Delco 981968

[Cruiseomatic]

This is project 3 of 3...But this one works. Pulled it out of a '59 Buick Electra. I was 15 and REALLY wanted it. The rest of the car was gone. Rust litteraly took it over. So I got what I could that wasn't destroyed and the radio was one of them. It surprisingly works, Need a major alignment. 740 comes up twice on the dial and the further left it goes, the more noise it gets, 740 comes in then 610 then 740 then 950. Needs some parts like the dial lamp holder needs replaced aswell as the cloth wire leading to it.
Before I recap, What could cause low volume and why is the audio louder before the output transformer and half volume after it?
And when the audio is up sometimes it sounds the speaker is overdriven at low volume. I know the speaker can handle any power this old radio can put out. Lastly, A transformer on the back "vibrates" and hums. It is being fed 12VDC just as if it were back in that Buick.
Anyone have any ideas or know where I can find a schematic? Tried Nostalia Air and nothing.
Thanks.
Pictures are coming.

[Reece]

Check your Silvertone thread: I posted a link to the Buick schematic there.

[Cruiseomatic]

Thanks. I got it.

[Jeffhs]

Originally Posted by Cruiseomatic

740 comes up twice on the dial and the further left it goes, the more noise it gets, 740 comes in then 610 then 740 then 950.

Yes, your radio's alignment is way off. I never heard of a situation like yours, in which the same station comes in between locals -- unless of course you are very close to a powerful station (I had that problem almost 40 years ago, when I lived one street over from the transmitter tower of a local FM station; it would blast in between the local Cleveland stations). Seems as if whomever had this radio before you must have fiddled with the alignment; I can't imagine normal component aging throwing the alignment off by that much.

I'm amazed you are consistently hearing 740 that far west of Toronto. The station's signal pattern is meant to cover the northeast United States and part of the Great Lakes, so even if the 740 frequency is devoid of local stations in your area, I would almost have to conclude that any reception of AM 740 would be a one-time fluke.

[Reece]

Jeffhs, he's talking about 740 KTRH in Houston, not Toronto.

How are you feeding the radio? There's a choke back there in the 12 volt power line. If you're feeding from a power supply with excess ripple, that could cause vibration. Best results would be to run it from your 12V car battery.

I would think that once recapped and aligned per the instructions on page 19 of that schematic, the stations will fall into place. This radio has a 262 Khz IF. A quick check would be to put the signal generator on 262 with modulated output and place the output lead near line 24 of the schematic. Should hear the tone. If not, IF may be off.

Another test: put an operating radio tuned to 1260 next to the Delco and tune the Delco around. If the Delco when tuned around 1000 produces a squeal or rush in the operating radio it means the oscillator is working, but with the stations not agreeing with the dial now, this may not happen.

Don't understand the statement about audio being louder before the output choke...as picked up by what? Signal tracer?

[Jeffhs]

Originally Posted by Reece

Jeffhs, he's talking about 740 KTRH in Houston, not Toronto.

Oooops! Thanks for the correction, Reece. I looked up 740 on RadioStationWorld.com and did in fact find KTRH, 740 kHz, in Houston. The station has a 50kW full-time signal, so Cruiseomatic's Delco radio is likely being swamped -- depending on how close to the station he is. As I mentioned, strong signal overload is a real problem in areas close to radio stations, even little 250-watt operations in small towns, so CoM's problem may not be the radio's alignment after all; it could be something as simple as KTRH's very strong signal generating harmonics across the dial. My grandmother had a 1948 GE clock radio in her summer cottage between Cleveland and Akron, Ohio. That radio would receive a 50kW local Cleveland AM station, whose transmitter was just about five miles down the road, on both its fundamental frequency (1220 kHz) and also around 600 kHz. I'm convinced the problem was simply overload from the station's high-power signal, although the fact that the radio was a TRF four-tube set probably had something to do with it as well. If Cruiseomatic's Philco radio is of the same or similar type (TRF, not necessarily four tubes), and the signal from KTRH is strong enough, overload may be the explanation of the problem.

[Cruiseomatic]

I recently moved os the only power supply I could find was a battery charger but now I found my actual 120VAC-12VDC converter, It no longer does it. Once everything gets settled, I'll begin the re-cep on it. Doing the oscillator test tonight once I get done with somethings and If I put a lead for the speaker before the choke, Its twice as loud as if I place it after.
Jeffhs, It's a Delco with RCA design. I have a Philco radio that was for a '66 mercury but it was trashed. Tuner was rusted up very badly and the buttons didn't work. Applied 12VDC and a spring in the back began to glow orange. It was completely shot so I took out all the electronics and use it for a night time decretive piece. That green backlight glow is nice.

