Music Man 2275-130 Tremolo Question.

[peterpicker]

I am new here. I have purchased an HD 210 130 MM amp. Apparently the amp works fine, but the tremolo isn't working. I have the schematic before me and it appears the oscillator is a network with a transistor and a FET. When I receive this amp and assertain that it isn't something obvious that is causing a problem, what would one generally look for in this circuitry that might be a common issue? I don't have scopes and fancy testing equipment, so it would have to be volt-ohmmeter stuff.

[Jazz P Bass]

Doesn't it need a footswitch to turn the Tremelo 'on'?

[Enzo]

Looks to me like it is free-running, and the FS grounds off the output after the JFET.

Hi, Peter, welcome to the forum. Always a good idea to link to a schematic when discussing anything. Here it is:
http://www.webphix.com/schematic%20h...6_2275-130.pdf

I see a simple oscillator driving the gate of a JFET. First isolate the problem, either the oscillator is not running or it cannot turn the JFET off and on. Turn the speed down qand measure voltage at the collector of the oscillator transistor. Steady DC (bad) or some sort of AC (good)? Power off, is the gate of the JFET shorted? Find the 150k resistor, does it seem to be "shorted"?

I tend to doubt the little caps are bad, maybe, but I don;t think so. You can verify the resistors easily enough. And check the two pots. Using the diode test function of your meter, check the oscillator transistor.

Power on, the thing runs on +16v and -16v. You got both on the circuit?

[peterpicker]

Looks to me like it is free-running, and the FS grounds off the output after the JFET.

Hi, Peter, welcome to the forum. Always a good idea to link to a schematic when discussing anything. Here it is:
http://www.webphix.com/schematic%20h...6_2275-130.pdf

I see a simple oscillator driving the gate of a JFET. First isolate the problem, either the oscillator is not running or it cannot turn the JFET off and on. Turn the speed down qand measure voltage at the collector of the oscillator transistor. Steady DC (bad) or some sort of AC (good)? Power off, is the gate of the JFET shorted? Find the 150k resistor, does it seem to be "shorted"?

I tend to doubt the little caps are bad, maybe, but I don;t think so. You can verify the resistors easily enough. And check the two pots. Using the diode test function of your meter, check the oscillator transistor.

Power on, the thing runs on +16v and -16v. You got both on the circuit?

Thank you for the replies. For the first one, yes it will have a footswitch and when I receive the amp next week, that is what I'll check first, to see if there is an issue with that. As far as the oscillator is concerned, it has been over 40 years since I was into this sort of thing. Could I have a brief overview on how the oscillator works? Maybe even a Youtube reference. The second question I have is if that oscillator circuit biasing the IC amp LM307?
Changing out the transistor and FET would be pretty cheap if nothing is obvious I suppose.

[loudthud]

Good info on the MusicMan tremolo in this thread: http://music-electronics-forum.com/t12356/

Included is a way to test JFETs with an ohm meter.

[peterpicker]

Loudthud, I opened up the amp today. Of course it physically doesn't look like the schematic, so I'm scratching around finding the points of interest. The details call for a 2N3391 transistor. If I choose to try another one, will a 2N3392 work equally, as that is what the local supplier has in stock. As far as the JFET is concerned, he doesn't have the ..91 in that but a ..92 also. Again, is this ok? I can only see one trim pot in the neighborhood, but one sitting in another area, so I'm not sure about that yet.

[loudthud]

The 2N3392 has lower beta than the 2N3391 so it may not oscillate or bias correctly. The 2N4092 has a slightly higher on resistance than the 2N4091 so tremolo might not be as deep. You could try a pair of 2N4092s in parallel if that is a problem.

[peterpicker]

The 2N3392 has lower beta than the 2N3391 so it may not oscillate or bias correctly. The 2N4092 has a slightly higher on resistance than the 2N4091 so tremolo might not be as deep. You could try a pair of 2N4092s in parallel if that is a problem.

I will try and get the original JFET and transistor to be sure. I have trouble visualizing the schematic versus the layout at present since the connections are underneath the screwed down board. It is confusing for me also to measure components since they will always be in parallel with something else to not give the correct resistance for example. Also are the zenor diodes and all that jazz suppling the correct voltages

[loudthud]

There are three wires from the board to the footswitch jacks. Unsolder these at the board after carefully noting which color wire goes to which eyelet. Now you can remove the screws and tilt the board up leaving all the wires connected to the pots to gain access to the backside.

The zener diodes regulate the +/- 46V down to +/- 16V. Since one side of each diode is grounded, you can simply measure the voltage across the diodes, it should be 16V +/- about 5%. The two 620 ohm 2 watt resistors will get quite hot. On my amp they have discolored the PCB slightly. I would suggest you replace them with 3 watt metal oxide or 5 watt ceramic types. Mount them up off the PCB for better heat dissipation. If you replace the zeners, they should also be mounted up off the board for the same reason. Since the amp seems to work, there probably isn't a problem in this area.

