have never posted photos before so I will most likely need help.l

Thanks!

Frank
 
F-2980 is the power supply. Specifically which resistors are fried? To post picture, click on the link that says "Upload a File" in the lower right corner below the text entry box. It will allow you to search files on your computer to select an image. There are some file size limits so you may have to size the image first.

Also, please post the serial number for the database. Click on the link in my signature block.

- Pete
 
R58, R70 look to be the specific ones that fried. R70 appears to be the main culprit however it is hard to tell from the condition. I can still get readings from R46, R56, and R68. TR20 checked ok. In the photo the resistors are located like the board is actually laid out. The layout in the service manual does not match the actual layout. Also the parts list for this board does not list these resistors unless I am missing something. See photos and thank you so much for your response.

I am not a professional tech nor do I have an electrical or electronics background. I have done repair to boards (replacing caps, transistors, trimmers, resistors, etc) so I am not a complete dummy.
 

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Image of F2980 board from the service manual. Note the mismatch between actual and the manual...nothing unusual but worth noting.
 

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  • Sansui G-7500 F2980 Parts List.pdf
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  • Sansui G-7500 F2980 Trouble Area.pdf
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TR20 is the DC protection transistor, the amp doesnt need that to operate, so you could leave that out whilst you troubleshoot.

Things to look at, driver transistors, Bias transistor, every resistor in the driver and output section basically all the emitter resistors.
You will also have bad output transistors

These driver boards are horrifying to work on, and they are very temperamental and when things go wrong, boy do they go wrong, cascades of destruction....

So work methodically through the whole thing, also install 100Ω 5 watt resistors in place of the 0.33Ω emitter resistors, this will protect the output section from running away on you when you first fire it up, once its stable then you can put the 0.33Ω resistors in.

You'll need to replace the bias and DC offset trimmers, and recap it whilst you are in there, its a big job which requires a lot of patience.

Let us know how you go...
 
You need to check the output transistors as well as the driver transistors with that much damage in the driver circuits. The drivers are mounted on those steel heat sinks.

By the mid to late 1970s, Sansui stopped listing common resistors and capacitors in the service manuals. They published a separate "Common Parts List." It's available in digital docs forum. It won't help though, it only lists values and part numbers. Short of having a board to compare it to, the best reference is the schematic.

- Pete
 
Thanks to all who have responded. Does anyone know if there is a parts list that has been created for this board that lists all the current parts. This would be like the one Mark the fixer has created for some of the pioneers. A lot of the parts on the board are obsolete and although I will attempt to find alternates I would appreciate help from those more knowledgeable.
 
I have a parts list for the F-2980 board in a G-6700, but do not know how many (if any) are the same. Will check when I get back.
 
Check for burnt traces on the foil side. The o/p bjts can be replaced with 200W TO-3P devices such as
NJW3281G (NPN)
NJW1302G (PNP)
the drivers can be
MJE15028, MJE15030 (NPN)
MJE15029, MJE15031 (PNP)
the VAS section, TO-126 devices can be KSC3503/KSA1381
 
Let me say from personal experience with this board: Going in to replace all (most) of the parts is a bad mistake!!!

You need to identify the cause and fix that first. Once you get it working you can replace the electrolytics one at a time. I would not replace anything else unless it fails or is out of spec. This board has a hair trigger, and it wants to have a fine balance between its components.

I'd replace the 100Ω single turn trimmer pots first, making sure that the replacements are orientated the same. That is, note which direction you turn them to increase the resistance.

Then I'd put it on the DBT and check the P/S voltages. These will change with different bulbs in the DBT, so take good notes.
 
Let me say from personal experience with this board: Going in to replace all (most) of the parts is a bad mistake!!!

You need to identify the cause and fix that first. Once you get it working you can replace the electrolytics one at a time. I would not replace anything else unless it fails or is out of spec. This board has a hair trigger, and it wants to have a fine balance between its components.

I'd replace the 100Ω single turn trimmer pots first, making sure that the replacements are orientated the same. That is, note which direction you turn them to increase the resistance.

Then I'd put it on the DBT and check the P/S voltages. These will change with different bulbs in the DBT, so take good notes.

I found some trimmer pots Bourn 3329H-1-101LF. Are these suitable?
 
Will they work? Probably, but I'd prefer a multi-turn trimmer instead of a single turn. 25 turn is the most prevalent, and the high # of turns should not be a concern. Hopefully you won't be doing this adjustment very often, but 25 turns gives you a very fine adjustment control where a single turn gives you very coarse control. ½ turn on a 25 turn pot gives you ~2Ω, and about 67Ω on a single turn (only get ~270°, 100Ω * 180/270 = 67).

The PV36Y101C01B00 would do fine, but most any Bourns style trimmer would do. You need a top adjust and I like the boxy blue ones. It is a simple task to bend the in-line pins to a triangle to match the hole pattern in the F-2980. Make sure that you insert it in the orientation such that a CW rotation will increase/decrease the resistance the same way that it does with the original single turn. This will make your refurbed board follow the adjustment procedure in the SM.
 
Always a good idea to ohm out everything and compare channels against each other with no power and then as said above, power up with a DBT to start with.
 
Thanks to all. I work on this as time allows so forgive me for slow responses. I was able to look at the unit again for a few minutes today and I pulled all the fuses on the board and checked continuity across the fuse holder terminals. All were fine except for the 2A 250. It has continuity across the terminals with the fuse pulled.
 
Would that be F601, F 602, F603, or all three?

Was the reading 0.00Ω?

All three. The reading for F601 was 2.3, F602 and F603 OL, and I also tested F604 and it was OL. So far I have pulled and tested all the transistors in the driver section and they test ok. Ive also pulled TR601 and TR602 one the power side and they test ok. I replaced the fried resistors and was getting ready to power up on the DBT when I remembered an AKAI R-R that I repaired had continuity across the fuse terminals so I thought I would just check them out. Turns out this board has the same issue. It was a bad transistor in the AKAI that was the problem. I am going to pull the output transistors this weekend and check them. I will also pull TR603 and TR604 and test them.
 
Look at your schematic. When you remove F601 and read 2.3Ω across the "empty" terminals, you are actually reading through three lamps in parallel and the 7.5V windings of the transformer.

Now, look at the display of your DMM when the probes are not connected to anything. It will probably show "0.L". I'm not sure what this stands for but you can consider it to be saying "Open Line". Analog meters used to show ∞ for infinity, but "infinite resistance" may not be technically correct.

I don't see a problem with your open fuse readings.
 
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