FenderFM212R reverb problem.

[hs5424]

Hi All,
Anyone out there did any fault finding on the above amp with no reverb. I suspect the spring reverb tank is faulty but want confirmation before ordering a new one. Scoped that audio is present at the i/p to the tank (sinewave gets distorted when the tank is connected). I also injected a tone on the o/p of the tank which I can hear on the amps speaker.
Any advice appreciated.

[CapLeaker]

could very well be that there is something wrong with the spring tank. I worked on a few of these, but never had a problem with the spring tank itself. you see anything on the output of the spring tank?

[hs5424]

Hi Capleaker,
Thanks for your reply and schematic.
I have a schematic and went through the normal process of signal tracing. I replaced U4 and U5 as I thought this would be a quick and cheap fix as these handle the i/p ond o/p of the tank reverb. I injected a tone on the o/p tank socket and this gets through to the speaker ok. I tried injecting a tone (experiment) on the tanks i/p but nothing appears at the other side. The i/p and o/p on these tanks have different impedances so I realise that my test may not be very scientific or conclusive. I found some DC resistance figures for the tank online which shows the i/p dc should read 36 ohms. It check out to be 66 ohms. The o/p should be 200 ohms, it checks out to be 176 ohms. I am suspect that the i/p side of the tank is the culprit so I have ordered a new one. What is really puzzling me is if I physically shake the tank I cannot hear the normal 'broken glass' sound at the speaker. Scoping the tanks o/p disconnected shows an AC signal.
I will update when the replacement arrives. Fingers crossed.

[CapLeaker]

Hm… did you open that spring tank up yet? If not, open it. I wonder if there is a wire that came undone.

[hs5424]

First thing I checked.
Both ends are closed circuit which I measured on the rca sockets.

[hs5424]

UPDATE.
New tank arrived and fitted but alas still no reverb so I had a look at the o/p side and found that the reverb pot was 50k and not 250k, so a man made fault. Now working.
For the life of me I cannot tell the difference in sound reverb no matter what way I connect the tank. The original unit has a red RCA on the side marked IN, whereas the new one has a red RCA marked as out. The original ones markings are obvious as the red RCA has signal coming from the preamp stages so I would call it i/p also.
Puzzling.

[CapLeaker]

Weird. So the amp works, The output and input of the reverb circuit works, installed a new tank, but still no reverb / echo sound? Did somebody maybe mod this speaker, or there is something broken in the reverb circuit.

[hs5424]

To clarify.
I had replaced the reverb potentiometer but it was marked with the incorrect value. Replaced with the correct value and reverb worked again. I also tried the original tank which is working also. It doesn't seem to make any difference which way the new tank is installed, ie in/out rca plugs.

[CapLeaker]

Ahh! Got it now! At least you got her going again. These pots inside these amps are problematic. The thin waver cracks inside and then same problem, or the -16V is gone. Good job!

[hs5424]

I found it difficult to source fender pots at a reasonable price for theses amps so I bought a load of different values from China. If you remove the mounting legs from the original pots and with a little bit of basic engineering you can fit the legs to the new pots. Job done for little money.

[CapLeaker]

I found matching ones on Ali. Works fine as kind.

[mon2]

Hey! Have one of these in the shop now. We are servicing more and more audio equipment locally in our city. As a FYI, you can purchase the replacement stereo and mono PCB mount jacks from Farnell (aka Avnet). Just replaced each of the 20 year old plastic worn out ones yesterday with the more rugged metal versions of the same. Drop in replacements from Amphenol.

Sharing the part #s for future readers:

ACJS-MHDRM
AMPHENOL AUDIO ACJS-MHDRM Phone Audio Connector, 3 Contacts, Jack, Panel Mount, Tin Plated Contacts
* this is with 6 contacts total (2 x 3) = stereo - there are 3 of these connectors on the 212R

ACJM-MHDRM
AMPHENOL AUDIO ACJM-MHDRM Phone Audio Connector, 1/4" Mono, R/A, 2 Contacts, Receptacle, Panel PCB Mount
* this is with 4 contacts total (2x2) = mono - there are 2 of these connectors on the 212R

Our unit in for repair was manhandled by other techs and the pots are with the wrong values. About to replace with Bourns pots (mix of log and linear style must be used). The Bourns do not offer the hind legs for the support but have to be better than the way too loose pots onboard.

Bourns we are sourcing from Digikey. Will update this thread after the repair.