Car poweramp SMPS repair TL594CN based design, help!

[Puggie]

I have an Earthquake UHC2020 (old class AB cheater amp rated at 2X20W@4ohm, but has 60V rails and an unregualted PSU ) basically I have it from a friend, he used the amp years ago and blew something, (PSU power transistors looking at the casing but he doesn't know) it was repaired and used for a year or so, then put into storage as a working amp. it was taken out of storage a few years later and is now dead, nothing has obviously blown no smoke charring, bit of board burned away etc apart from 1 resistor in series between the remote power on and SMPS chip (TL594CN).

So I want to repair this amp, I'm fairly confident from the history the fault is in the SMPS control circuitry somewhere. the 'blown' resistor still shows 40Ohm continuity and the outer surface is burned but not the board. My intention is to start by replacing the TL549CN chip (I have ordered a couple).

does anyone have any suggestions or advice? I do not have a schematic or parts list and my understanding of switch mode PSUs is the very basic fundamentals of how they work and nothing more. I have spare chips on the way, the datasheet for the TL594, a Scope and fully charged 12V battery to power the amp up for testing.

Thanks for any advice

[N-Channel]

Puggie,

It's rare to see a TL594 in a car amp- most use the lesser-precision TL494, which should work just as fine. It is also rare to have the PWM chip itself be a source of failure, especially a precision chip. Can you actually "read" the value on the resistor case, or is it too burned up to see? It is possible that the resistor just burned enough to alter its value (and keep the PWM from starting), but not enough to completely show an open circuit.

Have you tried googling Earthquake "UHC2020" to see what you get?

Hope this helps,

Steve

[Puggie]

Google doesn't bring much of any use up (its an old amp, early 90s IIRC.)

I hear what you are saying about the resistor, I'm trying to find someone else with the same amp to get a resistor code off.

[N-Channel]

Pugg,

In the mean time, If I were you, I'd remove the TL594 and replace it with a 16-pin DIP socket, this would make testing the PWM with a new chip ALOT easier, especially if the pc board is double-sided. For most of the PWM supplies I have done, I use the socket because it is much easier to unplug than unsolder......

Cheers,

Steve