AC 128/176

dr.partha

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Hello frieds!
I'm back here after a long 5years, needing your help once again.
Hope your are all doing well.

Now coming to the point. As you probably knew that I have a couple of HMV fiesta record players. Recently one of them has died due to GE transistor failure. Both the output AC 128/176 are showing collector-emitter short! And now I don't have any more spare.
I searched a lot but this time with no luck ( though I found a pair 5 year back). I found a few online with high priced tag. In some forums, they advised to replace these by 2n3904/3906 Si pair with bias modified. I worked a lot,having less knowledge in biasing, at last succeded to bring the audio out of it. But the sound is not up to the mark like original GE pair.And also the volume is low. As I'm not a tech expert so couldn't draw a circuit diagram, so I couldn't upload it.

Any help from your end is appreciated.

Thanks in advance.
 

reubensm

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Hello Dr.Partha, firstly did you actually use the 2N3904/3906 pair as a replacement? If yes, its not just with biasing but a few other circuit elements will also need to be redesigned. Yes, AC127/128, AC187/188 etc metal body transistors are not available these days. While they were famously used in small power amplifiers of portable audio and radio equipment in the 1970s and very early 1980s, their demanded reduced with he advent of small amplification ICs like the BEL700 (AM radio with inbuilt amplifier) and the BEL CA810 or Japanese TBA810 ICs (small power amplifier which could deliver 1 to 7 watts depending on the PSU and heatsink). BEL used to manufacture these transistors in India and has long stopped production. Knowing the history, here are your options:

1) Try for these in very old electronics spares shops (especially if the owner/proprietor is an elderly gentlemen). They may have a few of these left and the older generation shop owners would know these very well as they were commonly bought by students like me in those times
2) Locate some old radio repair shops, they may have a few discarded portable transistor radios or cassette tape recorders lying around from which these components can be salvaged
3) Drop a note in the 'wanted' section of our forum, some of our older forum members into DIY may have electronics junk boxes with a few of these

Of course, the current circumstances is a bit of a problem with couriers not functioning and most shops closed. Regards.

Edit: instead of AC176, you can also use AC127
 

dr.partha

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Thanks a lot. You have helped me again like before.
Really,due to this COVID -19 pandemic, I'm unable to rome about freely.
I wanted to know is there any direct Si replacement so that I won't have to change the circuit? As I have already mentioned, 2n3904/3906 worked but with very thin,high treble sound (? missing RIAA).

By the way, though I'm asking a separate topic in this thread, but I hope I'll get an answer as always .
I have a Philips hi q international which has also died (the amplifier, I mean) due to PT4/PT6 short circuit. Now I have removed the amp section and connected to a Sonodyne SC 3070 which bears a ceramic phono input. But there is terrible humming . I couldn't gound it properly. Please give me an insight how to ground this TT to the amp. I connected a thick copper wire between the ground pin of sonodyne and the ground point of the philips tt.
 

reubensm

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Thanks a lot. You have helped me again like before.
Really,due to this COVID -19 pandemic, I'm unable to rome about freely.
I wanted to know is there any direct Si replacement so that I won't have to change the circuit? As I have already mentioned, 2n3904/3906 worked but with very thin,high treble sound (? missing RIAA).

By the way, though I'm asking a separate topic in this thread, but I hope I'll get an answer as always .
I have a Philips hi q international which has also died (the amplifier, I mean) due to PT4/PT6 short circuit. Now I have removed the amp section and connected to a Sonodyne SC 3070 which bears a ceramic phono input. But there is terrible humming . I couldn't gound it properly. Please give me an insight how to ground this TT to the amp. I connected a thick copper wire between the ground pin of sonodyne and the ground point of the philips tt.

Kindly share a picture of the fiesta's circuit board and we'll take it from there. Is the 3904/3906 a metal cap pair? Sometime ago a friend of mine while fixing a small radio went to purchase AC187/188 and came back with BEL 187/188 in TO-92 plastic packaging. They did not work like the original metal cap ones and the sound was very low and screechy.

Regarding the Philips record player, PT4 and PT6 may be hard to find but again, that amp will sound really good if used with the original hi-q international high sensitivity speakers. Terribly humming means there is a break in the ground circuit between the cartridge and the amplifier. Check the circuit for continuity. Also for connecting the cartridge to your amplifier, always use shielded audio cable. Also the amplifier must be grounded with the turntable chassis.

I recall dad telling me back in the day that Philips used PT4 and PT6 output transistors in its Indian amplifiers while the equivalent European Philips amplifiers used AD161 and AD162 output transistors instead. They could produce 5 watts per channel output. These generally come up on ebay once in a while. Same with AC127/128 pairs or AC187/188 pairs. With a small investment, you can get these on ebay.