[peterpicker]

There are three wires from the board to the footswitch jacks. Unsolder these at the board after carefully noting which color wire goes to which eyelet. Now you can remove the screws and tilt the board up leaving all the wires connected to the pots to gain access to the backside.

The zener diodes regulate the +/- 46V down to +/- 16V. Since one side of each diode is grounded, you can simply measure the voltage across the diodes, it should be 16V +/- about 5%. The two 620 ohm 2 watt resistors will get quite hot. On my amp they have discolored the PCB slightly. I would suggest you replace them with 3 watt metal oxide or 5 watt ceramic types. Mount them up off the PCB for better heat dissipation. If you replace the zeners, they should also be mounted up off the board for the same reason. Since the amp seems to work, there probably isn't a problem in this area.

Hi, here's my analysis to date:

Transistor tested with diode checker showed B <-> E as 723 and OL. B <-> C as 719 and OL.
JFET tested with diode checker showed Gate <-> Source 740 and OL. Gate <-> Drain 739 and OL.

At the zener controlled area: The voltage from ground to top zener was 17.2 vdc. The voltage from ground to bottom zener was -7.3 vdc. This sounds bad as it should be -16.
Measuring the zeners with diode checker in circuit found the top one was 770 and 1591. The bottom one was 567 and OL. The two 620 resistors measured 668 and 664 in circuit.

The orange leads from the transformer were 60.8 vac.

Another issue is that with the master volume set at 10, as the volume controls in either channel reach around 5, there is an exponential increase in pretty loud white noise, like......... "surfs up dude".

[loudthud]

The transistor and JFET look ok. Hint: Its always a good idea to test transistors from E to C just to check for a short. A JFET will measure a low resistance from D to S. The resistance depends on the JFET, it would be anything from 1000 ohms to 5 or 10 ohms.

Something is drawing too much current pulling down the -16V supply. Since the opamps are in sockets, remove them all to see if the -16V comes back. Could be a capacitor across the supply or a bad zener.

If the -16V is ok with no opamps but fails again when you install the opamps, the problem could be a bad opamp, but an opamp usually draws current rail to rail so a bad one will usually kill both supplys unless something is shorting the output to ground. You might try pulling the opamps (with power off) one at a time to see if the minus 16V suddenly pops back to -16V.

The part that is drawing too much current might be getting hot. One way to find it is with a digital thermometer. Another low tech way is with thermal fax paper. Cut a piece about the size of the board, lay it on the board for a couple of minutes with power on, it will turn black near the hot part.

[peterpicker]

The transistor and JFET look ok. Hint: Its always a good idea to test transistors from E to C just to check for a short. A JFET will measure a low resistance from D to S. The resistance depends on the JFET, it would be anything from 1000 ohms to 5 or 10 ohms.

Something is drawing too much current pulling down the -16V supply. Since the opamps are in sockets, remove them all to see if the -16V comes back. Could be a capacitor across the supply or a bad zener.

If the -16V is ok with no opamps but fails again when you install the opamps, the problem could be a bad opamp, but an opamp usually draws current rail to rail so a bad one will usually kill both supplys unless something is shorting the output to ground. You might try pulling the opamps (with power off) one at a time to see if the minus 16V suddenly pops back to -16V.

The part that is drawing too much current might be getting hot. One way to find it is with a digital thermometer. Another low tech way is with thermal fax paper. Cut a piece about the size of the board, lay it on the board for a couple of minutes with power on, it will turn black near the hot part.

Hi again. Yes I did change the zeners and now the +/-16 volts are good and the tremolo works. I have ordered some electrolytics as many have said that old ones should be replaced for good measure. There is a lot of white noise with the master up to 10 and as the volumes are raised above 5 in either channel. This, I'm not sure why yet.

[jbltwin1]

YOu DO realize taht there is an adjustment pot on MM's JUST for the trem. Very common for them to get bumped, dirty, out of adjustment. I'm just sayin'. Mike.

[peterpicker]

YOu DO realize taht there is an adjustment pot on MM's JUST for the trem. Very common for them to get bumped, dirty, out of adjustment. I'm just sayin'. Mike.

Yes I saw that right away thanks and it did no good until I changed the zener. I even tweaked the bias trim pot since to get the 25mv suggested. I have some elecrolytics in the mail and will be swapping those too. Aside from the electronics, I have a small electroplating kit arriving to refinish the handle supports and the chassis straps. Luckily the Ernie Ball corners fit perfectly. This thing may never sound good, but it will look pretty.