Capacitors on a Dynacord Echocord Super62

[Noth]

Hi,

I'm repairing a Dynacord Super62, which a tube tape delay.
My unit is.. fully original, nothing has changed since 1962, so it's clearly time for a recap.

I'm not super familiar with vintage tube designs so I have a few questions:


- All electrolytics/paper capacitors in place use very high tension values (from 350V to 600V, usually 500V), even at some locations where there seems to be no high tension. Is there a reason for this or is it only because at the time, no lower tension capacitors were available?
- On the schematic, for some of them the tension is specified explicitely (ex. C3 4uF/ 350-385V), does this mean that for the others more generic types can be used (35V or 63V for example)?
- On the schematic there is a dual 32uF capacitor (for the power supply filter). Is there a reason for using such a specific component (RF shielding, thermal coupling..)? Can I eventually replace this with just two 32uF capacitors?
- The previous owner did replace the original power cable and added an IEC connector, which seems to be a nice idea. But.. the ground is not connected to the chassis! Is there a technical reason for this (except killing the guitarist of the band)?

Thanks for any advice!

 

[rock soderstrom]

All electrolytics/paper capacitors in place use very high tension values (from 350V to 600V, usually 500V), even at some locations where there seems to be no high tension. Is there a reason for this or is it only because at the time, no lower tension capacitors were available?

High voltage capacitors have always been more expensive, so it is safe to assume that these types were only installed when necessary. Therefore, I would only replace with equal values, unless you know what you are doing and you are 100% sure.

On the schematic, for some of them the tension is specified explicitely (ex. C3 4uF/ 350-385V), does this mean that for the others more generic types can be used (35V or 63V for example)?

The same applies as written above. Not all capacitors are described with voltage specifications in the schematic, many of them require high voltage models.

On the schematic there is a dual 32uF capacitor (for the power supply filter). Is there a reason for using such a specific component (RF shielding, thermal coupling..)? Can I eventually replace this with just two 32uF capacitors?

This was quite popular at the time, probably for space reasons. You can replace them with two single capacitors without any problems.
But there are still similar models for sale, I would ask Jan

The previous owner did replace the original power cable and added an IEC connector, which seems to be a nice idea. But.. the ground is not connected to the chassis! Is there a technical reason for this (except killing the guitarist of the band)?

There is no reason not to connect the protective earth conductor, it is required by law!
Check whether all metal housing parts have direct continuity.
Work safely, you are liable for the device after the repair.

Cheers

 

[Noth]

Thanks a lot for your answer!

I will replace with same capacitor voltages, anyway the price difference considering how the amount of these, doesn't really change much.

I was too very suprised with the grounding of the chassis! I really don't understand why someone would replace a power cable with a more modern connector and.. not solder the ground. There's plenty of screws and metal everywhere on the unit, that's a very trivial task..!

 

[scott2000]

There's a clearer drawing in the manual.
Voltages are listed throughout the schematic so you can get a pretty good idea what's going on. Like there's 160Vdc sitting at C3... possible 280 if stuff goes awry....

 

[Noth]

Oh.. I just noticed that the previous owner also removed the fuse when he added the IEC connector..

 

[rock soderstrom]

Oh.. I just noticed that the previous owner also removed the fuse when he added the IEC connector..

Yeah, at least the guy was consistent. Consistent illegal and dangerous.

 

[Whoops]

Therefore, I would only replace with equal values,

I will replace with same capacitor voltages,

Just to add to what's been said here,
for the capacitors you can replace them with the next higher uF value and also with an higher voltage. Never lower than the stated voltage, but higher is fine and preferable most of the times.

So for example an 4uf 385V cap, can be replaced with an 5,6uf 400V or 450V cap.

As for the IEC, I would connect it's ground Pin to the chassis.

And I would install a fuse.

 

[Noth]

Just to add to what's been said here,
for the capacitors you can replace them with the next higher uF value and also with an higher voltage. Never lower than the stated voltage, but higher is fine and preferable most of the times.

So for example an 4uf 385V cap, can be replaced with an 5,6uf 400V or 450V cap.

As for the IEC, I would connect it's ground Pin to the chassis.

And I would install a fuse.

Thanks for the advice, I'll do that!

 

[Noth]

Hi everyone,
Here's a quick follow up on my work on this Dynacord unit.
- I've replaced all electrolytic capacitors (using the closest values and proper voltages)
- I've replaced all film capacitors
- I've cleaned all potentiometers
- Added a fuse and ground
- I've cleaned (re-heat or complete resolder) every solder
- I've replaced some of the ceramic disk capacitors (most of them are not easily accessible)

So, the unit turns on, the motor seems to work fine and all tensions on the tubes seems fine according to the schematic (fine = "quite close")
Now, my next step is to simply use the "preamp" section from Mic1/Mic2/inst1/inst2/Line-in and get a dry signal on the output (I'll work on the delay/reverb section afterwards).



