Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim dresser on March 12, 2021, 04:43:15 pm
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When the reverb sw1 is off there is significant hum. However, when the sw1 is in the on position there is no hum. All ch 2 functions work ok. Trem is ok. Reverb is ok, but sounds very microphonic and sensitive which is typical. Could the sw1 cause this hum? Thanks for any ideas.
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When the reverb sw1 is off there is significant hum
when the sw1 is in the off position there is no hum
Off = no reverb?
anyway, check the ground from tank "out"
Ohm from the tank to chassis
verify the in is in and out is out
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Correction:
Reverb sw1 off = hum
Reverb sw1 on = NO hum
Additional findings and work done:
Loose reverb spring hold down string secured and reverb assy looks solid - Done
Verified reverb tank is grounded - Done
Reverb out is grounded per schematic, however someone added a 2.2M resistor across reverb out to gnd, removed this resistor - Done
Two 27K resistors in driver ckt drifted upward to 45k-54k, respectively, replaced both - Done
Tube V4 pin 2 missing 68K resistor to ground, added the 68k to gnd - Done
Reverb depth pot/sw dirty, cleaned with deoxit. Now sw and wiper working ok
Continuity verified chassis to all reverb ckt grounds and pt to pt signal path wiring
Will test again with correct plate resistors when parts arrive. After replacing the two 68K resistors R45, R46, all voltages including - bias measure reasonably close to schematic values. The hum is low with reverb sw off and eliminated with reverb sw on. Reverb is now more under control using the depth pot.
The 1484 is fun but a challenge to work on. Thanks to all on this forum for your support.
3/21/2021 Update:
The plate voltage measured at V4 pin 6 today is only 70v, schematic indicates 160v. Continuity is present V4 pin 6 all the way to the R59/C25C node. The reverb turns on at sw1 with a loud pop and the depth is nearly full on. Adjusting the pot clockwise balances the wet/dry mix, actually well. When ch 2 vol is zero the reverb is max!
Monitoring the plate voltage at V4 pin 6 during power on, voltage is in the range of 300+ volts and drops to 70v as the amp warms up. This location has measured 170v and 159v on different times. I don't have a capacitance meter and I suspect the 5uf in the cap can is not holding a charge. All other voltages elsewhere are close to indicated on the schematic.
It continues...
Update 3/22/2021
Above issue with low plate voltage was my errors. Low plate volts on V4 pin 6 caused by connecting the load resistor to wrong location. Now 170v. Other voltages are a little high, too so some resistors will have to be changed.
Will measure can cap values 3/23/2021.
The 2.2M resistor was added by someone to prevent the loud pop when turning on reverb sw1. It works.
So, I'll change some resistors, check cap for leaks, and live with the level of ambient noise for now.
Thanks
Update 3/24/2021
All cap values including cap can measured correct with cap meter. So after changing out only 4 resistors that were much greater than spec, there is no reduction in hiss or hum achieved with reverb sw1 off. Sw1 ON, no hum but hiss still present. All grounds are solid and ring to chassis. All inputs are shorting jacks with metal film 68K resistors on the jacks. The 1M resistors are carbon comp but measure correct value. Plate voltage at V1a,V1b is now 120v, schematic indicates 100v. I can change B+ dropping resistors to lower the plate voltage at V1a, V1b. The amp plays and sounds good, just hissy. A little too much hiss for recording close mic'd. I know noisy resistors can be detected with a signal tracer, but I don't have that device.
Suggestions?
3/25/2021 Update
The hissing was caused by R27, 220k resistor. Replaced this resistor and the hiss stopped. Hum is still present with reverb sw1 off. However, the hum can be reduced by carefully moving AC wires which connect the on/off sw4, fuse, and transformer, toward the center of the chassis . Hum eliminated when reverb sw1 in on. The work continues.
Thanks.
4/5/2021 update:
During power on troubleshooting, C27 and D2 failed. I had been discharging the caps without using a large value resistor so maybe I caused this to happen. The 4 caps and diodes in this ckt were replaced. The amp was checked for shorts, none were detected, and the amp was powered on through a current limiter. The amp plays in ch 1 and ch 2 and has normal trem speed and intensity. R58 and R59 were replaced with 82K resistors to reduce B+ by ~ 10v.
Now hum is gone but no reverb when sw1 turned on. There seemed to be a path for raw guitar signal through the tank to V4 pin 2 when sw1 is on. But no reverb! So examination of the tank assy shows much of the insulating material on the drive end brass terminal had flaked off. I used paper to replace this at the 4 locations.
Chop sticking the reverb tank spring assy, sw1 on, creates low volume splash noises at driver and recovery ends and along the spring. However, no typical verb effect is observed. C18, C19 read .02ufd on the LCR meter. V5 pin 1, 6 has 225v.
4/13/2021 update:
Each piezo measured a small capacitance value which indicates no shorts. Completely insulated the copper end brackets and the piezo transducers. The spring center was secured to the board and the ends were attached to the end brackets. The reverb began to function using the depth control. V4 pin 6 plate voltage is 164v. Voltage on R45, R46 is 225v.
Thanks
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besides what was already suggested, is the reverb switch jack isolated or connected directly to chassis? might could want to be isolated and have the ground connected to wherever the reverb driver/recovery stage is grounded
from what i understand (i wasn't around back then), folks were a lot more tolerant of hiss and hum back in the "good `ol days" (i've heard some folks say "that's how we knew it was on and working")