Want to replace blown amp board with generic on Fisher P294B Stereo

Thread Starter

izon

Joined Mar 17, 2013
217
HI,
I have this Fisher model P294B chassis 30 which has both audio driver / output channels blown. These two functions are on one seperate board so want to swap in whole new generic board. This is about 1970 model and seems to be no parts available any longer so replacing entire blown
board is only practical approach left.
I have some poor quality schematics of this board on the Fisher as well as the power supply used. I will attach them here. If they are just too hard
to read, I can give you the site to download the entire service manual. FISHER 30 35A 50 STEREO RECEIVER TURNTABLE SM Service Manual download, schematics, eeprom, repair info for electronics experts (elektrotanya.com)


Also I'm considering this board from Amazon Amazon.com: Clyxgs TPA3116D2 2.1 Placa de amplificador de potencia digital, 2x80W+100W (bajo) Audio Stereo AMP Módulo para sistema de audio DIY Altavoces 12-26V: Home Audio & Theater

Sorry this comes up in Spanish so will have to use the translator to go to English..thanks. Also at risk of losing all I have done here, I will not try to delete the duplications.

IMG_4898-1.PNGIMG_4898-1.PNGIMG_4898-1.PNGIMG_4897.PNG
If you get this far I have some questions regarding wattage, power supply etc.
Thanks very much ... this is my last hope.
 

Attachments

abrsvc

Joined Jun 16, 2018
90
What parts require replacement? Many of us still have "old stock" parts around for such a project. Please list what you need as they may be available. For example, the TRO2051 crosses to an ECG 106 (I have one in stock) and Amazon has one n stock too!
Update this with a list.
 

Irving

Joined Jan 30, 2016
2,058
There was a long thread on these 'generic' boards last year regarding their performance and reliability. Be aware that the quoted outputs are PMPO not RMS and the heat-sinking is insufficient for sustained high output levels.
 

Thread Starter

izon

Joined Mar 17, 2013
217
What parts require replacement? Many of us still have "old stock" parts around for such a project. Please list what you need as they may be available. For example, the TRO2051 crosses to an ECG 106 (I have one in stock) and Amazon has one n stock too!
Update this with a list.
Thanks for the offer to help but I will not use any subs or generics to replace transistors as they are not very reliable of compatible. Thanks again
 

Thread Starter

izon

Joined Mar 17, 2013
217
Thanks for the offer to help but I will not use any subs or generic since they are not that compatible or reliable. Thanks again.
 

Yaakov

Joined Jan 27, 2019
3,464
Thanks for the offer to help but I will not use any subs or generics to replace transistors as they are not very reliable of compatible. Thanks again
That's a little odd. When I was repairing stereo equipment, which was contemporary with what you have, ECG replacements were what we used. You didn't order from the OEM, you ordered from MCM Electronics, along with the belts and rollers, bulbs, and power cords.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Sorry about my not doing the reply properly......Yaakov....I know what you mean as I have serviced for years and that was true then but things are sorta backwards on some of this now where it works best with the original parts Thank you.
The stereo unit already has a preamp board so that won't an issue but thanks for the reminder. Also could someone point me to the thread discussing those amp boards...
If you can see the power supply board, it has a 42 volt output and also a 32 volt. I'm wondering if I can use a resistor to reduce the 32 volt down to about 24 volt which is what those boards usually require??????????? What value resistor and placement.
 

Yaakov

Joined Jan 27, 2019
3,464
Sorry about my not doing the reply properly......Yaakov....I know what you mean as I have serviced for years and that was true then but things are sorta backwards on some of this now where it works best with the original parts Thank you.
The stereo unit already has a preamp board so that won't an issue but thanks for the reminder. Also could someone point me to the thread discussing those amp boards...
If you can see the power supply board, it has a 42 volt output and also a 32 volt. I'm wondering if I can use a resistor to reduce the 32 volt down to about 24 volt which is what those boards usually require??????????? What value resistor and placement.
You probably don't want to do that. A 10V drop across a resistor would generate a lot of heat. You need a buck converter.
 

abrsvc

Joined Jun 16, 2018
90
Thanks for the offer to help but I will not use any subs or generic since they are not that compatible or reliable. Thanks again.
I gave you that option as an example. There is still a possibility that there are NOS parts around. The bottom line here is that you need to list what you need. TRO devices are not common and substitutes were often used to replace them. Note that in most audio circuits, the specific transistor is not critical as long as the specs are close. You want to use substitutes that are good quality and they should be available. I have been repairing audio gear for almost 40 years and have never had a problem with a substitute where there was no other option. I have replaced both channels to ensure that both behaved exactly the same way if there was any doubt.
 

