Silvertone 1484 Tremolo Help

[Helmholtz]

If tremolo works with power tubes in we're on the safe side, so put them in for now.

[misterc57]

Now pull both power tubes (this will increase DC voltages) and scope again.

Next test will be increasing V6 pin 2 drive by paralleling a 330k resistor to R54.

Alternatively increase V6b gain by tacking a 47µ cap across R56 (27k).

THese are tests to see if the roach is still useable.

What fun! Now test 1 (pull output tubes) no longer delivers a pulse on the scope at any intensity setting.

Test 2 brought R54 to a value of 210K. I tried this test with output tubes installed and output tubes removed. Either way, no pulsing on the scope.

V6 pin 6 pulsing voltage range is 145 V to 158 V with all tubes installed.

Is it time to dissect the roach? Anything else to try?

Thank you!

[Helmholtz]

Test 2 brought R54 to a value of 210K. I tried this test with output tubes installed and output tubes removed. Either way, no pulsing on the scope.

Ok, seems the LFO doesn't like the additional load. So leave R54 alone and try the cathode cap across R56 and verify 27k resistance.

[misterc57]

Ok, seems the LFO doesn't like the additional load. So leave R54 alone and try the cathode cap across R56 and verify 27k resistance.

R56 (27K) measures 78K in circuit. Should I lift a leg for a better reading? I will try the 47uf cap across R56.

We are doing this with output tubes installed?

Thank you

[Helmholtz]

R56 (27K) measures 78K in circuit. Should I lift a leg for a better reading? I will try the 47uf cap across R56.

We are doing this with output tubes installed?

Resistors can never read higher in circuit than out.
First replace R56 with correct 27k. If no tremolo try bypass cap. Put output tubes in.

[Enzo]

One thing I always do when measuring a resistor in circuit that gives me a very high reading: I quick reverse my meter leads. If I get the same high reading, then the part is at fault, if however I get a substantially different reading - or even a "negative resistance" reading - that means some small residual voltage is lurking in a cap in the circuit to confuse my meter.

[Helmholtz]

Good point, Enzo.
I was in a hurry and forgot to mention meter artefacts by residual voltage. But that can't happen with R56. So if it reads high in-cicuit it's either drifted high or Mr.57 had bad probe contact.

Point is that the circuit environment can only lower resultant resistance, not increase it.

Always a good idea to check for zero voltage before using an ohmmeter.

[misterc57]

Great tips on reading resistance. Thank you.

So, I can physically see that R56 is a 68K resistor. Yet another deviation from the schematic. So, should this really be a 27K?

Other deviations found from schematic.

There is no R57
-1.5V at R60 and R61 is wrong
V4 pin 6 connects to C25B, not C25C
V6 pin 1 should probably connect to C25B, not C25C

[misterc57]

I am attaching a different schematic that more closely matches what I am seeing in the tremolo circuit.

[Helmholtz]

So, I can physically see that R56 is a 68K resistor. Yet another deviation from the schematic. So, should this really be a 27K?

Fender uses 56k//25µ at the cathode and we know that circuit works. Try the bypass cap. With higher resistor value 25µ is fine.

New schematic looks better but voltages still seem inconsistent.

To bring the LFO bias closer to the specified -1.5V you may try to increase R61 to 4.7k. This will also increase plate (pin8) voltage.

[misterc57]

I attached a 20uF 50V that reads 22uF from V6 pin 3 to ground. Still no pulsing on the scope.

[misterc57]

I am trying to learn as much as possible about the tremolo circuit. Came upon this article http://www.valvewizard.co.uk/trem1.html

At the bottom of the page it describes adding a red led for "A visual frequency / rate indicator". Would this be helpful in my application? The 1484 circuit does not seem to match what they describe as there is no cathode resistor biasing.

That aside. Is it time to dissect the roach to try to visually see its flashing behavior?

Thank you

[Helmholtz]

That aside. Is it time to dissect the roach to try to visually see its flashing behavior?

If increasing the LFO grid bias to around -1.5V doesn't help, yes.

[misterc57]

To bring the LFO bias closer to the specified -1.5V you may try to increase R61 to 4.7k. This will also increase plate (pin8) voltage.

I missed this yesterday. Will give it a try. Thank you

[misterc57]

Fender uses 56k//25µ at the cathode and we know that circuit works. Try the bypass cap. With higher resistor value 25µ is fine.

New schematic looks better but voltages still seem inconsistent.

To bring the LFO bias closer to the specified -1.5V you may try to increase R61 to 4.7k. This will also increase plate (pin8) voltage.

No changes to tremolo issue. This was tested without the 25u cap at V6 pin 3 to ground. Which I had tried previously.

With original 2.2K R61

bias voltage - 0.68
pin 6 voltage 171 to 182

With 4.7K R61

bias voltage - 1.39
pin 6 voltage 180 to 189

I also tried to parallel a new 100uF cap at C30. There was no change in voltages. Is this because the old cap is still in there?

Note; New schematic has V6 pin 6 and 8 cross labelled.