Problem with Laney 320

[betweenthees]

Picked up a Laney tfx 320. Trying my best to troubleshoot it, was hoping for some more ideas to throw into the pot.

This amp is Tube pre-amp with solid state main. The tube is a ecc83 (similiar to the 12ax7)

PRoblems: about 3 sec after turning on, it begins to hum. The hum stays consistant as long as the amp is on. none of the controls effect this hum.

The amp still functions despite the hum. The main volume/high/mid/low controls work. BUT, the preamp seems to have no effect. none of the gain/drive conrols do anything, and also the LED's that indicate drive/gain stages do not light up.

Heres some thins ive looked at already:
1: All fuses are fine- double checked them all
2: Annode/plate voltage for the tube is at 317(max) which is below the max rating for the tube
3:tube heater V are 13v (high, but shoudl be fine)
4:tube hums the same with or without the tube (pulled the tube to check this)
5:drop resistors on the preamp baord run very hot. They are 2x 470 5w. They are part of the curcuit that takes 50v ->drops/filteres to 17v for preamp board supply.


My next move will be to replace the filter caps on the pre-amp. The current filter caps are only rated 16v, while it seems 17.5v are present. This will require me to totally dissasemble the amp to reach these compents.
Before i dive in i wanted to see if anyone here had some ideas. Any little quick check or extreme idea you can think of will be appreciated.
I do have a schematic of the amp. could post a link to the PDF if anyone would like to see it.

If there is any other information you need to assist your thought, let me know and ill post it for ya. Thanks for reading.

[J M Fahey]

I do have a schematic of the amp. could post a link to the PDF

Please do it or we are in the absolute darkness.

[betweenthees]

attached is the PDF of the schematic-

[betweenthees]

about 1/2 way down on the right of page 3 of the shematic you will see r54 and r53 (they will have the FP fire proof label on them). These are the resistors that I said were running hot

Directly to the left of the resistor their are caps c34, c35. These caps are rated 16v. I am reading 17.5 V and am considering replacing them with higher rated caps.

[J M Fahey]

Don't change parts at random, you'll never end that way.
1) Maybe I'm going blind but can't find the indicator Leds on the schematic.
Find them for me and tell me "they are to the right of R33 and on top of C45 pn page 3" or something like that.
2) Those 17.5V might indicate something, or not.
Your switching circuit is not working, it rests on "clean"
Check what voltage does U2 (CD4069) have on every pin.
It *should have 0V on pin 7 and +15V on pin 14, but post voltage on all.
Some voltages should change from +15V to 0 and viceversa depending on the footswitch or the channel change switch or the reverb switch.
Draw a small sketch , download the datasheet to be sure about the pinout.
Good luck.

[betweenthees]

thanks for the feedback. I checked out the datasheet for the cd4069, got the V and heres the results

1- 17
2-.1
3-0
4-17
5-17
6-0
7-0
8-17
9-0
10-17
11-0
12-17
13-.7
14-17

i could record the V on each lug for different swtich postions if that would help-

as for the LED indicators... i think i have located 2 of them. to the right of u2-d... it seems to be assumed to be between p10 and p11 right above where it says "drive 2"
also right below that, between p8 and p9. Thats where the LEDs acutally are when I follow the wires.

[J M Fahey]

Ok, let me "map" those voltages to Fet switches being on or off.
The fact that some pins are "high" (+15V) and some are "low" (0V) tells me that the IC seems to be working.
Why it does not change state when told so is the problem to analyze.

[betweenthees]

was taking some more V readings, and some of the pins do switch from "high" to "low" and vise versa when switched. The only 2 pins of note where 2 ,9, and 13. Pin 2 had a const V of .1 regaurdless of switch pos. and pins 9 and 13 had a "high" of only .8v  (low of 0).
The rest of the pins that did switch V went from 0->17 or 17->0. Some pins had a const V of 0 or 17.
You have inspired me to thourghly inspect this area today.

[betweenthees]

Im looking at how the values from cd4069 correspond to the fets. Based on the schem note that "switching fets. high gate V = off" it seems that some of the fets are "stucK" off. There are several places on the cd4069 with const high or low V.

