DIY Cosmic 5000 REPAIRof the VU meters, one speaker, and sound in the 'left' of A&B channels

[veekm]

Hi, so I've posted pics on Reddit but got no worthwhile reply:

https://www.reddit.com/r/diyaudio/comments/jftvrw

I have access to the Amp and need to repair the following (assuming it needs such a thing):
1. 1-Speaker - looks to be some issue internal to the case: what's the back of the speaker made off because I'll have to drill and cut it away from the hardwood case? (what wood was used)
2. VU meters light up but the needles don't jump about.
3. Sound on both channels is emitted only in 'A+B' mode by connecting one speaker to the 'Right' port of each channel. I can't run two speakers on the same Channel (A or B). The 'Left' port seems to be broken.

I have 0 experience fixing this/audio stuff BUT I do have some limited experience with multimeters, oscilloscopes and transistors. I still need a circuit diagram to poke around.

The amp is working somewhat (I can hook up my Philips speaker and get sound) and I am familiar with Bangalore/SP Road for parts.

Can I get any help from this forum in this regard (fixing the amp. circuit diagram) What's the amp worth in as-is condition? What if it was repaired - how much then? [I'll need to invest in tools: drill and some kind of jigsaw to cut along the glue line?] Any reading list for amps. Anyone from Bangalore? (just to be clear - this is a world class CRAPPY amp - the components are similar to what I see in old Indian V-Guard stabilizers and such but it's Indian so )

I don't know what Cosmic Speakers I have - the sticker is missing (tiny narrow rectangle). The upper sticker with COSMIC is there.

I'll try to post clear pics - assuming I get a reply

 

[veekm]

Just to be clear - I'm not insulting the amp/COSMIC - it works after all this time - but the tolerances on aged components are ? and it looks shabby.. and I'm not sure about the steel used in that gigantic transformer.. I've seen Philips transformers with shiny steel that looked new so.. and the PCB is not FR4.. what the heck is that material? (ah cotton impregnated phenolic resin)

https://www.researchgate.net/post/Which_type_of_polymer_is_used_in_base_boards_of_Printed_Circuit_Boards_PCBs

the substrate most commonly used in printed circuit boards is a glass fiber reinforced (fiberglass) epoxy resin with a copper foil bonded on to one or both sides. PCBs made from paper reinforced phenolic resin with a bonded copper foil are less expensive and are often used in household electrical devices.

 

[reubensm]

Don't mean to discourage you but you will have to be familiar with amplifier repairs to fix it, especially since you wont have schematics to refer to. It would be great to read up as much as you can online and then sit down to work on the amp. Anyways, here are some comments which can get you thinking:

1. 1-Speaker - looks to be some issue internal to the case: what's the back of the speaker made off because I'll have to drill and cut it away from the hardwood case? (what wood was used)
Reply -> you mentioned amp but this reference seems to be about the speaker. If you use the method you plan to use, the entire speaker cabinet will be ruined. Looks like Cosmic has used a sealed box with the speaker driver bolted in from the outside. You may want to carefully remove the speaker grill, unscrew and take out the driver and then investigate.

2. VU meters light up but the needles don't jump about.
Reply -> VU meters are usually powered by a small circuit or directly from the speaker outputs. You will have to investigate whether the VU meters are receiving volts from the amp circuitry. If the meters are receiving volts, then the meters themselves may be faulty, if they are not receiving volts, then you will have to check the circuits that feed them

3. Sound on both channels is emitted only in 'A+B' mode by connecting one speaker to the 'Right' port of each channel. I can't run two speakers on the same Channel (A or B). The 'Left' port seems to be broken.
Reply -> the speaker selector and the binding posts need to be checked

 

[veekm]

1. ah but how exactly do I remove the spkr grill? All I see is a cloth+frame.
2. Any articles/books [Elektor] on VU meter wiring, working, polarity. I found this: https://www.giangrandi.org/electronics/vu-meter/vu-meter.shtml
3. what's a good starting point for audio electronics? (not Horowitz and Hill)

 

[reubensm]

Let me tell you how i started - when i was a little, i used to sit and watch my father repairing or building stuff. I used to sit with him for hours listening to his tests and watch his calibration sessions. I used to ask him many questions (at times he used to get angry and chase me away, when some calibration steps were not falling in place, only to cool down and call me back when order was restored) and he used to graciously and meticulously explain everything to me. When I grew a little bigger, i think around the time when I was in third standard he would take me along to the British council library and let me roam around in the Radio and Hi-Fi section. He would let me take a book of my choice (often a circuit book) which I would sit and stare at for hours, and ask him questions about designs, etc. That's from where my knowledge came. Later on, I decided not to make electronics a career (he was very angry about this as he thought he was grooming the next big electronics engineer) but all this knowledge proved useful from a hobby perspective. Another important part of the hobby (and is often ignored) is that my father and I used to sit on the weekends and listen to record after record. I have come across a lot of folk into DIY who are more interested in sound and aren't really interested in music. What you hear is the outcome of your creation and that too should be enjoyed

Not to worry, even though these old school methods (and we are talking 1971 to around 1979 here) do not exist any more, you can create this virtual father-son scenario by using YouTube. There are so many informative videos out there that talk about almost everything around electronics.

Before actually looking up equipment, one must develop some basic skills - understanding of basic electronics, the ability to understand schematics and wiring, component identification (and with it, the ability to decipher component values), the knowledge of how to test (will involve an investment in some basic test equipment), how to solder and de-solder, etc. Also I often advise little kids to start building small LED based projects without transistors and then progress to transistor based LED projects and move up the chain with buzzer projects, etc. One has to progress through the ranks of learning, like a guitarist or cricketer. One cannot become a Joe Satriani after 10 minutes of guitar lessons and neither can one become a test cricketer with some basic training and net sessions. As knowledge and experience builds, you will eventually become very good at your trade.

With regard to amplifiers (this is where my expertise is). it is important to understand amplifier design philosophy first. What are the elements that make up typical amplifier (assuming we are talking about solid state here) - you will come across terms like preamplifier, power amplifier, power supply, integrated, discrete and if you drill down further, input selector, gain stages, tone stages, equalization, level and balance control stages, pre-driver stages, driver stages, output stages, protection stages, level indication stages, bias and a lot more.

Note: A person good at Amplifier repairs (or as a matter of fact, any kind of repair) is somewhat similar to a doctor. The capability of a good doctor lies in his/her ability to diagnose correctly and thereafter tie up the diagnosis with an effective prescription. We often go to more experienced doctors as their success rates for both diagnosis and prescription are much higher. Amplifier repair success can be achieved only with correct diagnosis and the associated remedies. I have noticed most modern techs are PCB replacement specialists (they will tell you that the board is gone and needs to be changed). This type of approach is of no use (these techs are only skilled with disconnecting and connecting wires). You should aim to develop component-level diagnostic and repair skills. Also I have come across repairers who shout "Eureka" on finding a defective component, replace the same, bolt up the box and declare the amplifier as 'fixed'. This is a wrong approach, though it may work in some cases, if the repairer is lucky. More often than not, failure may be due to a chain of events and the root cause/s would need to be identified and fixed as well. Always look to restore, never to repair. Then you should be able to repair any amplifier and be good at it

 

[veekm]

I've built kits, soldered, used an oscilloscope, megger, signal generator etc (so I'm familiar with some basic lab equipment) [guhh.. I did mention Horowitz and Hill]

1. I'm looking for an introduction to audio electronics. Reddit suggests Douglas Self and Cordell. Thought you guys might have a list.
2. How the heck do I get the cover/cloth-grill off the speakers? I tried wiggling it off and it moves a bit but.. not by much