Repairing a tape echo with tubes (Dynacord Echocord Super 62a)

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bigramp

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#1
hello !

i have an old Dynacord Echocord Super 62a which stood in an attic for loooong years and is a bit rusty. i'm interested mostly in its tube preamp.

here are some pics :
http://motthieu.free.fr/dynacord

- it turns on
- the 6 tubes are working
- the Magic Eye tube is working
- the engine works
- switches are working
- i've replaced 11 capacitors which were old / crackled and greasy
- most resistors have been tested OK

however
- reverb is not working (yes it does reverb too!). the knobs just add some noise
- i don't have tape so i don't know if the tape echo works. has i said earlier, what matters to me the most are the tubes...
- when i first got it, the sound worked on every inputs. it had a great tube tone (quickly overdriven though) but also a lot of hum and random noises. the dying capacitors had to be replaced so i did it. then the sound was very clean. no hum, no noise. then the unit turned off by itself. the fuse died. i replaced the fuse and it went on back. the unit worked for one hour. and suddenly the sound became lower, more distant. i turned it off then on, and the sound was even softer. today there's no sound at all.

the only way to get some sound through the unit is to uncorrectly plug a cable in the 2 instrument input, but it doesn't go through the tube preamp stange (straight out)

what's happening ?
i don't have any knowledge in electronic, i'm just replacing what's faulty... it feels like there's not much to do to get it back working but i don't know what to do

here's the schematic
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/dynacordechocord.jpg

thanks for any help !
bigramp
 
#4
bigramp,

No, this Echo unit have not any reverb device inbuilt, the knob "reverb" means that you can do a preset mix with all record heads. (with the three presets placed next to the heads)
So you must have a 51 cm tape loop, to get any echo or reverb sound.

There are two way to connect this machine:

1. Instrument input connector are a combined in and out.
(pin 1= input, pin 3= output, pin 2= ground

2. Use any input and use the separate output connector as main output.

Note that the tonecontrol knob have a pull-out mode, and in normal mode you have "dry and wet" signal mixed together, and in pull-out mode you have only "wet" signal (only echo)

--Bo
 
#5
Have you replaced the electrolytics in the power supply and while I am not sure what type of rectifiers are present be on the look out for selenium types which should be replaced with silicon types. (Selenium is toxic so treat as hazardous waste.)

Please compress your pix - even with dsl several of them took many minutes to download! Also please take pix of underside of chassis and post schematics if you can locate them.

I'd say that transformer has gotten a bit wet, hopefully it is ok..

Fuse blowing is never a good sign, when a fuse blows find problem first, do repair and then replace the fuse. Assume it is really there mainly to prevent a fire, and in many instances a lot of damage can occur before the fuse blows.

FYI units that have not been powered for a long time should not be plugged in and run until restoration work has been performed. You might be better off finding a technician qualified to do this work.
 
#6
thanks for your infos !

here's the schematic, the direct link to the image file didn't work
http://www.freeinfosociety.com/electronics/schemview.php?id=356

i've compressed the files on my ftp

i've already posted a pic of the underside of the chassis
http://motthieu.free.fr/before-replacing-the-capacitors.jpg

here's the same pic but with red squares on the replaced components.
http://motthieu.free.fr/replaced-components

so the big grey capacitor in the middle has been replaced. a guy in an electronic shop told me it was a dual capacitor which he didn't have, so he gave me two capacitors of correct value and told me how to connect them.

i've read about someone who can fix it but he's in germany and it would be a bit expensive. i got this unit for cheap and i don't really want to invest much money in it so i thought i would check to see what i can do myself... do you see any component that would obviously need to be replaced ?

i'll try what i can and then if it doesn't work i'll sell it back as it is.

thanks
 
#7
Hi,
I can see a few things, but was not necessarily able to find them in the pix. There is a bridge rectifier with a pair of yellow(?) wires connected to it, probably selenium which should be replaced with a silicon bridge - I was not able to identify it in the picture.

Also towards the top left corner are 3 or 4 blue electrolytic caps and one much larger yellow one, these should also be replaced.

Don't give up yet. You got it cheap, if you can get it working you got a good deal. These tend to go for pretty good money if in working order.
 
