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Unknown ST-70 Circuit Board

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Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Unknown ST-70 Circuit Board

Post by Sal on Wed Jul 27, 2011 9:02 pm

Does anyone know anything about this circuit board installed on a ST-70 I have in for repair. The ST-70 works but the volume is lower than usual. Voltages and Bias is OK. The tubes are 12BH7's. The feedback lines from the 16 ohm taps are not connected to the board.

If you have any documentation for this board, please let me know.

Thanks,
Sal



stewdan

Posts: 126
Join date: 2010-03-07
Age: 76
Location: Houston Texas

Re: Unknown ST-70 Circuit Board

Post by stewdan on Wed Jul 27, 2011 10:54 pm

Hi Sal --

Your board was made and marketed by Purist Audio Design in Clute Texas (near Lake Jackson), about 25 miles from Houston, Texas. There was an issue of Glass Audio (early 90's) were Roy Mottram reviewed the replacement boards that were available at the time. The PAD 70 was one of them.

I bought 2 of the boards for the ST70's that I had and installed them, I thought that they sounded very clean and nice. The version of the boards that I bought used a 12AX7 for each channel, not the 12BH7.

Oh, the Pad Board does not use the Negative Feedback from the Output Xformer.

I have documentation for my version of the board if that would help?
Schematic and Bottom Traces and hookup points.

If you have a fax, I can fax the sheets to you.

Let me know,
Stew


Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Re: Unknown ST-70 Circuit Board

Post by Sal on Wed Jul 27, 2011 11:06 pm

Hi Stew,
Thank you for your information, I knew someone out there seen this board. I don't have a fax machine as mine broke but my brother has one. I will pm you tomorrow with his fax number as I am not sure of it.

I wonder if the wrong tubes were installed. I can check you schematic to see if I have the same board.

Thanks again,
Sal

stewdan

Posts: 126
Join date: 2010-03-07
Age: 76
Location: Houston Texas

Re: Unknown ST-70 Circuit Board

Post by stewdan on Thu Jul 28, 2011 6:01 am

Hi Sal --

After I replied to your post, I compared the solder trace side in your picture with my solder trace hardcopy and it is indeed the same PCB.

The large capacitor and the resistor that are on the bottom of your PCB picture are mods against the original PCB. Probably to handle the 12BH7??

Anyway, let me know the fax number and I will send you the hardcopy.

Stew


Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Re: Unknown ST-70 Circuit Board

Post by Sal on Thu Jul 28, 2011 11:49 am

Stew,
I sent you a PM with a fax number.

Thanks,
Sal

stewdan

Posts: 126
Join date: 2010-03-07
Age: 76
Location: Houston Texas

Re: Unknown ST-70 Circuit Board

Post by stewdan on Thu Jul 28, 2011 1:54 pm

Hi Sal --

I faxed you 8 pages (cover page and what I had for the PAD70.)

Page 1 - Cover
Page 2 - Schematic
Page 3 - Figure 3 - Solder side of PCB with labels
Page 4 - Figure 2 - PAD70 hookup to ST70
Page 5 - Stock Dynaco Quad Cap Hookup Diagram for PAD70
Page 6-8 - PAD70 Instructions that I got with the PCB (has voltage readings for the 12ax7 circuit)

If anything is missing or unreadable, let me know and I will resend the item.

As I said in an earlier post, the large 100MF Capacitor that is wired to the solder trace side of the PCB is not standard fare for the PAD70.

Hope this helps!!

Stew

Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Re: Unknown ST-70 Circuit Board

Post by Sal on Thu Jul 28, 2011 6:14 pm

Thanks again Stew,
I will pass by my brothers house tomorrow and pick up the fax's you sent. I will pm you later in the evening.

Regards,
Sal

Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Re: Unknown ST-70 Circuit Board

Post by Sal on Fri Jul 29, 2011 7:21 pm

Stew,
I just picked up the faxes from my brother, thank you! They are very readable. The board installed in the ST-70 has 4 bias bots and 2 others which I don't see on the schematic. I will have to trace it out.

Thanks again,
Sal


Sal

Posts: 222
Join date: 2009-02-05
Location: Central New Jersey Dynaco-ST70.com

Re: Unknown ST-70 Circuit Board

Post by Sal on Mon Aug 01, 2011 8:37 pm

Stew,
I replaced the 12BH7 tubes with 12AX7 tubes and the volume doubled! Tht amplification factor between the 12BH7 tubes vs. 12AX7's are more than 5 to 1! 16.5 vs. 100..

Thanks again,

Sal
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