Marshall Valvestate 8004 - Blowing TDA1514A's

[walkman]

Does the TDA1514a need to be insulated from its heatsink or will heatsink grease like that used for computer CPUs work?

I picked up a faulty marshall 8040 and found that the chip wasn't even soldered! Someone else must have attempted to work on it before from what I can see. Is there any quick way to test if the chip is blown?

[Jazz P Bass]

The datasheet (Philips - datasheet pdf) indicates that the backplane is connected to -VP (the negative power supply).
So, yeah, it must be isolated from the heatsink, unless the heatsink is not connected to ground & is floating.
The CPU grease will help conduct heat but it is not to be used for isolation.
Sillipads or milar sheets are to used for that.

Usually when an IC such as this is 'blown', the output pin will be shorted to one or both of the power supply pins.

[sweatyk]

The CPU grease will help conduct heat but it is not to be used for isolation.
Sillipads or milar sheets are to used for that.

Usually when an IC such as this is 'blown', the output pin will be shorted to one or both of the power supply pins.

So are you saying that CPU grease is different from a silicone based heat sink compound? Never really looked in to that and didn't know there was a difference.

[g1]

They are both used for heat transfer. They are NOT used for insulation. You need mica spacers or silpads or something else to insulate the metal part from the heatsink.

[oc disorder]

8004.zip
D15 DIODE ZENER 15V 1.3 WATTS
Notes: ZD1;ZD2

D5 DI 6.8V ZENER VALVESTATE
Notes: ZD3;ZD4

Some exotic computer heat transfer grease is slightly conductive to electricity.
Also see my old post here
http://music-electronics-forum.com/t31030/
I resisted replacing the capacitors around the chip initially but after blowing 3 chips I succumbed. !!

[Jazz P Bass]

Yeah, I had one that kept biting me in the a**, in that case replacing the caps around the IC fixed it.

[walkman]

I have a new TDA1514a on order now. Will make sure to use an insulated spacer. From what I've been reading I will most likely replace the caps around it and e 6v8 Zener diode to.

[walkman]

New chip is in and the same issue as mullytron. what voltages should be present on each pin?

[J M Fahey]

the same issue as mullytron

1) Please state your symptoms.

2) Do you have an 8004 (the subject of this thread) or an 8040 ?

As Ènzo pointed, very different amps , different troubleshooting.

[Jazz P Bass]

New chip is in and the same issue as mullytron. what voltages should be present on each pin?

According to the datasheet, 30Vdc is the maximum that pin 6 should see.
Philips - datasheet pdf

Where did you purchase the 'new' ic?
This part has been out of production for some time now.

EBay seems to be flooded with knockoffs.

[walkman]

Ok I did some work on the amp over Christmas and new year, and some more last night.

It is a valvestate 8040. The symptoms are as follows

1. After playing for a few minutes with the volume up quite loud a faint second note appears in the background to the main note being played. After time the secondary note which is a harmony of some sort gets stronger but not as loud as the fundamental. It is also slightly destroyed it is almost like there is an effects pedal on.

After about 20 minutes of loud playing it starts to really freak out with loud distorted bursts hard to describe I will up load a link to a recording of this.

2. When played quietly there appears to be no problem at all and 20 minutes or more of playing and there is no noticeable secondary tone or other issue.

3. When turned off there is a barking noise from the speaker ( and I checked for the blckk dog in there ;-))

With the amp out of the combo I felt the temperature of the chip both when playing at loud volumes and quiet volumes, and the chip was almost too hot to touch after playing loudly. I wonder if it is overheating when in the combo as there is no venting in the chassis.

When I got it it wasn't working at all.

So far I have replace all the electrolytic capacitors in the power supply and around the TDA1514. Replaced the 6v8 zenners, replaced the broken channel switch, replaced the first preamp TL072 IC3 that was stuffed. And replaced R66 the 10 ohm resistor off the TDA1514. I have upgraded the speaker to a celestion silver series and used weather seal tap along the chassis to reduce any vibration of the chassis. I also put a 220k resistor across the gain control in the overdrive channel to reduce the amount of gain possible. I feel that is just about there and besides the issue sounds great.

When I replaced the TDA1514 I didn't use any heat sink grease and overlapped the mica spacers to cover the exposed metal of the chip. I haven't taken the voltages recently but they were around 20 ish the TDA.

I've checked the power amp in and power amp out and the issue still occurs at loud playing volumes when an external signal is put into the power amp in. I have swapped speakers and used other cabs too, as I replaced the wired in speaker leaders with an external speaker jack.

Any ideas I really want to get this working as it's come so far and I've learnt a lot.

Will add some pictures, voltage measurements and a sound clip to the next post shortly

[Jazz P Bass]

I think that you have a bogus output IC.

Your best bet is to redo the output to accept an LM3866 or a TDA7293.

[J M Fahey]

Agree, all fakes by now.

LM3866 is way easier than TDA7293 , yet still there's some brain surgery involved.

Post a picture showing TDA1514 still mounted, the heat sink, and the relevant part of the PCB showing the TDA1514 leg holes, because you'll have to:
a) redrill the heatsink for LM3886
b) add small wires matching new pins to old holes
c) add a new external/floating mute/standby circuit because it's incompatible with the old one

[52 Bill]

When I replaced the TDA1514 I didn't use any heat sink grease and overlapped the mica spacers to cover the exposed metal of the chip.

I would try correctly heatsinking the chip with grease and flat insulators before doing any mods. If the chip doesn't transfer heat very well, you will eventually have to replace it when it melts down.

[walkman]

Here is the sound it makes http://bit.ly/1Eqduba when the issue occurs.

I will put some heat sink grease and fix the spacers and see if that makes a difference.

The replacement was from a local electronics supplier not off ebay and looks identical to the original that I still have.

This site suggest that the fakes look different https://dandini.wordpress.com/2010/1...repair-issues/