Here is a typical AD161/162 project
 
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dr.partha

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Thank you very much. I'm grateful to you that you have spent your time for my questions.

Yes , the transistors are there and that's really great! Hopefully my hi q will be alive once again.

I've grounded the wire but didn't do it by shielded cable. That was the error. I'll bring it tomorrow. The continuity is ok,I have just checked it by multimeter. There is no breach of connection.

No, the transistors are like normal plastic cap transistors,not at all like BEL AC series.And of course, I'll post a few snaps of fiesta circuit.

One last favour, I've found a few circuit diagrams on internet (and also in a thread of this forum) to make a RIAA ceramic phono stage. I'm on my way to complete one of these simple circuit using BC549 npn. Please tell me if I connect my ceramic pickup to this phono and then connect the output to the input (line ) of any amp, would it be worthwhile? I mean to say that would it sound sweet like original fiesta amp?

Thanks once again.
 

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reubensm

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Thank you very much. I'm grateful to you that you have spent your time for my questions.

Yes , the transistors are there and that's really great! Hopefully my hi q will be alive once again.

I've grounded the wire but didn't do it by shielded cable. That was the error. I'll bring it tomorrow. The continuity is ok,I have just checked it by multimeter. There is no breach of connection.

No, the transistors are like normal plastic cap transistors,not at all like BEL AC series.And of course, I'll post a few snaps of fiesta circuit.

One last favour, I've found a few circuit diagrams on internet (and also in a thread of this forum) to make a RIAA ceramic phono stage. I'm on my way to complete one of these simple circuit using BC549 npn. Please tell me if I connect my ceramic pickup to this phono and then connect the output to the input (line ) of any amp, would it be worthwhile? I mean to say that would it sound sweet like original fiesta amp?

Thanks once again.

Always welcome.

The plastic types wont work when compared to the metal body AC series germanium transistors. You will have to use original replacements.

Regarding the turntable, every stereo cartridge has 4 pins, 2 are line pins for each of the channels and the other 2 are ground pins. Ideally there should be shielding between the cartridge pins and your amplifier input. If you are using a metal tubed tonearm, then ordinary wiring should be fine until the base of the tonearm. From there you'd need to run shielded cables to your amplifier input. The body of the tonearm must be grounded with the turntable chassis and a ground wire should run from the chassis to your amplifier's ground terminal.

Regarding the ceramic phonostage, i have built this exact same one for our forum member RPM. If your amplifier already has a ceramic input, this phonostage is not required. However if it doesn't, this can work very well. FM RPM uses it with his digital NAD D3020 amplifier and his feedback suggests that it sounds really good. I have built some others as well which received positive reviews.
 

dr.partha

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Thanks again..
Yes,the hum completely goes away after proper earthing the Sonodyne with Phillips hi q chasis.

Now, I'm posting the images of fiesta circuit board. Please see if any modifications of the resistance value can restore it. I have a strong emotional value attached with this small player. It was bought by my late grandfather. I wish to keep it as it was.

I have ordered those GE transistors as well. Don't know when will it arrive and in what condition! Till then, if any modifications work, I would love to do it.

Regards,
 

dr.partha

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Here is the pics..
 

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reubensm

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Thanks Dr. Partha, I can confirm for sure that those Plastic transistors won’t work. You’ll have to use the original metal body ac128/ac176 transistors. Alternately you can use ac128/ac127 or ac132/ac127 instead.

When repairing vintage equipment, personally we should restore rather than repair. These vintage pieces if owned by family can be extremely valuable and hence spending a few dollars on original spare parts is completely justified.Just do a complete recap as well and it will play just as how your grandfather would have listened to it. That experience is priceless.

I have purchased Germanium transistors from similar sellers and they’ve all arrived in perfect condition. They are basically new old stock carefully stored.
 
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reubensm

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On another note, ensure that the power transistors when you install them, are bolted to a metal plate or the chassis mounting of the fiesta. Heat sink is a must.
 

dr.partha

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Thanks. You have rightly valued my sentiments. I will wait for that pair to arrive. After successful restoration, I'll let you know.
Regards,
 

dr.partha

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Always welcome.

The plastic types wont work when compared to the metal body AC series germanium transistors. You will have to use original replacements.

Regarding the turntable, every stereo cartridge has 4 pins, 2 are line pins for each of the channels and the other 2 are ground pins. Ideally there should be shielding between the cartridge pins and your amplifier input. If you are using a metal tubed tonearm, then ordinary wiring should be fine until the base of the tonearm. From there you'd need to run shielded cables to your amplifier input. The body of the tonearm must be grounded with the turntable chassis and a ground wire should run from the chassis to your amplifier's ground terminal.

