Fender Hotrod Deluxe reverb problem

[Swampy]

Hey everyone

Customer complains about quite reverb...can only really dial in an atmosphere with the reverb control wound up past 3/4's.

Traced a signal through...

870mV pk to pk @ TP19
1.4 v pk to pk @ TP20

verified + and - 16v on U2, replaced C14, replaced U2, tried a different reverb tank with same specs, verified cables to tank are not partially / shorting, verified R33 and R34. Verified conductivity from pcb to tank and back.

Traced a signal with tank pulled
870mV pk to pk @ TP19
30V pk to pk @ TP20

Hot_Rod_Deluxe_schematic.pdf


kinda stumped with this one...any suggestions?

[Amp Kat]

Footswitch jack

[pdf64]

Those Vac readings are with a 4mV 1kHz sine at the input?
Pete

[Jazz P Bass]

A reading from TP 22 & 23 (reverb return) would help clarify where the issue is.

[MarkusBass]

That's funny - I have Hot Rod Deluxe with very simmilar problem. But the reverb circuit in this amp consists of intergrated circuits. It must be a different version. In my case the problem may be caused by the famous 5W resistors, which heat up to 120 degrees Celsius.
In you case I would check whether input and output to the reverb tank are not reversed. I had such a case and the symptoms were exactly as you have.

Mark

[Enzo]

Well, he linked two schematics, one for a Hot Rod Deluxe, and one for a 65 Deluxe Reverb. Then the title of the thread is Hot Rod Deluxe Reverb.

[Swampy]

Yes this is a Hotrod deluxe, 65 deluxe schem was an error.

So no footswitch jack for verb.

ran another wave though, this time readings are rms.

TP19 - 870mV rms

TP20 - 1.4 V rms

TP 21 less than 5mv...dirty waveform.
TP 22 44mv rms


also verified gain of U2B at approx 55 by disengaging tank, injecting signal into reverb return and measuring

TP21 43mV (clean waveform)
TP22 2.4V rms
(2.4 / 0.043 = 55).

[MarkusBass]

So you have the same amp as I have. But it is asembled and waiting for the customer to take it. Looking at what you write it seems that U2 cannot provide enough signal for the tank. Have you measure its resistance? Here are some parameters of this reverb tank: http://music-electronics-forum.com/t14283/#post112784 . Have you checked the input impedance? I would just put 600 Ohms resistor instead of the input section of the tank as check the voltages again. You have to get over 10V on the input of the reverb tank.
There is one very important issue with this amp: incorrect value (470 Ohms) of 5W resistors that drop 48V voltage to 16V. Each resistor disspated 2.2W power on a permanent basis. This is something that these resistors cannot handle - they heat up to 120 degrees Cesius and even sometimes desolder from the board. And the board starts changing its colour first to yellow and then to brown. Also the Zener diodes were very hot (over 100 degrees). I changed the resistors to 680 Ohms/5W and soldered them in a distance from the board (5mm). And now everything is fine - the temperature is below 50 degrees. Maybe it's not the case you have but some problems with this amp and its reverb are a result of desoldering of these two resistors (due to heat).
You may also check that the input connectors are insulated and the output connectors - one of them is grounded.

Mark

[Jazz P Bass]

What happened at TP20?
The original post stated 30V p-p.
O/k. About 11V RMS
In post#7 in went to 1.4V.
That IC may be bad.

[52 Bill]

What happened at TP20?
The original post stated 30V p-p.
O/k. About 11V RMS
In post#7 in went to 1.4V.
That IC may be bad.

I think that the 33 volts was with no tank loading the output of the chip, it drops to a few volts when loaded. He already replaced the chip earlier on.

I think that Mark was asking a good question regarding the tank, are we sure that the tank is the correct one for tha amp? If the input impedance is too low it will load down the signal, or if the input jack is grounded and not isolated it will kill the reverb.

[Swampy]

I think that the 33 volts was with no tank loading the output of the chip, it drops to a few volts when loaded. He already replaced the chip earlier on.

I think that Mark was asking a good question regarding the tank, are we sure that the tank is the correct one for tha amp? If the input impedance is too low it will load down the signal, or if the input jack is grounded and not isolated it will kill the reverb.

Ah yes, reverb tank input impedance is incorrect...should be 600 ohms, currently is 8 ohms.

[g1]

It should be a 4EB3C1B tank. 600 ohms is the impedance, but the DC resistance you measure with your meter will be low, more like what you are reading.
There is a chart about half way down this page that shows the impedance and resistance values: Spring Reverb Tanks Explained and Compared | Amplified Parts