Marshall jcm900 4501 higain dual reverb volume loss problem! Any help/ideas?

[HEADSMASHER]

hi forumites, can anybody shed any light on a problem i am having with a jcm900 hi gain dual reverb model 4501 please>

all voltages measure fine on all tubes, there are no obvious blown components, i get sound but it's quite a low volume. almost like the amp is stuck on low power setting,the high/low power switch doesnt seem to affect anything, i have replaced that switch.
r37 was open so i replaced that. there doesnt seem to be much in the half power circuit so i am now starting to wonder if that part of the circuit is working fine.

i have used another amplifier with the send and return to isolate the problem to the power amp section of the amp (the pre-amp works fine).

i have cleaned and checked the send/return sockets as they can cause problems.

i have substituted valves just in case some where bad but all voltages read fine.

bias is all good.

i cant think what else in the power amp section would reduce volume.

the output transformer has been tested fine.

it's annoying me as i am running out of parts to test and i just dont believe in gremlins!

anybody got any ideas?

i am just about to check wether some idiot has put el34's in an amp that was set up for 5881's but i have been told that it's run fine on el34's for years, i do know some of them had 5881's tho!

[jmaf]

The 5881 version has 470R screen resistors(vs. 1K), and a different resistor (or two?) on the power supply which feeds the inverter. After adjusting bias both kinds of tubes should work fine.

This is kinda dangerous if you don't have experience, but I like to touch the inverter tube grids to see what comes out. Should be a loud hum.

There is a pesky resistor that feeds +b to the inverter plate which arcs internally and often develops a cold solder, it works hot as a stove....

This one I removed from a Marshall myself years ago: Photo of a Arcing Resistor from Marshall Amplifier

It's R30 - On the EL34 version it's 22K, on the 5881 version it's 10K.

[g1]

Can you tell us which schematic you are using?
Is it this one? http://drtube.com/schematics/marshall/cd0112-iss7.pdf
The resistor JMAF mentioned is called R32 on the issue 7 schematic. You stated there were no blown components but also stated you replaced R37. Is that the 100R 5W right after the HT fuse? If so, whatever caused it to go may still have a problem.

[Chuck H]

Also, you say that all voltages read fine. Is that as per schematic? Did you test voltages on both power tubes, or just confirm on one tube? Does the schematic indicate voltages for all power tube pins? If not please do test voltage on all pins of on both sockets. If they test the same, list those voltages in a post here. If they don't test the same, list the differences as well.

[HEADSMASHER]

i am using the schematic for a marshall 4501 power amp from marshalls image bank. voltages are not listed on the schematic. i am going by the generally accepted voltages we normally find on el34's. from memory as i am not at home i get -40v on pin 5, 460v on pin 3 9plate) and 436v on pin 4, 6.9vac across pins 2 & 7 for the heaters. however i will go home and measure exact and post here tommorrow morning for both valves.
yes i replaced the 100r 5 watt as it was open, next to it r32 the 10r was fine, well it measured fine.
someone had been inside the amp before me shoving A 220K on pin 5 of one of the valves i guess in an attempt to balance up a mismatched pair! i removed that.
i have tried different valves in the amp all with the same result.

[Chuck H]

the output transformer has been tested fine.

How did you do this? The only way I know of is to apply an AC voltage at the secondary and measure the AC on the primary to calculate turns ratio. There are simpler tests that will tell you if an OT is just plain broken, as in grossley bad. But a few shorted turns can zap power and not show as a bad OT on simple tests.

[g1]

You also want to look at your phase splitter voltages, as the phase splitter is after the return jack and considered part of the power amp.

[Chuck H]

You also want to look at your phase splitter voltages, as the phase splitter is after the return jack and considered part of the power amp.

Actually, there is a bunch of circuitry past the return jack. Including the reverb circuit and associated recovery and mix circuits. It's only because of that open resistor that was found that I think we may be dealing with a power amp issue. Otherwise I'd blame it on the silicooties right off.

[g1]

It's only because of that open resistor that was found that I think we may be dealing with a power amp issue. Otherwise I'd blame it on the silicooties right off.

Completely agree Chuck. That resistor only feeds the phase splitter and preamp, and the FX send is good ruling out the preamp. So the phase splitter voltages should be checked. As far as "silicooties", those M5201 chips seem somewhat prone to faults in the 900 series.

[soundguruman]

The high/low power switch and related circuit tracks have been known to fail. Especially w/ EL34 installed.
You will need to change the screen grid resistors to 1 K- to run EL34s, and readjust the output bias.

[Chuck H]

Good call. I don't do repairs (yet) so I'm not too familiar with the weak links in many models. It's a fault in that area between the effects return and the power tubes most likely. Do check PI voltages. Then chase the silicooties.

[soundguruman]

I think I am getting into the mood to change that switch altogether, into an industrial beefier type switch.
I don't think the switch was really big enough for EL 34 output tubes...I mean it's cool to drop the power but I think the contacts are frying.
And maybe people are operating the switch with the standby on.