[dataway]

Anyone else ever attempt to re-cap their original Tempest/GTO AM radio?

I have the schematics and parts list, I've ordered the necessary parts .. going to dive in shortly. Radio "works" as is ... meaning it powers up, it will drive the speakers, it will just very faintly get a couple of very close by stations.

Very well could be other factors preventing it from receiving well ... but the 50+ year old caps have to be bad anyway, so I'll replace them first and move on from there.

The main board can be accessed without too much trouble .. about eight solder connections to remove so it can be flipped over to access the individual components.

There is one four-way capacitor can that I'm sure is no longer available, or if it is, it's 50 year old NOS so probably bad anyway. So that can will have to be replaced with four individual caps ... other than that it appears to be pretty straightforward component replacement.

I'll give it a shot, if it works, great, if it doesn't ... I'll get some replica am/fm/bluetooth unit.

[Stuart]

If reception is poor, do you have a good antenna and a good antenna cable connection?

[gto4ben]

A weak gain could be the result of an alignment shift. Have you tried to re-align the gain stages ,starting with the trimmer cap? In addition, Have you set the preamp stage bias pot to get 1.5V at the speaker output. Those pots get intermittent with age. A small turn can get the volume back.

[Shiny]

It's been several years but I replaced all the electrolytics in an AM/FM.

The cap replacement was straightforward but finding all the parts in the same format/size was a little challenging. I replaced the "multi-can" with discretes as you plan.

I did find an electrolytic cap that was so far gone it had burst and I'm sure many others were degraded.



IMO, replacement is wise and will only help.

I did buy a Sam's guide off eBay and it was well worth it. I had no hope without it.

Also keep your eye open for tin whiskers on component leads. They are fascinating! Probably not going to cause a catastrophic short in an old radio but hey, it could happen...

[dataway]

Tried several different antennas, and the socket and wiring in the radio looks good. It's got good volume, just lousy reception. I attempted to trim it some with the external trim above the tuning knob as outlined in the manual.

I figured it was pointless to attempt any serious tuning at this point considering it's probably got numerous bad caps. I'm replacing all the electrolytic and plastic covered paper, leaving all the ceramics in place.

I won't be using the same format .. in fact mostly axial leads (?) .. instead of the leads that come out the ends, so the electrolytics certainly won't appear OEM.

Radio is pretty nice inside, no obvious signs of damage, heat, water etc. I'll probably end up getting a signal generator if I have to tweak the tuning.

I downloaded a service manual/parts list for it. Took a good photo of the component side of the circuit board and superimposed it on a photo of the schematic and it works very nicely to identify components.

[Old Goat 67]

You are doing the right thing. You will win!

[Bill Hanlon]

GREAT idea on the superimposed pictures!

Having updated caps should allow for updated tunes.

[dataway]

I was having a heck of a time positively locating components using just service manuals picture of the back side of the circuit board .. not to mention some of the components may or may not be present depending on the board revisions. Helps to see both side at the same time

Just ordered a basic 60MHZ signal generator that should be suitable for adjusting it if needed. About $130 for the signal generator which I can find other uses for in the future.

[Shiny]

Originally Posted by Bill Hanlon

GREAT idea on the superimposed pictures! ...

Agree! Very clever!

[dataway]

If this all works out like I hope ( I give it 50/50), I'll post a list of the components I replaced, all were purchased from Allied Electronics (online), about $20.

I'm certainly no audio electronic expert so others could possibly select better components. I tried to shoot for the same capacitance, same or higher voltage, good tolerance and high durability. Most are general purpose caps, a couple are audio, and I think one listed as "automotive". Most are not similar in appearance or design.

BTW ... can the tuning get out of sync with what is shown on the dial? If so I assume there is a procedure in the manual for adjusting that?

[326HO-Lemans]

Where did you find the 850uf cap?
Everything I see is high voltage and too big.


