Fender Blues Deluxe Reissue channel switching issue.

[jvm]

I have a Blues Deluxe RI not switching channels properly. It has VERY low clean headroom in the normal channel. ( Low even for a Blues Deluxe). Test point 32 shows -6VDC instead of -16VDC, ch select in, and +16VDC instead of +7.5VDC. Test point 33 I have -13 VDC in "drive" mode and +16VDC in normal mode. Both low voltage supplies are fine. -16 and +16volts. I don't seem to find any component failures in either IC circuit, U3 or U4. I have replaced U3, U4 and K2. I have attached the complete service manual. I don't see any differences between the revisions as far as the switching circuitry. Any help appreciated.


Blues Deluxe Reissue Service Manual.pdf

[mozwell]

OK your +/-16V are ok
With power turned off & power supplies discharged, can you measure the coil resistance of each relay K2 and K1, they are in parallel, so i guess that's all we can measure. 24V signal relay may have a coil resistance of 1600 ohm. If you measure 800 ohm, then the two coils are ok. If the relay has a different resistance, the parallel figure will be different, The service manual doesn't list the relay part number, so check the board, google the data sheet & confirm the coil resistance

So lets measure some more stuff
1. Power supply on U3, do you have +/-16V
2. DC voltage at U3 pin 6, what does this change to when you press the "channel select" switch
3. DC volts at anode of D20 or TP31, do we get approx 1.4V
4. DC voltage of U4 pin 1, this should have a very quick signal change when you change channels, but should then go to -16V
5. DC volts on TP33, in clean & dirty channel modes
6. Instead of measuring TP32, it makes more sense to measure the DC volts across D16, 0V in clean, 24V or so in dirty

Lets see how we go from here, lets not worry about low headroom for now, get the channel switching working properly first

[jvm]

Thanks for the input, mozwell!
Here are the results:
1. I do have -16/+16v on U3
2. U3, pin 6 changes from 0v to 2.78v when ch switch activated
3. TP31 shows 1.39vdc
4. U4, pin 1. -15 to -14.9 when changing channels
5. TP33, +16.4v clean, -13.3v dirty ch
6. Voltage across D16, 0v clean, -20.9 dirty
I measured 966 ohms across the parallel relay coils.

[mozwell]

Hi JVM
Can you confirm what is printed on the actual relay itself, for now lets assume they are 24VDC coil relays
We have 20.9V across the relay coils, and the parallel resistance would indicate both coils are ok
So now lets look to see if the contacts are actually switching

We have 4 contacts to look at, these will be easiest to check with power on & a signal applied, set master R26 to minimum, and all tone controls to maximum
1. K1A, set R6 volume all the way down, change channels & measure from wiper of R6 to 0V, or across R52, we should change from 0VAC to some VAC depending on channel
2. K1B, set R6 volume to half way, change channels & measure across C53, we should change from 0VAC to some VAC depending on channel
3. K2A, change channels & measure TP13, we should change from 0VAC to some VAC depending on channel
4. K2B, set R7 drive to minimum, change channels & measure TP7, we should change from 0VAC to some VAC depending on channel

The measured VAC may be in the tens of mV range, depending on what signal you have applied, you should be able to use a 50mV 400Hz to 1kHz sine wave signal for this testing
We have the master turned down to zero, but you could disconnect the primary wires of the output transformer for this test, so you don't have to listen to the speakers while you test.
It may be a good idea to check all the soldering of the relay pins & any components around the relay contacts. It may be a bad solder joint & the relays may switch properly.

If all the relays check out ok, then we will need to inject a signal into the input of the amp & follow it through, the schematic has expected voltages all the way through the signal path that you can use to check against

good luck

[jvm]

They are 24V relays. I measure no AC voltage in either state at TP13. All other tests are as expected. Signal tracing, I have signal through R18 and R45. U1 has -16/+16 supply voltages. I have already replaced K2 so I don't suspect it to have a problem. Mute circuit? How can I easily disable the mute circuit to see if it is my problem?

[g1]

Are the test points in the channel switching area now correct? (tp32 that you mentioned in 1st post, etc.)
Until they are correct, there is no point looking at other things like the mute circuit.

[jvm]

No, g1. Nothing has changed regarding my TP readings.

[diydidi]

Hi. I would shotgun IC U3. Also check all diodes around the footswitch circuitry.

[pontiacpete]

This might not by the case for you but I just had one of these here that had switching problems caused by broken solder joints on the footswitch jack

[xtian]

Also remember that the bi-color LED is part of the switching circuit, and if it's broken, switching fails.

[jvm]

I appreciate the suggestions. LED is ok and footswitch jack connections are good.