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Post by mauro on Dec 13, 2018 23:16:51 GMT
I have repaired and rebuilt this amplifier with the help of Tony Millers book and searched the web on how to bench test it, as the jukebox is about 2 hours away, but really couldn't find all the info needed. I have a volume control and some bookshelf speakers connected and used an ipod for a signal. When I slowly powered up the variac it started cracking or static sound at about 80 volts so I backed it off. I did hear some very garbled music. Am I missing some jumpers? Or perhaps the ipod is not right to use here? I know many of you have done this before so I'm hoping, with some info, I can get it tested by the weekend. Thanks, Mauro
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Post by robnyc on Dec 14, 2018 0:45:33 GMT
Mauro, the garbled sound is due to the goofy way Seeburg designed the input stage. In order to get sound there --must-- be approx 560 ohms across each input. This is to complete the voltage divider for the first stage and bring it out of cutoff.
As a source, an ipod type device isn't ideal because it will be a bit too bassy due to the equalization in the preamp. It'll work well enough for test though. A t-t with magnetic cart can be use with no external resistors.
Set the volume control to approx half way, power the amp through a 40-60 watt bulb to limit current, bring up to full line with no drive and if no noises are occurring. If all is well bring up the ipod and see how it sounds.
You can watch the AGC voltage from the orange "mech" socket. It is the hole nearest to the heatsink. I can't exact voltage figures as i've modded this section on all the TSA-SHP amps. But you can check the attenuation by shorting that signal to chassis ground and noting the volume increase. A volt or two is around normal and approx 5 volts for squelch when the relay is pulled-in.
No extra jumpers are needed for test.
RobNYC
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Post by mauro on Dec 14, 2018 2:03:18 GMT
Hey Rob, Thanks for that explanation! Did as you said and when I brought the amp up close to line voltage the left channel crackled so I shut it down and replaced Q24 (predriver) and now all is good! All drivers and outputs already replaced. I set up the bias, as taught to me by Ron, and now it is playing on my bench. 2 days ahead of schedule!! You're awesome! Thanks again, Mauro
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Post by paradisecs on Dec 20, 2018 16:05:37 GMT
I'm trying to get my bench setup to test audio as well. I see that an IPOD isn't great but other than jukeboxes I haven't owned a turn table since CDs came out. Would the output from a computer sound card work if using line out (usually blue plug) and not the amp out (green plug)?
Stephen
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Post by robnyc on Dec 20, 2018 17:06:14 GMT
Stephen, various sources can be used for test but, Seeburg transistor amps -must- have a resistor placed across the input in order to bring the first transistor out of cutoff and thus have sound. I don't recall if i've forwarded this link already: www.flippers.com/pdfs/HudsonsBayWholesale_SeeburgSHP&TSA_ServiceNotes.pdf It is a good primer to transistor operation and refers to Seeburg amps in particular. Page 15 shows a simple two-resistor setup the both turns on the first transistor and reduces a line-level signal. The 560 ohm doesn't have to be exact as any from around 400 to 900 ohms will work. You can bypass the preamp-AGC board and go in at the volume control: www.flickr.com/photos/90641375@N06/25828588007/ This will only test the drivers and finals. No matching resistors are needed in this case. It is best to test from the phono input, but this will help determine which side of the volume control the problem is on. For speakers, just about any will do as long as they are no lower than 4 ohms and can handle some power. It is best to use speakers whose sound you are familiar with. Start all tests with volume control at minimum. Rob
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Post by Ron Rich on Dec 20, 2018 19:51:01 GMT
Stephen, With a little practice, you can use a cheap AM-FM radio. Set the radio volume, and the SHP volume, on minimum and use the ear phone jack for the input. bring both volume controls slowly--amp about half way --radio way lower, at first-- Ron Rich
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