Dull/soft HF response

nerdorama

nerdorama

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A friend brought a Fisher SA-100 to me complaining of a soft or dull high frequency. I checked the 10kHz square wave response and it definately has a quite rounded over leading edge. I can start digging in and looking for aged components and out of tolerance parts but though I might ask if anyone has experienced this sort of problem and ask what were the solutions or issues they found that caused it.

I thought I could start with the drifted resistor issues and then try backing off on the negative feedback and see what it looks like. The amp has a plate to grid capacitor on the voltage amp stage. It seems to me that it should have a reasonable frequency response by design and that the component that are in it for response shaping shouldn't have to be removed. There is likely some problem that is causing the dull high end.

Just trying to be a bit lazy and take the easy way out by asking here.

Thanks,
John
 
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This may sound ridiculous, but before you tear things apart...spray and exercise the AC Balance pots, the DC Balance pots, the Bias pots and the Input Level pots with a little Deoxit Fader Lube.

I had a dirty, intermittent AC Balance pot on one channel of my Fisher 680-A amp, the channel was going silent. So I sprayed both AC Balance pots last week and OMG did it wake up the amp. I'm going to spray all of my other Fisher amps in the near future.

If that doesn't help, try different input tubes.
 
Do you have a pic of the 10kHz square wave? I just re-built one (stock feedback components) and we could compare. I don't consider it dull at all on the high end--clarity and detail are there, but in a very non-fatiguing way. Frequency response to 100kHz is quite good.

Also seems odd that a component problem in the feedback or step networks would manifest in both sides simultaneously. Tubes OK?
 
Thanks for the comments. I'll get a picture of the 10k square today. Dave Gillespie posted a 10k square wave in his thread about this amp. That one looks very good compared to this one. I'll test the tubes and clean the pins and sockets today and see what happens. Didn't have much time with it yesterday but thought I would throw my line in the water before going to bed last night.
Input level pots were at full.
 
Problem solved. Someone along the way had added some 100k resistors at the inputs to make what appears to be a blended mono sub output using the center channel RCA jack. Each channel has a 100k in series with the input and then 100k to a tie point that blends the channels. The 100k series resistors were causing a LP with the input of the triode. When I shorted the series 100k resistors the 10k square waves look just like the one published by Dave G. in his SA-100 efb article.
Thanks for the suggestions. It was only when I had a little more time to stare at the guts and compare to the schematic that these resistors jumped out as something that wasn't original.
All the best,
John
 
epiloge:
Took it to a friend's house last night where the owner was meeting up and we played it on some big Tekton full range speakers. Once it was well warmed up it sounded really nice. Don't think it needs any more work right now. It already has the EFB mod.
Thanks again to all for the responses.
John
 
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