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Post by paradisecs on Mar 4, 2019 21:29:20 GMT
What would you use as a replacement for a bad bias thermistor? I found a few options but I can't tell from the schematics if they are NTC or PTC. Or do you have a part number for the replacement? Also, can I replace the stabistor with 2 diodes in series?
Stephen
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Post by Ron Rich on Mar 5, 2019 0:03:30 GMT
Hi Stephen,
Learn me something new !-- What does NTC-PTC stand for-- I still have a few of the original Seeburg supplied thirmistors----
Yes--two signal diodes in series are just fine. Ron Rich
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Post by robnyc on Mar 5, 2019 1:30:38 GMT
Ron, it means Negative (or positive) Temperature Coefficient. Here I agree that anything within the voltage rating will do. They aren't thermistors, just diodes.
Rob
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Post by Ron Rich on Mar 5, 2019 3:57:26 GMT
Rob,
Thanks--these ARE "thirmistors"--I don't know, if Pos., or neg., but do know heat makes them rise in resistance. They also have "stabistors" (Seeburgease ??), in that circuit--these are the "dual diodes"-- Ron Rich
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Post by robnyc on Mar 5, 2019 5:57:42 GMT
Ron, Stephen, yep there is a thermistor in there (should have looked at the schematic). It will be a NTC since you want the resistance to go down as it is heated.
It is not intimately coupled and to be Gepetto, if I had trouble with one, i'd just remove it and set bias w/out it. I use around 1.5 ma bias. IMO the diodes are enough when you start with such a low setting.
RobNYC
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Post by paradisecs on Mar 5, 2019 15:52:35 GMT
Thanks Rob, Ron. I thought I had read somewhere before that the diodes could replace the stabistor, but didn't want to blow up the circuit if I remembered wrong. As for the thermistor I would have thought you'd want the resistance to increase as the temperature went up. I'm glad I waited to order the replacements. Would 2 1n4148s be sufficient to use or should I use something a little bigger?
Stephen
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Post by Ron Rich on Mar 5, 2019 17:59:55 GMT
Gentz,
I just checked my stock of Seeburg thirmisters-- marked right on the bag, with the part number on it is "NTC"-- "50 @ 25C" -- will wonders never cease--had eye surgery yesterday, and can now see (with,or with out glasses !!)!!! lol Ron Rich
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Post by paradisecs on Mar 6, 2019 18:51:18 GMT
When setting the bias I can't get the voltage to go less than 1.6 across Q15-16 base or Q25-26 base, and that is with the pots turned all the way to limit. If I adjust the pots at all the other direction R76 and R98 start smoking and the heat sinks could fry and egg.
I don't think the lowest value of 1.6V is normal, at least on the couple of other SHPs I've redone. What in that circuit would push the voltage starting point so high?
Stephen
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Post by Ron Rich on Mar 6, 2019 23:20:00 GMT
Stephen,
What transistors are you using for the pre-drivers ? What transistors are you using for the drivers ? Ron Rich
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Post by paradisecs on Mar 7, 2019 2:21:35 GMT
On the driver board I put new MPSA06 and MPSA56, TIP31C & 32C, and for the pre-amp MPSA06 (I didn't put these on). And the big ones I used 2N3773G. The amp was dead when I got it and the four 100 ohm flameproofs were all cracked and the board burnt. I've replaced C65/66 with 3300 35V and made non polarized for C68/69 out of the same 3300s. New C67. New diodes on the chasis. All fuses put in fuse holders. New bias pots.
On the driver board nearly every component was replaced and I've double check resistor values to make sure I didn't use the wrong ones. I also replaced the resistors and stabistors under the heatsink.
This particular amp I've been working on for months. I did a SHP1 today using the same components and it worked fine so now I'm just confused.
Stephen
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Post by Ron Rich on Mar 7, 2019 4:04:24 GMT
Stephen,
What brand are the 3773's ? Ron Rich
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Post by paradisecs on Mar 7, 2019 4:13:52 GMT
ON Semiconductor. Any particular reason the brand would make a difference?
I think I may have just figured it out. I went back over my inventory of parts and I think I have a batch of MPSA06 and 56 that are the wrong case. I either didn't order the right ones or was sent the wrong ones and I never checked. I think I've been fighting every transistor C & E swapped. The SHP1 I did today used a different shipment. I'll swap them all and see what happens.
Stephen
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Post by Ron Rich on Mar 7, 2019 13:47:50 GMT
Stephen,
When SHP's were in testing--pre release,, we started to see failures that we could not understand-so-- we took the finals apart-- we were shocked at the differences in size of the chip and construction methods used, and CERTIFIED, by the manufacture, to meet our specs. At that time ONLY Motorola and GE-RCA, did so. We later found another source--a brand called Power Physics. I don't have any idea of what is inside the "ON" brand today. If your A06 and/or A56 transistors are "wrong", of course, that'll make a difference ! Ron Rich
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