77 JMP 2203 EL34 amp

[dan_tramble]

Hi guys I am looking for extreme help, I am LOST

I have a 77 JMP 2203 here that is very sick, I have low output in the amp On 10 (both volumes in the HIGH input its thin and very fizzy) I have tried good tubes from my own 2203 (no change) it has been recapped and the only thing different to my eye, the power tube sockets have been changed and there is 2.7K screen resistors on it These should be 1K 5 watt.... Initially I traced the board and the 100K going to pin 1 on V1 was almost open so I changed it out with a new resistor meters fine now....

I have performed a voltage test on the preamp tubes and it goes south on V3 (the phase inverter) I HAVE the heater voltage present, thats omitted

Pin 1- 0 VDC Pin 2- 20.64 VDC Pin 3-23.45 VDC Pin 6-180 VDC Pin 7-12.80 VDC Pin 8-23.95 VDC

My question, where should I be looking now? Please post to thread and THANK YOU in advance

Dan

[danman]

The zero volt reading at pin 1 is your likely culprit. It is connected to a 82k resistor on the board. Next to the 82k is a 100k that feeds pin 6. Check to see if the 82k is open or not. You should have a high voltage DC reading at the junction of these two resistors. Since you show voltage at pin 6 but not at pin 1, I would say the 82k has gone open.

[dan_tramble]

Danman,

I found a problem the plate wire off the 82K was partially shorted to the standoff This is a buddy of mines amp and he asked me to try and help him out.... It was in montreal and someone went through it already and put sharpie X on selcted resistors.... i initially ohmed all the resistors, and the V1 plate (100k) and the 82K read high i replaced both plus the wire to pin 1 in V3

The amp sounds very thin at 10 this guy pays LOUD jesus input 1 (clean) he drives the living hell out of this thing After my initial post I traced it and I replaced the 82K and ran a new wire to pin 1 on V3. After the repair with my hotplate on 16 dB reduction it sounds good, however it does not seem as loud as my metroamp 2203 master volume build.....

I will test it tomorrow loudly however I am resistant to say it is fixed..... I think or should I say (feel) it should be louder at that volume with my hotplate My 2203 is deafening loud with a hotplate.....

I measured 525 on the plates and set the bias at 32 mA i wonder if the 2.7K 5 W screens may be adding to this....

They are supposed to be 1K5 watt

What do you think?

Thank you DanMan I greatly appreciate all the help

Regards Dan

[dan_tramble]

Amp is fully repaired and sounds excellent

Thank You for the Help!

[neikeel]

Amp is fully repaired and sounds excellent

Thank You for the Help!

What else did you do?

Is it as good as your Metro now?

[dan_tramble]

Neilkeel

I replaced the V2A 820 cathode, the 82K plate resistor (and new wire) replaced V1A 100 k plate resistor, plus did a full recap on it I would like to replace the ceramic tube sockets as well, but he only has so much money and for now they are good.... i did remove the ground buss on 8-1 of the octals and put in 1 ohm 1 watt resistors as well to get full good BIAS readings as well...... He didn't mind me replacing those... I just wanted the amp to where I myself would be happy...

It is a different tone very similar however I don't have enough hours on mine yet! Call me crazy but I love the clean channel and I like the dirty one too but I find myself in the clean more often..... You can get a convincing malcolm Young tone out of the clean channel.....

The amp (mine) past 4-5 on the MV squawks a bit i suspected that off the hop i can deal with that i did one mod to it, I put a 5 K resistor on V1B cathode and removed the 10K Its got a touch more "more" if that makes sense it's awesome for boosting and it sounds awesome boosted its a killer amp

I did 3 2203 amps in the last 2 months had a buddies 83 CSA 800, (CAP JOB) and power tubes, Had another one (CSA cap job and general go over) and this 77 2203 he bought it off reverb and it was a touch dirty but I stuck with it and got it awesome

Thank You for all the help I really love building and repairing amps