Dynaco SCA-80Q I thought my troubles were over

[Ron Joiner]

SCA-80Q Blown Resistor (5)

• Ron Joiner
• Sep 13, 2016

Blown resistor

SCA-80Q Blown Resistor (4)

• Ron Joiner
• Sep 13, 2016

Blown reistor

This past spring I resurrected this old integrated amp from the basement. It immediately developed a loud hum. I addressed this by replacing many of the large caps specifically the 2 C7s (speaker caps), the C9 and the C11. I also had one cap replaced on PC-18 on the side opposite where I discovered my new problem. Through the summer months it ran very smoothly often 10-12 hours per day but for one channel cutting out from time to time which I attributed to an oxidized pot.

On the last stay of our summer vacation my wife alerted me to the fact that there was a strange electrical smell accompanied with a puff of smoke from the area of the amp. The amp was still powered on but instead of wonderful music there was a loud higher pitched hum. I switched it off and no doubt the smell was coming from the amp.

I opened up the case and found what I think is a blown resistor on one of the PC-18 boards + a very loose green wire with a partially melted casing and a scorched thingy under the resistor. I know a little about the inner workings but that's as far as it goes.

The question is this. Is it an easy fix? What else could go wrong? How much more money do I continue to put into this thing? Should I contact Dan at www.updatemydynaco.com and put in a new power amp assembly?

It does have sentimental value as I am the original builder.

Ron

 

[chicks]

Yeah, it's a very old and inefficient design, from an era before transistor designs were perfected. I gutted mine and replaced with a Renardson MJR-7 board, bypassed the crappy tone controls, and left out the phono pre. It's now a nice, cool running, great looking small amp.

The original transformer isn't center tapped on the secondary, so only a single supply rail is available. The updatemydynaco board is one of only a handful that will work on one rail at the voltage supplied, so it's an excellent choice.

 

[Ron Joiner]

http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=4223. Saw your post at this site. Very interesting and yes I will likely pursue Dan's design.

 

[slimecity]

If you are keen to pursue fixing this yourself it would be wise to download a schematic and find out what major component is "near" that resistor. It has blown for a reason. It appears to be the last component for a major connection going from the board (or the first component for a major connection coming in). If its the former, the red wire adjacent to that resistor could be going out to the power transistors on one channel? Meaning you have issues "downstream" - ie: one or more blown outputs.

 

[jackattack13]

Curious if you ever got this issue fixed. Mine started frying this exact resistor on the PC-18 board. Still trying to figure out what the cause is with no luck.