Fender London Reverb

I'm re-hab'ing a Fender London Reverb. The schematic states ±18 and ±15 from the low-voltage power supply. I’m measuring around ±15 and ±12, respectively, unloaded. The secondary of the transformer measures about 12-0-12VACrms. (The high-voltage supply is spot on at ±33). I’m wondering if this is normal. 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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Joined 2011
Owner's manual:
https://www.stratopastor.org.uk/str...fenders/LondonReverbMontreuxShowmanManual.pdf

Service manual:
https://www.stratopastor.org.uk/strato/amps/twoseriessolidstatefenders/London Reverb.pdf

page 1 - parts list
page 2 - power amp and eq schematics (eq also for Showman)
page 3 - preamp schematic (also for Montreux)
page 4 - logic schematic (also for Montreux and Showman)
page 5 - PCB and parts list for unknown section
page 6 - low voltage rectifier board PCB and parts list (also for Montreux and Showman)
page 7 - front panel (preamp and graphic eq) parts list
page 8 - front panel (preamp and graphic eq) PCB
page 9 - low voltage and high voltage rectifier schematic inc. power transformer (also for Montreux)
page 10 - PCB and parts list for unknown section
page 11 - power amp PCB
page 12 - blank
 
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If the transformer output is 12 Volts AC RMS

With diode drops etc etc

Around 15.5 DC sounds right.

Unless of course something is pulling it down.

You could check R1/R2 which could have drifted upward in resistance
or any type of shunt regulator in that circuit.

I see they shunt regulate a 6 volt voltage for some of the chips.
Believe its D9 Zener diode and a 560 ohm dropping resistor
from the 15 volt supply

Most likely not a issue if both Negative and positive low volt supply's measure the same.
 
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Yes, I think it would hard-pressed to get ±18VDC from a 12-0-12 secondary. The (lower) voltages I'm measuring are without any load. But I have a hunch: This thing uses TL60x P-MOS switches to route the signal chain. As written, the schematic has 18VDC as the control voltage for those switches. TI's datasheet said the control voltage could be up to VCC+, which in this case is 15VDC, so the control voltage already is too high. Moreover, TI's recommended operating conditions specified a maximum of 5.5VDC on the control pin. As we now know, those early switches were prone to failure, which Fender may have discovered early in the production run when the thing was actually running with ±18 and ±15 rails. Rather than design and manufacture an entire new batch of boards with lower control voltages, Fender simply may have substituted a new transformer with a lower secondary. The amp's got gain to spare, so even with the lower voltage on the pre-amp circuit it still can drive the output (which is powered entirely from the ±33VDC rails). That's why I'd like to see a service manual. If my hunch is correct, the manual should have tipped off the service tech to the lower voltages. Second best would be to see what voltage others are seeing in the wild.
 
OK, I don't see 18v getting to the control pins of the 604s. I see those control voltages coming from outputs of the CMOS chips. I only see 18v there as those same CMOS outputs also control the transistors driving indicator LEDs. The 18v is used for those LED circuits. So as I see it the 604 control voltage is limited to the supply of the CMOS ICs.


On the other forum I asked which PMOS you meant, I didn't connect the 604s to that in my head, sorry.

But question remains, what are you trying to fix here? I mean the amp has probably been running on these same voltages for close to 40 years now.
 
So as I see it the 604 control voltage is limited to the supply of the CMOS ICs.
Which is 18V (check the Vdd pin (14) of IC7).
This amp is new to me. I went in for a couple of reasons: 1. To replace the psu filter caps; 2. because the channel switching (which uses the TL604) is glitchy; and 3. because the reverb (which signal routes with a TL607) is anemic. To begin the troubleshooting, I first measured the psu voltages and compared them to the schematic. And here we are.