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Old 23rd December 2014, 01:22 PM   #1
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Default Replacement capacitor value

Hello all, I am looking for advice on replacing two leaking capacitors in the output stage of a vintage Telefunken Opus Studio MX5650. I had no output on the left channel.They are 2500 mfd with axial leads and mounted to the chassis with a band clamp. I am having a hard time finding one with the same value. Can I substitute one of another value without dramticlly altering the operation of the output. I can find lots of values close to 2500 mfd, 2300, 2400 etc. Is the function of this cap only to eliminate DC in the output. The cap is C1702 in the following schematic. Thanks in advance everyone.

Collin Stuart
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Last edited by collinstuart; 23rd December 2014 at 01:26 PM.
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Old 23rd December 2014, 01:32 PM   #2
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It does appear to be just a DC blocker. Any of the values you have listed will be fine.
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Old 23rd December 2014, 01:43 PM   #3
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Standard value would be 2200uF at 35v DC.
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Old 23rd December 2014, 01:51 PM   #4
Mooly is offline Mooly  United Kingdom
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As others have said, it is just a coupling cap. I would probably go for either a 2200, a 2700 or 3300uf. Modern types tend to be "Radials" with the two connections at the same end. You will also find modern caps seem very small compared to there older cousins.

Its well worth pointing out that something of this age should probably have all the "wet" electrolytics changed. That one failure is a warning sign.
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Old 23rd December 2014, 03:14 PM   #5
AndrewT is offline AndrewT  Scotland
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The output capacitor voltage rating needs to be just a bit higher than the worst case supply rail voltage?
Do you know your supply rail voltage?

The output capacitor should not act as a filter for the low bass frequencies.
2500uF and 8ohms will already be down by 3dB @ 8Hz and starts to roll off @ ~80Hz.
The filter should be good quality filtering type at the input to the amplifier. A 1uF polypropylene works well if Rin suits that value.

I would be trying both 3300uF and 4700uF and maybe even paralleling those to give 8000uF to compare the sound output for the three caps values.
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Old 23rd December 2014, 03:28 PM   #6
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Default Capacitor rare values

You could just connect capacitors in parellel. ie 2300mfd + 200mfd = 2500.

I have done this with polyprops for riaa filter settings.

Good luck
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Old 23rd December 2014, 04:26 PM   #7
DF96 is offline DF96  England
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As electrolytics often have 20% tolerance (and somewhat unstable values over time) there is no point in trying to exactly match a particular value. At most, ensure that the two stereo channels have similar values here.
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Old 23rd December 2014, 05:31 PM   #8
Mooly is offline Mooly  United Kingdom
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Another drawback of going to extreme in value could be a noticeable increase in switch on/off thump. The AD149 is exactly indestructible either Keep it real and in the same ballpark give or take.
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Old 23rd December 2014, 09:03 PM   #9
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Thanks so much for all of the replies gang. I will be replacing these ones first to make sure the left channel comes back online. If so i will invest the time to replace all of the rest of the caps. After that I will need to clean up the volume and tone controls. They are very noisy when rotated. I will repost once the caps are in with the results. Thanks again everyone.
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Old 27th December 2014, 07:52 PM   #10
Dave2 is offline Dave2  United States
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Kind of wondering what leads you to believe one of the output caps is responsible for the no-output situation. If the cap is leaky or shorted, I'd think you would still get something at the output.
I would just put anything you have laying around the house, even if it's like a 470F cap, that has the has a high enough voltage rating and see if you get any output. If you do, then dump some money on something decent. Yes, a small cap will make your bass response suck. But you just want to debug this and make sure the caps are to blame.
The quality of these coupling caps can make a big difference in your sound. These caps are decoupling into 8Ω, which means they have to be so big as to necessitate an electrolytic. But putting a poly-cap of smaller value(maybe like 100F) in parallel with the big electrolytic might help with things like high frequency clarity. And with those inter-stage coupling caps, it's more practical to use a single poly or PIO cap there.
I'd make the new electrolytic cap as big as you can. As was mentioned above, with 2500F, the bass is starting to roll-off and phase shift at 80Hz, which can give a soft or muddy bass.
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