Now here's my current status:
- all inputs works fine, the input pots behave as expected
- the magic eye works fine too
- I had a strange 56kHz signal with an amplitude around 100mV. I did unsolder the 330k resistors (R21-R22-R23-R24) after each preamp section and ended up to the conclusion that this signal seems to be coming from the two Mic inputs. So I did add a 22nF to filter that and the problem was solved
- When measured at TP1 (see schematic), I can see my test signal (sent on the line input) and it's "quite" clean
- When measured at TP2, well .. this is quite a mess

Here are a few captures:

No signal on the input, measured at TP1 (we can see that we a have a low frequency, low amplitute noise)


Still at TP1, but with a 800Hz test signal (the test signal is slightly modulated by the previous noise):


Measurement at TP2, without test signal (we have a huge low frequency noise):


And with a test signal (the amplitude of my test signal is quite low comparing to the observed noise):



So, my guess is that this large noise comes from the "green section", but since the play heads were not active (motor not running) nothing serious should be coming this way.
Tomorrow I will try to disconnect C13 to shut off the feedback from the green section to see if it helps.

If anyone has some advice, I would appreciate any help!

 

[Noth]

So, this morning I did change a few other ceramic disk capacitors tried to unconnect some parts of the circuit much not much success.
As a desperate move, I did try to remove the tubes one by one and came up to the following conclusions:
- The erase head tube Ro6 is the origin of the very annoying 56kHz noise. I did test with other replacement tubes and still had the same issue.
- The Play head tube Ro4 is the origin of the saturated 150Hz signal. I did test with other replacement tubes and still had the same issue.

So, this afternoon I'll focus on those sections!

 

[Noth]

Ok, so it appears that my 56kHz noise is actually generated by Ro6 and sent to the erase head (I'm not super familiar with tape circuit, so I guess it's obvious for anyone who knows a bit ).
So the question is : how is it that this signal ends up on the audio path?

 

[rdg]

Ok, so it appears that my 56kHz noise is actually generated by Ro6 and sent to the erase head (I'm not super familiar with tape circuit, so I guess it's obvious for anyone who knows a bit ).
So the question is : how is it that this signal ends up on the audio path?

Hey, did you ever get anywhere with this? I have a S62a and have a similar problem -- a ghost note or intermodulation that only seems to appear at your TP2.

 

[Noth]

Hey, did you ever get anywhere with this? I have a S62a and have a similar problem -- a ghost note or intermodulation that only seems to appear at your TP2.

Hi,

I ended up finding that, for a very unknown reason, a wire was connected on the erase head tube to the rest of the circuit. I can't remember exactly where unfortunately.
I did just reconnect this wire where it was supposed to be and.. that was fixed.

Don't ask me why..

 

[metalockpick]

Hi,

I'm repairing a Dynacord Super62, which a tube tape delay.
My unit is.. fully original, nothing has changed since 1962, so it's clearly time for a recap.

I'm not super familiar with vintage tube designs so I have a few questions:


- All electrolytics/paper capacitors in place use very high tension values (from 350V to 600V, usually 500V), even at some locations where there seems to be no high tension. Is there a reason for this or is it only because at the time, no lower tension capacitors were available?
- On the schematic, for some of them the tension is specified explicitely (ex. C3 4uF/ 350-385V), does this mean that for the others more generic types can be used (35V or 63V for example)?
- On the schematic there is a dual 32uF capacitor (for the power supply filter). Is there a reason for using such a specific component (RF shielding, thermal coupling..)? Can I eventually replace this with just two 32uF capacitors?
- The previous owner did replace the original power cable and added an IEC connector, which seems to be a nice idea. But.. the ground is not connected to the chassis! Is there a technical reason for this (except killing the guitarist of the band)?

Thanks for any advice!

All else equal, higher voltage rating is better.
If the signal caps aren't leaking voltage, and don't have negative voltage on one side and positive voltage on the other, I would not change them.
I own some tube gear from the '50s with 100% original capacitors that are working fine. I've seen gear from the '40s where the original capacitors were in good working order. Of course, if the capacitors aren't in good at working order, by all means replace them with quality parts.
As far as specs go, you want equal or greater voltage (AC and DC), temperature rating from at least -40 to 105° c (or higher), dielectric absorption as low as possible. All else equal, smaller size is better.
If you're looking for one brand that's a safe bet in capacitors, that'd be Panasonic.