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November 26, 2022, 01:28:14 PM

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Fender London Reverb

Started by GOR3, September 08, 2021, 06:01:36 PM

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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.

Jazz P Bass

What does the supply read for the power amp stage?
+33Vdc & -33Vdc is stated.

The 15 Vdc supply is for the opamps.
A 3 volt drop will not hurt.
As long as they are both the same. (+ & -)

The 18 Vdc supply is for the reverb & effects.
Again. No biggy as long as they are both the same.

Have you checked for Vac ripple?


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?


I don't follow.  Where are the PMOS you refer to?  I see a couple CMOS chips, but they are not using 18v as far as  can see.   And I don't get how if the supply voltages are low, how can the control voltages wind up too high?

But the real question is this:  what are you trying to fix?  What is the amp doing wrong?


Is the transformer marked 019252 ?
Is it getting 120V at the primary, with both primary windings connected in parallel, as shown on schematic?
For 12-0-12VAC winding, your voltages measured sound right (+/-15V).
So perhaps the 12-0-12 is not what should be there.

Are you using a footswitch?  Common faults on this series of amps were the push switches (pots) and bad solder on every pin of the board connectors.


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.


I've seen other products with schematics that mis-label +/-12V  as 15V.
That could be the case here as well.

If ch.2 always works well with no issues, I think your supply levels are ok.

Can we just call the fault for now as 'ch.1 cuts in and out' and take it from there?
Is that an accurate description of the symptom?


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.