On the right side of the schem at the top (below the chip diagrams) there are 4 transistors (pf3820). each one is associated with a,b,c, or d. These are points from the switch curcuit we have been exploring. Based on what im seeing, A is has a const Low V, and b,c,d have const high V. It would seem that the 3 of these transistors that have high V are stuck off, and they seem to be the ones associated with the drive/gain states. the 4th, associated with A is stuck on, which seems reasonable to my eye.

The huge assumption i made was that pf3820 is in fact a switching fet. If im reading this right, would that point to a bad cd4069 chip?

[J M Fahey]

Excellent analysis.
That's the way to solve problems, by analyzing *the function* it "should" do, check whether it does it or not, and only afer that start to guess what part might be bad.
The 4069 seems to be working properly (I had half expected all voltages = "0");
Check that Fsw or push switch do their job:
A Node is where many par6ts meet.
At the node D3/R9/R10 you must have either 5V (switches open) or 0V (either switch closed) Do you?
On TS1 collector you should have either 15V or 0V Do you? Does it react to switching?
That's the kind of questions you must answer.
If not, why?
Voltage on TS1 collector should change *and* related voltages on 4069 should too.
TS1 and all inverter gates on 4069 *are* inverters (duh!), meaning:
0V in=15V out
15Vin=0V out.;  in TS1= 5V in through R9 or 0.7V on its base, but the end result is the same.
You'll soon find why the gate voltage on switching Fets is stuck, probably a cracked solder or track , some failing connector or the needed +15V is not reaching TS1.
Apply logic, 1000 times faster (and easier) than replaciong at random.
Good luck.

[betweenthees]

This hour's developments....

Im going to call the node of d3,r9,r10 Node 1 for this post

There was a bulky soldeir joint close to node 1 that was just slightly touching it. This bulky soldier joint was a ground point, and thus Node1 was permently groudned=> this explains perfectly why i was stuck on the clean channel! So i got out my exacto-knife and gently cut the connection between node 1 and the bulky soldeir. Wala! Node 1 now had a 5v reading! this in turn swapped the values for A and B with the result of making the gain/drive channel active.

switching between the gain and drive works correctly. Those were the C and D values going to ts6 and ts8. Not at the end yet though. Now im stuck on the gain/drive channels. but i know why before i even start to look.
Heres the logic-
When node 1 was perm grounded, it was on the clean channel. after the fix, Node 1 is perm ungrounded, and stuck on the gain/drive channels. Makes me think the Channel switch (labeled clean/drive and is just to the left of node 1 on the schematic) is not working properly.
Thats where i am now- next move is to wrap my head around that switch.

[J M Fahey]

EX-CEL-LENT !!!!! 
Way to go !!! 
Now you only have a "mechanical" problem.(and you already are *sure* that the electronics part works, specially the 4069 and the FETs).
Follow the tracks , wires and connectors.
To confirm what you just found,  ground Node 1 with a small piece of wire, it should switch channels back and forth.
Congratulations.

[betweenthees]

The switch has been repaired! all swiches are now working properly and all channells are avalible for use! so we have conquered the first problem on this laney. With the switching curcuit repaired, its time to move onto the hum.

[joecool85]

The switch has been repaired! all swiches are now working properly and all channells are avalible for use! so we have conquered the first problem on this laney. With the switching curcuit repaired, its time to move onto the hum.

Excellent!  Keep us posted with your work.  It's great to see repairs done on equipment that might have otherwise been tossed.

[betweenthees]

Ran out of time to dedicate to this project I had a 5 day gap between my other work and was hoping to get further on this project.

I have altered the preamp switching curuit so that the channels can be selected. However, after a few more hours of tracing around, i think i have tracked down a possbile grounding issue on the effects board.

Turns out the switch was stuck on ground, not stuck on open. I altered the switching curuit to work, but afterwards found out that it wasnt acually the switch! that is when i traced it back to the fx baord-
Now my inbox has been re-flooded with alternative fuel research needs. Im not done! but by progess will be much slower from here on out.