What could be wrong?

[Anand A]

Hi all,
A noob to HiFi. I had picked up this Rotel 412 (class AB, mid 70s make) a year back on Olx in a whim and the sound quality from it was something that I haven’t experienced before. Ofcourse it was my first serious audio equipment I must admit. I enjoyed the warmth of the sounds it produced with the floorstanders I have producing a good vocal representation from the media I streamed though a CCA device. Now, for a while the amp started giving out a cracking noise, even when the volumes are way down. I’m not sure if it is due to earthing, or some other factors. I had the amp checked with a reputed antique audio equipment repair shop near Pammal, Chennai and the gentlemen said he had found the defect and replaced some faulty/aged capacitors. He had it running for a while as well before I picked it up. After coming back it worked well for a day without issues, but again started the same issue the next day, although less pronounced than before. What could be issue? Is it the end of road for this beauty? Pls check the video

 

[Anand A]

https://imgur.com/a/DdVODJv

video for reference

 

[Anand A]

Tried the amp paired with another set of micca bookshelves too. Same problem as before. Before the repair the issue was seen with both channels, not it seems limited to the right channel in both test cases.

https://imgur.com/a/5MD7PdF

 

[Yelamanchili manohar]

Tried the amp paired with another set of micca bookshelves too. Same problem as before. Before the repair the issue was seen with both channels, not it seems limited to the right channel in both test cases.

https://imgur.com/a/5MD7PdF

I could'nt watch the entire video due to my limited internet access. But from what i heard, it is definitely a crackle. And not a hum, so that rules out ground issues. It sounds like a loose connection or dirty pots or issue with caps. On a more serious note, it could be power source components on the right channel including transistors. The serious issues are beyond our DIY and will need a qualified techie to peep into.

Below are the two easy things you can DIY, after doing your morning pooja and then praying for the best

Please switch off the amp, and then try the below :

1. Inspect the insides carefully to identify any loose contacts at wire ends. Some old solder might have come loose. So ideally try moving the wires with a pencil to see if they are still rigidly connected.
2. Get a can of WD-40 from any hard ware store. Pop open the lid of the amp. Now starting with one knob at a time, spray the WD-40 generously from both outside and inside, and turn the knob full range continuously . Complete every knob and every switch on the front panel. Now give 30 min for the WD-40 to completely dry (It actually dries immediately, but still wait a little ).
3. Now switch on the amp. If we still have a crackle, then it is either capacitors or transistors, mostly in power line to the channel having the crackling noise. And better to take it back to the techie.

All the best

 

[Anand A]

Thanks for the responses and updates. Now any of you have a good reference to a technitian in Chennai to who can be entrusted?

 

[ajuvignesh]

Please try Torvin (power electronics), Ritchie street, Chennai.

 

[VSTEREO]

Hi Anand,

You can try Power Electronics, Besant Nagar. 97907 02046 ( Lingeswaran).

Btw, I do stay in Pammal.

 

[Anand A]

Thank you @VSTEREO good to know! I gave it initially to Chennai Musical Antiques in Pammal. They seem to outsource the work to some technitian who was not accessible to me. Hence thought I’ll check with someone more reliable. Thanks for the tip!

 

[reubensm]

Nice amplifier. From the evidence, technicians would tell you that it could be dry solder joints or failing capacitors. However, from my personal experience, it could also be failing transistors especially in the preamplifier and driver stages of the power amplifier sections. The key to fixing these issues is to be able to isolate the problem and an experience technician can help you with that. Folks like me working on amplifier repair or restoration jobs used to focus mostly on shifting values of resistors, electrolytic capacitor replacements and pots/switch lubing. However, in the recent past, I have also realized the merits of replacing older transistors, especially ones that run warm or hot, over a period of time. I have noticed that some of the transistors, especially vintage silicon types from the early to mid 1970s tend to crackle a lot over time. Some time ago for example, an Ahuja transistor amplifier from the 1970s was brought to me. Had the same crackling problem. A local mechanic had diagnosed the problem to be a sparking power switch but gave back the amplifier saying that replacement cannot be got . What I did was, did a carpet-replacement transistor job, essentially upgraded all the old SOT-25 case type BEL BC148B transistors with more modern BC548B transistors and the problem was solved.

 

[availlyrics]

@reubensm
I have similar problems with my old Norge 2025 ,(single supply). Disconnected all volume , tone and switches, still the problem persists. I got replacement caps, will check transistors too

 

[IndianEars]

Nice amplifier. From the evidence, technicians would tell you that it could be dry solder joints or failing capacitors. However, from my personal experience, it could also be failing transistors especially in the preamplifier and driver stages of the power amplifier sections. The key to fixing these issues is to be able to isolate the problem and an experience technician can help you with that. Folks like me working on amplifier repair or restoration jobs used to focus mostly on shifting values of resistors, electrolytic capacitor replacements and pots/switch lubing. However, in the recent past, I have also realized the merits of replacing older transistors, especially ones that run warm or hot, over a period of time. I have noticed that some of the transistors, especially vintage silicon types from the early to mid 1970s tend to crackle a lot over time. Some time ago for example, an Ahuja transistor amplifier from the 1970s was brought to me. Had the same crackling problem. A local mechanic had diagnosed the problem to be a sparking power switch but gave back the amplifier saying that replacement cannot be got . What I did was, did a carpet-replacement transistor job, essentially upgraded all the old SOT-25 case type BEL BC148B transistors with more modern BC548B transistors and the problem was solved.

