Advice on repairing clamshell Rega Elex (Brio/Mira/Elicit)

[mashley_nad]

Hi - I am replacing the faulty speaker relay in my Rega Elex MkII (see pics). I have a couple of questions hopefully someone can help me with.

To unsolder the relay I had to lift up the PCB which is held to the casing by two allen key screws through the black u-shaped bar which also clamps the output transistors onto mounts in the casing.

Under the PCB is a layer of insulation with holes for the output transistor mounts.

It seems there is some (brown) thermal insulating tape between the transistors and the mounts. Should I replace this

Also, there are a couple of blobs of some glue-like but not very sticky substance that seems to stop the front panel insert and the PCB rattling around. What could I use to replace that with? Indeed, what is it exactly

Have to say I'm not very impressed with the quality of construction, but then I guess all the quality went into the price, not the product

[sbrads]

The thermal pads should be OK if they're still soft and look undamaged, but after assembly check that the transistors aren't shorted to the chassis. Personally, I would put a tiny smear of thermal paste on these pads as insurance if they can't be replaced. The glue blob is fairly standard assembly practice these days. If it doesn't get hot there then hot glue would be OK to use and is my preference because it's easy to get off if needed. If it does get hot then a blob of RTV would do but is harder to get off cleanly.

[mashley_nad]

Thanks for the tips, but I'm not sure what you mean by RTV. Is this the stuff?
Silicone 401 RTV

[sbrads]

Originally Posted by mashley_nad

Thanks for the tips, but I'm not sure what you mean by RTV. Is this the stuff?
Silicone 401 RTV

That will do, or anything similar - it's only a blob on the edge of the board after all.

This is the proper stuff by the way.....
http://www.alibaba.com/product-gs/48...retardant.html

[madtecchy]

Hi mashley.
The glue like substance is most definatley of the hot melt varity. also notice in the same piccy as the glue, two transistors marked T1 they seem to have thermal jointing paste on them does somthing make physical contact with them when the unit is assembled? It may just be to bond the two smal ltransistors thermally rather than for cooling . Just a word of caution using silicone substances, Speaking from expirience this stuff can be conductive at audio frequencys.
Regards Ian

[sbrads]

Originally Posted by madtecchy

Hi mashley.
Speaking from experience this stuff can be conductive at audio frequencys.
Regards Ian

It must depend on the type of RTV then as we use it at work where it passes 100MΩ insulation requirements at 1Kv DC (and AC when required) and usually measures over 1GΩ.

[madtecchy]

Originally Posted by sbrads

It must depend on the type of RTV then as we use it at work where it passes 100MΩ insulation requirements at 1Kv DC (and AC when required) and usually measures over 1GΩ.

A friend of mine decided to fill the xlr connectors on his speaker leads with silicon to aid with durability and prevent moister ingress the next time he took the rig out it blew the output protection fuses on all his amplifiers. i tested all the leads with a multimeter expecting to find dead shorts but none could be found . i finally managed to open the xlr connectors removed the silicon and all was well after that . all i can remember about the silicon he used is that it was of a clear type. I guess there are many diffrent types so it would be wise to apply caution when using this stuff.
regards ian

[mashley_nad]

Originally Posted by madtecchy

two transistors marked T1 they seem to have thermal jointing paste on them does somthing make physical contact with them when the unit is assembled?
Regards Ian

Well spotted, but no that's just some colour coding; most of the little transistors have some blob of colour on them.

[mashley_nad]

The relay was successfully replaced with an exact replacement Finder model 50.52.7.024.0000 (24 V DC, DPDT, 5A).

The two glue globs did not need to be replaced afterall, as I found they effectively just "clipped" back into position.

The heatsink insulator strips were also in good shape, so other than giving the output transistors a wipe clean, I made no changes here.

The thing with these Rega amps is that a lot of heat comes off the driver transistors (which I think must operate in class A) and it is these which heat the cabinet rather than the output transistors themselves.

Lack of proper ventilation to cool the interior is a design flaw, but despite this, the amps seem to work well and don't seem to overheat despite running very warm.

All back together and working well!