Jet City JCA50H buzzing - Sorted!

brownovich

Hey everyone, just wondered if anyone might be able to help with an issue I’ve encountered. 

I was just about to ship my Jet City JCA50H to another member of the forum when I decided to do a last minute check to make sure all is well. I had tested it a few weeks back at low volume but decided to give it a bit of a blast.

When I turned up the volume there was a very loud, low frequency buzzing noise on both channels, far beyond anything you’d ordinarily expect with natural hum. The buzz was present both with and without the guitar plugged in and with the channel gains turned down to a minimum. Tried several guitars (w/ buckers), power leads and speaker cables, no difference.

Feel terrible but I immediately refunded and explained the situation. Still feel bad though!

My question is does anyone have any indication as to what this might be and if it’s likely to be an easy fix? It is NOT and expensive amp and frankly if it’s gonna cost more than a few quid to sort I’d rather just scrap it and save myself the trouble.

Thanks everyone.

ICBM

If the buzz didn't appear until you turned up the volumes, it's coming from the preamp. Try replacing each preamp valve in turn with a known good one. If you don't have one, swap V1 for V2 and test again, and if the noise doesn't stop or change, swap (what is now) V2 for V3, test again, and if still no change swap (what is now V3) for V4. If none of this stops or changes the noise at all, it's not a valve problem in which case you need a tech.

It may not be an expensive amp, but it's a very good one - well made and not especially difficult to work on, so it's unlikely to be expensive to fix.

brownovich

Thanks! I don’t have a spare valve so I’ll try swapping them around tomorrow.

Yeah it would be a real pity to scrap it, it’s just given the value it’s a very small window to make a repair economically viable.

ICBM

The amp is probably worth £150 or so, which is a pretty big window for a simple repair. At a guess the most likely fault if it's not a valve is a filter cap failure, which should be no more than a £50 job unless you know a very expensive tech...

brownovich

Great, thanks! I had visions of taking it in and being quoted the same as its value to diagnose and repair! Hopefully it’s just a valve!

brownovich

Bad news, doesn’t look like it’s the valves that are the problem, tried the test as described above, no joy. Also tried tapping each of the tubes, no success.

Just spoke to my local tech and basically it’s gonna be £50 to get it diagnosed and likely another £50-£100 for a repair.

Think this is likely going to be a “for parts/not working” job!

ecc83

Whilst I strongly agree with a tech charging a non-returnable deposit to deter time wasters and prevent the build up of junk, I think a nifty is a bit steep?

I know costs have gone ape-S since I was 'in the trade' but telephone is FAR cheaper and email is free!

Dave.

simonk

Just make sure you've actually got to V1. On my JCA20HV V1 is tucked right behind one of the transformers and it's almost impossible to see it, let alone reach it, without taking the chassis out.

brownovich

simonk said:

Just make sure you've actually got to V1. On my JCA20HV V1 is tucked right behind one of the transformers and it's almost impossible to see it, let alone reach it, without taking the chassis out.

Yeah its the same on the 50 watt, had to use my left hand to access it as it was behind the transformer.

HHwarner

Its not something to do with the vicinity you are testing it, ie florescent lights etc

brownovich

No, Ive tried several different amps in the same area. I do get terrible hum in my house for some reason anyway, but this is in a different league. Also tried it in several different rooms too

DJH83004

Its a real shame to be honest as they are a great amp and it may well be a very straightforward fix. I suspect your tech's charges reflect his location and his overheads etc. It is a pity you have not got a known good pre-amp valve to do a substitution test. ICs swap test recommendation is the best compromise when you haven't got a spare, but you can transfer the fault, (lesson to to all you valve amp guys out there, buy a JJ ECC83 / 12AX7 test valve they are tenner and invaluable in circumstances like these !) 
Would be happy to help out if you were a bit closer but not a lot more can be suggested remotely.   

ICBM

brownovich said:

Just spoke to my local tech and basically it’s gonna be £50 to get it diagnosed and likely another £50-£100 for a repair.

That's steep. While I don't disagree with a £50 minimum charge, that should also cover many simple jobs without adding on the same or more as well. If this is a broken connection or a single failed filter cap - the most likely causes of hum from the preamp, if it's not a valve - that should be easily covered. This type of cap is less than a fiver, anything else will be less or just labour.

I would find another tech.

brownovich

Yeah thanks fellas. I’m away on holiday week after next but maybe when I’ll get back I’ll see if I can find another tech that may be a bit cheaper.

In the meantime I’ll order a spare 12AX7 and see if that brings any joy.

Thing is I do understand this particular tech are busy and they are very good, but yeah it does feel on the high end of things.

Maynehead

Have you tried the amp in a different location? Sometimes lights (especially ones with dimmers) can cause terrible hum.

brownovich

Maynehead said:

Have you tried the amp in a different location? Sometimes lights (especially ones with dimmers) can cause terrible hum.

Yeah several different rooms (even in the cabin at the end of my garden!). Hum is way louder than interference. Definately an amp problem unfortunately.

brownovich

Maybe some postive news, Ive found another local tech who seems a lot more reasonably priced! Fingers crossed

Chalky

Interested to hear what it is!

Modulus_Amps

Normally the standard fee covers the first hour of work and allot of small issues would be/should be sorted in that time frame

DJH83004

Modulus_Amps said:

Normally the standard fee covers the first hour of work and allot of small issues would be/should be sorted in that time frame

Agreed Mike, You should have a pretty good idea of the problem in the first hour and very often the job can be completed in that time, so it becomes one hours labour plus any parts.

brownovich

DJH83004 said:

Modulus_Amps said:

Normally the standard fee covers the first hour of work and allot of small issues would be/should be sorted in that time frame

Agreed Mike, You should have a pretty good idea of the problem in the first hour and very often the job can be completed in that time, so it becomes one hours labour plus any parts.

Yeah that’s what the guy I’ve spoken to has said, for £35 plus parts it should either be diagnosed/fixed or I’ll know it’s a gonner!