Dynaco re & re

taj

diyAudio Member
2005-02-23 8:49 pm
Dynaco re & re

I picked up a Dynaco Stereo 80 (~40 wpc @ 8) recently (in beautiful shape!) and I need to replace the power cord, which is all hacked up. It's a 2 cond. cable and I was thinking of putting on a 3 cond. grounded cable.

I see that the neutral line from the wall power is connected to the chassis in a star near the power supply. Should I attach the ground prong from the wall outlet to the chassis as well, or will I toast something and/or kill myself (not necessarily in that order)?

I'm electronically challenged, but I'm learning. This unit is a good learning amp.

Also, since the unit is about 35 years old (1969?) which parts will have deteriorated by now, or might be technically improved by now, such that it's worthwhile to replace them?

link to schematics/specs

Any advice is muchly appreciated.

TAJ
 

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Looking at the schematic of the PSU wiring, the line cord black wire (hot) goes to the fuse. The white wire (neutral) goes to the switch. I don't see the neutral connected to ground anywhere. Suggest that you verify you have traced the wires correctly.

In a transformer powered design with a two wire power cord it would be unusual to connect neutral to ground. If you inserted the power cord wall plug reversed, the chassies would be hot and if you touched the chassies and a ground at the same time you would be shocked, literally and figurativly.

A three wire power cord with a green ground wire should have the green wire connected to the chassis. Make the connection as short as possible.
Regards,
Ray
 

taj

diyAudio Member
2005-02-23 8:49 pm
I stand corrected.

You're absolutely right Ray. The ground star is on the secondary side of the supply. Not sure what I was looking at the first time.

So, just connecting the green wire to the chassis is fine then.

Taj
 
The Stereo 80 had the same amplifier circuit as the Dynaco SCA-80Q and is nearly idenetical to the amplifier circuit in the Stereo 120. I have both of those. The components most likely to have age related problems are the elctrolytic capacitors. I have had to change several of those. The input capacitor C1 has failed in all units i have.

Greg
 

taj

diyAudio Member
2005-02-23 8:49 pm
cap replacements

Thanks for responding, Greg.

Does it matter much (sonically) which brand of electrolytics I use to replace existing ones? I don't plan to spend $200 a pop for Blackgates for basically a garage sale amp. But if it really makes a difference, then I will certainly spend some extra bucks on better parts -- assuming someone can tell me what those better [and readily available] brands might be.

Also, I don't know which circuits, or parts of circuits benefit from better quality caps. Filter caps? Bypass caps?

Do the output transistors deteriorate? or would metal film resistors be of any benefit? Are there better drop-in replacement transistors than the ones in the unit?

Please excuse all the questions, I'm new at this.

..Todd
 

taj

diyAudio Member
2005-02-23 8:49 pm
Thanks

As I mentioned, I'm just doing this to learn. It's a great amp for learning. Even if I rebuilt it completely, it wouldn't cost too much. There's not many parts in it. But the experience and knowledge gained would be well worth the effort.

I have plenty of good amps here for listening, just haven't built, tweaked or destroyed any yet. :rolleyes: Gotta start somewhere!

..TAJ
 
I wouldn't waste my money on caps like the Balck Gates on this amp. It is a fairly early transistor design that even with the best parts would not be a great amplifier. But it is a good learning tool. The topology of the amp gives the input/VAS stage fairly poor power supply rejection, so enalarging C11 could provide some benfit. Even better would be to have a C11 for each channel and double the value of R21 to each to obtain the same voltage drop. This would isolate the two input/VAS stages from each other.

I also find that the carbon composition resistors used throughout drift pretty badly with age. This can upset the DC bias point and cause channel gain imbalances. Check the actual values R4 and R9 if the two channels do not have the same gain. I have seen this on mine.

Greg
 

taj

diyAudio Member
2005-02-23 8:49 pm
enlargement?

Greg, Could you explain this? I'm just learning electronics, and I don't know what this means (yet).

>>>The topology of the amp gives the input/VAS stage fairly poor power supply rejection, so enalarging C11 could provide some benfit.


And 'enlarging' means a higher uF value component or high voltage rating?


Thanks,
TAJ
 

taj

diyAudio Member
2005-02-23 8:49 pm
Greg B. said:
It is a fairly early transistor design that even with the best parts would not be a great amplifier. But it is a good learning tool. The topology of the amp gives the input/VAS stage fairly poor power supply rejection, so enalarging C11 could provide some benfit. Even better would be to have a C11 for each channel and double the value of R21 to each to obtain the same voltage drop. This would isolate the two input/VAS stages from each other.

I also find that the carbon composition resistors used throughout drift pretty badly with age. This can upset the DC bias point and cause channel gain imbalances. Check the actual values R4 and R9 if the two channels do not have the same gain. I have seen this on mine.
Greg

A newbie question for the gurus here...

I finally fixed some major power supply issues with my Dynaco Stereo 80 so it runs now. One channel works fine, the other is very ill. Very low volume with lots of hums and noise.

I measured the voltage at all the test points (see link below) and the bad channel is reading about double the voltage of the good channel on most rail points. Test points #3 & 4 (on PCB-19) should both be the same (36v), but #3 is about 70 volts. Points #5 & 6 should both be 37.5 volts, but #5 is about 70 volts. I can't figure out why though. The power supply is offering up the same supply connections through resistors 16 and 17 to each channel. Could C6 or D2/D3 be causing this problem?

Schematics: http://home.indy.net/~gregdunn/dynaco/components/ST80/index.html

..TAJ