#10
bigramp said:
forget it ! what i thought was a rectifier was just a simple switch... 🙂

here it is

http://motthieu.free.fr/truerectifier.jpg
Click to expand...

Yes, that's the one and it is definitely a selenium type. Several German companies made these, the ones I am most familiar with were made by Siemens. (Widely used in US hifi gear in the 1950s - 1960s.) This rectifier will almost definitely be bad at this point, and running it may let out the magic smoke which is very, very toxic. Snip the wires, do not unsolder it. This is hazardous waste and should be carefully bagged and given to a disposal agency that accepts hazardous electronic waste.

Replacing this part will probably elicit real signs of life... 😀
Use a silicon bridge with at least an 800PIV/1A rating. You can increase the value of R67 slightly if the voltage is more than about 5% high at nominal line voltage. You might not have to do anything..
 
#11
great !

i just come back from the electronic shop.
they told me they hadn't got those blue capacitors, and also to come back on saturday for the yellow capacitor and the rectifier (the guy didn't know where his boss left them)

i'll keep you updated

thanks !
 
#12
It is a nice machine, with multiple recording heads and with microphone input transformers. I highly respect Dynacord's designs, they always sounded nice and were very robust and convenient, but nice looking!

I would start from replacing of electrolytic capacitors and of course of that Selenium rectifier! Electrolytics are white can, blue cans, and one yellow can. Yellow in plastic capacitors are probably still good. White one contains couple of 32 uF capacitors inside, you may use pair of modern capacitors of 50 uF each instead of it, they are smaller.

I hope that the power transformer is still alive, despite of rust.

To find a noise source I would pull off all tubes except the last one, and going from output to inputs inserting tubes checked all stages and tubes such a way.
 
#14
bigramp said:
hello

i don't see any white capacitors in the machine. where do you see them ?
http://motthieu.free.fr/before-replacing-the-capacitors.jpg
Click to expand...


Right in the middle. Ok, it is not white, it is "silver".

also, i can't find any 500 mF or 4mF capacitors. only 3,3 mF / 4,7 mF and 470 mF / 560 mF

can i replace a 500 mF capacitor with a 560 mF one ? a 4 mF with a 4,7 mF ?
Click to expand...

Yes!
Be sure to find capacitors rated for the same or higher voltages.
 
#16
Okay !
went to the shop this morning and came back with everything i needed : 4 capacitors and a rectifier.
i soldered everything then put the machine in garden and turned it on. no explosion.
plugged it, and it's a semi-victory : the sound is back, but again, it's very weak. but the sound goes through the input tubes, i can set the gain and tone, and the magic eye seems to indicate the sound coming IN is healthy. problem might be on the output area

also, there's no sound on instr 3 input (the one that doesn't have gain + tone). maybe my cable doesn't work with this input ? dunno

don't know what's going on... maybe the output tube is dying ? i don't know it's my first experience with tubes.

the machine has 5 X ECC83 for inputs, and 1 X ECC82 for output. physically they all appear to be in good condition, and they all lighten up.
i guess if i remove the ECC82, i'll get no sound at all ?

here's an MP3. testing every inputs here. i've normalised so you can hear what's going on.
instr 1 > instr 2 > mic 1 > mic 2
(tone settings might not be the same for each input)
http://motthieu.free.fr/echocordtest.mp3

and here's an old mp3 when the unit was working (before the fuse blowed), with gain at max, to give you an idea...
http://motthieu.free.fr/echocordbefore.mp3

7n7is : thanks but first i'm trying to get the preamp working, i'll get to the reverb later 🙂 the reverb knobs still give me noise when set at max. and then if i push the front panel buttons, i get huge POPs. maybe the reverb area is causing trouble to the preamp ?
 
#18
another discovery :
as Bo Hansén said, instrument input 1 is special :

pin 1= input, pin 2= ground, pin 3= output

i made a special cable to test this. din > double RCA

plugged left RCA to my guitar, and right RCA to my amp

it worked, sound came thru and was processed by the tube with gain + tone working. but the sound was the same as you can hear on the echocordtest.mp3 .

so the problem might not be at the output stage !
 
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