My Dynaco SCA35 died tonight - well, a part of it

tri_pod

Active Member
Hello Everyone

As the subject said and I need some help for a dufus, please.

I wanted to listen to some youtube stuff and got no sound. I played a record (selected Phono) and got sound. For youtube and other computer sound, I use "Spare" which is the problem.

I touched the cover and it felt very hot - much hotter than usually - more or less directly above those 6 tubes. And I switched off immediately and removed the wallplug.

This amp was rebuilt about 1 year ago by a local tech who imo did a very good job. Somewhere on AK are pics of the restore and about a month ago, I heard some very faint crackling a minute or so after switch on. This sound disappeared after about 2minutes after switch on and maybe I should have taken this as a pre-warning of something going south.

I am a total novice diagnostically but can solder very well and replace parts I can visually see overheated and the tech which did the rebuild for me is no longer available.

All the fitted tubes are still Dynaco-branded originals, btw.

Tomorrow, in daytime, I will take off the bottom cover, have a look and take some pics incase I can see the origin of overheating so much.

Please help.

Cheers all

tri_pod
 
Could be many things but first and easiest thing to check is the tubes, specifically the output tubes. Since they are original they are nearly 60 y.o. You can test them and see where they measure, but really if you don't have any back up tubes, I would get a matched quad of EL84M or 6P14P-EB.
 
yes, it does sound like a tube issue, especially if the tubes are still the originals, especially the crackling, which more often than not is tube related. Also, as you mentioned, take some inside shots so that we can see what was rebuilt in your SCA-35.
Having a complete spare set of tubes for your SCA-35 is good insurance, so as has already been suggested if you do not have a spare set, buy a set and keep them for when things go south!
 
Hello Everyone

As the subject said and I need some help for a dufus, please.

I wanted to listen to some youtube stuff and got no sound. I played a record (selected Phono) and got sound. For youtube and other computer sound, I use "Spare" which is the problem.

I touched the cover and it felt very hot - much hotter than usually - more or less directly above those 6 tubes. And I switched off immediately and removed the wallplug.

This amp was rebuilt about 1 year ago by a local tech who imo did a very good job. Somewhere on AK are pics of the restore and about a month ago, I heard some very faint crackling a minute or so after switch on. This sound disappeared after about 2minutes after switch on and maybe I should have taken this as a pre-warning of something going south.

I am a total novice diagnostically but can solder very well and replace parts I can visually see overheated and the tech which did the rebuild for me is no longer available.

All the fitted tubes are still Dynaco-branded originals, btw.

Tomorrow, in daytime, I will take off the bottom cover, have a look and take some pics incase I can see the origin of overheating so much.

Please help.

Cheers all

tri_pod
SCA35 is very hot as all 6 tubes are close together.
Does any of the el84 has redhot anodes ? If not i'd suggest you continue to use the amp and figure out
if it sounds as usual.
Do check voltages as per the manual, and if reasonable close close the issue.
 
Hi tri_pod, As I think I know what amp you're talking about, I'm posting a link to another site that shows your amp after work by the tech. http://www.antiqueradios.com/forums/viewtopic.php?f=13&t=258926

If I'm mistaken, I apologise. What those pics show is the good work done by your tech in replacing the power supply caps, etc, and that he replaced all the caps on the other boards.

Could you post pics of above and below to check any visual issues? What you describe could be caused by the old boards or old tube sockets. If so, a rebuild using Dave G's new boards is in order. (Just needs basic soldering skills as he gives clear instructions.)
 
Hi Guys

Thank you for your generous replies. The guesstimate of the problem being a tube issue seems on target and I'm making some enquiries for a quad set locally.

In the meantime, I show below (from left to right, amp controls facing) the tubes presently fitted. All are Dynaco branded.

7199 69-43 (274) 69-43 (274)
7199 6BQ5 (6-43, 274) 6BQ5 (69-43, 274)

Apart from the 2X7199's, are the other 4 tubes all the same tube - ie either a quad set of EL84M or 6P14P - EB?

@Dandy
The Antique Radio link is quite correct. That is my Dynaco.

Much obliged

tri_pod
 
Hi Guys

Thank you for your generous replies. The guesstimate of the problem being a tube issue seems on target and I'm making some enquiries for a quad set locally.

In the meantime, I show below (from left to right, amp controls facing) the tubes presently fitted. All are Dynaco branded.

7199 69-43 (274) 69-43 (274)
7199 6BQ5 (6-43, 274) 6BQ5 (69-43, 274)

Apart from the 2X7199's, are the other 4 tubes all the same tube - ie either a quad set of EL84M or 6P14P - EB?

@Dandy
The Antique Radio link is quite correct. That is my Dynaco.

Much obliged

tri_pod
The 4 EL84 should be a matched quad.

The 7199 is ok, keep them.

Are you shure the amp is broken ? If no tubes "redplate" and amp sounds ok then what is the problem ?
 
I wanted to listen to some youtube stuff and got no sound. I played a record (selected Phono) and got sound. For youtube and other computer sound, I use "Spare" which is the problem.

I touched the cover and it felt very hot - much hotter than usually - more or less directly above those 6 tubes.

I heard some very faint crackling a minute or so after switch on. This sound disappeared after about 2minutes after switch on


Not sure what you mean by "Spare" is the problem. The input you use has nothing to do with how hot the amp runs.

