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mcgill1964 29th September 2012 10:57 AM

GM delco eight track 1970s 80bfmt1 problem
Hello,(thanks for letting me in ).
I,ll try and keep to point.
I have a GM Delco 80bfmt1 8 track/am/fm radio.(pics below).On following the wiring diagram printed upside down on the top(i hope it was upside down) I tried to"just" power up on a 3 amp psu.
On turning the radio on,there was no radio backlight(bulb hollder, bulb all ok).
i stuck a simple tester ciruit tester (bulb) to where the power should leave the radio to power the antenna,still nothing.
I did hear a considerable difference in noise from the 3 amp psu,when radio is turned on position.
I assumed the radio was too much for the 3 amp psu and proceed to a 7 amp psu.
On switching on this time still the same,no light and no power from power ant output. However something did happen,so i turned it off and unplugged.I got that strong smell of something hot and a touch of smoke..
I took it apart and repowered to find out what was smoking,and located it to a blue with white stripe capacitor or resistor (my knowledge is minimal).Where the on/off volume switch sits on the board.Doesnt melt on the 3 amp psu...
Im am after knowing a few things at this stage,
1) On the sockets at rear,one terminal,next to power ant output,states LAMP.what goes to or from?
2) What is the blue and white item that melts,arrow pointing to item in bottom left photo. I,ll be needing another one.
3) Is there a proceedure i can follow to get it to power up,so i can see what works and what doesnt.
Many thanks for your time,
I would like to add its not a case of its dead...I have to bring it back to life...
all the best

sofaspud 29th September 2012 11:58 AM

1) I don't know, but I'm curious to know what it is. It shouldn't affect operation and troubleshooting, though. My best guess is it connects to the dash dimmer.
2) Is that an identical blue & white part behind the white wire? They look like tantalum caps but it's hard to tell for sure.
3) You're basically on the right track. I'd stick with the 3 amp psu; that should be plenty for radio operation at moderate volume. Strong local AM stations may not even need an external antenna. You have checked for fuses? Use your tester to verify that power is actually making it to the circuit; I'm not sure I'd trust the Ant out. And old caps may be dried up and open. A multimeter would be helpful; a go-no go tester can only take you so far.

kevinahcc20 29th September 2012 12:56 PM

The lamp in the radio is not powered from the radio power feed, but from a separate feed that is controlled by the light switch in the vehicle. That is the LAMP connector you asked about.

mcgill1964 29th September 2012 03:11 PM

Thanks,spud/kev been trying to get unit lit up only to find " it is" illuminated via power from dash lights...ya right .

Yes there "was" an identical blue and white part behind white wire, and searching Tantalum caps,im sure thats what they were.On closer inspection they look green,coated purple,with a white paint mark.
I take it the white line was to notifty which side positive..There was no markings,numbers letters or anything else. Ive obviously got to find the correct ones,so how do i know which ones ?
I also managed to pop a vertical capicitor,yellow one below in pic,what number do i use ?
numbers listed
7898283 , 900uf,0-16vdc,7743h
all the best

mcgill1964 29th September 2012 04:11 PM

Thank you Kev ,Spud, bulb powered up ,via lights :)
You can see a identical tantalum cap in pic,i searched google and thats what they look like,closer inspection shows theyre green ,coated purple,with a white painted assuming that inotifies what side was positive ???
However that one blew too,there was no numbers letters or markings on them so how do i know which ones to get?
Ive also just blown the tall yellow capicitor,
Delco part no.7898283,900uf,0-16vdc,7743h......Unfortuately when i type in part number it comes back as a delco 900"mf"......can this be used?anyone know where to get these parts
all the best

sofaspud 29th September 2012 05:35 PM

I'd replace that yellow electrolytic with a 1000F, 25V radial part.
I'm rusty on the tantalum color code. I'll try to dig up that info a little later.
I'm clueless about original Delco components. but generic replacements won't be hard to find.

mcgill1964 29th September 2012 11:00 PM

spud, ive ordered the 1000uf,25v,as for the tantalum,i found a code listing on Marvac.
With the violet over green,i work it out to be 7uf,16v.......but cannot find any,have i worked it out correct,???? thanks all for ya time

geraldfryjr 29th September 2012 11:38 PM

According to this calculator it is a 750PF,

Tantalum Capacitor Color Code Calculator

But it seems that it is a 7.5 uf

I Can't see all of the colors but I am assuming a 9v or 16v

tantalum capacitor polarity - Google Search

this one says 7.5pf,

The first two colors are read as 75 in picofarads the white dot that runs into a stripe should be the multiplier of .1 unless there is another dot that we don't see in the photo.

So 7.5 pf seems to be the correct value Typically these caps shouldn't be bad in this circuit depending on what it is used for.
It could be just used in the feedback loop to keep the amp from oscillating as it is a very small value.

jer :)

sofaspud 30th September 2012 07:37 AM

7.5pF seems to me to be a bit small for a tantalum cap. It's surrounded by ceramic discs and it makes some sense that GM would have just stuck another couple in there at those positions. Unfortunately, 7.5F isn't a standard value. 6.8F and 8.2F are closest, and either ought to work acceptably, or two could be placed in parallel eg 6.8F & 1F for 7.8F. No less than 16V rating in any case.

mcgill1964 2nd October 2012 11:52 PM

Hi all,just like to say i really appriciate the time you,ve given me on here.
I know im a dumbass when it comes to electronics.Let me clue you up to my situation.
In the last two years i have learnt welding ,panel beating,leatherwork,mechanics,vehicle wiring, paintwork,and lpg conversions,(petrol being $10 a gallon here).I have removed all rust,changed every nut ,bolt,rubber and wire on my late fathers 1969 v8.Myself and my young son.
Im pleased with how its came out,its been a steep learning curve,it,s now come to this final peice,this 8 track radio.Ive applied the same principle to this radio,as i did the car,and i cannot get my head around it.I am in complete awe of you and your understanding of all this.
Logic states i should farm this unit out to be fixed,or just purchase another unit the same,but something inside me nags at me saying,ive come so far,i have to fix it,its dads old radio..
Im having trouble locating the tantalum beads to the values in the above thread,lowest i can find is 10uf,16v.,and as i have no idea and am in completely in your hands.
I also assume that once the above thread components are changed,others may fail too.
What causes these parts to fail?and what function of the unit do these parts work?
I bought a decent multimeter but i have no idea what im looking for...
Many thanks for all you time and brain grey matter

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