Shebro Carcom 3 Amplifier strange fault

[Joe_Lorenz]

Hello,

right now I have a 12V three component vehicle stereo unit (receiver, cassette-player, equalizer-amplifier) on the bench that is almost driving me crazy. Unfortunately I haven't got a schematic. There are four NEC µpc 1181 H3 amplifiers as main components. It came in with one channel silent, the other working perfectly. I found out one of these ics was hot, so I replaced it (had a new one at hand) and replaced all big coupling electrolytics, too.

What's the outcome? If I apply the power supply, raising voltage slowly, this will happen: At about 9V / 100 mA the receiver starts to tune in stations, works well. At about 10V both channels work well, cassette player can be used and works fine. Further raising voltage increases current and loudness as expected. At about 11.5 V current rapidly raises well beyond 2000 mA followed by a popping noise, driving my power supply into safety cut out. That can be repeated quickly and forever. Measuring voltages of all pins at all four µpcs @ 10.5V when it runs stable has shown not any significant difference!

I have been able to trace the fault to one channel, changing input signals or speakers has no effect. Now I am wondering if I should start changing every single transistor around there or try to find a new set of all four µpcs. Which may turn out to be no easy task as I think they are a bit outdated after 40 years now.

Has anybody already been-there-done-that? Any advice is welcome. I try to add some photo shots here.

Regards, Joe

[Silicon]

Have you looked at the amplifier outputs with an oscilloscope to see if there are any ultrasonic signals that should not be there?

Have you checked the ESR of the Electrolytic capacitors?

[PJC58-Hythe]

I agree with Silicon you need to look at the outputs with a scope as it could be high frequency oscillation. This is a 4 channel stereo with 2 amps for front (left & right) and 2 amps for rear (left and right), have you got all 4 channels terminated in a suitable load? The only information I could find on this unit is a specification sheet (attached) but at least it shows the connection of the loudspeakers.

[Mr 1936]

Hi Joe

One of the things I remember about working on 12 volt car amplifiers is that they are characterised by very high audio circulating currents. This means that good earth (ground) connections are vital to avoid interaction between the outputs and the sensitive earlier stages.

I suggest you carefully check all ground connections, both soldered joints on PCBs and hardware like nuts, bolts and tags. Maybe there is a dry joint or a loose/corroded tag somewhere ? Re-make the joints and dismantle/clean/reassemble the hardware if anything is suspicious.

Do the loudspeaker outputs have Zobel (resistor + capacitor) networks on the PCB to aid supersonic stability ? Check these out too.

[G6ONEDave]

What are you sing as a power supply, it will most likely need to be capable of 5 amp or preferably higher loading. If it's only rated at 2 amp, it probably won't be high enough to power this sort of equipment.

Although a bit awkward it might be worth disabling the plus supply to 2 of the output chips lets say rear left and right and then see if everything behaves. After that if all is good reconnect the 2 disconnected output chips and disconnect the other 2 output chips. Do the same test again. If all is well then it suggests that the power supply is not up to the job.

It's just my thoughts and could be wrong.

Dave

[Joe_Lorenz]

Hello gentlemen,

first of all thank you for all those thoughts. These are very useful hints for me now!

- I am not (yet) in such a desperate state of mind that I would fire up the trusted Tek 581 to monitor the outputs. But maybe I will soon.

- The big coupling electrolytics have been substituted by new ones last week, but I still have to check ESR of the tiny ones.

- That specification sheet is helpful, too. 4.5 Amps @ 14.4 V means I have set the current limiter of my psu too low at 2 Amp. I am used to set it at 2 Amps when running such gear as it is ok for most car radios. I think I will raise that gradually and watch out for components getting hot.

- Ground connections: I will have a close look now! Thanks for remembering me!

- Disabling the + supply of one chip at a time is a good suggestion, too, as there is no easy good-bad testing of these chips. I am so puzzled by the fact that above 11.5 V the raise of current is quite rapid.

We will see, I will post my results. After all, I admit a bright glowing anode of a PL 504 and a steaming cathode capacitor are easier for me to fix than dealing with semiconductors!

Regards, Joe

[Ed_Dinning]

Hi Joe, another thought; these chips need good decoupling or they can oscillate at high frequencies. Have a look at the caps local to each chip and decoupling the supply locally

Ed

[Joe_Lorenz]

Hello gentlemen,

problem solved! Mods, please can you move this to "success stories", please?

Using my best reading glasses I finally spotted a most tiny copper whisker across two tracks near the input of one of the amplifier ICs. It was not even fixed in solder but must have been there from new. After brushing it off the radio resumed normal operation.
If I had cleaned the pcb before attempting repair I would have saved some time.

Now I record about 480 mA current consumption at 12.6 V, radio idling.

Regards, Joe