Regarding the ceramic phonostage, i have built this exact same one for our forum member RPM. If your amplifier already has a ceramic input, this phonostage is not required. However if it doesn't, this can work very well. FM RPM uses it with his digital NAD D3020 amplifier and his feedback suggests that it sounds really good. I have built some others as well which received positive reviews.
Yes, you are right. Today this small project has been completed. I've tested the circuit in different amplifiers I have. It really produces a RIAA equalization and hence a sweet sound with good bass. I astonished to see even a ceramic pickup can produce such a sweetness!
Thanks for your reference thread in thus forum. It has helped me a lot. I will stick to this phono until my replacement of pt4 pt6 arrives.
 

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reubensm

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Yes indeed Dr.Partha, many enthusiasts have written pretty nasty things about ceramic carts over the years. My father always used to say, ceramic and crystal pickups can be made to sound like expensive cartridges, if the right phonostage with equalization is provided. People write unfavourably about ceramic cartridges as they've often heard them on small record players, playing through small inbuilt speakers, or they've attempted to use ceramic carts with regular amplifier inputs obviously without equalization. You'll also hear stories about how ceramic cartridges with sapphire styli can severely damage records, again this comes from the use of ceramic carts on low end record players with heavy tonearms (tracking at well over 8 grams or even the entire weight of the tonearm in some cases). My father extensively used a ceramic cartridge (EEI CS2000) tracking at 5 grams on a Garrard SP25 mk1 deck for decades and none of his records were damaged. They play superbly well on my rigs with MM carts tracking at 1.5 grams.
 
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reubensm

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For your reference, one of the greatest sounding vintage solid state low power amplifiers in its era based on AD161/162 output pair. This circuit was commonly copied or leveraged. The ceramic phonostage is outstanding in my opinion. We had a DIY version of this amp which was used by my father. Its now with my brother's father-in-law. The schematic of one channel is shown below:
22rh580.png
 

dr.partha

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Thanks..Though the circuit is bit complex for DIY(at least for me).but I'll definitely try.
By the way, can you tell me where can I get a new GP 215 Philips cartridge?
Another question , as it's a ceramic cart, can I replace it with a MM cart? I have to change the headshell as well.But I'm not sure whether a new headshell will fit in the 'S' shaped tone arm or not. I'm bit confused to buy a headshell. Can you provide any insight regarding it?
Regards,
 

reubensm

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Thanks..Though the circuit is bit complex for DIY(at least for me).but I'll definitely try.
By the way, can you tell me where can I get a new GP 215 Philips cartridge?
Another question , as it's a ceramic cart, can I replace it with a MM cart? I have to change the headshell as well.But I'm not sure whether a new headshell will fit in the 'S' shaped tone arm or not. I'm bit confused to buy a headshell. Can you provide any insight regarding it?
Regards,

Now that's the hard part. Philips used non-standard headshells and cartridges with mounts that only suited these headshells. Also the circuit design was better suited to these cartridges and hence an OEM replacement is the way to go. However, original spares, though available are expensive and have to be sourced from abroad. Here is an example of the cart which you mentioned. Investment-wise, it will cost over Rs.2500 to import one into India through this path. Checking with old radio shops may help. One can always innovate and use other carts with the Philips headshell but in my opinion, always go with OEM spares, even if it costs a little more. The end results will be closest to the original.
 

dr.partha

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Luckily, got one spare in Kolkata in NOS . Really this city rarely deprived me regarding turntables and spares. I'm lucky!
Yes ,you are right. Phillips is always "far from normalcy ". When I wanted to upgrade my 242 cartridge, it had given me much trouble. Finaly, an elderly person (I used to call him 'Ali Chacha' and he was in this buisness for last 30+ years) who loved me a lot ( unfortunately he passed away last year) helped me with Shure N78S stylus and one MM cartridge (company name forgotten) . Thus finaly that player was upgraded. Here I'm giving a few pics..
 

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reubensm

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Luckily, got one spare in Kolkata in NOS . Really this city rarely deprived me regarding turntables and spares. I'm lucky!
Yes ,you are right. Phillips is always "far from normalcy ". When I wanted to upgrade my 242 cartridge, it had given me much trouble. Finaly, an elderly person (I used to call him 'Ali Chacha' and he was in this buisness for last 30+ years) who loved me a lot ( unfortunately he passed away last year) helped me with Shure N78S stylus and one MM cartridge (company name forgotten) . Thus finaly that player was upgraded. Here I'm giving a few pics..

The shure N78s as the number suggests, is a 2.5mil stylus for 78s. I would not use it for microgroove records :)
Getting NoS is really great. They are hard to find.
 
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dr.partha

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The shure N78s as the number suggests, is a 2.5mil stylus for 78s. I would not use it for microgroove records :)
Getting NoS is really great. There are hard to find.


Oh really!! I didn't know it!
Though this is player is hardly used, yet I'll stop it's use. Then what stylus should I use ? Any suggestions please?
 
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