Hi All,
I've had an opportunity this weekend to return to another project I had on the go. I bought this old 1963 (or there abouts) old instrument amplifier some while ago. Since these pics were taken I've removed and replaced all the Hunts caps, the plessey red/black electrolytics and the resistors that were well and truly out of spec. I'm now stuck as the output volume is very low indeed and heavily distorted but its not a pleasant musical distortion by any means. Being just a person that can follow instructions to build electrical equipment or to replace parts, my electronic technical knowledge is very much lacking with regard to problem solving. I have checked valve pin voltages on the ECC83, EL84 & EZ80 and all appears ok. I have reformed the smoothing cap too.
I am stuck what to do next?
Kindest Regards
Rob
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Have you tried an external speaker?
Also have you tried the third jack socket?
I am thinking of a bad switch contact in the third jack socket or a bad speaker.
The valve heaters also look a bit bright as if it is set to 220V.
I had one of these to repair several years ago, exactly the same symptoms. It turned out to be the output transformer with shorted turns. At the time I could only determine that by substitution. A tranny from a valve radio would suffice since it is driving a 3 ohm speaker.
Dave
Many thanks
Just as an update - the pic of the valves was before the recap etc so the valves dont light up like Christmas tree lights now. I have tried all of the jack plug inputs with no change.
I dont have a 3ohm speaker to hand. is there any means of testing the output transformer at all??
KR
Rob
Just about any speaker will do. From 3 to 16 ohms though avoid the fairly rare high impedance speakers which may be 35 ohms or more. The match hardly matters. It will only affect maximum power to the speaker.
Note that this is generally true for valve but not transistor amps. For solid state amps you need to take care if you're using a lower impedance speaker than specified as it can overload the amp.
My understanding of the suggestion in post #3 is that you try a spare transformer, not speaker. For a quick test almost anything will do.
Don't forget that a speaker that has an intermittent open circuit can also cause damage to the transformer, through high voltage stress on the primary, recently been there.....
Amie.
Amie, that's quite a subtle one. Many of us know that disconnecting the speaker from a valve amplifier is a bad idea but an intermittent o/c is really bad luck.
Yes. Bad luck indeed! The tails from tags to cone had started to break up
What are you using as the input? Too much IP can cause distortion of the OP but wouldn't really explain the low OP level. You say you've checked DC voltages but check again the grids in relation to ground but also check the grids in relation to the cathodes. the ECC83 should be around 1-2v and the EL84 around 7v.
Have you a scope? A comparison of the input waveform to the OP is beneficial, as is having a look at the signal on each grid and anode. Make sure you don't exceed your scopes IP Vmax. You can use a 0.1u 630v cap or any other cap of around that value on your scope probe to block high voltage DC to be on the safe side.
Finally you can check the OPT with a DC resistance check. Unplug the EL84 then check DC resistance from pin 6 to ground, then with the speaker disconnected check the secondary. Primary DC R should be around 100r ish +, the secondary a few ohms. There might be a HV cap on the primary side and possibly a resistor, make sure these aren't effecting your reading. You can also have a listen and a sniff around the OPT, any buzzing or,nasty smell is indicative. You can also check the OPT with a screwdriver, put the plaggy end against your ear, metal end on the OPT laminations.
Andy.
PS, including a schematic is alus a good idea when posting a query.
In case it's not immediately clear, the grids should be NEGATIVE with respect to the cathodes.