I TOTALLY Agree.

Identifying the specific transistor can be a painful and time consuming affair. Carpet replacement of all the small signal transistors is often the most effective approach to overcoming "Popcorn Noise"

1st identify if the problem is Before or after the volume control. For this, simply turn the Volume control to Zero (Lowest Volume). If the Popcorn bursts still continue, it is originating from After the volume control ... Usually the input stage of the power amplifier.

Otherwise, its from the Pre amp section.

You can do this little test at home... to identify which half of the amplifier is creating the sounds.

 

[Anand A]

Thanks. I had already sensed it’s coming even while the volume is kept zero. So it’s probably after that @IndianEars

 

[MaSh]

Well this is the concern we all have while buying vintage. Hope you can find a good technician who can perform a complete restoration on the amp. There is little value in trying to diagnose and rectify one single issue at a time. Touch something today, something else will fail in a month.

If there is no Technician handy, best option is to move on to another modern amp, while still working on the vintage.

MaSh

 

[A K Bhattacharjee]

Hi all,
A noob to HiFi. I had picked up this Rotel 412 (class AB, mid 70s make) a year back on Olx in a whim and the sound quality from it was something that I haven’t experienced before. Ofcourse it was my first serious audio equipment I must admit. I enjoyed the warmth of the sounds it produced with the floorstanders I have producing a good vocal representation from the media I streamed though a CCA device. Now, for a while the amp started giving out a cracking noise, even when the volumes are way down. I’m not sure if it is due to earthing, or some other factors. I had the amp checked with a reputed antique audio equipment repair shop near Pammal, Chennai and the gentlemen said he had found the defect and replaced some faulty/aged capacitors. He had it running for a while as well before I picked it up. After coming back it worked well for a day without issues, but again started the same issue the next day, although less pronounced than before. What could be issue? Is it the end of road for this beauty? Pls check the video

I have faced the same problem with my Philips 900 series amp fa931. You can go through my thread on this topic. After thorough investigation for a few days, decided to change the selectors ic. It worked smoothly for 2/3 months after that it became silent. No output. Again started research. This time we decided to change the memory ic because rotary volume control was not working properly. Vol control knob was going to the max position whenever a function is selected. Anyway, the ics were changed(ics were purchased from overseas) .
Amp started working but the rotary vol control didn't. At last I decided to disconnect the rotary function of the vol control switch & made it manual. The amp is working fine since then.
Is your amp also have a rotary vol control? If so then disconnect it & see the result before going through the above process. Good luck.

Regards

 

[raj12345]

As per the video, issue seems to be due to failing small signal transistors either in Preamplifier section or in poweramplifier section. If this amp have pre-out & mains inn jumper at back then try connecting rotel as per & use another power amp. If still rotel creates noise while using in preamp mode then change small signal transistors of preamp. If it operates noise free while using it as preamp then we needs to change power amp board small signal transistors. Mostly this type of noise independent of volume control is due to failing transistors in preamp board ( tone board & equaliser board. Another method, try using Tape In or Tape Monitor Input (bypassing toneboard), if it operates noise free then tone control board's transistors is failing. To isolate a single failing transistors usually a freezing spray method is used just Google that spray method. Alternatively you can also try cleaning all small-transistors legs with ear-buds dipped in isopropyl alcohol. Also do clean back of PCB with isopropyl alcohol or any PCB cleaning chemical of your choice.

 

[Anand A]

As per the video, issue seems to be due to failing small signal transistors either in Preamplifier section or in poweramplifier section. If this amp have pre-out & mains inn jumper at back then try connecting rotel as per & use another power amp. If still rotel creates noise while using in preamp mode then change small signal transistors of preamp. If it operates noise free while using it as preamp then we needs to change power amp board small signal transistors. Mostly this type of noise independent of volume control is due to failing transistors in preamp board ( tone board & equaliser board. Another method, try using Tape In or Tape Monitor Input (bypassing toneboard), if it operates noise free then tone control board's transistors is failing. To isolate a single failing transistors usually a freezing spray method is used just Google that spray method. Alternatively you can also try cleaning all small-transistors legs with ear-buds dipped in isopropyl alcohol. Also do clean back of PCB with isopropyl alcohol or any PCB cleaning chemical of your choice.

Thank you for the detailed instructions. I guess I’d find a good technitian (got a few references from FM already)