Peavey CS800X4 Channel 1 blown output/DDT issues

[Delta362]

I'm working on a Peavey CS800X4 where Channel 1 is not working. It came in with its DDT light and no output. Upon inspection, I found Q106 and Q108 shorted, along with D113 shorted. I was not able to find the schematic depicted 2SC3281 and 2SA1302, nor the 2SC5949 or 2SA2121 that were installed in this unit. I ended up replacing Q106 with a MJL3281AG and Q107 with MJL1302AG, and Q108/Q109 with the called for HUF75639G3. I also replaced the MURS160T3 for D113/D114 with SURS8160T3.

Thinking I found all of the bad parts in Channel 1, I reassembled and powered back on. I am still getting the DDT light on channel 1, but I was able to check the newly installed transistors from the backside of the board and they read the same values as Channel 2 equivalents so I am confident that those new transistors did not short.

I then started following the signal between Channel 1 and Channel 2 to see if I could find a difference along the way. Channel 1 has no signal at the output of U102. D109 has 16.2V on the anode and 15.5 on the cathode. Channel 2 has signal on U102 output. D209 has -17.1V on the anode and -17.3 on the cathode.

U105 pin 2 has 0V on it for Channel 1, while pin 12 has -1.5V on the equivalent pin 15.


I uploaded the schematic in 3 sections as the original one I found had 18 pages as was 14MB. Pages 7-12 have Channel 1 and Channel 2, but the other sheet of interest would be page 2 or 15. Page 9 would be similar, except it's for Channels 3/4.

Any ideas on what components or help understanding the DDT circuit as to where I should look next? Thank you!

[Jazz P Bass]

"Any ideas on what components or help understanding the DDT circuit as to where I should look next?"
Can you sort through all of the schematics and show the DDT circuit?

[Delta362]

Thanks for the suggestion, Jazz! I was able to print the wild mess of the original pdf that I had into sequential pages since there were repeats in the original 18 page document. It does not contain sheet 1 of 8. I've pulled out channel 1 and 2 since they do share a couple components.

I don't see anything labeled DDT on the schematic, but on sheet 3 the DDT LED is LED100 (yellow), with the clipped signal being LED101 (blue) and LED102 being the green power light. When powering on the unit and no signal applied, Channels 2-4 have all 3 light up, then the DDT and signal LEDs fade off after a few seconds leaving the power on light there. Channel 1 only lights up the DDT LED and it remains on the entire time. These three LEDs appear to be driven by the Status and Clip signals from sheet 1, as well as the AMP_1_OUT signal that's on the output of Channel 1's relay RL101.

I'll need to compare the Status, Clip, and AMP OUT signals on Channel 1 vs Channel 2 to see what is different there.

[Delta362]

I went back and measured some more points:
At the 6 pin P4 connector for Channel 1 and Channel 2 to the Display board (I also measured values for the equivalent Ch3/4 connector just to confirm Ch2)
Pin 1: 0V
Pin 2: 0.184, but this is the temp out and isn't carried thru
Pin 3: 1.1
Pin 4: -1.9 Clip (vs 1.1 for Channels 2-4)
Pin 5: 3
Pin 6: 0.1 Status (vs 3 for Channels 2-4)

I then went looking for the origin of the Status signal. On sheet 1, it looks like an out near the top left, with an input being in the middle top feeding the collector of Q102. But they are directly connected on this page. I believe this controls the RL100, but it looks like it is drawn as an out arrow. Relay coil resistance measures around 600 ohms, same as the other 3 relays. Flyback diode measures ok. I was unable to find another Status output in the schematic to determine where to look next.

Other voltages measured. Numbers in () correspond to Ch2 equivalent component values
Q100
B: -0.6 (-16)
E: 0.1 (28)
C: 0 (0)

Q101
B: -16.5 (-16.1)
E: -17.3 (-17.3)
C: -0.3 (-16.7)

Q102
B: 30.4 (26.6)
E: 29.9 (28.0)
C: -0.1 (27.3)

Q103
B: 0.66 (7.6)
E: 0.4 (6.3)
C: 30.8 (6.9)

The LO_RL+ both measure 32V, while the RELAY_24 both measure 29.7.

Where to check next? Thanks again for your help!

[The Dude]

Do you have DC on the output of that amp channel?

[Delta362]

Do you have DC on the output of that amp channel?

Good call! Seems I forgot to check the obvious on that. Yes, there is 9.75VDC on the L100 output inductor before the relay. Definitely not good. Any ideas where that could be originating from?

[Delta362]

To get better access to the components for measurements, I pulled the heatsink and then reinstalled the board. Q108 and Q109 voltages match those with the working Channel 3 (since I knew the component orientations on the board are identical). The difference I found was the base and emitter for Q106 and Q107 measure the 9.76V, whereas on the others it's 0.

I then checked all voltages on the U103 and U303 (STK442-130) since Channels 1 and 2 share device U103, and Channels 3 and 4 share device U303. All voltages are the same, except for pins 4 and 5, which are the output of this for Channel 1.

Given the originally shorted components for when I got the amp, does an approximate 10V output make sense as a failure mode for this channel (pins 4 and 5) of the STK442-130? If not, what are some other possibilities that I'm missing?

Thank you!

[Delta362]

I had hoped to isolate pins 4 and 5 of U103 to see if they were the cause of the DC voltage, but I wasn't able to get any heat shrink material thin enough to keep those pins isolated from the board. I ended up pulling U103 and measuring voltages on all of the pins of Q106/Q107 and Q206/Q207 to compare. With U103 completely out of the board, the base and emitters of all four have the approximate +10VDC on them, instead of just Channel 1 previously. The collectors have either +32V or-32V appropriately. I have not been able to reach anyone at Peavey the last few days so I went ahead and ordered one from B&D since they have a couple remaining in stock.

[Delta362]

I received the STK442-130 from B&D Enterprise earlier this week. I was able to get it installed without issue. That solved the last bit of power amp still in DDT protect mode.

Then I found that the blue signal LED for Channel 1 was not operating at all. I checked the voltages on the 6 pin connector and compared to Channel 2, noticing that there was a difference between them.

I then swapped the connectors on the board so that Channel 1/2 drove Channel 3/4 LEDs and pots, and vice versa. The problem stayed with Channel 1 LED. Then while the amp was powered, I flexed the board slightly and the LED turned on. After reflowing the solder to the LEDs, resistors and transistor, it's now working properly.