Peavey XR 696F - I made a booboo

[jondoe]

The postal Gods are being kind to me, the cheap 2nd class post parts I ordered yesterday came the next day, I have my transistors.

I have put them in the board and I no longer have bright bulb on with all the power supply connectors plugged in, just a faint and dim bulb. The large transistors are no longer warming up. The large resistors for the AMP IC supply R290/R291 are still way way too hot to touch, which worries me a little, but they are raised well off the board, so maybe they are designed to run hot?

[glebert]

The large resistors for the AMP IC supply R290/R291 are still way way too hot to touch, which worries me a little, but they are raised well off the board, so maybe they are designed to run hot?

They are designed to drop the 10V difference between the +/-25V and the +/- 15V set by the zener diodes. 10V across 220 ohms should only be about 1/2 a watt (using V^2/R), which to me shouldn't be that hot. Can you check the voltages on either side of those? The 25V may be low if you have the light bulb limiter still in place.

Glad you seem to be making progress.

[jondoe]

They are designed to drop the 10V difference between the +/-25V and the +/- 15V set by the zener diodes. 10V across 220 ohms should only be about 1/2 a watt (using V^2/R), which to me shouldn't be that hot. Can you check the voltages on either side of those? The 25V may be low if you have the light bulb limiter still in place.

Glad you seem to be making progress.

Using the chassis as a ground point, the readings on either side of the resistors are as follows.

R291 is 26.5v and 0.8v
R290 is -27.7v and -2.2v

[glebert]

So you have something on the +/-15V rails that are pulling them way down. Looks like the the +/-15 that goes to the XR header is off from proper voltage regulators V100 and V101 so the AMP IC supply is only for local devices. I would start checking diodes like D116, D117 for shorts and the ICs on this board like U100-U102 and see if anything feels hot. If any of the ICs are socketed you could pull them to get them out of circuit, but I think the days of Peavey socketing ICs is probably long past.

D134, 136 and ZD104, 105 too.

[jondoe]

So you have something on the +/-15V rails that are pulling them way down. Looks like the the +/-15 that goes to the XR header is off from proper voltage regulators V100 and V101 so the AMP IC supply is only for local devices. I would start checking diodes like D116, D117 for shorts and the ICs on this board like U100-U102 and see if anything feels hot. If any of the ICs are socketed you could pull them to get them out of circuit, but I think the days of Peavey socketing ICs is probably long past.

D134, 136 and ZD104, 105 too.

Brilliant, thank you for pointing out the problem with the 15v rails I've just started looking over the board with a magnifying glass and found that part of the top of Q2 is missing part of its casing, I couldn't quite see it with the naked eye. U102 has very low resistance between pins 4 and 5, I might swap that out for a new one although I'll check to see if it's warm. I'll check the parts you suggested too.

[glebert]

You may not get too much warmth at the IC since the voltage there is so low, and those resistors are actually dissipating most of the power.

[jondoe]

ZD104, ZD105 are shorted, which is a surprise as I was sure I'd tested them previously, but never the less, they are goners. I've ordered some new ones

To be continued!

[Enzo]

They use short versions of part numbers all the time. No, there are not two different parts with the same number in here. You might see 6357 transistors which are really SJ6357. MPS6533 becomes 6533. It just reduces clutter on the drawing. Technicians are expected to know. The schematic file includes a parts list. If a common part has a 300v rating and you only need a 200v part, how much time and effort should one put into finding a less common part? I don't see 300v as overkill if the part is exposed to 115v.
WHen you are checking voltages at idle, you are seeing only that...idle voltages. But the transistors in the power amp are often exposed to the entire signal swing. So you would really have to scope it while fully cranked.

[nickb]

They use short versions of part numbers all the time. No, there are not two different parts with the same number in here. You might see 6357 transistors which are really SJ6357. MPS6533 becomes 6533. It just reduces clutter on the drawing. Technicians are expected to know. The schematic file includes a parts list. If a common part has a 300v rating and you only need a 200v part, how much time and effort should one put into finding a less common part? I don't see 300v as overkill if the part is exposed to 115v.
WHen you are checking voltages at idle, you are seeing only that...idle voltages. But the transistors in the power amp are often exposed to the entire signal swing. So you would really have to scope it while fully cranked.

