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Messages - GOR3

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Amplifier Discussion / Re: Fender London Reverb
« on: September 17, 2021, 02:32:15 PM »
Yes and no.  By which I mean, that is the fault that caused me to open up the amp, but it's not the reason for my post.  The reason for my post was to see if the low supply voltages were "normal."  That is, were the voltages I measured the same as others who had had this amp on the bench.  I'm confident (perhaps foolishly) that I can remedy the intermittent channel.

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Amplifier Discussion / Re: Fender London Reverb
« on: September 13, 2021, 10:56:30 AM »
Thanks, g1.  The transformer itself is marked 019252.  That matches the part number on the schematic I have.  The primary voltage is good at about 120VAC and the primaries are wired correctly (in parallel and using the 120V windings).  I too believe the 12-0-12 is low. 
The fault I'm finding is that when I try to switch channels, Ch. 1 often -- but not always -- is silent; Ch. 2 always works.  The fault occurs whether the channel-select push-button on the Ch. 2 volume control is used or the footswitch.  The LEDs that indicate which channel is selected always work properly, i.e., the Ch. 1 LED lights when Ch. 1 is selected; the Ch. 2 LED lights when Ch. 2 is selected.  So I have to believe the trouble spot is elsewhere.
George

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Amplifier Discussion / Re: Fender London Reverb
« on: September 09, 2021, 11:07:43 AM »
Thanks, Jazz.  The power amp voltages match the schematic at ±33.  I haven't measured Vrip.  If you're wonder about the filter caps, they've all been replaced recently by fresh, name-brand components purchased from Mouser.  And bear in mind, that the unloaded VAC to the board is 12-0-12.  As mentioned, that won't get you ±18VDC.
I understand that this amp will -- and does (but see below) -- work at the lower voltages.  But the bigger question is, what's going on here?  Why are all of the low-voltage supply voltages down by more than 15%?  And the inquiry isn't just academic.  The 18V supply also is used as the control voltage on the P-MOS switches.  This is an issue for two reasons:  First, TI says that control voltage may be up to Vcc.  Here it exceeds Vcc whether you use the schematic values or the "as measured" values.  Second, although TI says the control voltage may be up to Vcc, TI recommends that the control voltage be no more than 5.5V.  This is a concern because these switches are prone to failure.  Indeed, the reason I got into this was because the channel switch function is glitchy.
Any thoughts?
George

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Amplifier Discussion / Fender London Reverb
« on: September 08, 2021, 06:01:36 PM »
I'm re-hab'ing a Fender London Reverb. The schematic states ±18VDC and ±15VDC from the low-voltage power supply. I’m measuring around ±15 and ±12, respectively, unloaded, so 3V low on all the rails. (The high-voltage supply matches the schematic at ±33). The secondary of the transformer for the low-voltage supply measures about 12-0-12VACrms, again unloaded. I don't see how that makes it to ±18VDC.
I’m wondering if ±15 and ±12 actually is normal.  That is, that after the schematic was drawn and published, Fender decided to drop the low-voltage supplies by 3 volts and just didn't update the schematic (or at least the schematic I have).  I contacted Fender for the service manual, but they demurred. I also searched the web; no joy. If you’ve been inside one of these things, I’d be curious what voltages you found.
Thanks,
George

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