TR4/4A TR4 Bendix Radio

[KVH]

I have an old "am" five button one that looks like it says June 10 66.

It looks good and clean inside, but I know zip about electronics and the radio seems totally dead. No sound or light when switched on.

How can I confirm if it was, indeed, negative ground? It's stamped 4TBTR 009031.

Any advice or suggestions before I pack it well and send it off to Florida for repair?

 

[TR3driver]

Looks to be positive ground only, from the schematic in SAMS AR-26.

 

[KVH]

OK, thanks.

Looks like I'll be putting it on EBAY.

I don't assume it can easily be converted to negative ground can it?

 

[mrv8q]

Some radios have a little disk that be switched between + and -. With Randall's info, you can test it with your battery.....

Saw a nice rebuilt complete Triumph brand pos/neg ground 5 push button AM radio this morning at a swap meet for $150.

 

[TR3driver]

I don't assume it can easily be converted to negative ground can it?

Might not be totally out of the question. Looking at the SAMS, I think it might be possible to isolate the ground(s) for the circuit board and then switch the power wires. The antenna uses a coupling transformer so it will be OK; and the speaker isn't grounded anyway. But it won't be as simple as switching a plug around.

 

[HerronScott]

I'll have to check mine but I thought it might have had a switch as Kevin indicated. I do recall that if you accidentally grounded the case of the main power amplifier/transistor (which was external mounted) that you would fry it. Had to replace it twice when I was first learning about my car. Luckily it was cheap in those days to repair.

Scott

 

[TR3driver]

Another alternative, if you are so inclined, might be to buy or build a DC-DC converter that would put out -12v. I've not actually gotten into it (yet), but there are all sorts of ICs these days for building switching power supplies. Add a suitable transformer, power FET & flyback diode; shouldn't be too hard to build something.

Here's one that might be suitable, sells for about $30 @ Mouser:
https://www.murata-ps.com/data/power/mdc_uei-15w.pdf

 

[Geo Hahn]

I have a 5-button AM Bendix positive ground in my TR4. FWIW -- the prior owner said it was dead but in fact he had the battery in backwards and had inadvertently converted the car to negative ground. Amazingly when I switched the battery back to + ground the radio worked fine... had not been fried by the reverse current.

Also KVH -- if after you test it you find it needs work we have a TR3/4 owner here in Tucson who also restores old radios. He has done some for club members in case you need an alternative to 'ship it to Florida'.

 

[KVH]

Geo,

I'm forgetting, I guess. I thought I saw your TR4 with negative ground? No?

You can't just switch that around can you?

I told you I don't get electrics! What makes the difference between + and - ground?

(I'll understand if the answer is too long to expect in an email)

 

[KVH]

I guess this link says it all: https://www.spritespot.com/Downloads/Positive%20to%20Negative%20Ground%20Conversion.pdf

But, Geo, I'm still curious if your TR4 is currently positive ground.

 

[HerronScott]

I checked mine this evening and it's a slightly different (newer?) part number 6BTR 003928 and it does have a switch for negative or positive ground. You can just barely make out the writing showing negative to the left or bottom and positive ground to the right or top depending on which picture you are looking at at.

Scott

 

[TR3driver]

What makes the difference between + and - ground?

Basically, it is just which pole of the battery (+ or -) is connected to the car body & frame.

Many devices don't care about polarity, so it's relatively easy to change the earlier TRs to negative ground. But solid state electronics (like transistors and diodes) tend to be very picky about polarity; and the vast majority of them assume negative ground. So if you have installed a modern stereo or alternator, negative ground is pretty much a requirement.

 

[TR3driver]

I checked mine this evening and it's a slightly different (newer?) part number 6BTR 003928 and it does have a switch for negative or positive ground.

Yup, 6BTR is slightly later (1966 according to SAMS) and does have the switch. That was just about the time that Triumph was making the change to negative ground, and I'm sure the switch made it easier to stock the right radio. I vaguely recall that the TR4A and Spitfire were negative ground in 66, but the Herald was still positive ground. I'm sure someone will correct me if I've gotten that wrong

 

[Andrew Mace]

My factory manual tells me that the Spitfire only went negative ground from the Mk3's introduction in March 1967. But virtually all sources I've read seem to feel that the TR4A models were negative ground, while TR4s had been positive.

Supposedly the Herald 1200 went negative ground in around March 1968, but that was well after imports to the US had ended. (All Herald 13/60s were also negative ground, but there were only a very, very few of these built to US spec's for Puerto Rico of all places!)

 

[KVH]

Hmmm.

Mine has no switch and must be older. Mine also has some kind of "rail" on the side that makes it appear to be from another model, maybe a Spitfire.

 

[KVH]

Here's another pic.

 

[KVH]

And another:

 

[HerronScott]

Someone was trying to sell one like yours (with black buttons versus chrome ones) on Craigslist a while back for some crazy amount of money. I would suspect the large transitor above the left-hand knob as I managed to short that out twice. Back then it was easy to find an electronic repair place that would replace it for $5-$10.

Scott

 

[Rrbbeerrttoo]

My negative ground radio in my TR4a is numbered 6BTR 005059 and it looks exactly as the 3 pictures shown above by Herron Scott. It still has the inspection tag stamped 249 66 . I bought mine in 1977 from someone claiming it was from a '67 Triumph Tr4A.

 

[TR3driver]

Entirely possible, I didn't mean to say they were only used in 66. That's just the date listed in the SAMS (which is service information for the radios). And since radios were generally dealer items in the US (otherwise too many disappeared during shipment), the dealer might have done almost anything. Plus of course the cars were frequently registered as a year (or even two years) later than when they were made.

SAMS also lists model 6SMTR for 66 TR4, Herald 1200 and Spitfire 4; plus model 7SMTR for 67 TR4, Herald & Spit. Apparently they are the same electrically though, as there is only one set of service instructions, schematic, etc. And these are the models with the plug you rotate to set polarity, rather than the slide switch like the 6BTR has.