Wurlitzer 200A Help Repair/Troubleshooting

[fenderguy81]

I have a Wurlitzer 200a on the bench that sounds like it is exhibiting a bit of cross over distortion. The tail end of the notes are distorted and playing full chords sounds distorted. I have already replaced all of the electrolytic capacitors, transistors, bias resistor(s) and even put in replacement speakers. I will provide the voltages of the transistors and IC below. I'm not sure how specific some of the tolerances should be but nothing really jumped out at me. I am thinking it could be a faulty coupling cap or noise resistor or? Any help would be greatly appreciated! Thanks!

TR	E	B	C	IC (pins 1-14)
1	1.923	2.447	3.98	0
2	3.356	3.988	8.45	14.08
3	.636	1.187	OL	14
4	0	.636	OL	7
5	x	x	x	7
6	x	x	x	7
7	.619	.019	-23.88	0
8	.619	.015	-24.47	0
9	-.602	.007	.579	7.66

10	-.003	.579	23.71	14.08
11	24.33	23.80	-.002	24.47
12	-.010	-.601	-23.89	24.47
13	-24.46	-23.88	-.010	15.33
14	-23.43	-23.84	-.601	0
15	.992	1.539	3.760	x
16	2.580	3.761	7.980	x

Bias resistor R34 (x for paralleled R58): 205 ohms
4.9mv across R37
4.8mv across R38

 

[CrisNich]

And the AUX output? If you can plug in to a mixer, and listen... is also distorted sound?

 

[fenderguy81]

And the AUX output? If you can plug in to a mixer, and listen... is also distorted sound?

I tested the headphone and AUX outputs this morning through a guitar amp. The headphone out sounded even more distorted than the Wurlitzer's internal speakers and the AUX output was a little low in volume so it was hard to tell if the internal speakers were distorting or the AUX. I temporarily disconnected the Wurlitzer's internal speakers and the signal sounded clear through the AUX output.

I came across another forum post on this site that sounds like a similar issue: Troubleshooting Wurlitzer 200a amp board for bias and crossover notch distortion

One user stated that they figured out a temporary solution by using a 1k trim pot across R58 with a 270r resistor on R34. I tempted in a 300r resistor on R34 with a 1k pot on R58 so I could adjust the bias in real time to see if I could hear any audible difference. Once the Wurlitzer was powered on the resistance jumped from the ~205r setting I had set the pot to, to ~8.6M. I slowly swept the pot up and down and did not hear any difference so I do not know if my problem is necessarily in the bias network like the other user's problem was linked above?

 

[CrisNich]

Remove the capacitor "8", and connect that point to your guitar amp. See if have distortion, now.

 

[fenderguy81]

Remove the capacitor "8", and connect that point to your guitar amp. See if have distortion, now.

I removed the 4.7uf cap "8" and the aux signal is still clear on the guitar amp.

 

[CrisNich]

If you remove the cap 8,and signal is ok, so is not from before this point, the distortion.

 

[CrisNich]

maybe you can post here a little take of this sound, the distorted sound ?

 

[tommia]

There is a youtube movie that explains what to do. The crossover distortion on the output amp is not caibrated and all over the place. I think it was from vintage vibe or vintage keys, has been a while

 

[fenderguy81]

maybe you can post here a little take of this sound, the distorted sound ?

View attachment Wurlitzer copy.mov
It is most noticeable on lower chords. I checked every resistor and all are within 5% . I installed a small 1k trim pot to make the bias easy to adjust and reflowed all of the solder joints on the main board. I then cleaned all the traces with iso alcohol to help avoid any solder bridges or irregularities that these boards are prone to. Still no luck...

 

[fenderguy81]

There is a youtube movie that explains what to do. The crossover distortion on the output amp is not caibrated and all over the place. I think it was from vintage vibe or vintage keys, has been a while

Vintage Vibe has a few videos on Youtube that I have checked out regarding cross over distortion. They provide a good explanation of the bias network and how it relates to cross over distortion. They also show the use of a scope with a signal generator which I think is what I probably need to better diagnose/troubleshoot this problem. I've tried a wide selection of values and now have a trim pot to easily adjust the bias, yet I still can't seem to fine tune to a sweet spot that cleans up the signal. The schematic calls for 1-10mv across r37 & r38. I have them closely matched at 4.9mv and 4.8mv respectively with the bias resistor set at ~205r.