[coffee123]

Those radios have vibrators in them dont they? no really...not joking, the 6v motorola's ive seen have vibrators on them, its a device that has something inside that moves back and forth in a coil of wire, via induction u get the high voltage for the tubes....im callin it normal? lol

Mark

[Electronic M]

I'm thinking that who ever started using the word vibrator for what folks these days associate with it must have worked with tube car radios.

If you have ever played with relays you would probably know that a relay can be made oscilate by wireing it such that the coil will be energised through it's switch contacts when ever it becomes unenergised. And if you know about coils then you know that they don't like changes in voltage (or is it curent?), and respond every time the switch opens by generating a strong magnetic pulse to try and counter the change. Another coil can be wound around the coil of our self oscillating relay to make that magnetic pulse into the sets unfiltered (often unrectified) B+ voltage.

I believe that is basically how a car radio vibrator works (correct me if I'm wrong).

Tom C.

[Reece]

The Delco radio under examination uses tubes that work on 12VDC of B voltage, with a transistor output. This was a hybrid design that existed during the transition from all tubes to all transistors in the late fifties early sixties. Since it needs no high voltage, there is no vibrator in the set. Any vibration that was noted on that rear choke was due to ripple from the battery charger. The best test for that radio would be to hook it to a car battery so it gets a proper smooth supply.

I always respect a car battery for the power that it contains. A short can cause a big flash and burns or in the worst case the battery and its acid blows up. I make my connections carefully and make certain that there are none that might short, recalling that the chassis of the car is negative, too. Good idea to put a fuse in the test line, also.

I haven't worked on one of these hybrids so not sure, but the speaker may have to have a certain impedance. Do you know if the speaker under test is the right one for this set? The schematic shows that there is a shorting switch which apparently operates automatically when you unplug the speaker in order to save the transistor from running with no load. If you pull the speaker plug and then test before and after the choke I would imagine that any output that it still had would be louder before the choke since there would be a dead short after it, although with a dead short I wouldn't expect to hear very much of anything. Maybe one of the car radio gurus will chime in. If no one here, there are some over on ARF.

[Cruiseomatic]

Did the oscillator test. Got opposite results. The functioning radio is the one that got affected. I did get the delco somewhat aligned. However, It did produce magic smoke last night. Audio has always been low when the control was at full, Didn't think it could be more than one of the caps on the volume control. Well, just listening to Man cow on it this morning, My power supply started to really hum and something under the PCB started to smoke. She kept playing though. Shut it down and turned it back on later. Nothing but a faint attempt through the speaker. So I began to look. No obvious damage. Started checking resistors and some were out of range but not bad. One that is a white box and is labeled "Fuse Resistor" has no continuity whatsoever. Jumped it and she started playing again, However, The choke on the back started smoking. So I shut it down and put it all back together. Looked at the schematic I have, Not much help. It's number 59 in "Most-Often-Needed". Doesn't list a value or anything. Same with most parts. Just numbers. Maybe I'm reading it wrong? If any of y'all have any insight I'd really appreciate it. This is currently my only working tube radio. Haven't got the money to do the Emerson and the Silvertone turned out to be a basket case. Sorry for taking so long to get back to this, Been busy with college and work.

[Reece]

When doing the oscillator test, the functioning radio IS the one that should squeal when you tune the "broken" radio around.

You have a short somewhere. Following the schematic, check everywhere that 12V+ could be shorting out to 12V-. You'll find it. Don't run the radio by shorting the resistor as it opened up for a reason. Nothing should get hot or smoke!

[Cruiseomatic]

Found the short. It was the speaker. Where do I find a .47 ohm fuse resistor? Also, Once I get that replaced, What can I do to increase the weak volume?

[Einar72]

Thought I'd better pipe up before you fry your output transistor . The .47 or .68 ohm fusible resistors are loooong out of production, Delco having stopped using this design in the late 70's. I doubt the vendor even exists anymore, my best guess being it was IRC. There are other fusibles out there, but I don't see the right ones on the bay right now. Pure Unobtanium at its rarest.

Speaking of which, your output transistor is currently on the bay for 35 bucks. Best cruise the thrift stores and get a heap of similar Delcos (like I do) for donors.

Watch out when buying speakers - you will almost never find a true 10-ohm Delco speaker, so find an 8-ohm. NEVER hook a 4-ohm to one of these. Unfortunately, most of the imported crud out there IS 4-ohm these days.

[bandersen]

Perhaps you could put a .47 ohm resistor in series with a fuse ? For that matter, maybe just a fuse since the resistance is so low.