Audioguru again

Joined Oct 21, 2019
3,505
A resistor reduces the current, then the reduced current reduces the voltage. Then the voltage is much too high when the amplifier plays softly and the voltage is much too low when the amplifier plays loudly.

The original amplifier produced about 22 Real Watts per channel into 8 ohm speakers with its 42VDC power supply. The new 2.1 TPA3116 amplifier has a subwoofer channel that you do not need, buy a 2.0 TPA3116 stereo amplifier board instead. Parts Express has a TPA3116 stereo (2.0) amplifier board for 10 American dollars.

The new amplifier needs a 22VDC power supply then it can produce about 25 real Watts into 8 ohm speakers. The power supply for the new stereo amplifier must be rated at 63W which is 63W/22V= 2.9A or more.
25W will sound the same loudness as the original 22W.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Thanks for the great information AudioGuru I really didn't intend to use the amp with the subwoofer out. I couldn't find the one I intend to use till now. Look at the review on this YouTube link: TPA3116 class D stereo amplifier board review - best one yet? - YouTube
I think the one being reviewed is this one on Amazon.
Amazon.com: Docooler TPA3116 TDA3116D2 - Amplificador digital de alta potencia con dos canales (12 a 24 V): Computers & Accessories

Could you please give me your comments on this one. What wattage do you think it is and maybe I would need to buy a boxed
AC adapter power supply.... what wattage or current rating?

Any additional input is much appreciated !
 

Audioguru again

Joined Oct 21, 2019
3,505
Yes, the cheap stereo amplifier is from Amazon.
Of course you need a 24V power supply at 3A if your speakers are 8 ohms each. Then the maximum undistorted power is shown in the video to be 28.9W per channel.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Do you think this AC adapter from Amazon would do the job for the amplifier board?
Amazon.com: Adaptador de CA universal 24V 3A Fuente de Alimentación 0 ~ 3000mA Compatible AC110V a DC24V Controlador de Alimentación con 8 Puntas de Enchufe Convertidor Transformador Adaptador AC-DC (DC24V-3A): Electronics

Or could you spot another at a low price that would also work for this application? Thanks again... you have been of great
assistance and I do appreciate your time and all who contributed to solve my problem !
 

Audioguru again

Joined Oct 21, 2019
3,505
The small size and very low price of the Chinese power supply indicates very poor quality. It might not produce any where near 3A and probably will produce a whining sound in the audio. Why not buy a good one made by a reputable manufacturer in a country that you can trust?
 

Audioguru again

Joined Oct 21, 2019
3,505
I do not know where in the world you are.
I do not buy fake or defective junk from ebay, AliExpress, Banggood or Amazon.
I live in Canada and buy Name-Brand electronic items from local stores or online from Digikey.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Sorry I failed to mention I'm in USA...... Minnesota, on the Canadian border... forgot about DigiKey where I have an account.
Thank you ! I have no "local" stores in my small rural town that handles electronics.
 

Thread Starter

izon

Joined Mar 17, 2013
217
I'm rethinking this whole situation. If the original audio output board could run from the existing power supply where it used only the 42 volt tap from the supply, why can't a replacement amp board (like the one I've suggested) with the proper voltage from that supply also be adequate?

So what I'm saying is why can't a DC to DC converter get that 42 volt supply down to either a 12 or 24 volt output to operate the new board? That 42 volts is/was only used for the output amp so disconnecting should free up some
load and allow for a voltage drop method to work...........If the original supply could produce whatever power it was supposed to why not the small new amplifier being (when adapted/modified) connected?

Hope you follow what I'm getting at. I don't want of try and jam an extra power supply along with a new amp into that small space beneath the turntable?
 
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