Sent from my iPhone using Tapatalk

[dataway]

I used an 820uf, 16v .. figure 820 is well inside the typical 20% tolerance. Bear in mind you can use voltage way higher if you want ... they don't perform any better, but no worse either. The voltage rating for a cap is the maximum, they really don't have a minimum. Also if need be you can run caps in parallel to achieve the value you want. These old AM radios are the most basic and primitive types, pretty much any modern component you use, if it's anywhere close to the original value is going to work better and last longer than the original parts.

[dataway]

In the alignment procedure in post #3 (thanks ben) does anyone know if a speaker should be attached during the process? It just mentions an AC meter across the speaker leads .. hesitant to use full volume with no speaker attached.

And I assume tuning to the "high end stop" means turning the tuning dial all the way to the right.

[Old Goat 67]

I wouldn't chance not having a load on the output, JUST in case!

[dataway]

That's what I was thinking .. since I'm only reading AC voltage having a speaker attached shouldn't effect the output voltage much.

[dataway]

I'm happy to report that phase one is complete and was a success. Much to my amazement this project was not a failure. Just testing on my bench, holding a cheapo antenna in my hand the reception was orders of magnitude better than before. From one weak station to half a dozen strong ones and multitude of weak ones ... and this is without the antenna bolted to a car and grounded as intended.

Volume is good, tone is much improved (replaced a tone cap), nothing smoked, no bad smells. The power transistor got a bit warmer than I expected, but it wasn't bolted to the metal radio case as it normally is, and I have no idea how warm they get in normal operation.

My knowledge of analog audio electronics is only the most basic. I know the functions of the various components, but have zero knowledge of circuit design etc. .. so I was very surprised this all worked ... but then it was just component replacement, no modifications.

Phase two .... I'm waiting for new Siglent signal generator and Oscilloscope to arrive to do the tuning adjustments, yes a considerable expense .. but I've always wanted those tools anyway.

So ... if any of you have an OEM radio that at least powers up and makes noises, and is not totally trashed inside ... this may well be the cure for a lot of ills. The various caps I replaced were in both the audio circuits and the power conditioning circuits ... so could perhaps solve a range of problems. Pictured below are the parts I removed including the can the held a 4-way tapped capacitor. Replacement parts were about $20 ... and I bought an extra one of every component in case I screwed something up.

How long will it work? How long would it play at full volume? .... I don't know, but it works well at the moment. I'd be happy to provide the parts list, photos etc. to anyone that wants to try it themselves.

By the way, some of the replacement parts were much, much smaller than the originals ... but, they seem to work ... a farad is a farad. I can only assume in the 50 years since this radio was made they got much better at making things smaller.

[Old Goat 67]

Good Job!

Looks like you have it well in hand. Fifty year old electrolytic caps have outlived their life expectancy. I've had great success just changing those before.

I think you have it well in hand. Get that power transistor bolted down to get it heat synced and it should last good.

[Shiny]

Congrats! Hope it's rewarding for you!

Add new heat sink grease under the power transistor if that's not already planned. These amp designs apparently run a high current continuously and they DO get hot.

You also need plastic tools to adjust the coils when you do the alignment (you probably know this). I bought a cheap set off eBay and they worked well.

As to having a speaker connected or not during alignment, I don't remember. But cranking full volume with a speaker attached sounds weird. Find a EE or read up on the internet before you commit. A quick call to a radio repair shop would also answer it. The guy at Turnswitch (Electro-Tech) was super helpful and generous when I repaired mine and needed Delco parts that are no longer made.

[dataway]

At the very least I'm going to remount the power transistor heat sink to the outer case with heat sink compound .. which it didn't have originally.

Waiting on the plastic tools from Amazon ... and no I didn't know that until I read it this week

I must have attempted repair of vintage audio equipment a dozen times in my life, this is the first time it worked. But then this is first time I had a schematic and parts list to use. Yes the personal reward was worth it ... cause the radio itself certainly isn't

[Shiny]

You are learning a lot, I'm sure, and will have more confidence on your next electronic adventure!

Given the state of today's electronics and packaging, it's amazing to me that leaded components are even available. Good job hunting down all the parts and glad you are having success.