| Frankenstein Head: replacement cap values [message #24244] |
Tue, 22 December 2015 13:33  |
IdahoPicker
Messages: 7
Registered: October 2015
Location: North Idaho
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Junior Member |
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Hi all,
I posted a while back about a Frankenstein head, the help I received was fantastic! I have another Frank head on the bench (same customer). Here's what's up: It was distorting at all volumes (sounded like bad speaker). I replaced the 500uf cap on the Driver Regulator. It cleaned things up quite a bit. I've tested every transistor in the pre-amps and the driver regulator, every electrolytic cap as well. Everything is measuring within range. The only cap that looked questionable is a 90uf electrolytic in the driver regulator. All I have on hand are 100uf caps. Is this a safe replacement for 90uf in these amps?
The amp sounds decent, but it strikes me as a little underpowered, and it begins to distort a little bit with volume at 3 o'clock (both channels)
I have an image of the scope display, but I'm having trouble getting images on here. The waveform looks good on the top half, but the bottom shows a deviation. The deviation increases with volume.
I'm (obviously) pretty new to these amps, and I appreciate your advice!
Thanks!
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| Re: Frankenstein Head: replacement cap values [message #24245 is a reply to message #24244] |
Tue, 22 December 2015 13:46  |
IdahoPicker
Messages: 7
Registered: October 2015
Location: North Idaho
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Junior Member |
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I just answered my own question about the cap values.javascript: insertTag(document.post_form.msg_body, '', '  '); I reread the previous post I made about the first Frank head I worked on. I did some research on the assymetrical waveform, it looks like a 3rd harmonic on the negative phase only. I'm pretty new to analyzing signals with a scope, i'm not sure what this is pointing to.
Thanks, and any advice, please, I'm all ears!
[Updated on: Tue, 22 December 2015 14:03] Report message to a moderator |
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| Re: Frankenstein Head: replacement cap values [message #24246 is a reply to message #24244] |
Tue, 22 December 2015 15:38 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Yes, caps can be replaced with a fairly wide range of values and still be within spec. These are guitar amps, not rocket ships or medical devices.
As for your distortion, have you checked all of the resistors connected to the output transistors? As there is no direct connection to the driver stage due to the transformer, the outputs are biased by all of those resistors. I find then drifted all over the place. Also have you compared the voltages that you have on the outputs with the ones posted on the schematic?
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| Re: Frankenstein Head: replacement cap values [message #24248 is a reply to message #24244] |
Wed, 23 December 2015 06:33 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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Change those output stage resistors , all of them and up the 5 watters to 7 or even 10 watt values !
If need be order up or use resistors in parallel or series to get dead on the value needed as stated on the schematic.
It does not take much resistance drift to knock near 40 watts of RMS power off of these amps from what I have seen, and on the non wire wound resistors do use carbon comp type use metal film.
[Updated on: Wed, 23 December 2015 06:34] Report message to a moderator |
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| Re: Frankenstein Head: replacement cap values [message #24286 is a reply to message #24267] |
Thu, 31 December 2015 14:00 |
IdahoPicker
Messages: 7
Registered: October 2015
Location: North Idaho
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Junior Member |
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I replaced c203 (500uf) cap on the driver board and cleaned the pots. I didn't get a chance to test the resistors, the customer was happy with the initial repair. I have yet ANOTHER Frankenstein head on the bench! You'll hear about it soon...
Thanks for your help.
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