Vintage Hoffman Stereo Amp Problem - fump, fump, fump

[tcd5m]

I recently acquired an early 60's Hoffman tube based console stereo. I'd like to retrofit the amp to use it my NAD receiver; preamp out to the Hoffman amp.

Problem: I've added chassis mounted RCA plugs and wired them to replace the original 3 pin DIN audio input. When I use the RCA's I get a loud fump, fump fump sound. If I run the NAD as line-input into the Hoffman preamp and use the preamp-to-amp DIN connection it works fine.

Can anyone offer suggestions on how to track down the problem?

I don't have schematics yet, waiting for radiomuseum.org membership to go through, but here is what I know about the amp:

Hoffman Amp: 1143 chassis, 3 channel amplifier
Tubes: 5U4GB, 7199 Phase Inverter, ECC83 Pre, 4 x EL84
Left and Right channels appear to be driven by the ECC83 and 2 of the EL84's
Center channel appears to be driven by the 7199 and the other 2 EL84's

Thanks,
John

 

[M Gregg]

Hi,

Sounds like motor-boating " low freq oscillation".

I hope the unit has a mains Tx and does not rely on neutral to Gnd the chassis<<if it doesn't dont use it!

All the usual safety requirements apply.

If it works OK with the Din input then there is no fault with the amp. Have you got an earth loop created with the RCA sockets, ie do you have a common ground with the chassis earth to the NAD via the mains supply and a Gnd connection with the screen of both RCA connections to the NAD?
To try this; disconnect the gnd connections to the RCA connectors at the Amp end (or isolate them from chassis)(or remove them from the chassis and hang them loose for a test) and see if it still does it!
Do not remove chassis earth if there is one!

Regards
M. Gregg

 

[TheGimp]

No proper chassis ground. 2 prong non-polarized cord by the looks of it.

By the looks of the left unit (which appears to have the PS for the right unit and ac out for phono) it has a power transformer and choke. In which case it is relatively safe but still needs a fuse and 3 prong power cord properly installed.

Once that is done, proceed with isolating the fault. The proper three prong grounded power cord may actually solve your problem.

You have used isolated RCA connectors so they are not grounded at the entry point unless you added a ground. That should help break any ground loop. You may find a bypass cap to ground at the RCA connector helps.

 

[tcd5m]

Here's what time would permit me to try:
1) Disconnected the ground from the RCA's at the Hoffman Amp Chassis. Noise is still there.
2) Verified no short in the RCA plugs with chassis ground

This weekend I'll try installing a 3 pronged power cord. Also, there is already a fuse, but wasn't shown in pics.

Attached is a pic of the underside showing RCA w/ground disconnected and a cheesy schematic of the input.

Thanks for the replies, it gives me something to move ahead with,
John

 

[VictoriaGuy]

Just dropping in here (late to the conversation).
The amp was working fine using the DIN inputs from the preamp?
The working theory is that adding the RCA connections caused the 'thump-thump' to start?

Perhaps it is just a coincidence, and another problem is surfacing here? Or you have disturbed a connection while wiring in the RCAs?

Disconnect the remaining RCA yellow (signal) wires from the DIN jack...ie go back to the 'before RCA' stage.

Does the amp work properly now?
If 'Yes' then the amp is looking for 2 signal wires with common ground tied to chassis at the DIN jack. And the DIN cable should be connecting back to the preamp the same way the RCAs would.......
Start adding back the RCAs- ground first (tie the two RCA lugs together then to DIN gnd with the shortest wire possible.). Test with input on DIN.
The add short jumpers from the RCA signal connectors to the DIN.
Test.
Short the RCA inputs to ground. (RCA shorting plugs are handy and can be made in a minute from an old/cheap interconnect.)
Test
Connect RCA cables to preamp- preamp power off.
Test

etc

etc

 

[tcd5m]

Thanks all! The suggestions helped narrow down the problem. The problem seems to have been caused by the long ground wire running from the RCA plugs to the DIN ground connection. I used a single short wire and the problem went away.

Thanks again,
John