It does make it confusing. In the BOM the 7204 is called out as "70467204 SDS-7204". The cross ref matches SDS-7024 with Peavey p/n 70407204. The substitutes are 2N6557, MPS-U10 and later on 2N3439 and MJE-3439 with different packages. Finally the board is stuffed with MPSW42. It's a big mess.


Having looked at the schematic yet again, flying rails is always a brain tease but add switched power rails into the mix and it just hurts, as far as can see the emitter is always close to ground and the collector never goes above 25V, so it's not 205V or 115V but just... 25V! That does lend credence to a cost saving exercise theory although it's surprising that Peavey wouldn't have have a more suitably spec'd part used in another product on the shelf anyway.

Anyways, this as all a bit of a digression. I'd say jondoe did the best thing and replaced like with like. I do hope they are not fake. He seems to be a great job of working through the issues too.

[jondoe]

Hi all, me again!

The zener diodes have arrived and been fitted to the board, with mixed results. Upon first power up we had a dim bulb, I tested with just the HDR100 and on the "AMP IC SUPPLY" it had +16.7v/-16.4v and +27.5v /-28.2v, so not bad. I plugged in the power amp AC supplies and some 5 or 6 seconds later, it went very bright, then dim, throbbed a little and then kinda stuck some where between bright and dim. I turned it off, checked my soldering and part placing. I plugged it all back in again and now I just have a constant dim bulb, regardless of what AC feeds and plugged. The only other observation is that Q110/Q107 are quite warm to the touch, Q110 gets VERY hot after a short while, where as all the other transistors are stone cold.

I have yet to replace the transistors on the mute circuit Q1/Q2, they have been removed. Would that cause me a problem?

[jondoe]

I've taken some voltage readings on the "good" side, that is the side that doesn't overheat at idle, voltages are indicated on the pads in the screen grab;



I compared the above, the two that are getting hot (Q110/Q107) on the other side have a lower voltage, as pictured, the hottest one has the lowest voltage on the collector;



Do you think the side that appears to be good, is actually good? i.e. can I use it as a reference?

EDIT:

Q117 is as dead as a door nail, it's a MPSW92.... here we go again part finding!

[jondoe]

After a short break away from home I'm finally back to tinker, the MPSW92's have been delivered and the dead one replaced, the problem has now moved along, a little. With the new MPSW92 I no longer have hot Q110/Q107 transistors, but I now have a bright bulb with the J107/HDR100 power feeds plugged into the board, the AMP IC SUPPLY is sagging very low, so I guess I have a problem on that rail somewhere. With J202/HDR100 plugged in or with just HDR100 plugged in I have a dim bulb.

I'm still waiting on the replacement SOT23 Q1/Q2 for the removed OFF MUTE section on page 1, I can't tell if this is part of the problem.

I'll spend some more time on this tomorrow!

[jondoe]

It's a cold start to the day in the shed, I think I need long johns

I've removed Q112, tested it off the board and it appears to be dead, my transistor tester thinks it's just a resistor. It's marked on the schematic as 2N6534S, google isn't helping me with that part number. Bill of materials lists it as 2N6533/34 PNP (SOT23) XSISTOR SMT, google returns a giant TO66 or not quite as big TO92's, I'm a little stumped. I have dropped an email to Peavey USA to see if they can offer and alternative.

The part number printed on the SOT23 is R2K and after some googling I've found that might be a MMST8598 DSA-317172.pdf but that could be a red herring.

Fun and games!

[glebert]

Looking at the schematic it looks to me like Q112 and Q109 are implementing some negative feedback from the last output stage back to the driver stage. The voltages and currents seen by these transistors should be relatively small. The BE voltages are across the 0.22 ohm resistors on the output stage, so that should be quite low voltage, the collector voltages come off the emitters of Q106 and Q117 which seems like they have to be limited by the +/-25V rails. Looking at the available SOT-23 package BJT parts it seems like most are 40-60V parts. I don't think these require special parts, but I'm sure someone will tell me if I am wrong. Obviously you have to watch the pinout as you can't bend the legs around on SMT parts.

[jondoe]

I'm happy to report I had a response from Peavey, they quoted the replacement part as;

30418033 - 2N6533/34 PNP SOT-23 TRAN

2N6533 is the same unhelpful part number, but if i cross reference the Peavey part number 30418033 I find it's a MPS6533, the plot thickens!