Fender Hot Rod Deluxe Reverb issue

[spuddo]

Hi all , first time poster.
The amp i have is a MIM 2007 , 240volt version.
The only schematic i have been able to get is dated May '96 , i'm sure there have been some changes made since 2007.
I have googled a lot about this amp and been given sage advice.
First thing i did was replace R78 , 79 & 97 , Zeners CR13 & 14.
They had fried the board and traces , lost 4 pads and had to hard wire as required. All ok now.
Trimpot R82 was playing up (intermittent open cct) so put in new cermet and changed R77 from 68k to 47k. The bias now set to 60mv.
The reverb is not working.
The following is some tests i have done:
Reverb pan
Tranducers , Input 59 ohms , 82 mH
Output 235ohms , 324mH
All leads and sockets checked out OK.

Test point readings for reverb section came up odd.

Tp19----15.8 mv
Tp20----16.6 mv
Tp21---- -o.26mv
Tp22---- -3.68mv

Any thoughts.
Thanks for listening

[Enzo]

Your reverb has two cables, one for drive and the other for return to the amp.

Pull the two cords from the reverb pan, turn on the amp with the reverb control midway. Now touch the tip of each cable. One of them should make hum. Does it? If it does, then plug in into the OUTPUT jack on the pan. At this point sshaking the pan ought to make spring crashing noises out the speaker. Does it? If all that is yes, then the return circuit works and drive is missing. If neither cord makes hum when touching tip, then return is missing.

[pdf64]

Here’s a 2001 manual from another mef thread http://badraven.com/Fender_Manuals/H...d%20Deluxe.pdf

[pdf64]

...
Test point readings for reverb section came up odd.

Tp19----15.8 mv
Tp20----16.6 mv
Tp21---- -o.26mv
Tp22---- -3.68mv
...

I suspect you may be measuring DC?
Voltages in square boxes are DC, oval are AC rms with the noted input and control settings.

[spuddo]

Your reverb has two cables, one for drive and the other for return to the amp.

Pull the two cords from the reverb pan, turn on the amp with the reverb control midway. Now touch the tip of each cable. One of them should make hum. Does it? If it does, then plug in into the OUTPUT jack on the pan. At this point sshaking the pan ought to make spring crashing noises out the speaker. Does it? If all that is yes, then the return circuit works and drive is missing. If neither cord makes hum when touching tip, then return is missing.

Hi Enzo , it was the former.
My guess would be U1 op amp is cactus.
Thousands of $ of test gear on my bench , and all i needed was my index finger.

Thanks for your input.
Best regards

[spuddo]

I suspect you may be measuring DC?
Voltages in square boxes are DC, oval are AC rms with the noted input and control settings.

HI pdf64

DOH , yes fell asleep at the wheel.

The new AC measurements were just as way off.

TP19------- 41mv
TP20------- 16 mv
TP21------- 0 .41mv
TP22------- 0 .41mv

I suspect U1 is cactus---any thoughts.

The schematic you sent is the same i have.

I found an schematic that gives an update and explains the components on my board that didn't exist according to '96 schematic.

Tried to attach but it appears i'm breaking forum rules

Thanks your reply and interest.

[The Dude]

If we're using the schematic in post #3, the reverb op amp is U2 (A for send B for return). "It was the former" isn't enough info for me to tell what you have going on. Did you get hum through the amp when you touched the cables? At any rate, I'd check voltages on U2 if you suspect a bad op amp. Generally speaking, if an op amp is shorted, it will have supply or something close on pins other than the supply pins (4&8).

[spuddo]

Hi "The Dude".

My bad , it is as you say U2.

The voltages i measured (second time) were AC and they were way off.

"it was the former " referred to Enzo's 1st test. It did as suggested.


"Now touch the tip of each cable. One of them should make hum. Does it? If it does, then plug in into the OUTPUT jack on the pan. At this point sshaking the pan ought to make spring crashing noises out the speaker. Does it? If all that is yes, then the return circuit works and drive is missing."

U2 pin volts , DC to ground

P1-----16.7mv
P2-----16.7mv
P3-----15.6mv
P4----- - 17v
P5----- -0.05mv
P6----- 0.28mv
P7----- -3.7v
P8----- + 17.8v

Thanks for your input

[pdf64]

...
P7----- -3.7v
...

That seems weird, but the return circuit is working (if I understood the ‘former’ reference?).

[spuddo]

Hi pdf64.

I had a bad hair day yesterday , stuffed up AC vs DC and quoted the wrong chip number.

The Enzo test suggested that if i got a hum from one connector , that was the input , if i shake rattled and rolled the reverb i would hear spring crashing noises , and i did .

The inference was that the "return circuit works and the drive is missing"

The AC measurements i made was.
Pin7---- 41mv
As per schematic
Pin7 (TP22) should read --- 3.43v.

The U2 chip i have is not as shown in the schematic , a Tl072 , it is a 4560 , both pin compatible and i believe the the 4560 has a higher slew rate.

I appreciate the help both you and Enzo have offered.

I promise not to use "former or latter" in any further dialogue.

Best regards

[Helmholtz]

TP19------- 41mv
TP20------- 16 mv
TP21------- 0 .41mv
TP22------- 0 .41mv

Just as an example: The reference AC levels at TP19 (870mV) and TP 20 (11.6V) tell us that U2A is supposed to have a gain of 11.6/0.87 = 13.

So if you have 41mVAC at TP19, you should get roughly 0.5VAC at TP20.

Measurement requires 1kHz test signal (TP20 reading depends on frequency).

With pan input disconnected U2A gain will increase to 83 (see schematic Note 6).

[g1]

With pan input disconnected U2A gain will increase to 83 (see schematic Note 6).

The note 6 references U2B. I thought the unloaded gain of U2A would be 1 + (4700/47), which comes to 101 ?

[g1]

The Enzo test suggested that if i got a hum from one connector , that was the input , if i shake rattled and rolled the reverb i would hear spring crashing noises , and i did .

The inference was that the "return circuit works and the drive is missing"

No, he said the one that hums goes to the output. If that was not a typo on your part, go back and connect that cable (the one that hums) to the tank output and make sure you still get the crashing sounds from the output side of the tank.
Yes, the inference that the issue is in the drive circuitry is correct.
The means we can stop looking at the U2B side and concentrate on the U2A side and what comes before it.

[The Dude]

Also check RCA cables to tank. It could be as simple as a bad cable.

[Enzo]

I specifically said the one that hums plugs into the OUTPUT jack on the PAN. One could consider it the input to the amp chassis I suppose, but I don't use that form.