A hot cover on SCA-35 is normal. It's a tube amp. Like peterh said above, look at the EL84 tubes (the taller ones) and if any of them are glowing cherry red, THEN there is a problem.

Faint crackling at turn on. Was this coming through the speaker? Both channels?

Or was it in the amp itself. Some mechanical tinkling/crackling noise is normal as the tubes heat up and cool down.

red-plate.jpg
 
Hi PeterH

The Dynaco is exclusively used for 78rpm playback and recording, listening to youtube stuff and occasionally, CD or DVD playback from my computer drives.

Last night, when I used the amp, I could play shellac but youtube stuff and CD playback from my computer was dead - no sound.

The amp is located behind me and I couldn't see if any of the EL84's were redplating - instead, I touched the cover which was much, much hotter than what I'm used to and I immediately switched off and pulled the wallplug.

The centre of the cover's heat was concentrated directly above those 4 EL84's.

I have located a firm nearby who has 4 EL-84's in stock. Whether they will be able to match them, is unknown. I'll check tomorrow. Two other things - the tubes they have are Sovtek - no problem with that as I've used Sovtek's before in my Leak Stereo 20 which I recently sold. THEY DON'T HAVE EL84M's - only 'normal' EL84. Will this be OK as replacements?

Regds

tri_pod
 
I've had the Dynaco for a very long time and I'm quite used to the covers being hot. The heat I felt last night, was VERY MUCH hotter than usual, the hotter place directly above those EL84's.

tri_pod
 
It is hot above the tubes.
If it works for 78 but not from computer it's a problem with the computer or cables.
 
I've had the Dynaco for a very long time and I'm quite used to the covers being hot. The heat I felt last night, was VERY MUCH hotter than usual, the hotter place directly above those EL84's.

tri_pod
Well, it seems like you pay attention to your equipment and if you noticed somethiing different then you are smart to pull the plug and check it out. The main reason your unit would get hotter is if a tube started to runaway and redplate. And your new matched quad of el84 should fix that. But, If you unit has the original circuit boards which are notoriously fragile after 50+ years, a problem with a bad connection around a solder connection, the dreaded cold sold joint can also create odd noises. If after installing a new set of tubes and you still have the odd noise problem you should really consider upgrading the circuit board to a new fiberglass with better traces.
 
Regarding your tubes question, normal EL84s will be fine. This does not run 7189 tubes, like some Pioneer receivers. The assembly manual gives a plate (anode, pin 7) voltage of 360V and screen (pin 9) voltage of 365V.

It may help to post a picture of the main PCB from below. I have found that just the act of carefully taking pictures helps me to look more closely at the connections. No harm in reflowing the solder joints.

Have you tried running the amp with input to 'radio' rather than 'spare'?

And, have you cleaned the sockets and spraying the pots with Servisol or a similar switch cleaner?

My guess is that your SCA-35 hasn't died, but just needs a visit to the health spa for some pampering.
 
Check the voltage at R42, the power tubes cathode resistor. It should be about 1.86Vdc at idle. If it's much higher, you have bad output tubes. It could also be the cathode bypass cap C24 shorting.
 
I wanted to listen to some youtube stuff and got no sound. I played a record (selected Phono) and got sound. For youtube and other computer sound, I use "Spare" which is the problem.

Color me officially confused on the trend in this thread. Trying to figure out how it can be a power tube issue if you get good sound out of phono.

My first inclination would be to switch the computer connections to tape or radio. Not sure if either of those have any special EQ circuitry attached, but they should work, just maybe not as well as a direct connection.

And ya - pick up some more power tubes if you find a good deal - always good to have spares.
 
Hi Guys

I under-estimated my stupidity. The trouble started with a micro mouse USB receiver I wanted to plug into a CCTV camera DVR...

My desk where my computer gears are on, is a bent pipe affair. As a result, this desk is kinda unstable and two Sundays ago, while busy with the USB thingy, the desk toppled over. All the gear on the table slid off and ended up on me and then on the floor.

All the items were totally unmarked from the fall but two were broken - a flatscreen monitor and my Roland Edirol soundcard.

Both items were replaced and a tech came to my house and installed the driver for a new Behringer U-phoria UMC204HD interface which worked correctly for 1 day only. I could play my 78's, listen to radio, playback CD's on my computer's drives etc.

The next day, I could only listen to records, using the SCA35.

Since I have been ill for most of 2017 and rarely used the Dynaco AND having been away from my house for 2 months, my memory was a bit dim on how hot the amp gets and IMAGINED the cover area above those EL84's was hotter than usual.

So anyway, today I bought a quad set of Sovtek's and replaced the originals with the Ruskis.

No change. I could only playback using the Dynaco with direct sources, ie my turntable, radio etc.

What had actually happened?? I was listening to the Realtek soundcard and not the new Behringer U-phoria. The Realtek software installed itself inplace of the Edirol USB soundcard which fell off my desk and got broken.

So there was really nothing wrong with the SCA35's tubes, no redplating but just my own Keystone Kop heritage.

I found all this out late yesterday afternoon by accessing my computer's sound setup.

tri_pod
 
Repeat after me ...

dOH! :D

In hospital for two months? Gotcha beat there - four months here. :oops:
 
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